JPS622500A - Small-dia. stationary wave linear accelerator structural body - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention] The present invention relates to standing wave linear particle beam accelerators.

In detail, a charged particle beam accelerator and its method,
The present invention relates to the use of coaxial coupling structures to create small diameter effective electron accelerators for radiotherapy and industrial X-ray imaging.

Prior art electron linear accelerators with energies up to 50 MeV have been widely used in radiotherapy and industrial radiography since the early 1960's. At present, emphasis is placed on efficient, compact and cost-effective designs. Regarding the Pi/2 mode linear accelerator with a standing waveform, the coupling cavity takes into account the flexibility of the columns, but since they are not excited in stable operating conditions, the standing wave accelerator coupling with zero excitation is desirable. Mo caves can be arranged into four general types. namely, on-axis coaxial, lateral cavity, and annular ring structures. These four structures are schematically shown in FIG.

Since the side cavity structure is off-axis, it does not affect the design of the acceleration cell and allows the side-coupled accelerator to achieve high efficiency. However, the side-coupled structure has the disadvantage of increasing the effective diameter of the accelerator guide 1 and increasing the number of required assembly steps. The cylindrical symmetrical hollow, on-axis, coaxial and annular ring design has the advantage of being machined directly onto opposite sides of the acceleration cell, thereby eliminating multi-piece assembly and pre-blazing. sound. Construction costs can be practically reduced. However, all existing designs have disadvantages. The radius of the on-axis coupling cavity coincides with the radius of the acceleration cavity. However, the structure is susceptible to parasitics and beam blowup modes, reducing overall accelerator efficiency and beam stability. (J, f', Labrie and J, Mckeo
wn's 'The 0oaxjal Coupled L
lnac8structure' Nuclear
Instruments and Methods@
1' of t*19B (1982), 437-44
4 - On-axis structure also results from thermal deformation of the web between the acceleration cells. It is also sensitive to thermal detuning. (J.

McKeown and J. P. Labrie's 'Hea'
tTransfer.

Thermal 5tress Analysis a
and the Dynamic Behavior of
14ight Power RF 8structure
s'IE-EB Transactions on N
uclear 8ience.

Vol, MS-3+1. P of N14 (1983)
Although the coaxial structure eliminates direct interaction of the electron beam with the coupling cavity, it increases the effective guide diameter by 80% from 60 inches in prior art designs. Prior art designs consist of narrow cylindrical cavities sandwiched between acceleration cells.

Coaxial T'Mo+. Works in a mode like . (For example, αFuhrmann Other 0haracter
latique de dispersion et.
Impedances 8hunt de Trots 5
Structures Biperiodlques A
cceleratr! cesen Bande S',
Nucl”ear Instruments an
dMethods in Physics Re5ea
rch, P of Nu227 (1984), 196-
2 (14 and Le, M., Laszewskjand L.
A. Hoffswell's '0oaxlal-Ooup
ledLinac 8structure for
Low Gradlentλpplicatlon
a” Proceedings of the
Linear Accelerator 0onfer
ence (1984) P, 177-179.

) The annular ring design in the prior art has the same size disadvantage as the existing coaxial arrangement, along with increased machining.

SUMMARY OF THE INVENTION Coaxial coupling cavities extend into zero field regions between adjacent acceleration cavities, thereby reducing the efficiency of the accelerator. However, the coaxially coupled structure achieves a higher percentage of the theoretical shunt impedance 0 (8,0,8c
hriber o@λccelera+torStru
ture Development for &om
-TemperatureLlnacs' IBEE
Trans, Nuclear 8ciance.

Vol, N8-28. m3 (June 1981) P,
See 3440-3444. ) By the way, if the web between the acceleration cells does not increase by more than a few millimeters.

Acceleration efficiencies comparable to those of side-coupled structures can be obtained. The size disadvantages of annular rings and existing coaxial designs are that 1) they have a diameter that limits the acceleration cavity, and 2) they do not significantly increase web thickness.

3) Illustrate the problem of developing new coaxial designs with strong nearest neighbor couplings with small next nearest neighbor couplings. In the present invention, a novel coaxial cavity design is disclosed.

