Patent Grant
8674630
This application claims the benefit of U.S. Provisional Application No. 61/628,329, filed Oct. 28, 2011.
The present invention relates to electromagnetic charged particle accelerators. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98
The electromagnetic acceleration of charged particles such as protons, electrons and ions, has practical applications in the fields of medicine, industry, and scientific research, particularly including experimental research in nuclear particle physics. In recent decades, high-energy particle accelerators have been advanced by the use of superconducting technologies to achieve ultra-low electrical resistivity and associated reductions in power losses.
Electromagnetic particle accelerators utilize one or more resonant cavities to accelerate charged particles. Such particles are typically accelerated in bunches as they travel through a series of resonant cavities, each of which accelerates the particles to successively higher velocities by interaction with a resonant radiofrequency (rf) signal present within the cavities.
In order to accelerate a beam of charged particles, the electromagnetic fields associated with a resonant signal inside a cavity must have sufficient magnitude to efficiently transfer energy from the resonating signal into the beam particles. The signals in the cavities are produced by introducing a rf signal into the cavity from a high-powered rf source, which is generally located some distance from the accelerator. The rf signal is transmitted from the rf source to the accelerator via a coaxial transmission line or a waveguide transmission line. At the point where the coaxial transmission line or waveguide joins the accelerator, the rf signal has typically been introduced into the accelerator cavities with a “loop,” an “iris,” or an electric-field probe, depending on the type of accelerator cavity being used. Introducing the rf signal from the coaxial cable or waveguide into the accelerator cavities is promoted by a device known as a power coupler. One such device is the subject of the present invention.
A power coupler converts the rf signal in the source transmission line into a form that matches the electromagnetic field configuration in the accelerator cavities. Power couplers come in two varieties depending on their interaction with the accelerator cavities. Electric power couplers primarily interact with the electric fields in the cavities. Magnetic power couplers interact primarily with the magnetic fields in the cavities. The choice of coupler depends on the configuration of the accelerator cavities. The rf signal transmitted along the transmission line is transformed by the coupler into either electric field components or magnetic field components, which penetrate the volume of the cavity and introduce energy into the cavity. Because of the resonant nature of the cavity, much of the energy introduced into the cavity is stored in the resonating electromagnetic field in the cavity. As the stored energy increases, the magnitude of the electromagnetic field inside the cavity increases. Thus the cavities become operable to accelerate a beam of particles when these fields reach a sufficient operating level.
Coupling of a rf signal from the source with a resonant rf signal in a cavity is necessary in particle accelerators as well as in other applications. At frequencies above 300 MHz, hollow rectangular waveguides are most often used to transmit a rf power signal from an rf generator into a radiation-shielded area containing the accelerator. The most common method for introducing the rf signal into the accelerator cavities is to connect the rectangular waveguide to a coaxial transmission line that penetrates the vacuum wall and protrudes into the accelerator cavity. Such a transmission line is terminated with a bar that short-circuits the center conductor to the outer wall of the coaxial transmission line, effectively forming a “loop.” RF current flowing through the loop produces an rf magnetic field that introduces energy into the accelerator cavity by magnetic field coupling with the resonant magnetic field signal in the cavity. When the magnetic flux density through the loop matches the flux density at the same location in the accelerator cavity, the loop becomes “critically coupled” to the cavity and rf power flows unimpeded from the rf generator through the transmission line and into the accelerator cavity.
Alternatively, a waveguide transmission line can be attached directly to the accelerator cavity. Typically, part of the cross section of the waveguide is occluded with an “iris” to match the electric field magnitude in the waveguide to the electric field magnitude at the same location in the accelerator cavity, which is known as electric field coupling. As with the critical magnetic coupling described above, when the electric field in the iris matches the corresponding electric field inside the accelerator cavity, the rf generator is “critically coupled” to the accelerator cavity, with the result that rf power flows unimpeded from the rf generator through the transmission line and into the accelerator cavity.
Thus in the case of both magnetic and electric field coupling, optimization is achieved when the rf generator is critically coupled to the accelerator, in part because reflection of the rf power signal at the interface is minimized.
