Patent Application
20090310731
1. Field of the Invention
In a general sense, the invention is related to fusion power systems. More particular the invention is related to single-pass, heavy ion fusion, system and methods.
2. Background Information
The heavy ion driver defined in 1975 by R. L. Martin and A. W. Maschke used the known abilities of high-energy RF (radiofrequency) accelerator systems to store megajoule quantities of ion beam energy and to focus this stored energy on very small spots. They saw that the short stopping distance of nuclei with high atomic number (Z) at one-half the speed of light meant being able to create the energy density in small targets containing fusion fuel as needed to ignite small clean-fusion explosions and produce fusion power. And they showed that the continuous stored beams could be rearranged into multiple bunches, compressed in length, and delivered to the targets in short duration pulses as required by the dynamics of the fusion ignition and burn processes.
Beams of protons can be accumulated—and stored—over a long period of time, as the protons resist processes that cause them to wander from their controlled paths, such as knock-on or multiple scattering, and have low probability of changing their charge to 0 (neutral) or negative (H−). On the other hand, the probability of the charge state of a heavy ion changing by collision with an atom remaining even in a very high vacuum requires ignition pulses be generated in a fraction of a second. This is consistent with the need for an ICF (inertial confinement fusion) power plant to pulse frequently, and pulsing many times per second is routine for accelerator systems. However, the need to generate an ignition pulse within a limited time places a constraint on the accelerator technology that eliminates slow pulsing machines like synchrotrons.
Thus, at the inception of heavy ion fusion (HIF), a few principles were established:
Accelerating heavy ions solved the problem of depositing the megajoules of beam energy in small fusion targets. The beam energy also must be delivered to the targets in pulses with the short durations, e.g. of the order of 10 nanoseconds, consistent with the timescale of igniting small fusion explosions by rapidly compressing and heating to ignition so that fusion burn is effected before the compressed and heated fuel is able to fly apart. Using processes verifiable by the same analytical tools at the root of the design of all successful accelerators, Martin, Maschke, and others defined examples of systems to reconfigure the beams and deliver them to the target on this time scale.
Key to the repeatable, reliable, and efficient generation of tightly focusable beams of high-energy ions are physical “conservation laws” as firm as the more familiar relationships E=mc2 or F=ma. These physics constraints, stemming from the same basis as the well-known field of thermodynamics, are summarized in the statement that the final focusability of the beams can not be better than that defined by the volume of “6-dimensional phase space” defined at the start of the beam generation process. Four of the six dimensions of this “space” are the two conventional, Euclidean dimensions transverse to the direction of the beam, coupled with the angles of the trajectories relative to a nominal ion on the axis and moving parallel to it. The two other dimensions are the difference between the energy of a particle and the nominal (ideal) energy and the width of the phase space ellipse on the time axis.
“Ballistic” focusing of charged particle beams is analogous to focusing beams of light: the spot size depends on the parallelism of the particle's paths coming into electromagnetic lens, the aperture of the lens, and aberrations. The effect of focusing a particle beam that has a range of momentum per particle is similar to the “chromatic” aberration of focusing light with a variety of wavelengths (or photon energies, or “colors”), shown graphically in the spectrum from a prism, and the term chromatic aberration also is used in “particle beam optics”.
The individual current amplification processes and proposed HIF “point” designs were intensely vetted from 1975-80. Validation of the beam compaction processes led to a shorthand manner of summarizing their individual contributions by the following equation showing how the total beam current delivered to the target results is built up from the current produced by a single ion source:
I
target
=I
source
×N
sources
×N
injection
×N
compression
×N
beams
on
target. (1)
The beam power on the target is the product of the current of particles and the energy per particle. Ignitor pulse power of about 1 PW (one petaWatt or one billion megawatts) is needed for ignition. That can be provided, for example, by 20 GeV ions with a total current of 50 kA (kiloamperes), divided among some number of beams.
Another means of amplifying the eventual current (introduced in 1978 by Burke) accelerates ions of multiple isotopes. This method effectively multiplies the 6-dimensional phase space available to the designer, since the physics constraint applies separately to each isotope. The motivation for the multiple isotope technique was to gain design margin and relieve pressure on other techniques for beam amplification/compression/compaction. However, the potential ways to use this additional design factor to best advantage were not aggressively explored, and only formally adopted in an internationally vetted “point” design in 1995-97.
A single-pass heavy-ion fusion system includes a new arrangement of current multiplying processes that employs multiple isotopes to achieve the desired effect of distributing the task of amplifying the current among all the various processes, to relieve stress on any one process, and to increase margin of safety for assured ICF (inertial confinement fusion) power production. Energy and power of the ignition-driver pulses are greatly increased, thus increasing intensity of target heating and rendering reliable ignition readily attainable. The present design eliminates the need for storage rings. Further innovations are to give the HIF (heavy ion fusion) Driver flexibility to drive multiple chambers in the most general case of different total distances between the linac output and each of the various chambers. Using multiple chambers steeply decreases the pro-rata capital investment and operating costs per power production unit, in turn decreasing the cost of power to users.
A single-pass heavy-ion fusion system includes a new arrangement of current multiplying processes that employs multiple isotopes to achieve the desired effect of distributing the task of amplifying the current among all the various processes, to relieve stress on any one process, and to increase margin of safety for assured ICF (inertial confinement fusion) power production. Energy and power of the ignition-driver pulses are greatly increased, thus increasing intensity of target heating and rendering reliable ignition readily attainable. The present design eliminates the need for storage rings. Further innovations are to give the HIF (heavy ion fusion) Driver flexibility to drive multiple chambers in the most general case of different total distances between the linac output and each of the various chambers. Using multiple chambers steeply decreases the pro-rata capital investment and operating costs per power production unit, in turn decreasing the cost of power to users.
The evidence now is overwhelming that a new source of energy that is both clean and abundant must begin replacing fossil fuels in about ten years, and have scaling properties that allow worldwide build-out to meet energy and environmental needs by 2050.
Properly sized fusion sources each will produce the equivalent energy flow of a “supergiant” oilfield. The cost per unit of energy product from the HIF heat source will be affordable, clean energy at or below the current cost of coal, as one capital intensive fusion Driver will serve a multiplicity of power chambers. The cost of a HIF heat source will be comparable to the cost of developing a large oil field, of which very few rise to the supergiant category, and have a rule-of-thumb annual operating cost of 10% of capital cost, over a long lifetime. The cost of fuel raw materials is negligible, and fuel cost is associated with the capital cost of the fuel processing systems.
At least three phenomena comprise the basis of the Fusion Energy Enterprise.
The physics of dynamic, inertially-confined fusion ignition and burn are known technology. Scaling laws specify the requirements of power deposition intensity, the amount of energy input in a given time to a given mass of target material. Scaling down requires increasing degrees of fuel compression. Compression is challenging, but experiential data confirms computer models. ICF (inertial confinement fusion) Drivers using beams of high-energy heavy ions (the HIF Driver) avoid whole arenas of issues. One major arena is the interaction of intense laser light with matter. Similarly, the limits of pulse energy, repetition rate, and efficiency are factors of ten greater for high-energy heavy ions as for laser beams.
