SMALL DIAMETER STANDING-WAVE LINEAR ACCELERATOR STRUCTURE

Abstract: A compact, small diameter, standing-wave linear acceler­ ator structure suitable for industrial and medical applications is disclosed. The novel structure utilizes a new type of cou­ pling cavity for Pi/2 mode, standing-wave operation. The cou­ pling cavity fits into the webs between the accelerating cavi­ ties substantially within the diameter of the accelerating cavi­ ties. This is made possible by keeping the center section of the cavity thin to concentrate the electric field vector at the center of a section of the cavity and by enlarging the ends of a section of the coupling cavity to accommodate the magnetic field vector. This structure offers a significant reduction in overall diameter over the side-coupled, annular ring, and existing coaxial coupled structures, while maintaining a high shunt impedance and large nearest neighbor coupling (high group velocity). A prototype 4 MeV, 36 cm long, S-band accelerator incorporating the new structure has been built and tested.

Description: The present invention relates generally to standing-wave linear particle beam accelerators and more particularly to charged particle beam accelerators and methods wherein a coaxial coupled structure is used to build a small-diameter efficient electron accelerator for radiation therapy and industrial radiography. Electron linear accelerators with energies up to 50 MeV have been widely used for radiation therapy and industrial radiography since early 1960. Currently an emphasis is being placed on more efficient, compact, and cost-effective designs. For standing-wave, Pi/2 mode linear accelerators, the coupling cavities allow for a flexibility of design, since they are unexcited in steady state operation. Existing standing-wave accelerator coupling cavities can be placed in four general design types: on-axis, coaxial, side cavity, and annular ring structures. These four structures are shown schematically in FIG. 1. Since the side cavity structures are off-axis, they do not influence the design of the accelerating cells, enabling side coupled accelerators to attain high efficiencies. Side coupled structures, however, have the disadvantages of inereasing the effective diameter of the accelerator guide and the number of machining and assembly steps required. Cylindrically symmetric cavities, the on-axis, coaxial, and annular ring designs, have the advantage of being machined directly into the opposite side of an accelerating cell, thereby eliminating multipiece assembly and prebrazing. Construction costs can be substantially reduced. Existing designs, however, all have disadvantages. The radius of an on-axis coupling cavity is comparable to the radius of the accelerating cavity. The structure is susceptible, however, to the excitation of parasitic and beam blowup modes, which reduce the overall accelerator efficiency and beam stability. (See J. P. Labrie and J. McKeown, "The Coaxial Coupled Linac Structure", Nuclear Instruments and Methods, No. 193, pp. 437-444, 1982). On-axis structures are also sensitive- to thermal detuning, a result of the thermal deformation of the web between the accelerating cells. (See: J. McKeown and J. P. Labrie, "Heat Transfer, Thermal Stress Analysis and the Dynamic Behavior of High Power RF Structures", IEEE Transactions on Nuclear Science, Vol. NS-30, No. 4, pp. 3593-3595, 1983). Coaxial structures eliminate the direct interaction of the electron beam with the coupling cavity, but designs of the prior art increase the effective guide diameter 60% to 80%. Prior art designs consist of narrow cylindrical cavities sandwiched between accelerating cells, which operate at a coaxial TM010-like mode. (See for example: C. Fuhrmann et al, "Characteristiques de Dispersion et Impedances Shunt de Trois Structures Biperiodiques Acceleratrices en Bande S", Nuclear Instruments and Methods in Physics Research, No. 227, pp. 196-204, 1984 and R. M. Laszewski and R. A. Hoffswell, "Coaxial-Coupled Linac Structure for Low Gradient Applications", in Proceedings of the Linear Accelerator. Conference 1984, pp. 177-179). Annular ring designs in the prior art have the same size disadvantage as the existing coaxial structures, along with increased machining complexity. The invention is defined in Claim 1. In one example a coaxial coupling cavity extends the zero field region between adjacent accelerating cavities, thereby reducing the efficiency of the accelerator. Coaxially coupled structures, however, attain a higher percentage of theoretical shunt impedance. (See: S. O. Schriber, "Accelerator Structure Development for Room-Temperature Linacs", IEEE Trans. Nuclear Science, Vol. NS-28, No. 3, pp. 3440-3444, June 1981). Consequently, accelerator efficiencies comparable to that of side coupled structures can be obtained if the web between accelerating cells is not increased more than several millimeters. The size