However, critical coupling is an unstable condition. Acceleration of charged beam particles transfers energy from an accelerator cavity into the beam, thereby decreasing the energy in the accelerator cavity. In this regard, the energy efficiency of a resonant cavity is typically described by its quality factor (Q), which is defined as the electromagnetic energy stored in the cavity divided by the energy loss per rf cycle (and further in this regard, the operational bandwidth of a resonant cavity at the half-power points on either side of the maximum is equal to the resonant frequency divided by Q). As the energy stored in the cavity is absorbed and thus decreased by the accelerated beam, the ratio of stored electromagnetic field energy to rf drive power decreases. A critically coupled rf drive thus becomes undercoupled because the electromagnetic fields at the interface between the rf power source and the resonant cavity are no longer equal. This inequality, or impedance mismatch, causes rf power to be reflected from the coupler back toward the rf source. This additional loss of drive power further depletes the rf energy stored in the accelerator cavity and further decreases the coupling factor until the rf generator trips off as a result of the high reflected power levels.
This unstable condition is mitigated by deliberately overcoupling the rf drive to the accelerator cavity, or providing slightly more power than is necessary to equalize the flux/fields at the power coupler. When the accelerating beam depletes the stored energy, the match between the rf generator and the accelerator cavity improves and the additional power that was initially reflected in the overcoupled condition enters the accelerator cavity and maintains the stored energy at the desired level. As a practical matter, all accelerator cavities are deliberately “overcoupled” by a small amount in this manner to stabilize their operation. The coupling factor between the rf drive and the accelerator cavities is adjusted by changing the size and/or orientation of the drive loops in the case of magnetic coupling, or by changing the size and/or location of the iris in the case of electric coupling.
A complication with this approach arises with the use of cryogenically cooled superconducting cavities. In particular, the coupling of rf power signals with resonant signals in cryogenically cooled cavities is more challenging than in room-temperature copper cavities. Room-temperature cavities become less stable as the energy stored in the beam approaches the energy stored in the accelerator cavities. Superconducting accelerator cavities, however, are characterized by extremely high unloaded Q values (≈1010), such that only a few Watts of rf power are needed to produce large accelerating gradients. This low power level required for superconducting cavity excitation is in stark contrast with the one Watt of rf power required to increase the energy of one milliamp of accelerated beam by one keV in a nonsuperconducting cavity. Hence in superconducting systems, the rf power required to accelerate the beam greatly exceeds the rf power needed to maintain the electromagnetic fields in the cavities. It is this contrast, between the energy required to energize superconducting cavities compared with the energy required to energize room-temperature copper cavities, that makes superconducting accelerators extremely efficient, but also complicates the control and stabilization of their operation.
To stabilize the operation of superconducting cavities, the rf power couplers are deliberately and significantly overcoupled to the cavities in order to decrease their effective Q. This decrease in Q increases the operational bandwidth of the accelerator so that the rf source can match the resonant frequency of the cavity. Without the coupler loading the cavity, the operational bandwidth, which is the resonant frequency divided by the Q-factor, required of the rf source would be ≦1 Hz. This bandwidth is well below the capabilities of modern rf sources. Lowering the Q with the power coupler enables matching the rf source to the accelerator cavities. With a significantly overcoupled rf system, most of the rf power is reflected until the accelerating beam depletes the energy stored in the cavities. The optimal coupling factor depends on the accelerating beam current. Hence the coupling factor needs to be adjusted during operation to optimize the transfer of rf energy from the rf power source into the beam.
Beam bunches passing through a superconducting particle accelerator induce electric fields and currents in the metallic walls of the accelerator components, which are generally known as wakefields and image currents. The induced currents can excite unwanted resonant modes in the superconducting cavities. The induced energy associated with such unwanted resonant modes, typically less than a Watt, needs to be removed from the cavity before they accumulate enough stored energy to affect the properties of the beam. The unwanted modes always have higher resonant frequencies than the accelerating mode and can be separated with a frequency filter that passes only the higher-order mode (HOM) frequencies to a resistive component that functions as a damper to absorb and dissipate the induced energy. A variety of HOM dampers have previously been incorporated into superconducting systems. However, previous HOM dampers have been universally mechanically separated from the rf power coupler. This separation requires additional penetrations into the cavity and provides additional locations for problems such as vacuum leaks, contamination, electron multipactor, and high voltage sparking to occur.