Beams of high-energy heavy ions (the HIF Driver) have been known since the 1970s to be capable of meeting the requirements for fusion energy release. Over the intervening three decades, the advance of the state of the art in a wide variety of technologies, electronics, control software, modeling and design software, and more, has removed time from the to-go schedule for fusion power production.
Although much cleaner and more abundant in principle than fission energy, fusion energy is nuclear energy. Design of the reaction vessel must avoid the disadvantages of materials activation and degradation by neutrons from the fusion reactions. A crucial feature of the ICF (inertial confinement fusion) approach is the stand-off distance achieved by beaming ignitor pulses via energy carriers, such as ions of high atomic number or photons. The complex means of achieving fusion energy release are physically separated from the reaction vessel. This freedom of design for the containment vessel has enabled configurations that avoid materials activation and degradation to the extent that lifetimes will be at least thirty years.
New terms are coined where indicated to facilitate description by removing the ambiguity that is unavoidable as a result of using existing terms for new purposes. In particular, “beam compression”, “beam compaction”, and the like apply to the whole beam generation process and to each of the steps that contributes to the process. Where new terminology is used, the convention will be to capitalize the terms. In addition to the novel terminology, the following lexicon includes some conventional terms to clarify possibly subtle meanings and as a convenience for the reader.
Density×Length=rho·R=0.2−0.5 gm/cm̂2
The length parameter decreases as density increases. For spherical geometry (similar for cylindrical), the mass that must first be heated to ignition if propagating burn is to start is:
Mass=Volume×Density=(4/3)πR̂3·rho
The parameter has key implications, most centrally the required degree of fuel compression.
In terms of the propagating burn parameter, the mass is:
R̂3·rho=(rho·R)̂3/rhô2
Thus,
Mass=Constant·rhô2.
In terms of the characteristic dimension, of interest relative to technological capabilities for expediting propagating burn:
R̂3·rho=R̂2·(rho*R)
Thus,
Mass=Constant/R̂2.
The energy that must be deposited to raise the burning fuel is ˜kT times the number of particles in the plasma fuel, in standard fashion. To reduce the amount of fuel that must be ignited, to bootstrap surrounding fuel into propagating burn, increasing the density is the mechanism.
From these relationships, a critical advantage accrues for heavy ions to accomplish Fast Ignition with Telescoping Beams. For instance, the Isotopic Species for the Fast Ignition Pulse may be selected to heat a tailored mass of precompressed fuel.
With the conventional definitions for the longitudinal phase space, the horizontal axis represents time and the vertical axis represents momentum. The phase space of a collection of particles (in this case, heavy ions) is “a constant of the motion”. In an RF accelerator, the phase space of the bunches evolves as in an elliptical shape that can be squished on one axis and will respond by stretching on the other axis.
If a bunch is tall and skinny (as all are in the above graphic), that means the momentum spread is at a relatively large value and the time spread must be correspondingly at a relatively small value. Momentum spread results in chromatic aberrations, which must be within some limit (like 1%) if the bunch is focused to a small spot. If the momentum spread is too large, the chromatic aberrations may be the parameter that determines spot size.
If a phase space ellipse is left alone to drift, the higher momentum particles will move ahead and the lower momentum particles will fall behind. The effect is that the ellipse will shear along the axis.
Whereas “bunch rotation” connotes “laying the bunch down” on the time axis to minimize the momentum spread at the expense of time spread, bunch reflection rotates the bunch into its mirror image in either axis. Since it is not physical to reset the position of the bunch in time, physically, the reflection is done by shearing the bunch via the applied electric field—that means that the leading tip that is at the highest momentum spread is sent down through the axis to an equally negative momentum spread. Thus, the particle at the leading tip which has been fastest becomes the slowest and begins falling toward the back, while the particle at the rear that was the slowest becomes the fastest and begins moving toward the front.
For illustration, the HIDIF design rotates the bunch after it shears in phase space during a drift distance of 160 m. With the same parameters, a reflector would be needed every 320 m. It will be a bit easier technologically to reflect the bunches more frequently, as the HIDIF pushes the phase width of the bunch at the time when rotation is applied to the extent that they have to fabricate a sawtooth waveform to knock the ellipse down—i.e., to rotate it. They do that to get the longest length along the time axis, and therefore the lowest momentum spread. What we want to accomplish can be done with much simpler demands on the RF waveshape.
The HIF Driver delivers an Ignitor Pulse via a practical number of beams to the entrance ports into the Reaction Chamber (e.g. eight beams total, with four on each of two sides). The salient features of the chamber embody precautions taken to convert the 14 MeV neutron energy to heat without reaching the chamber 2000 walls. As shown in
The lithium sabots 3000 also shield the fusion fuel targets at cryogenic temperatures from the elevated temperature in the reaction chamber. The fuel-transporting sabots may be variously shaped and configured, with appropriate access holes 3001 for the heavy ion beams. In the embodiment of
The reaction chamber 2000 can have various shapes from spherical to cylindrical to composite shapes of various conic surfaces.
The heated lithium cools from a plasma state and eventually condenses in a series of phases, and the chamber is back to its ‘cool’ state ready for another reaction to take place in a fraction of a second. This requires pumping tons of lithium per pulse to cool and protect the chamber walls, e.g. approximately five tons for fusion releases of two BOE (barrel of oil equivalents) each, or 50 tons for twenty BOE releases. The heated lithium goes through the heat exchangers and returns as cool fluid to cool the chamber and re-establish the vacuum (low gas density) necessary for the ignitor beam to propagate across the chamber radius to ignite the next fuel target.
The total mass of lithium for each fusion pulse, injected into the chamber at flow rates tailored along the chamber's length for the desired temperature history, is sized according to the integrated scheme of fuel sabot injection, ignitor beam passage, fusion energy containment and conversion, expansion of the lithium, extinguishing the plasma, further cooling to heat transfer temperatures, and restoring the required pre-pulse environment. These phases compare to the processes of an internal combustion engine operating on chemical combustion:
Neutrons are insulated from the chamber walls by flows and sprays of low temperature lithium returned from the heat exchanger 3001. A large chamber for producing 100 BOE, or more, per minute provides adequate gas dynamic expansion. The volume of the plasma that forms upon ignition of the fuel pellet at the center of the Lithium may be about 1440 cubic meters. Microseconds after the pellet undergoes Fusion the lithium surrounding the fuel pellet has vaporized to become Plasma whose energy is being harvested by direct conversion to electromagnetic fields and electric currents.
