A novel method and apparatus for accelerating a charged particle beam to a desired energy is disclosed. The accelerator and the methods can be used to accelerate any type of charged particle to form an energetic beam. One example of an application is to accelerate a beam of electrons which in turn may be used to produce an intense photon beam through the bremsstrahlung process.
Particle accelerators generally are grouped into different categories according to their fundamental concepts:
1) Those that use constant electrostatic fields such as Van de Graaff accelerators;
2) Those that make use of radiofrequency cavities in a straight line such as linear accelerators;
3) Those that use the electric fields induced by a time varying magnetic field to accelerate a particle such as the betatron; and
4) Circular accelerators that recirculate the beam of particles through a radiofrequency cavity to reach a desired energy such as a cyclotron, synchrotron, microtron, racetrack microtron or Rhodotron™.
Different names have been used to describe different combinations of the ideas represented by these groups and the concepts they represent as they have been perceived to be advantageous in different applications. Many are discussed in books about accelerator design such as M. S. Livingston and J. P. Blewett, “Particle Accelerators”, McGraw Hill Book Company, Inc., New York, 1962. They all apply the fundamental Maxwell equations and particle dynamics in magnetic and electric fields to accelerate particles and form accelerated beams.
The accelerator and associated methods disclosed herein also use the governing rules of Maxwell's equations, but in a novel approach that cannot be equated with any of the concepts or applications of the conventional particle accelerator groups listed above. The essential elements of this accelerator are:
1) A magnetic core that can accommodate a time varying B-field;
2) A power supply that can provide suitable voltages and currents.
3) An electrically conductive vacuum chamber that encircles a portion of the magnetic core and that has a non-conducting gap; and
4) A magnetic guide field to guide the particles around the interior of the vacuum chamber in stable orbits as they gain energy.
According to the methods and systems described in detail hereinbelow, any charged particle can be accelerated, and any energy within wide limits is possible, the limits being imposed only by the practical limits of the state-of-the-art for electrical insulation, power supply capabilities, magnets, etc. The method achieves large beam currents at high duty cycles approaching 100%. No radio frequency power generators feeding tuned cavities are required. A voltage supply may provide the energy to the beam. Energy is delivered to the particles via coupling to an electric field that possesses a Curl at a gap.
The type of accelerator disclosed herein is different from the accelerator classes mentioned above. Compared to 1) no static electric field with a divergence is used for acceleration, thus high energies can be achieved without extreme voltages. Compared to 2) and unlike a Linac, high radiofrequency electromagnetic fields in tuned cavities are not required to achieve high energies. The electron beam need not be bunched matching the RF fields in the cavities for acceleration. Compared to 3), the induction core with its time varying magnetic field is used to provide a self inductance that allows a voltage across the insulated accelerating gap to be maintained by a power supply with relatively low currents from the driving power supply. Since the acceleration cycle occurs in a time that is short compared to L/R, (where the self inductance of the accelerating chamber is L and R is the resistive impedance of the accelerating chamber and the power supply system), the accelerating electric field at the insulating gap possesses a curl and allows cumulative acceleration on successive turns in an acceleration chamber. Also, unlike the betatron example used in 3), the magnetic fields that guide the beam in orbits enclosing the induction core are static whereas, in the betatron, the fields that guide the beam are time varying and strictly related to the instantaneous magnetic field in the induction core. Compared to 4), there are no RF power supplies feeding tuned RF cavities and there are no bunched beams synchronized to the RF frequency to achieve acceleration. As mentioned above and as will be discussed later, the maximum length of time for an acceleration cycle for the accelerator disclosed herein is limited only by L/R. This time is typically many microseconds to milliseconds.
The embodiments described herein are exemplary of the possible applications of the technology and methods disclosed herein for the acceleration of charged particles. Those experienced in the art will recognize that there are extensions, modifications and other arrangements of the important elements disclosed that can be implemented and they are intended to be encompassed within the scope of this disclosure.