The conventional approach to introducing rf power into a superconducting cavity is through a coaxial transmission line that enters the beam tube perpendicular to the axis near the rf cavity. Adjusting the penetration of the center conductor of the coupler into the beam tube changes the coupling factor. An unfortunate side effect of this approach is that the penetrating coaxial line breaks the cylindrical symmetry of the beam tube and can lead to deflection and even disruption of the beam. To address this problem, Veshcherevich et al. described a dual power coupler for the 1300 MHz cavities at Cornell that uses two couplers on opposing sides of the beam tube to minimize the perturbation. This approach reduces the dipole rf field components, but has the disadvantage of potentially introducing quadrupole field components. Dipole rf fields can deflect the beam to one side or the other, or can “shear” the beam envelope by affecting the beam particles on one side in a manner that is different from the effect produced on the other side. Quadrupole fields are generally less disruptive, but can nevertheless affect the focusing of the beam and can shear the beam envelope into four sections. The preferred approach is to maintain the cylindrical symmetry of the superconducting cavities and beam tube.
Most input power couplers reported in the prior art utilize a conventional side-mount configuration. A review of such couplers has been published by A. Variola (“High Power Couplers for Linear Accelerators,” in Proc. LINAC06, Knoxville Tenn., 2006, p. 531). Analyses of the performance of, and the problems associated, with side-mount couplers have been published by Jenhani et al. (H. Jenhani, A. Variola, L. Grandsire, T. Garvey, M. Lacroix, W. Kaabi, B. Mercier, C. Prevost, and S. Cavalier, “Studies of Input Couplers for Superconducting Cavities,” in Proc LINAC08, Victoria B C, p. 972), and by Kako et al. (E. Kako, H. Hayano, S. Noguchi, T. Shishido, K. Watanabe, and Y. Yamamoto, “High Power Input Couplers for the STF Baseline Cavity System at KEK,” in Proc Superconducting RF Workshop, 2007, Bejing China, p. 270).
Kashiwagi et al. make reference to an L-band coupler being fabricated at Fermi National Accelerator Laboratory (S. Kashiwagi, R. Kato, G. Isoyama, H. Hayano, T. Muto, J. Urakawa, and M. Kuriki, “Development of a Photocathode rf Gun for an L-Band Electron LINAC,” in Proc. LINAC08, Victoria BC, p 621).
Veshcherevich et al. describe high power testing of the Cornell ERL injector (V. Veshcherevich, S. Belomestnykh, P. Quibleh, J. Reilly, and J. Sears, “High Power Tests of Input Couplers for Cornell ERL Injector,” in Proc Superconducing RF Workshop, 2007, Bejing China, p 517), and Veshcherevich and Belomestnykh describe the single-sided input coupler for the main linear accelerator at Cornell. (V. Veshcherevich and S. Belomestnykh, “Input coupler for Main Linac of Cornell ERL,” in Proc. Superconducting RF Workshop, 2009, Berlin, Germany, p. 543).
Also, Veshcherevich et al. have reported on the performance of a two-sided coupler system at Cornell (V. Veshcherevich, I. Bazarov, S. Belomestnykh, M. Liepe, H. Padamsee, and V. Shemelin, “A High Power CW Input Coupler for CORNELL ERL Injector Cavities,” in Proc. 11th Workshop of RF Superconductivity, Lubeck Germany, 2003, p. 722)
References to on-axis coupler configurations are set forth in the publication of Kunze (M. Kunze, W. F. O. Muller, T. Weiland, M. Brunken, H. -D. Graf, and A. Richter, “Electromagnetic Design of New RF Power Couplers for the S-DALINAC,” in Proc. 2004 LINAC Conference, Lubeck Germany, 2004, p. 736); and in the publication by Cee et al. (R. Cee, M. Krassilnikov, S. Setzer, T. Weiland, “Beam Dynamics Simulations for the PITZ rf-Gun,” in Proc. EPAC02, Paris, France, p. 1622).
