RU2808701C1 - Accelerating resonator - Google Patents

Abstract

FIELD: resonator.

SUBSTANCE: charged particle accelerator technique intended for use in linear accelerators for accelerating hydrogen ions in the energy range from 4 MeV to 200 MeV. The accelerating resonator is a high-frequency (HF) resonator operating on the TM010 oscillation, including a housing, drift tubes without focusing elements that are placed outside the resonator, and rods for attaching the drift tubes to the resonator body. The number of acceleration periods in the resonator is determined from the conditions of the dynamics of the accelerated particles and the power of the HF power sources. In order to reduce the influence of TE111 vibrations on the performance characteristics of the resonator by increasing the frequency, a conductor is inserted into the middle of the resonator in a plane perpendicular to the axis of the resonator, its ends fixed to the resonator body, symmetrical relative to the plane of the rods, and the length of the conductor is selected in the range from 0.42λ to 0.495λ, where λ is the wavelength of the operating vibration.

EFFECT: providing greater freedom to realize the optimum performance of the resonator.

3 cl, 11 dwg

Description

The invention relates to charged particle accelerator technology and is intended for use in linear accelerators of hydrogen ions at medium energies.

Linear accelerators of hydrogen ions for average, less than 200 MeV, energies of accelerated particles are used either as injectors into complexes of cyclic accelerators that accelerate particles to energies of tens of GeV or higher, or as the initial parts of linear accelerators of intense beams up to energies of the order of 1 GeV.

Currently, an integral part of ion accelerators is an accelerator with Spatially Uniform Quadrupole Focusing (POKF, [I.M. Kapchinsky, V.A. Teplyakov, PTE, N2, p. 19, 1970]), which captures particles in the acceleration mode and accelerates them up to energy (3-5) MeV.

To accelerate hydrogen ions to higher energies, on the order of 100-200 MeV, the Alvarez accelerating structure has traditionally been used ([L.W. Alvarez, Physical Review, v. 70, p. 799, 1946]). The Alvarez structure is a long resonator operating on a TM010 type vibration and containing up to several dozen drift tubes inside, which house focusing elements - quadrupole lenses. The options, characteristics and features of Alvarets resonators are discussed in detail in the book [B.P. Murin. and others. Linear ion accelerators, Atomizdat, Moscow, 1978, T. 2, paragraph 2.5, Accelerating structure with drift tubes and its modifications]. In modern foreign literature, the Alvarez structure is classified as Drift Tube Linac (DTL). The main difficulties and limitations in the creation and operation of the structure are caused by the large size of the resonator housings, the need to have elements for stabilizing the distribution of the accelerating field in a long resonator, and high requirements for the accuracy of the alignment of the drift tubes in which the focusing elements are located.

To eliminate the shortcomings of the Alvarez structure, an accelerating structure SDTL - Separated Drift Tube Linac, [T. Kato, Proposal of a Separated-type Drift Tube Linac for a Medium-Energy Structure, KEK report 92-10, 1992]. The SDTL resonator circuit is shown in Fig. 1. Focusing elements - quadrupole lenses or lens doublets, are located outside the resonator. This makes it possible to reduce the diameter of the drift tubes and optimize their shape, both to increase the RF efficiency, determined by the value of the effective shunt resistance, and to significantly simplify the manufacturing technology. The absence of focusing elements in the drift tubes radically weakens the requirements for the accuracy of alignment of the drift tubes when they are installed, by means of fastening rods, on the resonator body. Focusing elements located outside the resonator are more accessible for adjustment and maintenance. Like Alvarez resonators, SDTL resonators operate on a TM010 type oscillation and the length of the acceleration period - the distance between the centers of adjacent accelerating gaps - is equal to D=βλ, where β is the relative speed of the accelerated particles, and λ is the wavelength of the operating oscillation. The number N of acceleration periods in the resonator is selected both from the conditions of particle dynamics to ensure stable conditions for the transverse and longitudinal motions of particles, and from the conditions of available RF power. Therefore, SDTL resonators contain a relatively small number of acceleration periods N<10 and their length, L=Nβλ, is small compared to Alvarez resonators, which simplifies the process of manufacturing resonator bodies. Relatively short resonators operating on TM010 type oscillations do not require additional elements to stabilize the distribution of the accelerating field, and such elements are not used in SDTL resonators, which significantly simplifies the procedure for tuning the resonators and further simplifies the design of the resonator body.

At an operating frequency of 324 MHz, the SDTL structure is implemented in the linear accelerator of the J-PARC complex (Japan Research Accelerators Complex) in the H ion energy range ^- from 50 MeV to 191.5 MeV, [T. Ito et. al., RF Characteristics of the SDTL for J-PARC, Proceedings of LINAC 2006, Knoxville, Tennessee, USA, 2006].

A modification of SDTL is the CCDTL - Side Coupled Drift Tube Linac structure, developed based on the structure proposed in [J. Billen et. al., A New RF Structure for Intermediate-Velocities Particles, Proceedings of LINAC 1994, Tsukuba, Japan, 1994]. The CCDTL circuit is shown in Fig. 2. To be powered by one powerful RF source, several SDTL resonators are combined by communication cells into a single one. At an operating frequency of 352 MHz, the CCDTL structure is implemented in the Linac 4 linear accelerator of the CERN accelerators, [A.G. Tribendis et.al., Construction and RF Conditioning of the Coupled-Cell Drift Tube Linac (CCDTL)for Linac4 at CERN, Proceedings of LINAC 2014, Geneva, Switzerland, 2014].