The newly developed coupling cavity is placed entirely within the copper web between the acceleration cells of a standing wave linear electron accelerator operating in PI/2 mode. The coupling cavity is remote from the beam axis of the accelerator. The outer diameter of the coupling cavity is approximately equal to the outer diameter of the acceleration cavity resonant at the same frequency, distinguishing the design from prior art coupling structures that are not open to direct electromagnetic interaction with the accelerated electron beam. The regions of the cavity near the inner and outer diameters are enlarged to form triangular cross-sectional volumes, and the intermediate region consists of a pair of narrow, separate parallel plates. Therefore, the magnetic and electrical components of the fundamental mode electromagnetic resonance within the cavity are separated, i.e. by concentrating the magnetic field in the inductive end region of the coupling cavity and the electric field in the capacitive region between the parallel plates. do.

Coupling is accomplished through a pair of coupling slots cut 180 apart in the web between the coupling cavity and the acceleration cavity. This maintains symmetry about the beam axis and minimizes beam perturbations. Since the magnetic field in the coupling cavity is concentrated within this region and the electric field is negligible, the magnetic coupling is severe while the electrical coupling is at a minimum. This is the optimal coupling condition for high efficiency operation. A relatively small slot blocks enough flux for coupling. This minimizes the effect of the coupling on the distribution of the electric field in the accelerating cavity. Furthermore, by rotating the coupling slot tt - 90 revolutions in each half-acceleration cavity, the coupling slot is at maximum separation, thereby further reducing the direct coupling between the acceleration cavities through the slot.

This reduction increases power flow and accelerator stability.

Also, since the cavity is separated from direct interaction with the beam, transverse beam dispersion modes and ineffective parasitic modes cannot be excited by beam-cavity interactions.

These and other structural and operational features of the invention include:
It will become clearer upon reference to the following detailed description and accompanying drawings in which preferred embodiments and further aspects are illustrated.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawings, reference numerals are used to designate parts throughout the various views, and FIG. 2 shows a short partial cross-section of a structure in accordance with the present invention. It consists of a small radius coaxial structure 10, which has a coupling cavity 1 provided in the webs 1, 4 between the acceleration cavities 16.
2 to increase the magnetic casting conductivity in areas near the inner and outer diameters of the coupling cavity. Essentially a material like %TM6to whose bonding structure increases the intrinsic field distribution of a simple coaxial cavity while making the cavity smaller in all directions. Thin flat area 18 between the widened aura areas 200
acts as an effective capacitor.

The electric field is concentrated in a flat region 18H away from the coupling slot 22 as shown in FIG. The concentration of magnetic fields within the widened region 20 is an ideal coupling opportunity. The expanded region 20 is more triangular in shape than shown, but hemispherical or elliptical shapes may also be used. The two slots 22 are cut 180 degrees oppositely to each other with respect to the beam axis entering each acceleration cavity 16, thereby.

Symmetry about the beam axis is maintained. A relatively small slot can provide adequate membrane bonding of 1 and the first order membrane bond Km can be reduced to a negligible amount by rotating the slot 90 times around the beam axis on opposite sides of each web 14 of each cell. It can be made small. Its design is also. By increasing the slot width and the length of the arc, a very high value of 1 can be obtained while keeping the value of ◇ to an acceptable value. O8 inside
- The 10-type design of the band accelerator structure has several size constraints.First, the outer diameter of the coupling cavity should be approximately equal to the outer diameter of the acceleration cavity.Second. In order to maintain mechanical tolerance, parallel plate spacing 1 is 311I.
I couldn't make it less than m. Third, the minimum wall thickness of 3111 m for the S-band cavity was to be maintained in all respects for mechanical stability and thermal conductivity.

Before the coupled cavity prototype was designed, an acceleration cavity with Kuepp thickness of %9su* was optimized for maximum shunt impedance using the cavity program LALA. (
'ComputerDe' by H, 0, Hoyt et al.
signed 805MHz Proton Lin
ac Oav eye tea'.

ILlevel of 8cient1fic
Instruments, Vol.