More specifically, Kunze et al. have described twin on-axis coaxial input couplers for the superconducting Darmstadt electron linear accelerator (S-DALINAC) (M. Kunze, W. F. O. Muller, T. Weiland, M. Brunken, H. -D. Graf, and A. Richter, “Electromagnetic Design of New RF Power Couplers for the S-DALINAC,” in Proc. 2004 LINAC Conference, Lubeck Germany, 2004, p. 736). The twin waveguide-to-coax transition configuration appears to be working well on the S_DALINAC and on the room-temperature PITZ photocathode rf gun (J. Bahr, I. Bohnet, D. Lipka, H. Ludecke, F. Stephan, Q. Zhao, K. Flottmann, and I. Tsakov, “Diagnostics for the Photoinjector Test Facility in DESY Zeuthen,” in Proc. DIPAC 2001, Grenoble France, p. 154).
Sekutowicz, et al. describe a coaxial coupler/higher-order mode (HOM) damper originally developed for HERA, but has since been scaled down and adapted to the TESLA cavities (J. Sekutowicz, “Higher Order Mode Coupler for TESLA,” in Proc. 6th Workshop on RF Superconductivity, JLAB, Newport News, Va. 1993, p. 426). The configuration of this system is a short coaxial cylinder that “floats” between superconducting cavities. The drive power is coupled to this cylinder via conventional side-couplers and the HOM power is dissipated in a pair of HOM dampers located ˜120° from the rf drive coupler. The coupling factor of such a configuration is fixed and cannot be adjusted without breaking the vacuum in the cavity.
Accordingly, it is the object and purpose of the present invention to provide an on-axis rf coupler for a superconducting particle accelerator. More specifically, it is an object and purpose to provide an on-axis rf coupler that does not break the central cylindrical symmetry of a centrally symmetric superconducting accelerator cavity.
It is another object and purpose of the present invention to provide an on-axis rf coupler that also functions to damp and dissipate higher-order-mode (HOM) resonant rf signals that may be induced by a beam passing through a superconducting accelerator cavity.
It is yet another object of the present invention to provide an on-axis coupler that enables the coupling factor to be adjusted during operation so as to optimize the transfer of rf energy from the rf power source into the beam, without requiring that the vacuum in the accelerator cavity be broken.
The present invention provides an on-axis rf power coupler for a superconducting particle accelerator having a superconducting cavity. The coupler includes a rf waveguide stub for receiving an rf power signal transmitted from a rf power source through an rf waveguide. The waveguide stub is operable to convert the rf power signal from a transverse electric mode into a transverse electromagnetic mode. The stub has aligned openings in opposing side walls thereof, which openings are alignable with the axis of a particle beam line of a superconducting particle accelerator. The waveguide stub further includes a ceramic tube that functions as tubular waveguide window, and which connects the aligned openings of the stub walls in sealing relationship and extends coaxially with the beam line.
An electrically conductive coupler tube extends coaxially through the openings in the walls of the waveguide stub and through enclosed the ceramic tube. One end of the coupler tube extends into and is connected to a tubular vacuum bellows assembly affixed to an outside wall of the waveguide stub, preferably the wall of the stub that is downstream with respect to the direction of beam travel. The other end of the coupler tube extends through the opening in the opposite wall of the waveguide stub, and preferably into a conductive vacuum tube that extends from the upstream wall of the stub, and by which the coupler can be connected to a superconducting cavity. The coupler tube and the conductive vacuum tube have diameters that make them collectively function as a coaxial transmission line for transmitting rf power into the accelerator cavity.
The bellows assembly includes a linear drive translator that operates to selectively move the coupler tube in translation coaxially along the axis of the beam line. The power load in the to cavity and the coupling between the rf input signal and the resonant signal in the accelerator cavity can be selectively varied by extension or retraction of the coupler tube, so as to achieve an overcoupled condition, or an undercoupled condition, or a balanced power load condition, and is thereby effective to achieve optimum particle acceleration and energy efficiency and prevention of beam disruptions during operation of the accelerator, without breaking the vacuum of the accelerator cavity or disturbing the central axial symmetry of the particle beam.
In a preferred embodiment the coupler includes a doorknob mode converter in the downstream wall opening of the waveguide stub, which functions to convert the TE mode of the incoming rf power signal into the TEM mode, such that the rf signal can be transmitted coaxially into the accelerator cavity along the movable coupler tube. The coupler also preferably includes a circular rf choke joint formed in the inside circumferential wall of the doorknob converter. The choke joint is sized relative to the exterior surface of the coupler tube so as to pass higher-order-mode (HOM) signals out of the waveguide stub and away from the accelerator cavity, while containing and reflecting lower fundamental rf signals in the accelerator cavity.