For a better understanding of the present disclosure together with other and further objects thereof, reference is made to the accompanying drawings and the following detailed descriptions of selected embodiments.
As an aid to understanding the operation of the embodiment in
Still considering the idealized situation, a charged particle (charge q) traversing the non-conducting gap 108 in the vacuum chamber 104 will be accelerated with an energy gain of qV. This particle is guided around the induction core 102 inside the vacuum chamber 104 by an appropriate magnetic guide field 134. The particle experiences no retarding fields in the vacuum chamber 104 because all fields (except for the static magnetic guide field as discussed below) are zero except for those induced on the walls by the charge of the particle itself. As the particle travels around the induction core 102 it reenters and traverses the non-conducting gap 108 in the vacuum chamber 104 and its energy is increased by qV again. If it makes n circuits (or turns through the gap) it gains a total energy nqV. The path integral around the inside of the vacuum chamber 104 of E·dl in one complete path is V. Here, E is the electric field in the vacuum chamber 104 and dl represents the path length differential for the beam path (bold quantities are used to represent vectors). E is zero in the conductive portion 106 and is equal to EG in the non-conducting gap 108. It should be recognized that EG is a complex function of position in the region of the non-conducting gap and not a constant as implied by the approximate relation EG=V/d. It is not described in detail herein for the purposes of simplifying the discussion. However, regardless of this complex variation, most of the field EG is located in the vicinity of the non-conducting gap and the path integral of E·dl in one complete path is rigorously V. That is, this electric field has a Curl for its vector character. This distinguishes this electric field from an electrostatic field where the integral of E·dl around a closed path is zero. Conventional means (not shown) are employed for injecting and/or extracting the beam 116 into/from the vacuum chamber 104 according to techniques that will be well known to those familiar with the art.
Thus there are two very distinct electromagnetic field regions in this idealized situation. One is inside the vacuum chamber 104 where the only fields are those created by V in the region of the non-conducting gap 108, those induced by the particle charge q on the inner walls of the conductive portion 106 of the vacuum chamber 104, and those constituting the magnetic guide fields. The other field is outside the conductive portion 106 of the vacuum chamber 104 where the current IO 130 from dIO/dt=V/L travels along the outside surface of the conductive portion 106. These two regions are coupled only via the non-conducting gap 108.
Still considering the idealized situation, an induced image charge on the inner surface of the conductive portion 106 of the vacuum chamber 104 forms current II 132 and travels along the inner surface in the same direction as the path of the particle(s) in the beam 116. Current II 132 is equal to the rate of flow of charge of the particle(s) in magnitude and opposite in sign. When the particle(s) is for example an electron(s) this image charge is positive. When the particle(s) in the beam 116 reaches the end 118 of the conductive portion 106 at the non-conducting gap 108 it simply crosses the non-conducting gap 108 in the vacuum and gains energy qV. However, the induced image charge (and thus the current II 132) has no alternative but to come to the outer surface of the conductive portion 106. Upon reaching the outer surface at the end 118, the current II 132 travels through electrical leads 128 and through the power supply 122, which has an ideally zero impedance. Thus, in this example, the current II 132 resulting from the image charge flows through the power supply 122, electrical leads 128, and enters the inner wall of the conductive portion 106 of vacuum chamber 104 at the end 120, adjacent to the non-conducting gap 108 with the voltage +V and exits at the inner wall of the conductive portion 106 at the end 118, where the voltage is zero, and returns to the power supply 122. The image charge flow provides an additional current II 132 flow into the power supply equal to the current flow of the beam 116. The image charge flow is an image current. Thus the power supply provides power to energize the induction core 102 and additionally it provides power to the beam 116 via this coupling with the image charge or image current.