disadvantage of the annular ring and existing coaxial designs exemplifies the problem of developing a new coaxial design which 1) has a diameter comparable to an accelerating cavity, 2) does not significantly increase the web thickness, and 3) has strong nearest neighbor coupling with small next nearest neighbor coupling. In this invention, a new coaxial cavity design is disclosed. The newly developed coupling cavity is located entirely within the copper web between the accelerating cells of a standing wave, linear, electron accelerator operated at the Pi/2 mode. The coupling cavity is isolated from the beam axis of the accelerator. The outer radius of the coupling cavity is approximately equal to that of the accelerating cavities resonating at the same frequency, distinguishing the design from prior art coupling structures not open to direct electromagnetic interaction with the accelerated electron beam. The regions of the cavity near the inner and outer radii are enlarged to form triangular sectioned volumes, while the middle region consists of a pair of narrowly separated parallel plates. Consequently, the magnetic and electric components of the fundamental mode electromagnetic field resonating in the cavity are separated, by concentrating the magnetic field in the inductive end regions of the coupling cavity and the electric field in the capacitive region between the parallel planes. Coupling is accomplished through a pair of coupling slots 1800 apart cut into the web between the coupling and accelerating cavities. This preserves symmetry about the beam axis, minimizing the beam perturbation. Because the magnetic field in the coupling cavity is concentrated in this region and the electric field is negligible, the magnetic coupling is maximized while the electric coupling is minimized. This is an optimum coupling situation for high efficiency operation. Relatively small slots intercept sufficient flux for coupling. This minimizes the effect of coupling on the electric field distribution of the accelerating cavities. Further, by rotating the coupling slots 900 at each half accelerating cavity, the coupling slots are at maximum separation, thereby further reducing the direct coupling between accelerating cavities through the slots. This reduction increases the power flow and stability of the accelerator. Also, because the cavity is isolated from direct interaction with the beam, transverse beam break-up modes and inefficient parasitic modes cannot be excited by beam-cavity interaction. These and further constructional and operational characteristics of the invention will be more evident from the detailed description given hereinafter with reference to the figures of the accompanying drawings which illustrate preferred embodiments and alternatives by way of non-limiting examples. FIG. 1 shows partial longitudinal cross-sections and end views of four general types of designs of standing-wave linear accelerators in the prior art: FIG. la on-axis coupled structure, FIG. ib coaxial coupled structure, FIG. lc side coupled structure, FIG. ld annular coupled structure. FIG. 2 shows a partial longitudinal cross-section in FIG. 2a and an end view in FIG. 2b of the standingwave linear accelerator according to the invention. FIG. 3 shows a section through a coupling cavity according to the invention in which the dotted lines represent the electric field vector. FIG. 4 shows theoretical energy spectra for an accelerator according to the invention. FIG. 5 shows a longitudinal cross-section of a complete accelerator according to the invention. FIG. 6 shows measured and theoretical dispersion curves for an accelerator according to the invention. Referring now to the drawings wherein reference numerals are used to designate parts throughout the various figures thereof, there is shown in Fig. 2 a short section of the structure. It consists of a small radius, coaxial structure 10 with coupling cavities 12 which are located in the webs 14 between accelerating cavities 16 and increase the magnetic induction in those regions near the inner and outer radii of the coupling cavity. In essence, the geometry enhances the intrinsic field distribution of a simple coaxial cavity in the TM010-like mode, while reducing the cavity to smaller overall dimensions. The thin flattened regions 18 between the enlarged end regions 20 act as an effective capacitor and concentrate the electric field in the flattened regions 18 as shown in FIG. 3,'away from the coupling slots 22. The concentration of the magnetic field in the enlarged regions 20 provides an ideal coupling opportunity. The shape pf the enlarged regions 20 was selected to be in the triangular form shown in the drawings but enlarged shapes can be used, such as a hemispherical or oval type shape. Two slots 22 are cut 1800 opposite each other about the beam axis into each accelerating cavity 16, thereby preserving symmetry about the beam axis. Relatively