In another aspect of the invention, ferrite tiles are attached to the exterior surface of the coupler tube, inside the bellows assembly, such that HOM signals passing through the choke joint and out of the waveguide stub are absorbed and dissipated by the ferrite tiles.
The coupler is preferably oriented with the free end of the coupler tube extending upstream along the beam path toward the accelerator cavity, such that the open end of the coupler tube separates out HOM signals for isolation, absorption and dissipation by the ferrite tiles, without disrupting the particle beam inside the coupler tube.
The power coupler of the present design maintains the cylindrical symmetry of the system and shields the beam from the non-symmetric perturbations produced by the waveguide. In addition, the HOM damper is an integral part of the power coupler so that additional connections to the superconducting cavities are not required. The rf electric field from the end of the coupler tube of the coaxial line couples directly into the electric field inside the accelerator cavity. The distance from the end of the coupler tube to the inside wall of the superconducting cavity determines the coupling factor from the coupler into the cavity. Adjusting this distance changes the coupling factor and can be used to optimize the performance of the accelerator. The required coupling depends on several dynamic factors and real-time adjustment is a considerable advantage compared with non-adjustable couplers.
The accompanying Figures are incorporated in and form a part of this specification. In the Drawings:
a is a magnified view of the area shown as encircled in
a and 7b illustrate the coupler of
The accompanying drawings illustrate the construction and function of the present invention particularly when taken with the followed detailed description of the invention.
In the description that follows, orientations and positions of various elements are in some cases described with respect to the common longitudinal axis of the coupler 10 and the accelerator cavity to which the coupler is attached, and by reference to the direction of flow of the accelerated particles along such axis, i.e., as either the downstream direction or the upstream direction. In the accompanying Figure the direction flow of the particle beam is shown by arrows.
Referring to
The doorknob mode converter 12 surrounds, and is coaxial with, an electrically conductive coupler tube 13, preferably made of copper, which extends through opposing upstream and downstream side walls of the waveguide stub 11. Coupler tube 13 is movable in translation axially therein, as further described below, passing through the doorknob converter 12.
Both the waveguide stub 11 and the accelerator cavity to which it is attached (not shown in
Both the ceramic tube 14 and the vacuum tube 16 are significantly larger in diameter than the coupler tube 13, such that the coupler tube 13 and the vacuum tube 16 form a coaxial rf transmission line that functions to transmit rf power from the waveguide stub 11 upstream to the accelerator cavity 17. Briefly, translational movement of the coupler tube 13 changes the loading of the accelerator cavity 17, so as to either introduce power into, or withdraw power from, accelerator cavity 17.
The position of the coupler tube 13 is adjustable by a bellows assembly 19 that may be expanded or contracted so as to move the coupler tube 13 along its axis, in translation relative to the stationary waveguide stub 11 and the stationary accelerator cavity 17. The coupler tube 13 is thus movable and positionable in both directions along the common axis of the coupler 10 and the accelerator cavity 17. As described further below, the bellows assembly 19 enables the coupler tube 13 to be selectively moved and positioned relative to the waveguide stub 11 and the accelerator cavity 17 so as to adjustably couple the incoming rf power signal with a resonant rf signal in the accelerator cavity 17.
Still referring to
The downstream end of coupler tube 13 is connected to a coupler flange 22 (also shown in
The movable bellows flange 21, the attached coupler flange 22, and the coupler tube 13 are all driven in axial translation by a commercially available linear piezoelectric drive translator 23 that connects the bellows flange 21 to the waveguide stub 11. Commercially available piezoelectric translators are simple, robust, and compatible with high vacuum environments, cryogenic operations, and class-100 clean-room standards. In addition, such piezoelectric drives have micron-level position resolution over many centimeters of travel.
The flanges 21 and 22 are guided and supported by a set of four rigid guide rods 24, which extend from the downstream wall of the waveguide stub 11 and which are parallel to the axis of the coupler 10 and the accelerator cavity 17. Guide rods 24 thus also function to support and guide the coupler tube 13 as it is driven in translation through the walls of the waveguide stub 11.