Further cooling and chamber wall protection is accomplished by filling the chamber volume with sprays of liquid lithium droplets. Out to a certain distance from the fusion burn, this lithium becomes part of the plasma. Further out, lithium is even vaporized. Lithium covering the walls protects the walls by ablation, and the lithium beneath the ablation boundary maintains the walls at the modest temperature of the lithium returned from the heat exchanger 3001 subsystem. Heat is not extracted through the main walls of the chamber, as the bulk of the heat flows towards the ends of the cylindrical expansion volume. The lithium working fluid progressively cools by interaction with lithium sprays along the axis of the cylindrical chamber, and condenses beyond the direct conversion zone. Condensed, hot lithium comes in contact with the primary heat exchanger 3001 and heat is transferred to a secondary fluid for use in processes located outside the primary containment, defined as the lithium boundary.
Exhaust of fusion reaction products concerns primarily the helium and tritium produced. Tritium is needed to fuel later D-T (deuterium-tritium) pulses. Tritium containment also is the chief radiological hazard of the entire HIF power system. The large body of knowledge regarding tritium safety is clear on the engineering requirements. The HIF chamber system economically accommodates several layers of redundant features to assure tritium safety.
Prior to the next energy release, the low temperature lithium acts as a getter pump to scavenge lithium vapor left behind by the power and exhaust dynamics.
The temperature of the lithium progressively decreases as it functions to:
to convert energy to electricity in non-contacting, direct-conversion processes.
Lithium in liquid form at different positions in the reaction chamber experiences temperatures as low as 200 degrees Celsius to temperatures as high as 1200 degrees Celsius each time a pellet ignites, not counting the room temperature lithium of the fuel sabot or the temperatures of this and immediately surrounding lithium during the plasma state. This heat flux, along with the electrical energy extracted by direct conversion, is the major product of the fusion reaction. Secondary heat exchangers convert this heat to other products such as hydrogen gas for use in producing synthetic fuels, steam for use in conventional steam turbines, and heat for the desalinization of water by evaporation.
An external view 9000 of a cylindrical reaction chamber 9001 and its primary heat exchange system 9002 is shown in
Because tritium is released to the working fluid during the reaction it must be recovered to meet governmental radiation safety standards and to provide the Tritium necessary for subsequent reactions. To assure that no Tritium is accidentally released to the environment, the whole of the reaction vessel and its heat exchangers is typically enclosed in a secondary containment vessel. This vessel may be filled with a gas that is not reactive with Lithium, for example Argon. Supporting activities for the reaction vessel 2000 include:
Telescoping is exploited, e.g. 10 Isotopes for tenfold increase in working volume of 6-dimensional phase space. State of the art source technology is used.
A State of the art Preaccelerator HVDC of ˜1 MV is used, cf. Argonne National Laboratory 1976-80. A Linac emits multiple parallel beams, e.g. four.
Stacking in transverse phase space uses a low number, e..g. two in each transverse plane. Ignitor Pulses are generated with once-through accelerators and beamlines. Storage rings are not used. Microbunch structure is maintained all the way to the fusion fuel target, i.e., identity and integrity of each RF microbunch of ions is maintained. Macropulses of individual isotopes, called Slugs, contract (called Snug) due to differential acceleration in Snuggers, e.g. ±5% to ±10% of the nominal speed, using successive blocks of linear accelerator tanks operating at progressively higher frequencies, e.g. from 400 Hz for first block and 4 GHz for the last block.
The last sections of the Snugger, called the Snug Stopper, reverse the sense of the input Snugging voltage to return the nominal speed of all microbunches to the nominal speed of the Isotopic Slug. The beam passes through a Helical Delay Line that removes space from between Slug centroids by magnetically switching out successive Slugs from successive coils of the Helix, at programmed times such that, when they are reinjected into common beamlines, they take the next programmed-step of power amplification.
This set of beamlines, e.g., four beamlines, continues to switch points that route the beams to one of the multiple fusion chambers. The differential distance to multiple fusion chambers is accommodated by the central timing program for computer-controlled operation. To provide two-sided target illumination, a set of two Slug Trains, each comprising a Compression Pulse and a Fast Ignition Pulse, are produced in series by the target for both Slug Trains. The accelerator may be timed such that drift distances and other parameters for Snugging and Telescoping simultaneously achieve maximum intensity timed in coordination with fuel target timing.
A low factor of emittance multiplication, e.g., 2.5×, realizes a step-change improvement for low emittance at the fusion fuel target. The Fast Ignition requirement of small spot diameter is enabled by the smaller emittance. Chromatic aberrations are controlled within practical limits by conservation of longitudinal phase space RF of the beam structure at the microbunch level, e.g., 1% momentum spread in the final focus lens.
Overall RF-based coordination produces and delivers Ignitor Pulses to fusion targets on absolute, end-to-end timing to the accuracy of a fraction of an RF period. Substantial timing errors are permissible, as the limit of the capability exceeds foreseeable requirements.
Programmed timing of the pulsing of the array of ion sources, HVDC, and RF power provides the large flexibility (bandwidth) of the design concept to dial-in the sequence of beam generation processes in the computer control program.
It is instructive to regard the Driver design from the vantage point of the controls system. This especially aids design illumination by providing a common framework to describe the manner in which the individual processes function and the requirements to coordinate them. Referring now to
The above design provides the timing accuracy to cause the various dynamic processes of beam generation to culminate at fusion fuel targets with including power profile and aiming, at fusion targets power profile and to meet the targets as they move through the target zone. The design also provides the timing flexibility required to achieve t specified Ignitor Pulse parameters, in Multiple Chambers. Overall Ignitor Pulse programming is able to vary the spacing of Isotopes based first on the speeds of the different ions a table of Isotopic Species. The timing for source gating is derived from the master clock of the RF synchronizer.
More will be said about each of the above components as the Description proceeds.
A beam diagnostics and accelerator controls system establishes accuracy of the arrival of the Ignitor Pulse to timescales for the Ignitor Pulse's temporal waveform, e.g. nanoseconds to tenths of nanoseconds. The accuracy of the absolute (“ZULU”) arrival time of the Ignitor Pulse is determined by the speed and rotation rate of the fusion fuel target as it falls through the bullseye.
The Driver is computer operated, using centralized Master Timing via the coordinating effect of synchronizing RF waveforms. Distributed timing control provides realtime corrective responses, using for example the ability (provided by the ionic speeds being less than control signal propagation speeds) to feed-forward data about the beam position and other parameters. The state of the art for the precise timing and control of RF fields extends to approximately one part in ten thousand.
Delivery of a high current short duration pulse to the fusion pellet target located in each of many chambers at various distances from the source is depends on the pulse structure of the ion source. The precise timing of each beam to each chamber is unique and accounts for the distance to the chamber for the specific beam, the properties of all of the switches and accelerators in the beam path, and the precise lengths of each of the delay paths. It also may take into account the differences in mass of the individual isotopic species used in the ion beam.