Thus far in this discussion the conductive portion 106 has been considered as ideal with no resistive impedance. In the real (non-idealized) situation, finite resistance must be considered in the working embodiments of this disclosure. This situation is well treated in many texts on electromagnetic theory. Referring to the book by J. D. Jackson (“Classical Electrodynamics”, Third Edition, John Wiley & Sons, 1999) the subject is treated in several places. In particular, in Chapters 5 and 8 it is shown that the main effect of the finite conductivity is to localize the currents and fields to a region of the surface called the “skin thickness”. This means that fields that vanished at the surface of the idealized perfect conductor now penetrate the real conductor of this working embodiment, but die away as e−x/δ where x is the distance perpendicular to the surface and δ is the skin thickness. The value of δ depends on the resistivity of the conductive portion 106 of the vacuum chamber 104 and the frequency of the external electromagnetic fields considered. As an example, at 2.5 kHz for copper, δ is approximately 1.3 mm. By assuring that the wall thickness w 112 of the conductive portion 106 is considerably larger than δ, the inner and outer regions of the vacuum chamber remain effectively decoupled electromagnetically. The non-conducting gap 108, however, still causes the flow of the image charge current II 132 from the +V side of the power supply 122 into the inner surface of the conductive portion 106 of the vacuum chamber 104 and the flow of the image charge current II 132 out of the inner surface of the conductive portion 106 into the low potential side of the power supply 122. In the real situation, the Ohmic resistance to the flow of the current II 132 and the current IO 130 are no longer zero (as in the idealized situation discussed above) in the conductive portion 106, but can be evaluated using standard expressions of current flow through a medium with resistivity ρ with the current distributed in the skin thicknesses of the inner and outer surfaces as described above. Generally, for good conductors such as copper and for geometries and values of δ at the frequencies considered herein, these losses may be low compared to power consumption by other elements.
The coupling of the power supply 122 to the beam 116 in the vacuum chamber 104 through the image charge flowing into the vacuum chamber 104 via the ends 118, 120 of the conductive portion 106 at the non-conducting gap 108 cannot be represented by standard fixed electrical circuit parameters. However, an equivalent electrical circuit can be constructed to illustrate the functional behavior described herein. This is shown in
V−LdIO/dt−IORO=0 (Equation 1)
(Of course, for the special idealized case where RO=0, as discussed above this reduces to the expression V−LdIO/dt=0, or dIO/dt=V/L. In addition, even when RO≠0, for times short compared to L/RO, the relation dIO/dt=V/L remains sufficiently accurate.) The energy dissipation of the induced image current II 132 in the inside of the conductive portion is noted by the current, II, flowing through a resistance given by the symbol RI in schematic 200. The symbol CBP denotes the beam coupling of the beam 116 to the power supply 122 via the induced image current II 132 on the inside of the conductive portion 106. This induced image current is given by II=IB, where IB is the circulating beam current inside the vacuum chamber 104 due to the beam 116. The image current II 132 is supplied by the power supply 122 via the beam coupling CBP through the non-conducting gap 108. The total power supply 122 current is:
I=IO+II=IO+IB (Equation 2)
Thus the total current from the power supply 122 is the sum of the current Io 130 exciting a magnetic flux in the induction core 102 and the current IB due to the beam 116. The power supply 122 supplies energy to the magnetic field in the induction core 102 and to the beam 116. If the beam 116 is not present, only the magnetic energy is supplied. The power supplied by the power supply 122 is given by P=V(IO+IB). In any practical situation, the losses due to the dissipation in RO and RI are small compared to the dissipation in the magnetic induction core 102 due to hysteresis and internal currents and therefore the Ohmic losses may be neglected. The dissipation in RI causes a decrease in the energy gain of the circulating beam 116. In general this decrease is much smaller than the qV beam energy gain for each cycle and may again be neglected in terms of beam dynamics except in evaluating the final particle energy.