Coupler tube 13 is entirely supported by the guide rods 24 and flanges 21 and 22, such that it passes through both walls of the waveguide stub 13 without making contact with the interior surfaces of either the surrounding doorknob converter 12 or pass-through 15.
In this regard, the diameter of pass-through 15 is significantly larger than that of the coupler tube 13, so as to enable the coupler tube 13 and the vacuum tube 16 to function as a coaxial rf transmission line. However, the inside diameter of the doorknob converter 12 is only slightly larger than the outside diameter of coupler tube 13, so as to permit installation of a choke joint 28 that is effective to damp and dissipate HOM signals.
Choke joint 28 is illustrated in
Also as a consequence of the coupler tube 13 passing freely through the walls of the waveguide stub 11, the interior of the coupler tube 13 and the annular volume surrounding it, as well as the space between the bellows 20 and the coupler tube 13, are in communication with one another as well as with the vacuum space of the accelerator cavity 17, which in the case of superconducting accelerators must normally be maintained at a vacuum on the order 10−9 torr and must also be maintained at or near class-10 clean room particulate cleanliness levels. However, the interior volume of the waveguide stub 11 is isolated from the vacuum space within the accelerator cavity 17 by ceramic tube 14, such that the vacuum space of the accelerator cavity 17 is isolated from that of the waveguide stub 17, which is evacuated through port 11a.
Thus
The second bellows 25 shown in
Referring to
a and 7b show the coupler 10 of
The rf couplers 31 and 32 are shown as being attached to opposite ends of cavities 33 and 34, respectively, but one of these couplers could be mounted in the center, between the two cavities, rather than at the ends as shown in
Two segments of WR-650 waveguide, 37 and 38, penetrate the vacuum tank 35 and connect to the associated waveguide stubs of the couplers 31 and 32. Ion pumps 41 and 42 evacuate the waveguide stubs of couplers and 31 and 32, respectively. The direction of the particle beam is denoted by arrow 18. The couplers 31 and 32 include vacuum bellows assemblies 19 that are the same as that shown in
Referring particularly to
The depth of penetration of the coupler tube 13 into the accelerator cavity 17 is adjusted by moving flange 21 in the axial direction by action of the piezoelectric linear translator 23, over the range of motion indicated by
It should be noted that
A key feature of present invention is the quarter-wave rf choke joint 28 in the doorknob converter 12. The radial gap between coupler tube 13 and the surrounding doorknob converter 12, which is necessary to the operation of the choke joint 25, also functions to eliminate physical contact between the movable coupler tube 13 and the doorknob mode converter 12. Eliminating physical contact between the sliding surfaces of these components eliminates the potential for producing metallic dust that could migrate into the superconducting rf cells and compromise their performance. Thus choke joint 28 operates to reflect the fundamental rf mode while passing unwanted HOM modes to the ferrite tiles 29, and additionally reduces the potential for sparking and erosion of metal components.
It is also important to note that the choke joint 28 is effective primarily over the fundamental accelerator frequency range commonly used in superconducting accelerators, for example the 1500 MHz frequency employed in certain Jefferson Laboratory accelerator cavities and the 1300 MHz frequency proposed for the proposed International Linear Collider cavities. HOM signals have frequencies higher than these fundamental frequencies and thus pass readily through the choke joint. The thermal contact between the ferrite tiles and the coupler tube 13 ensures a low thermal resistance path so as to keep the ferrite tiles 29 cool.
The geometry of the coupler design is also compatible with cryogenic operation. As shown in
The coupler 10 of the present invention readily operates at the low temperatures of around 2 degrees K typically maintained in superconducting accelerator cavities. The thermal break between the surrounding room temperature and the cryogenic temperatures maintained is typically located in the rf waveguide upstream of the WR-650 waveguide stub 11 shown in the Figures. This approach separates the problem of minimizing thermal loads from the problem of maximizing thermal conductance to maintain low operating temperatures.