When the properties of the pulse at the target are defined by the energy release needs of the fuel pellet, the challenge is to amplify the source ion current via the pulse structure and the accelerator properties to the magnitude required by the ignition parameters at the target.
This amplification is dependent upon cascading a series of steps of current amplification as described in subsequent sections, but it is all dependent on the ion source current parameters and their precise timing structure as they leave the sources. The timing within the pulse structure 1202 that evolves as a result of the beam generation processes is set by the release of ions via grid gating at the source 1201. The heaviest ions are released first and are followed sequentially by each of the lighter species in descending isotopic mass order. One source for each of the isotopes is integrated into a compact structure called a Source Hotel 1201, as shown in
The ion source within a Source Hotel is gated to release identical duration macropulses 1300,
The sequence of Isotope Slugs for the Fast Ignition (Fl) pulse is emitted first (i.e., using heavier ions for the Fl Pulse than for the Compression Pulse), with the first Slug containing the heaviest isotope. Next, the Slugs for the Compression Pulse are released after a pause in time determined by the velocity differences between the Fl ions and the lengths of beamline determined by details of the series of beam generation processes. The timed release of each of the different Isotopic Slugs follows in descending isotopic mass order, with a schedule of delays between Slugs that is determined by the ion mass (which determines its speed in a series of isotopes by the Telescoping Condition of equal magnetic rigidity), the accelerator length, and the length of the beamline to a fusion target in a given reaction Chamber.
Each complete series of Isotopic Slugs forms a non-overlapping sequence of Slugs called a Slug Train. The total release duration for each Slug for the Compression Pulse (which many times the total energy as the Fast Ignition Pulse) is nominally 10 μsec and the overall release time Slug Train lies between 400 μsec and 500 μsec, depending upon the distance to the most distant reaction chamber.
In the first RF accelerator section, the Slugs continue to be accelerated as parallel beams with the Source Hotel's array. All the accelerating channels are on, regardless of which channel a Slug is in at a given axial location and time. Visualized end-on, the emission of Slugs from the individual channels is similar to a theatre Marquee with only one light blinking at a time in a pattern with complex but specific timing.
Immediately downstream from the Preaccelerator, each macropulse enters the first section of the RF accelerator and is imprinted with the micropulse structure. The strength of the accelerating field over the entire linear accelerator is higher for Slugs with higher mass, to accelerate the higher mass to an equal speed at each point along the linac.
Referring now to
The first RF accelerator is a multi-channel radiofrequency quadupole, or RFQ, which integrates RF quadrupole electric focusing and acceleration. The RF field in the initia section of the RFQ provides strong focusing fields and smoothly increasing accelerating field to approach isentropic conversion of the DC incoming Slug beam into microbunches (μbunches) in a continuous stream at the RF frequency. For illustration, each μbunch contains a number of ions of the order of ten billion. An entire Ignition Pulse (e.g. carrying a total of 20 MJ of ions that carry 20 GeV (3.2 nanoJoules) each) contains about eighty thousand of these elemental, μbunch groups of the energy-carrying heavy ions. The purpose for continuing the Marquee in the first stage of RF acceleration is to delay bending the beam until the speed of the ions is able to efficiently use magnetic focusing to handle the space charge forces associated with high beam current. The initial speeds of the heavy ions for HIF Drivers (i.e., in the front end) are especially slow because, to achieve the brightest beam, the preferred choice is for the ions to be singly charged.
After the ion speed is raised in the RF accelerator section with the Marquee array of parallel Isotopic Slugs, the beam is fed to an accelerator section operating at twice the frequency of the RF Marquee, e.g. 12.5 MHz. Between the two RF structures, the beams from the Marquee are Aligned for insertion into the 25 MHz structure as a collinear Slug Train. The array of the Aligner's magnetic beamlines, e.g. sixteen (nominally ten for the Compression Pulse and six for the Fast Ignition Pulse), are routed, one each, to a corresponding series of AC switch magnets (one on the Aligned beamline for each Slug) that bend the Slugs into a common, Aligned magnetic transport channel, in a Slug Train with the specified time structure. Prior art also describes an alignment process that integrates the interleaving (or funneling) of microbunches at the frequency doublings. Prior art further describes a process of interleaving two beams that smoothly integrates with the design of an RFQ accelerator. Using this concept, the Aligner also doubles the average current of a Slug.
The beams emerge in the higher frequency RF structure downstream operating at 25 MHz (e.g., a second RFQ) with twice as many micropulses in each Slug, and half the number of parallel beams. The beams continue into the next structure and upon emergence are interleaved with an adjacent beam once again thus again doubling the number of micropulses and halving the number of beams that need to enter the next linac section. After each subsequent acceleration section the beams continue to have their micropulses doubled by interleaving until four beams remain at the end of the 200 MHz accelerator.
With interleaving repeated at each of the frequency steps, e.g. five, the current of each Slug multiplies by a factor of thirty-two.
The final portion of this linac section, called the Telescoper, has a pulsed switch magnet for each of the Slugs. The switches are located where the Slug in questionreaches the specified Common Beam Rigidity. Once that magnetic stiffness is reached, they are removed from the accelerator and fed into a Telescoping beamline, i.e., a magnetic beamline in which Slugs of the same stiffness but different speed are able to catch up to each other. The following (faster) Slugs for the Fast Ignition Pulse are fed into an accelerator with twice the frequency (e.g., 400 MHz), but are not interleaved, and continue as four parallel beams of Slugs with RF-synchronized microbunch structures. The final portion of this linac section is, again, a Telescoper, integrating a pulsed switch magnet (between linac tanks) for each of the at the point where the Slug in question reaches the specified Common Beam Rigidity, which is identical with Rigidity of the ions in the group of slower Slugs.
Once all slugs are out of the telescoper, the four beam lines are merged to form one beam line with four times The current. The radiofrequency microstructure of the merged beam is the same as for each of the pre-merged parallel beams, as is the SubSlug structure.
Next, alternating SubSlugs from the merged beam line are immediately switched into the start of a new beamline, which is bent into 360 degree loop, to arrive in RF synchronism with the next SubSlug. This Loop Stacking will use a series of two loops (sending four parallel beams downstream), or one (sending two parallel beams downstream). The result of Loop Stacking is to position multiple SubSlugs at precisely equal distances from the fusion target.
Downstream, the Slugs are the length of a SubSlug, and the SubSlug timing feature goes away. The number of parallel beams in parallel beamlines at this point (i.e., either two or four, in this illustration) continues to the Chamber and the fusion target, with one of the two SlugTrains magnetically switched into one or the other of two sets of the beamlines for two-sided target heating.
All operations beyond the Telescoper may take into account the fact that the Slugs are moving at different velocities relative to each other and thus are getting progressively closer together at the same time that the RF frequency of the Snugger is bringing the micropulse structure to higher and higher frequency. The Snug Stopper freezes the microstructure, but the Slugs continue to drift together until, at the target, they all arrive on their pre-programed schedule.