small slots can provide adequate nearest neighbor coupling, K1, and the next nearest neighbor coupling, K2, can be made negligibly small by rotating the slots 90* about the beam axis at the opposite side of each web 14 at each cell. The design also allows for a very high K1 to be obtained while keeping K2 to an acceptable value, by increasing the slot width and arc length. The coupling cavity sits in the web between two accelerating cavities. Several dimensional constraints were imposed upon the prototype design for an S-band. accelerator structure. First, the coupling cavity outer diameter was to be approximately equal to that of the accelerating cavity. Second, the parallel plate gap could not be less than 3 mm to maintain reasonable mechanical tolerances. Third, a minimum wall thickness of 3 mm for S-band cavities was to be maintained at all points for mechanical stability and thermal conduction. Before the coupling cavity prototype was designed, an accelerating cavity with a 9 mm web thickness was optimized for maximum shunt impedance, using the cavity program LALA. (See: H. C. Hoyt et al, "Computer Designed 805 MHz Proton Linac Cavities", Review of Scientific Instruments, Vol. 37, p. 755, 1966.) A cavity with inner radius 3.58 cm and theoretical shunt impedance per unit length of 124 M-ohm/m was developed. The cavity code LACC was then used to design the coaxial cavity, subject to the constraints listed above. (See: A. Konrad, "A Linear Accelerator Cavity Code Based on the Finite Element Method", Computer Physics Communications, No. 13, pp. 349-362, 1978.) The program was used to arrive at a cavity 5% higher in frequency than the operation frequency, because of the anticipated effect of the coupling slots. The size and location of the coupling slots were determined using the LACC magnetic field values. A coupling slot 22 of arc length 450 and width 5 mm was selected and located along the outer edge of the accelerating cavity 16. Substantially smaller and larger slots are workable. The prototype coupling cavity shown in FIG. 2 resonates at 3160 MHz without the coupling slots and 3015 MHz with the slots. In the dissembled accelerator, however, the coupling slots are rotated 900 and this lowers the full cavity frequency to 3000 MHz. Machine tuning to within + .2 MHz of the desired frequency was easily accomplished by increasing the diameter to lower the frequency or the capacitive gap to increase the frequency. The prototype accelerator was designed to match the performance characteristics of an existing side coupled structure for comparison, the L1000-A accelerator built by Varian Associates. It consist of 7-1/2 accelerating cavities and was designed for optimum performance at 4 MeV output energy. A beam simulation program was used to develop the buncher configuration for the guide, using the LALA field profiles. An injection voltage of 15 kV was used, with variable field gradients. A three-cell buncher with cell length 44.8 mm was selected. The resulting output energy spectrums are given in FIG. 4. The overall length of the guide is 35.9 cm. RF power from a magnetron is inputted at the 4th full accelerating cavity. The peak rf power delivered at the guide is 2.3 MW, with a 4.3 microsecond pulse width. Table I summarizes the accelerator design parameters. TABLE I PERFORMANCE SUMMARY Accelerator Length 35.9 cm Number of Cavities 7 1/2 Frequency 2997 MHz Coupling: Nearest Neighbor (K1) 3.3% Nearest Neighbor (K2) .03 RF Peak Power 2.3 MW RF Pulsewidth 4.3 microsecond E peak/EO 8.1 Transit Time Factor .916 Theoretical Qo 16,000 Theoretical ZT2/L 124 M-ohm/m Design Measured To 14,400 13,500 Qext 7,200 6,600 Beta, = Qo/Qext 2.0 2.05 ZT2/L 111 M-ohm/m 104 M-ohm/m Both the coupling and accelerating cells were accurately machined of oxygen-free high conductivity (OHFC) copper to make post-braze tuning of the guide unlikely. The accelerating cells were tuned in separate halves to within.+ 1 MHz of the desired frequency. The desired frequencies were determined from the dispersion measurements of successive stacks of 2, 4, and 6 half cells. The coupling cavities were tuned in separate halves to within + .2 MHz of 3009 MHz, which gave full coupling cell frequencies of approximately 2994 MHz. Because of the sensitivity of the large capacitive region of the coupling cavity to gap length, the effect of the braze had to be allowed for. The full coupling cavity frequency varies 240 MHz/mm of additional spacing between half cells. The braze process adds 20 microns of copper between cells on average, resulting in an approximately 5 MHz increase. The prototype.accelerator constructed to test the new coupling cavity is shown in FIG. 5. A series of identical half-cavity pieces 30 of OFHC copper are brazed together alternately back to back and front to front as shown. Slightly modified coupler half cells 32 are used to admit microwave energy. Slightly shorter buncher pieces 34 are