Finally, it will be noted that the vacuum system inside the ceramic tube 14 of the coaxial coupler 10 and superconducting cavity 17 is separated from the vacuum system of the waveguide stub 11 by ceramic tube 14. In this regard, the coupler 10 is fed rf power through a conventional WR-650 waveguide and associated rf window (not shown), located upstream of the waveguide stub 11, so that the stub 11 can be evacuated to very low pressures comparable to those in an adjacent accelerator. As a result, the cylindrical ceramic tube 14 need not support the vacuum of the superconducting cavity against ambient air pressure, enabling its thickness to be minimized. Minimizing the thickness of the ceramic tube 14 minimizes rf power losses and hence also minimizes the thermal management issues that often plague other window designs. Additionally, any dust or debris associated with connecting or disconnecting the waveguide and/or waveguide window falls to the bottom of the waveguide stub 11 and does not contaminate the beam vacuum or the superconducting cavity cells 17b.
It should also be noted that the coupler 10 may be installed in an accelerator such that beam particles travel in a direction through the coupler 10 that is opposite from the direction illustrated and explained above with respect to
In general the configuration of coupler 32, as also shown in
The important features of the on-axis rf coupler of the present invention include: 1) a conventional cylindrical rf window, 2) on-axis variable rf coupling, 3) a choke joint that passes HOM signals while eliminating sliding contacts that could produce metallic dust), 4) an integral HOM damper with sufficient thermal contact to cooled surfaces, 5) cookie-cutter geometry to control wakefields, 6) a dual window design (a tubular ceramic window in conjunction with waveguide window), and 7) a high-precision linear insertion drive mechanism.
The on-axis rf coupler of the present invention offers significant improvements in the operation of superconducting accelerators. Variable on-axis coupling preserves the cylindrical symmetry of the beamline and the accelerator cavity and associated resonant cells, while simultaneously enabling a large in situ variation of the coupling factor between the rf source and the superconducting cavity. The geometry of the on-axis coupler is well suited to installation in a cryogenic container and maintains isolation of the beamline vacuum from the rf waveguide vacuum, thereby preventing contamination of accelerator cavities during installation and maintenance.
A number of factors will be addressed in the ordinary course of the detailed design of the coupler, all of which are within scope of one ordinary skill in the art of rf superconducting accelerator design and engineering. These factors include: 1) the efficient conversion of waveguide TE-mode energy into TEM coaxial energy with minimal reflected power and minimal conversion into undesired modes, 2) the electrical, mechanical, and thermal design of the tubular ceramic window, 3) the detailed design of the choke-joint, 4) issues associated with cryogenic design and thermal management, 5) the design of the mechanical support and motion control of the coupler tube, 6) efficient collection and safe dissipation of HOM energy, and 7) the elimination of the propensity for electron multipacting that often plagues superconducting accelerator systems.
Multipacting describes the process where a single electron can be accelerated by the rf fields and impact the metallic walls of the structure either in the same location as originally emitted or in another location. Each electron impact has the potential to liberate other electrons whose eventual impact liberates still more electrons in an avalanche process. Large numbers of impacting electrons can absorb a significant amount of rf energy and deposit that energy in a relatively concentrated location. Absorption of rf energy decreases the energy available for accelerating the beam. Also, depositing such energy in a concentrated location can quickly destroy the superconductivity at the metal surface, causing the cavity to “quench” and thereby lose all superconductivity. When a quench occurs, the rf power source must be turned off before the cavity is permanently damaged.
The design of the transition from WR-650 waveguide to a coaxial transmission line is a critical part of the overall design, particularly since the system operates at 2° K. The final coupler design is a compromise between the rf design, mechanical and thermal design, and the potential for electron multipacting.
It will also be noted that the coupler tube 13 extends some distance from its mounting flange 22 and thus a sound mechanical design is essential to maintain the rigidity of the coupler tube 13, to ensure that it remains precisely aligned with the beam path and does not contact the doornknob converter 12, particularly when being moved in translation by linear translator 23. This may addressed in part by counterweighting of the coupler tube 13 by extending its length beyond the coupler tube flange 22, as shown for example in
A sound thermal design is also essential if the system is to operate reliably at 2° K. Not all of the features required for isolating the cryogenic components from the ambient temperatures are shown in the Figures. Others that will be apparent to one skilled in the art of cryogenic design may be utilized.
The present invention is described and illustrated herein by reference to a preferred embodiment and the best mode known to the inventor. However, various alterations, substitutions and modifications that may be apparent to one of ordinary skill in the art may be made without departing from the essential invention. Accordingly, the scope of the present invention is defined by the following claims.
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