Specified RF waveforms are generated at low power by a Master and Subordinate Arbitrary Waveform generators. The Driver's RF Master Clock communicates with the Chamber controls, in particular those concerned with the dynamic injection of fuel charges in their protective sabots.
The total duration of beam emitted by the linear accelerator for each ignition pulse is, for example 200 μsec. Blank spaces in the overall beam profile are needed for a number of purposes, including:
Certain processes can exploit the same time gap as certain others. Thus, the required sum the time gaps may be less than the sum of the times of the gaps for processes individually. Prominent features of the design are specifically for the purpose of removing these gaps, including Telescoping of Multiple Ion Species and by the action of the Helical Delay Line.
The following list is in the approximate order in which the processes occur during generation of an Ignitor Pulse:
Generation of the Fast Ignition Pulse mostly uses the same Driver hardware as the Compression Pulse. Separate hardware is used for processes that are affected by a large difference in ion (and microbunch) velocity, which can provide important advantages for overall ignition efficiency:
The parameters that characterize acceleration in the linac follow the proven prior art, established by operating machines and designs using standard, industrial design tools. Linac output current is increased by using multiple, parallel, RF-synchonized output beams, e.g., four. Linac output further is increased at the front end by using established ion source and high DC voltage technology, e.g., Argonne National Laboratory 1977-1980.
The new arrangement of current multiplying processes makes strong use of accelerating multiple isotopes. The effect of using Multiple Isotopes, alternatively known as “Telescoping Beams”, can be appreciated by adding another multiplicative factor to the previously existing line-up of processes. However, ramifications of the present approach to exploiting beam telescoping lead to distinctly different types of current multiplier processes. Occurring in the driver system “downstream” (after) the linear accelerator, and under the constraints of the 6-D phase space as previously discussed, the different beam restructuring, beam compaction/intensification/overall current-amplification also favorably affect the ultimate focusing on the fusion target.
The primary new mechanisms are employed for the compaction of the beam after it leaves the linac. The new design also involves changes in features of the linac, which complement the improved beam reconfiguration design. Most novel are the features related to the use of a larger number of different Isotopic Species than previous HIF driver designs.
The Ion Source Hotel integrates many isotopic sources into a compact cluster of one for each Species, including both the Species for the Compression Pulse and for the Fast Ignition Pulse (if employed). The output pulses from individual isotopic sources are synchronized via a gate voltage in a programmed series to produce the basic building block of Slug beamlets in the specified sequence. The compact array of beams enables the HVDC column to continue the specified array of apertures.
HVDC source technology in excess of 1 MeV, e.g., 1.5 MeV demonstrated by prior art, viz., Argonne National Laboratory 1976-80. In conventional design practice, the peak current limit for transport in a strong focusing magnetic beamline increases with the five-thirds power of the momentum. Using commercial ion source technology and commercial HVDC sources, this feature contributes an important factor to increasing the peak current of each beam at the output of the linear accelerator. The compact array of beams enables the following Marquee RF Linac to continue the specified array of apertures.
Marquee R F Linac: The Marquee Linac facilitates acceleration of the space-charge dominated, low velocity beam by not significantly bending the beams at the lowest velocities where magnetic focusing fields are less effective. The Marquee linac structure has an array of parallel bore tubes matching the bore hole pattern of the Source Hotel and the accelerating column in the HVDC preaccelerator. Each tube in the Marquee carries only one Isotopic Species of beam. The pulsed beams of specified Isotopic Species ( also referred to as Sluggetts) occur in the array of bore tubes in the programmed temporal sequence imprinted at the ion sources.
Marquee Collapser (Aligner): After exiting the Marquee, the beams in temporal sequence exiting from parallel beam tubes in the Marquee are fed into a single beam tube, i.e., one tube per Marquee by a series of moderately fast switch magnets. The risetime of these magnetic switches is one of the chief determinants of the gap between Slugs. After the Collapser (Aligner), all of the accelerated isotopes in the specified order of Slugs a transported in a common line.
Telescoper: The multiplicity of isotopes is distinctively greater than the prior art. The internally consistent, end-to-end design is predicated on using many isotopes, e.g. ten. When an Isotope reaches the Common Rigidity, that Slug is switched into a Telescoping Beamline, i.e., a beamline in which Slugs get closer together as they move forward. Heavier isotopes are switched out of the Telescoper first. The isotopic masses of the multiple isotopes range approximately ±5%, subject to the bandwidth limitations of downstream RF beam handling processes.
Timing features of the beam pulse structure are provided by generating a specified RF waveform covering each Ignitor Pulse, according to the overall distance from the ion sources to the fusion fuel targets in Multiple Chambers at different distances from the ion source, arriving according to a specified sequence that provides the desired Ignitor Pulse power profile. Gated emission of the various Isotopes from their respective is coordinated with the master RF waveform.
For illustration, at the linac output, each of four active beam tubes emits 1.25 A.
Merging: The multiple beams exiting the linac are merged in transverse phase space, amplifying the current in a single beam by the number of linac outlet beams, e.g., four. Merging may be effected in a two-step process, which may be illustrated by using the example of four linac beams: (1)Merge beams two at a time into two downstream beams in one plane of transverse phase space, and (2) merge the resulting two beams into one using the other plane of transverse phase space.
The Merge (plus dilution factor) is the last process that necessarily increases the transverse emittance of the beam after its exit from the linear accelerator. Beams may be merged with economical use of phase, at a beam focus.
This introduces substantial improvement in the tightness of focusing of the beams on fusion targets compared to the prior art. Although maximum target heating is the first priority, reduced beam emittance alternatively may be exploited to give relief to the parameters of the final magnetic lens system.
Loop Stacking: The purpose of Loop Stacking is to balance the burden of overall current multiplication between processes that operate in the transverse phase space and those that operate in the longitudinal phase space. Loop Stacking sorts successive sections of beam into parallel beamlines, in synchronism at the level of the individual microbunches in the beam sections in parallel beamlines, as needed for microbunch structure to be maintained in common RF structures with multiple bores for the parallel beams.
The following illustrates a case of Loop Stacking. The structure of the beam emitted by the accelerator is specified with each Slug subdivided into four SubSlugs, separated by time gaps adequate suited for the risetime of switch magnets. The first SubSlug is switched into a beam line that completes a circle to return the SubSlug to the vicinity of the switch and into parallelism with the original beamline. This first set of two parallel beams are switched into a second loop with twice the circumference of the first, which joins in parallel with the beamlines containing the the third and fourth SubSlugs.
The total instantaneous current of the multiple propagating Slugs has been increased, and the space between Slugs has been increased. The enlarged space will be removed by the Helical Delay Line (HDL). Beam amplification has been accomplished by using the transverse phase space. The longitudinal phase space is unchanged in principle, and growth by dilution will be determined by the precision of the RF fields that maintain the microbunch structure.