37.1966, page 755) Inner radius 3.58
A cavity with a theoretical shunt impedance of 124 M-o hrr 7 m per unit length was developed. next,
Cavity code L Person 00 was used to design a coaxial cavity subject to the constraints described above. (Konra
d. “ALLnear Accelerator Ca
Vity 0ode Baaed on the Fin
目e Element Method', Oompu
ter Pl+ysicaOormiunlcatio
The program was run at a frequency 5% higher than the operating frequency in the cavity due to the expected effect of the coupling slot. The size and location of the coupling slots were determined using the Lm00 magnetic field values. A coupling slot 22 with an arc length of 45 and a width of 5 squares was selected and placed along the outer edge of the acceleration cavity 16. In practice, smaller and larger slots can be utilized.

The prototype coupling cavity shown in Figure 2 is 3160MHz without coupling slot and 3Q with coupling slot.
Resonates at 15MHz. However, in the assembled accelerator, the coupling slot is rotated 90 times, which brings the total sinus frequency t-3000 MHz down to ±0.0 of the desired frequency.
Mechanical tuning to within 2 MHx of the prototype accelerator can be easily done by increasing the diameter and decreasing the frequency, and increasing the capacitive gap to increase the frequency, compared to existing prototype accelerators.

Lateral binding structure (Varian As5ociatea
Made by LlooO.

-A accelerator) designed to suit the performance characteristics of
.

It was. It was designed to have the best performance with an output energy of 9.4 MeV consisting of a 7-% acceleration cavity.

Beam simulation program.

We developed a puncher for guides using the LALA field graph file. An incident voltage of 15 kV is applied to a variable field gradle.
nta). Three-cell puncher with a cell length of 44.81m
her) was selected.

The resulting output energy spectrum is shown in FIG. The total length of the guide is 35.9 ffi. The F output from the magnetron is input to the fourth full acceleration cavity. Beak RF power supplied to the guide is 2.3MW
It has a pulse of 4.3 microseconds.

Table 1 shows the accelerator design parameters.

Table 1 Performance summary Accelerator length 35.9 crIM number of cavities, A frequency 2997 h Coupling: Nearest neighbor (Ks) 3.3 Chi nearest neighbor (K bird)
O, Oa% RP peak output 2.3MW RF pulse width 4.3 microseconds E, e□/g,
8.1 Travel time coefficient
0.916 Theoretical Qo 16.
000 Theoretical Z T /L f
24 M-ohrry'm Design value Measured value Q, 14,400 13.5
00Qex, ? , 200
6.600Beta6 =Qn/Qext 2
-0 2.05Z T/L 1
11 M-ohrrym 104M-ohrrV/m
Both the coupling and acceleration cells are made of oxygen-free high conductivity (oxygen-f) to create an unlikely guide post-blaze tuning.
ree high conductivity (OH
FO)) Precisely machined from copper. The acceleration cell is separated in half and tuned to within ±0.1 MI (z) of the desired frequency. The desired frequency is determined by the scattering amount of successive stacks (4 tacks) of 2.4 and 6 half waves. .The coupling cavity is separated in half and ± for 3QQ9MHz.
Adjusted within 0.2mm, which is approximately 2994MH
Give the fully connected cell frequency of z. Because of the sensitivity of the bulk region of the coupling cavity to the gap length, the effect of the blaze had to be acceptable. The total combined cavity frequency varies by 240 Mllz/III of the additional space between half cells. The blaze procedure adds an average of 20 microns of copper between the cells, resulting in an increase of approximately 5 MHz. A prototype accelerator built to test the new coupling cavity is illustrated in FIG. A series of semicircular pieces 30 of 0FHO steel are placed back to back to each other as shown.
Blaze alternately with facing each other. A specially modified coupler half cell 32 is charged with microwave energy. A slightly shorter buffing strip 34 is used to increase the beam velocity more than to match the phase of the acceleration section. Beam source 36 directs the beam into the puncher. A high energy beam impinges on a target or window 38 at the end opposite the end of the beam source.

and the measured dispersion curve of the blazed guide.

The theoretical version is shown in FIG. The theoretical curve is f
= 2996.69MHz and '*yfuvyy =
Assume a double periodic structure with 3001.5 MHz acceleration. Bead drop data is shown in FIG. The guide is 2.05 V8WR (Be111.)
with.

Measured Qo of 13,500 and Qext of 6.580
has. 1 for nearest neighbor coupling is 3.3%.

The next nearest neighbor coupling was 0.04%. Coupling cavity frequency is 3001.5FvlHz±1.5MHz
Met.