Referring again to
The time for full acceleration is denoted as tA, while the time of one-half cycle is denoted as T. A beam 116 at full energy is available for the time interval T−tA and the beam 116 at full energy may be continually extracted starting after the acceleration time tA. During the interval T the voltage will be +V across the conductive portion 106 of the vacuum chamber 104 and reverses to −V for times T<t<2T to give the current a negative slope. This cycle can be repeated as often as the acceleration cycle is desired. Of course, it will also be possible, by setting V=0 at any time, to hold a rotating pulse or beam of particles at a fixed energy or range of energies. This may facilitate studies of beam dynamics or the delivery of the beam over an extended period. It will also be recognized by anyone skilled in the art that by reversing the beam injection direction and guide field direction, that acceleration may be achieved during the excursion of the current IO from −I to +I as well as the excursion from +I to −I, where I is the maximal amplitude of the current IO.
An approximate equivalent circuit of this embodiment is illustrated in
In this embodiment the current in the secondary is determined by the current of the beam 116. This is coupled as an equal current (in the case of a one-to-one turn ratio) in the primary coil 404 connected to the power supply 402. In addition, in the primary coil 404 there is the current required to store magnetic energy in the induction core 102 and the induced losses in the induction core 102. RI and RO provide the resistive loss due to the flow of the image current in the walls of the vacuum chamber 104. Losses in the internal impedance of the power supply 402 must also be included. CBI represents the beam coupling of the beam 116 to the induced current II 406 flowing in the walls of the conductive portion 106 of the vacuum chamber 104.
The choice between the various embodiments may be based on considerations such as the voltages and currents required to be provided by power supplies, the desired geometric arrangement of system components, cost and electromagnetic shielding.
In all embodiments there are additional couplings of the currents flowing in the walls of the coil and/or conductive portion 106 of the vacuum chamber 104 to the conductive and magnetic guide field elements in the system. These couplings are mitigated by the techniques already discussed such as the use of conductive shields that do not form a closed loop around the induction core 102, yet shield the aforementioned guide elements and the use of non-conducting magnetic materials for the magnets providing the guide fields.
An additional concern is the leakage of magnetic fields from the induction core 102 to nearby magnetic elements such as those forming the guide fields. Such leakage can result if the reluctance of the induction core 102 is not very small compared to that of the leakage paths. As anyone experienced in the art will recognize, this leakage can be reduced by judicious use of conductive shields (not shown) placed between the affected elements and the sources of the fields or by the technique of flux forcing whereby the current driving the induction core 102 is distributed along the length of the induction core 102 by suitably connected conductive material driven in parallel to the conductive portion 106 of the vacuum chamber 104 in the embodiment shown in
Important to the embodiments in this disclosure are the properties of the magnetic materials used to construct the induction core 102. The functioning of these materials with respect to hysteresis loss and losses due to induced currents affect the performance of the accelerator. Likewise, the permeability of the induction core material and the value of the induction core saturation magnetic flux are important. A high permeability is desirable as is a high saturation flux. The use of amorphous magnetic materials with microcrystalline character and of ferrite materials are included as part of this disclosure to allow the use of high frequency switching of the magnetic field in the induction core 102, but conventional magnetic materials may be used in appropriate applications of this disclosure as well.
Included in the disclosure of these embodiments is the use of magnetic guide fields indicated only schematically in
Although the methods and systems have been described relative to specific embodiments thereof, they are not so limited. Obviously many modifications and variations may become apparent in light of the above teachings.
While the systems and methods disclosed herein have been particularly shown and described with references to exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the disclosure. It should be realized this disclosure is also capable of a wide variety of further and other embodiments within the spirit of the disclosure. Those skilled in the art will recognize or be able to ascertain using no more than routine experimentation, many equivalents to the exemplary embodiments described specifically herein. Such equivalents are intended to be encompassed in the scope of the present disclosure.
This present application claims priority to and the benefit of U.S. Provisional Patent Application Ser. No. 61/019,944 entitled “Method for Accelerating Particles Using Induction to Generate an Electric Field with a Curl Localized at a Gap” which was filed on Jan. 9, 2008 by William Bertozzi, Stephen E. Korbly and Robert J. Ledoux, and which is hereby incorporated by reference.
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