The multiple beam configuration established by Loop Stacking continues to the fusion fuel target, with Slug Trains routed to arrive at the target from a specified number of directions, e.g., two. The choice for the location of the stacking loops from a number of possible positions along the beamline depends on the technology trade-offs associated with propagating a single beam (viz. after Merging the multiple beams from the linac) or as multiple parallel beams (viz. as created by Loop Stacking). This consideration is relevant to the beam configuration input to the Snugger and the Helical Delay Line.
As shown in the diagram of
The microbunches inside each Slug are virtually identical at the input to the Snugger, which imparts a progressive speed differential amounting to, for example, ±5% to ±10%, to the first and last microbunches relative to the unchanged speed of the center bunch. When Snugging has reached practical technological limits, the Snugging process is reversed and the speed differential is removed in the Snug Stopper.
As shown in
Both differential acceleration and differential deceleration result from the Snugger's RF field being offset slightly from the bunch frequency. To add the differential velocity, the Snugger RF frequency is higher than the bunch frequency at a given point on the beam path. To remove the differential velocity, the RF frequency is a specific amount less than the bunch frequency at that point in the beam path.
The Snug Stopper is shorter than the Snugger because its RF frequency is higher, e.g. 10×, and the higher RF frequency structures support an accelerating voltage gradient that is higher as approximately defined by the Kilpatrick limit. For the example of 10× Snugging with equal increase in RF frequency, the gradient of the Snug Stopper is about three-times higher than in the first section of the Snugger.
As shown in the diagram 2100 of
The empty space that grows between the slugs subsequently will be removed via the Helical Delay Line, subject to the risetime of the switching magnet and downstream timing requirements for ignition in Multiple Chambers.
No net power is added to a Slug by Snugging. Excitation of the accelerator structure is the primary power requirement. However, beam energy flows to the RF fields during deceleration, and from the RF fields to the beam during acceleration. A modest part of the shifting energy may be recycled by RF system design refinements, but the energy consumed by the Snugger in excess of the excitation “copper loss” will be a small fraction, e.g. 1-5%, of the energy consumed by the primary linear accelerator.
The efficiency of using the provided RF accelerating field strength gains when ions experience the amplitude near the peak of the sine wave. In opposition to this argument for using a large excursion of phase angles is the desirability of a linear progression of the differential acceleration of successive microbunches. For illustration, nearly linear progressive increased acceleration/deceleration would restrict the phase width to ±30 degrees. A larger phase shift will decrease the peak RF voltage and/or the length of the Snugger accelerator. The Snugging uses the rising side of the sine wave, which provides the phase stability effect.
Cradling is a feature incorporated into the control of the RF waveforms to increase the usable phase width in Snuggers and Slickers. The Cradling effect shifts the RF sine waveform to compensate for the curvature of the sine wave as the differential speeds increase in the microbunches as the Slug passes through a Snugger, or to a much lesser extent in the Slicker. Control of the waveform for Cradling is integrated with parameters from detailed design and modeling. Cradling increases the efficiency of the Snugger and Slicker accelerators, primarily to reduce cost, although the power used by these components is a small fraction of the total required to run the Driver.
When the Snugging action reaches a technical limit or otherwise desirable stopping point, the Snug Stopper removes the differential energy spread by reversing the differential acceleration process. A primary technical consideration is the existence of high power RF sources at the frequencies of the Stopper. Another primary design restriction is the diameter of the bore tube, which decreases with increasing RF frequency. For illustration, starting the Snug with a 400 MHz RF and stopping the Snug with 4 GHz RF will shorten the Slug by a factor of ten, and transmission through a bore diameter on the order of 2 cm.
Microbunches enter the Snugger at the bunch frequency emitted by the linac, as defined by the linac's RF output or penultimate frequency. The highest bunch and RF frequency in the Snug Stopper will be approximately 4 GHz.
Timing and waveform control in the Snugger provides the synchronized sequence of RF frequencies that are are progressively increased in blocks of accelerator sections, and increase in each accelerator section to accommodate successive Slugs with progressively higher nominal speeds. These required bandwidths correlate with the range of speeds of the Multiple Isotopic Species.
One design-optimization trade-off concerns the number of different RF frequencies used. For any given frequency, individual microbunches move toward the zero crossing point of the RF waveform, and experience a smaller fraction of the peak accelerating (or decelerating) voltage gradient. By increasing the RF frequency of succeeding Snugger sections, the voltage gradient experienced by the first and last microbunches can be periodically reset to the original phase angle. Thus, the utility of many frequencies is to achieve more efficient use of a length of Snugger and the RF power that drives it.
The state of the art of accelerator structure and RF power design and manufacturing makes it practical and economical to use a substantial number of discrete frequencies. However, the multiplicity of frequency changes will experience diminishing returns, and the number of frequency changes used is a question appropriate for detailed design.
Control of the waveform for Cradling is integrated with parameters from detailed design and modeling.
Snug Stopping removes the velocity differential when the process has reached the practical limit set by the diameter of the bore-tube that the beam must pass through. Beam scraping is to be avoided, and simulations of particle beams famously cannot model beam “halo”, however it is noted that the high quality beams will be focused to millimeter and submillimeter diameters downstream. The workhorse S-band structure of SLAC's 2-mile linac is an appropriate illustration. The structure's bore is about 2 centimeters, which seems ample for clean passage of the heavy ion beam.
Microbunches progressively compress axially to fit similarly on RF waves with decreasing RF periods. The momentum spread within microbunches increases proportionally. However, after the microbunches are released from phase focusing after the Slicker, they shear in longitudinal phase space, the phase space ellipses stretch in the time dimension, and their instantaneous momentum spread shrinks. This behavior is exploited by the Slicker, at a later point on the beam path.
Shown in the diagram 2200 of
The length of each coil 2203 of the HDL is of the order of the distance between the centers of successive Slugs. However, timing of the magnets 2204 for switching individual Slugs out of the HDL accommodates any Slug spacing greater than the time of the orbit around the circumference of one coil of the HDL. The first Slug in a Slug Train traverses the full length of the Helical Delay Line before its exit point. Successive Slugs of progressively faster ions exit the HDL sequentially, after traversing progressively fewer turns of the HDL. The exits 2205 for the various Slugs are approximately at the same azimuthal point on the HDL 2201.
Large fractions of the inter-Slug gaps, including the enlargement of the gaps due to Snugging, are removed when the Slugs exiting the HDL are switched back into the common beamlines that continue to the Chambers.
The microbunch spacing is static from the Snug Stopper downstream to the Slicker associated with each of the Multiple Chambers, to accommodate:
Locating the Snug Stopper upstream from the HDL 2201 allows the HDL to transport beam with the small momentum spread inside individual microbunches.
Maintaining the microbunch structure and preserving the 6-dimensional phase space of individual bunches is a hallmark feature of the new Driver design.