The guide dimensions before and after praising showed a greater than average increase per cell, approximately 30 microns. This accounts for the high coupling cavity frequencies.

It is evident in the dispersion curve that zero acceleration cells are ±0.0 of the fixed frequency. It is maintained tuned to within I MHz. These frequency changes were acceptable and the guides were not subjected to typical boss blaze tuning.

The invention is not limited to the preferred embodiments described so far, but modifications thereof may be made without departing from the scope of the invention, and the features of the invention are summarized in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a partial cross-sectional view and an end view of a four standing wave linear accelerator in the prior art. Figure 1a shows the on-axis bond structure. Figure 1b shows the coaxial coupling structure. Figure 1c shows the lateral joint structure. Figure 1d shows a cyclic bond structure. FIG. 2A is a partial vertical cross-sectional view of a standing wave linear accelerator according to the present invention. FIG. 2B is an end view of a standing wave linear accelerator according to the present invention. The third factor indicates the part passing through the joint tunnel according to the present invention, in which point a' indicates the field vector. FIG. 4 is a theoretical energy spectrum for an accelerator according to the present invention. FIG. 5 is a longitudinal cross-sectional view of a complete accelerator according to the invention. FIG. 6 shows measured and theoretical dispersion curves for nine accelerators according to the present invention. Description of main symbols 12 - one bonding slot 14 - web 16 - acceleration slot 18 - one flat area 20 - one widened area 22 - one coupling slot 36 - beam source Patent Applicant Parian Assoc. Incoholated

Claims (1)

[Claims] 1. A linear standing wave charged particle beam accelerator for accelerating a particle beam source, comprising: a) a plurality of standing wave electromagnetic coupling acceleration cavities connected in series and having approximately the same resonance frequency; the accelerating cavities are positioned such that the particle beams travel longitudinally through the cavities forming a beam axis, the accelerating cavities being of a convolutional shape about a beam axis having a diameter of the cavity, The cavity is an acceleration cavity where it is electromagnetically coupled. b) a coupling cavity equidistant between said acceleration cavities and substantially within said cavity diameter, said coupling cavity having an inner rim forming an inner diameter and an outer rim forming an outer diameter; in the form of a flat annular ring, the coupling cavity being coaxial with the beam axis, and the gap between the flat annulus intermediate the inner and outer diameters being substantially increased by the gap at the inner and outer rims. c) means for coupling the acceleration cavity to the coupling cavity, the coupling cavity separating the cavity from excitation by the beam; c) means for coupling the acceleration cavity to the coupling cavity; an accelerator that has the means for 2. The apparatus of claim 1, wherein the coupling means comprises a first pair of slots coupling a first main cavity to the outer rim of the coupling cavity; each of which is in the shape of an arc of a circle about the beam axis, the slots being generally 180° apart from each other with respect to the beam axis; a second pair of slots between said first pair of slots, the second pair of slots being similar in shape and location to said first pair of slots. 3. The device of claim 1, wherein the gap at the outer rim of the coupling cavity substantially widens to the gap at the inner rim. 4. The apparatus of claim 2, wherein the second pair of slots is 90° apart from the first pair of slots with respect to the beam axis. Device. 5. The device of claim 1, wherein the coupling cavity in the form of a flat annular ring enters the cross-section of the generally triangular inner and outer diameter cross-sections from the center. 6. The device according to claim 5, wherein the triangular shape is an isosceles triangle, and vertices between the equilateral sides of the triangular shape are formed, and the apexes of the triangular shape meet the cross section of the coupling cavity. device that is oriented in the direction closest to the center of the 7. An apparatus as claimed in claim 1, comprising means for maintaining a support wall of sufficient thickness to dissipate heat. 8. An improved narrow standing wave charged particle beam accelerator having a beam source, a power source, an acceleration cavity forming the largest diameter of the acceleration cavities, and a coupling cavity, wherein the accelerating cavity has a coupling cavity in the web between the acceleration cavities. forming a coupling cavity, the coupling cavity having an annular ring shape having a center, an inner diameter and an outer diameter, the outer diameter being substantially the largest diameter of the coupling cavity; An accelerator forming an annular ring gap whose inner and outer diameters are substantially larger than the middle.