used to increase the beam velocity to match the phase of the accelerating section. A beam source 36 inserts a beam into the buncher. The high energy beam strikes a target or window 38 at the end opposite to that of the beam source. The measured and theoretical dispersion curves for the brazed guide are shown in FIG. 6. The theoretical curve assumes a biperiodic structure with faccelerating = 2996.69 MHz and fcoupling = 3001.5 MHz. The bead drop data is shown in Figure 6. The guide had a measured QO of 13,500 and Qext of 6,580, witch a VSWR (Beta) of 2.05. The nearest neighbor coupling, K1 was 3.3% and the next nearest neighbor coupling was .048. The coupling cavity frequencies were 3001.5 MHz, + 1.5 MHz. Before and after brazing length measurements of the guide indicated a greater than average increase per cell, approximately 30 microns. This explains the high coupling cavity frequency, which is apparent in the dispersion curve. The accelerating cells remained tuned to within + .1 MHz of a fixed frequency. These frequency variations were acceptable and no post-braze tuning of the guide was done. This invention is not limited to the preferred embodiments heretofore described, to which variations and improvements may be made, without leaving the scope of protection of the present patent, the characteristics of which are summarized in the following claims.

Claims: Claims 1. A linear standing-wave charged-particle beam accelerator for accelerating a source of a beam of particles, said accelerator comprising: plural cascaded standing wave electromag netinally coupled accelerating cavities with approximately the same resonant frequency, said accelerating cavities being positioned so that a particle beam propagates longitudinally through them defining a beam axis, said accelerating cavities being figures of rotation around the beam axis having a cavity diameter, adjacent ones of said accelerating cavities being electromagnetically coupled, coupling cavities located equidistant between said accelerating cavities and substantially within said cavity diameter, each said coupling cavity being in the form of a hollow flattened annular ring having an inner rim defining an inner diameter and an outer rim defining an outer diameter, said coupling cavities being coaxial with the beam axis, the gap of said flattened annulus at the mean of said inner and outer diameters being much less than the diameter of said outer rim with significantly increased gap at said inner and outer rims, means for coupling said accelerating cavities to said coupling cavities and said coupling cavity including means for isolating said coupling cavity from excitation by a beam. 2. The accelerator of claim 1 wherein said coupling means includes a first pair of slots connecting a first main cavity to a coupling cavity near said outer rim of said coupling cavity, each of said slots being in the form of an arc of a circle around said beam axis, said slots being generally 180 degrees from each other with respect to said beam axis, and a second pair of slots between said coupling cavity and a second main cavity, said second pair of slots being similar in shape and placement to said first pair of slots. 3. The accelerator of claim 1 wherein the gap at said outer rim of said coupling cavity is substantially increased over the gap at said inner rim. 4. The accelerator of claim 2 wherein each of said second pair of slots is 90 degrees from each of said first pair of slots with respect to said beam axis. 5. The accelerator of claim 1 wherein said coupling cavity in the form of a flattened annular ring includes in a section from a center to said inner and outer diameter sectional portions of generally triangular shape. 6. The accelerator of claim 5 wherein said triangular shapes are those of isocelles triangles defining apexes between equiateral sides of said triangles, said triangular shapes being oriented to place said apexes closest to a center of said section of said coupling cavity. J 7. An improvement in linear standing-wave chargedparticle beam accelerators having a beam source, a power source, accelerating cavities defining a largest diameter of said accelerating cavity and coupling cavities, the improvement comprising: forming coupling cavities in webs between said accelerating cavities, said coupling cavities being the shape of an annular ring having a center, an inner diameter and an outer diameter, the outer diameter being substantially within the largest diameter of said coupling cavity, the gap of the annular ring being substantially greater at said inner and said outer diameters than inbetween said inner and outer diameters. 8. The accelerator of claim 1 including means for maintaining sufficient thickness of supporting walls to dissipate heat.

Inventors: BAYER, MATTHEW, TANABE, EIJI, TRAIL, MARK
Applicants: VARIAN ASSOCIATES, INC.
Application year: 1986
Main classification: H05H9
IPC codes: H05H9, H05H7