Telescoping and Snugging are the key dynamic beam generation processes. Telescoping is grossly programmed to culminate at Multiple Chambers via appropriate differences in the timing of emission from Multiple Isotopic ion sources. Precise timing is provided by the RF waveform control. Absolute timing of the arrival of a Slug at the target thus is extended to a fraction of the RF period of the lowest frequency RF accelerator, for example, control to 1% of the 100 nsec period of a 10 MHz Marquee Linac would give 1 nsec control of the Ignitor Pulse Profile.
The Snug Stopper permits microbunches to maintain relative positions as a Slug traverses the distance to one of the Chambers. At a specific location on the beamline before the target Chamber, the differential motion of the microbunches is restarted by the Slick process, which is similar to the RF process for Snugging.
At specified distances upstream from each of the Multiple Chambers, Slicking imparts specified, smaller velocity differentials back into microbunches of the various Slugs. Slicking is similar to starting the Snugging process but differs in that, after the Slicker imparts the speed differentials, the microbunches are released from the axial length constraint of phase focusing and the Slicking action is not stopped.
Conserving the longitudinal phase space area, the Microbunches stretch in time and narrow in instantaneous momentum spread as the various Slugs proceed toward Telescoping into the desired beam power profile at the fusion target.
The differential speeds imparted to the microbunches by the Slicker are initially specified so that all microbunches arrive at the target simultaneously, or with a desired spacing. Any effects of space charge to change the inter-bunch speed differential may partly be overcome by corresponding increase in the accelerating voltage of the Slicker. Space charge effects and errors in the Slicker's RF waveform will be responsible for any growth of the longitudinal emittance.
The effective minimum, total momentum spread is illustrated in
Ignitor Pulses are switched from the Manifold Beam Line into beamlines that terminate in the individual Chambers. Each of these terminal sections of beamline requires an individual Slicker. The Slicker imparts much smaller differential speeds, and individual Slickers (nominally the same for all chambers) for each Chamber is a small cost item.
The purpose of the RF Wobbler concept is to swirl the beam spot rapidly around an annular target, for purposes of smooth energy deposition density in the target. The RF Wobbler is located upstream of the final focusing lenses, where the beam diameter is small to correspond with the Wobber aperture. Where Isotopic Species that have a large percentage speed difference are used, particularly for the sequential processes of Compression and Fast Ignition, the block of Slugs for Compression must experience the Wobbler effect (for the spot to illuminate an annular shape), while the Wobbler effect must be off when the block of Slugs for Fast Ignition pass through, as the Fl pulse is aimed at the center of the target.
Using slower ions for the Fl Pulse, compared to the speed of the Compression Pulse ions, provides a space in time between the two pulses that can be used to turn the Wobbler on or off. For Cylindrical Targets in particular, the required Fl Peak Power decreases approximately linearly with the ion range. The range of energy deposition shortens with higher Z and lower kinetic energy. The sensitivity of design optimization to the choice of ions is not great, and choices of the relative mass of the Fl and Compression ions are driven by the practical consideration of immediate availability of the hardware, i.e., known and readily made technology.
For illustration, volumetric plasma xenon sources is commercial technology (ANL used this technology in key current and brightness demonstrations 1976-80.) Using xenon at Z=53 for the Compression Pulse, a number of heavier ions are good candidates. If lead is used for the Fl ions, and 20 GeV is the nominal energy of the multiple xenon isotopes for the Compression Pulse, the Telescoping Condition requires the energy of the lead isotopes to be in a range near 13 GeV. The shortening of the range in the pre-compressed fuel, of this example, is a factor of 6×-7×. The volume of the Fl heated mass of pre-compressed fuel may be made to be approximately the minimum (spherical) physical volume, containing the minimum mass to be Fl-heated. Quantitatively, the reduced Fl Pulse peak power requirement that results from the more optimum depth of the Fl-heated zone is a major reason for confidence in the operability of the new Driver design. Integrated optimization of the parameters for the Fl and Compression Pulses will achieve significant cost avoidance.
For illustration, the spot size required for the Ignitor Pulse Beams is found from the propagating burn parameter, rho-R, for example 0.5 g/cm̂2 (a conservative value). For fuel precompressed to 100 g/cm̂3 (a relatively safe requirement), the radius of the Fl-heated spot diameter needs to be at least 50 μm. Larger spots require more peak ignitor beam power and energy. Smaller spots require more compression, and higher beam brightness.
The Fl spot requirement is approximately a factor of ten tighter than for the Compression Pulse, as has been shown by reliable simulations. Prior HIF art held the Compression spot to be achievable, but hard to improve on. The use of the expanded volume in 6-D phase space provided by using a multiplicity of isotopes achieves the desired improvements, and makes the advantages of Fast Ignition safely within reach of the technology.
The large difference in speeds between the Compression and Fast Ignition pulses results in a substantial gap between them at the Wobbler. This gap accommodates the Wobbler's risetime 2701, as illustrated in
The risetime 2701 of the RF Wobbler field is of importance regarding separate pulses for Compression and Fast Ignition (Fl). Wobbling enables heating an annulus along the axial direction. But the Fast Ignition Pulse needs to be on axis, with two considerations: (1) If the total cross-sectional area of the pre-compressed fuel is larger than the minimum set by the propagating burn parameter, the Fl beam may be correspondingly off-axis. (2) If, economically, the power of the Fl pulse may be greater than the optimized minimum, the Fl pulse may have a larger spot area than the minimum, which may be off-axis and still cover the optimal minimum area of the precompressed fuel.
Compared to the prior art, the new current multiplying processes result in improvement of the beam parameters that define the intensity of target heating and the target response. Higher total beam energy, reduced spot sizes will increase power deposition density and drive targets providing higher energy gain from the fusion reactions. Power deposition density in the target will increase in proportion to the square of the spot diameter. Ignition calculations for fuel target design are planned to exploit these improvements.
Heat deposition uniformity is important for good target performance. Wobbling Telescoping Species smoothes the heat deposition by displacing the instantaneous spots hit by different Species. Due to their different speeds, ions at corresponding points along the different Slugs pass through the Wobbler at some distance upstream from the target (e.g., 30 meters) at different phases of the Wobbler RF field, and ions at different axial positions along a Slug penetrate the heated annulus at different azimuthal points.
During the passage of a Slug through a cylindrical target, a Wobbled beam flies forward with the fixed shape of a helical coil spring. The thickness of the coils is the diameter of the beam spot. During passage of this helical shape through the target, the instantaneous heating at each point in the cylindrical annulus corresponds to the helical shape of the heat source. Heating of the entire annulus is not instantaneously uniform. The time-averaged heating smoothes out over passage of the whole Slug.
With Telescoping, the helical-spring shape of different Slugs in the target is rotated relative to each other, around the common axis. For illustration, if the SlugTrain timing is specified for all Slugs to arrive at the target simultaneously (or with another specified timing, such as to provide a desirable Ignitor Pulse Power Profile), the tips of the different beam helices enter the annulus being heated at different azimuthal locations. The interspersed helical Slugs of the Multiple Isotopes fit into the helical spaces (the helical pitch minus spot diameter), netting a smoothing factor improvement equal to the number of Multiple Isotopes. Different Slugs may be timed for different overlapping arrangements.
The stretching of individual microbunches by the Slicker adds a further smoothing effect. The ions in a given microbunch differ in speed by, e.g. 0.1%. This results in ions that experience the Wobbler fields at the same time arriving at the target at different times. The effect is to flatten the cross section of the instantaneous beam.
First single-pass HIF driver to use conventional accelerator technology;
Makes strong use of multi-species for telescoping beams at fusion target;
Eliminates storage rings, removing difficult/expensive technical issue;
Loosend requirement for beam emittance of individual ion sources; and
reduces aggregate total solid angle of igniter beam input-port apertures.
Multiple fusion chambers with one robust accelerator/ignitor (10-100+BPOE pulses);
sacrificial lithium fuel-charge sabot, neutron moderator, T-breeder, ultra-high Temperature hot working fluid;
Lithium droplets and fog sprays muffle blast;
Lithium droplets and fog spray ultra-fast, inter-pulse, fusion chamber vacuum pump;
pulsed, very high-flow rate lithium pump (10s of tons per second in earliest chamgers)
Multi-ion species source hotel;
Micro-bunch snugging system preserves RF temporal structure and timing of ion beam;
Helical, serial-species delay and re-timing line;
Fewer beamlines and final focus lenses into fusion chambers; and
Direct conversion of fusion energy carried by both charbed particles and neutrons.
Improved ignitor pulse focusing properties (by exploiting 6-D phase space of multiple species);
More intense target heating, with classical “Bohr” ion stopping in matter;
More uniform target heating;
Ten times more ignitor pulse energy than the National Ignition Facility;
Fast Ignition (Fl) with Fl ion species chosen to maximize ignition vigor;
Timing for Multiple Fusion Power Chambers;
Driver duty factor in Pulsed RF range; and
Relieved vacuum requirements.
The new beam processes do not call for multi-turn injection into storage rings. This avoids areas of prior technical concern, significant design effort, and major hardware demonstrations of issues peculiar to storage rings. Removing these concerns shortens the schedule for HIF by removing the need for a time-consuming validation project, necessitating hardware with size, capabilities, and costs similar to those of the storage rings and linac that would be used in a power producing system.
The new processes may be expressed in terms of a line-up of beam multiplication processes.
I
target
=I
source
×N
isotopes
×N
sources
×N
snug
×N
slick
×N
sides
For illustration, treating either Compression or Fl pulse. Compression parameters shown:
Increasing the Ignitor Pulse current out of the linac results in the linac being on a relatively short on-time per ignition pulse, e.g., 300 microseconds. Using ten pulses per second, e.g. to drive ten Multiple Chambers, the RF duty factor is 0.003, safely inside the range classified as pulsed RF power. The benefits of pulsed RF are higher peak power per source and lower cost per peak power Watt.
The new set of processes for compacting the current produced by the linac minimizes the time the beam dwells in any section of the beam tube, and achieves the important case of a single pass system. Generating the pulse in a minimum of time increases the required RF peak power, but reduces the RF duty factor below the threshold of a fraction of 1%, where peak RF power costs substantially less peak Watt than continuous RF power. For purposes of illustration, Table 2 illustrates this cost consideration based on engineering estimates scaled from state of the art HIF design and costs in the current state of the art of RF power systems:
With 5A at 20 GeV, the RF feeds 100 GW into the beam during the pulse. The power to excite the accelerator is a factor of several less than the beam power, but is not shown. With this caveat, the illustration is instructive for consideration of the economics of HIF power production.
The new design features exploit the large increase in the total 6D phase space made available by the use of Multiple Isotopes. The smallest area that can be illuminated at the surface of the target and, therefore, the smallest volume into which the beam energy can be deposited, is governed by the conservation law of physics known as Liouville's Theorem. The essence of Driver design is to work with the 6D phase space defined at the point of origination of the entire number of beam ions, which total about 10 peta-particles, ten million billion, for each Ignitor Pulse.
HIF Driver designs in the prior art are considered stressed, in terms of the capabilities of known technology. Characteristically, the stress is expressible by pressure on the brightness of ion sources to put the required number of ions into a small enough volume of 6D phase space so that the processes that constitute Ignitor Pulse generation deliver the beam parameters that ignition calls for to the fuel target. Transverse emittance benefits the most, by limiting stacking in transverse phase space. The factor, e.g. 2.5× (including dilution), by which transverse emittance grows in each plane, as a result of Merging multiple beams emitted by the Linac, is the only one of the series of beam conditioning processes that employs the transverse (4D) phase space. Smaller transverse emittance enables achievement of smaller beam spots on the target, which increases heating intensity as the inverse of the diameter squared. For illustration, a spot diameter five times smaller will increase the intensity twenty five times. Prior art indicates that this much improvement is not required, but the potential is important for confident development of fusion power. Preservation of the microbunch structure and integrity in phase space offers, in principle, to deliver the smallest emittances to the target. The Snug and Slick effects capitalize on microbunch maintenance to conserve longitudinal phase space by systematically moving inter-bunch spaces to the adjacent inter-Slug spaces, which subsequently are largely removed (according to pulse timing specifications) by the Helical Delay Line. This process compacts the beam without damaging the longitudinal emittance, resulting in lower chromatic aberration at the target.
Generation of Ignitor Pulses by a single pass through the system relaxes the vacuum requirements. This avoids cost and adds safety margin to the design. The new beam processes do not call for multi-turn injection into storage rings. This avoids areas of prior technical concern, significant design effort, and major hardware demonstrations of issues peculiar to storage rings. Removing these concerns shortens the schedule for HIF by removing the need for a time-consuming validation project, necessitating hardware with size, capabilities, and costs similar to those of the storage rings and linac that would be used in a power producing system.
For an illustrative comparison to the prior art, the new Driver concept combines 5-10× higher total Ignitor Pulse energy (or more); as high or higher total Ignitor Pulse power; smaller spot sizes on targets; appropriate pulse power shaping at the target; Fast Ignition that is optimizable by choice of Ion Species for the Slugs in the Fast Ignition Pulse. The combination of improvements to the prior state of the art results in the system meeting all known or theorized requirements for the economical production of fusion power.
In the foregoing specification, the invention has been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative sense rather than a restrictive sense.
This application claims benefit of U.S. provisional patent application Ser. No. 61/061,593, filed Jun. 13, 2008, titled “Heavy Ion Fusion” (attorney docket no. ARCA0002PR), the entirety of which is incorporated herein by this reference thereto.
| Number | Date | Country | |
|---|---|---|---|
| 61061593 | Jun 2008 | US |