Studies have shown that the maximum RF efficiency of the SDTL structure occurs in the region of hydrogen ion energy of the order of 20 MeV, [S. Bourat, S. Perraudin, A DTL with Short Tanks and External Focusing for High Power CW Linac, Proceedings of 1997 Particle Accelerators Conference, Vancouver , Canada] and the structure is applicable at lower ion energies. For use at an operating frequency of 3 GHz in linear accelerators for proton therapy, the SDTL structure is designed for energies above (5-7) MeV and in foreign literature is designated as SCDTL (Side Coupled Drift Tube Linac), [L. Picardi et.al., The First Module of the 3 GHz Side Coupled Drift Tube Linac - Numerical Studies of RF Properties and Cold Test Results, Proceedings of EPAC 2000, Vienna, Austria, 2000]. The SCDTL circuit and the accelerating module of coupled SDTL cavities, powered by a single klystron, are shown in Fig. 3.

The SCDTL and CCDTL modifications are based on SDTL accelerating resonators with focusing elements located outside the resonators, and the SDTL structure is applicable in a wide range of operating frequencies and in the energy range of hydrogen ions from 5 MeV to 200 MeV.

The dispersion properties of SDTL resonators are considered in [V.V. Paramonov, Dispersion Properties of the SDTL Accelerating Structure, p. 299, Collection of Scientific Proceedings of the LX International Conference LALLA3-2023, National Research Nuclear University MEPhI, Moscow, 2023].

In a cylindrical waveguide, the main, lowest frequency wave is TE11. In a cylindrical resonator like the SDTL, the oscillation frequency of TE111 can be qualitatively estimated by the relation:

where c is the speed of light, ν ₁₁ is the first root of the equation J' ₁ (ν)=0 for the Bessel function J ₁ and R is the resonator radius. In a resonator with drift tubes, the rods for attaching the tubes are a violation of axial symmetry, which leads to splitting of the doubly degenerate vibration TE111. The lowest frequency of the split TE111 modes is the oscillation with the antinode of the magnetic field component H _z in the plane of the rods. The vector distribution of the magnetic field of the lowest mode TE111 in the resonator is shown in Fig. 4, where:

1 - drift tubes;

2 - rods.

The distribution of the electromagnetic field in the resonators, the determination of the dimensions of the elements and the determination of operating frequencies were simulated using the COMSOL software package.

A measure of the energy efficiency of the accelerating element is the value of the effective shunt resistance Ze. In FIG. Figure 5 shows graphs of the oscillation frequencies of TE111 and TM011, in units of operating frequency, for SDTL resonators with N=5, the dimensions of the elements of which are optimized to obtain the maximum value of Ze in the energy range of hydrogen ions from 5 MeV to 200 MeV. It can be seen that with increasing particle energy, the TE111 oscillation frequency quickly approaches the operating frequency of the resonator, and in the range from 20 MeV to 80 MeV, the TE111 mode is in close frequency proximity to the operating mode TM010. As the ion energy increases, the optimal resonator radius to ensure the maximum value of Ze decreases and the TE111 oscillation frequency increases. If the oscillations are close in frequency, the phenomenon of mode interaction appears [V.B. Steinshleger, Phenomena of interaction of waves in electromagnetic resonators, Moscow, State Publishing House of the Defense Industry, 1955]. In the general case of taking into account the influence of many fluctuations in the book [Murin B.P. and others. Linear ion accelerators. Atomizdat, Moscow, 1978, T. 2 paragraph 2.3, Irregularities of field distribution in long resonators] it is shown that the field E in a resonator with a perturbation δV consists of the field of an unperturbed resonator E ₀ with the addition of other modes of the resonator _Em ,

where μ ₀ and ε ₀ are the magnetic and dielectric constants of vacuum, W ₀ and W _m are the stored vibration energies. When the frequencies are close additives in the distribution of the working field increase and the design performance characteristics of the resonators are not achieved.

Therefore, the possibility of approximation, or even coincidence in frequency, in a certain energy range of ions, oscillations of TE111 and working TM010 is unacceptable and is an additional limiting factor that must be taken into account when developing SDTL resonators.

The purpose of the invention is to eliminate the possible influence of the TE111 oscillation on the performance characteristics of SDTL resonators, which occurs when this mode interacts with the working one, by shifting this oscillation in frequency.

The essence of the invention is illustrated below by the accompanying drawings.

An additional element is introduced into the design of the SDTL resonator in a plane perpendicular to the resonator z axis - a conductor, the ends of which are attached to the resonator body, as illustrated in Fig. 6, where:

1 - drift tubes;

2 - rods for fastening drift tubes;

3 - resonator body;

4 - inserted conductor;

5 - places where the conductor is attached to the resonator body.

Various insertion conductor configurations are possible, as shown in FIG. 7, where:

1 - drift tubes;

2 - rods for fastening drift tubes;

3 - resonator body;

4 - inserted conductor;

5 - places where the conductor is attached to the resonator body; '

6 - plane of the rods - plane of symmetry of the resonator;

S is the area of the closed loop formed by the introduced conductor and part of the cylindrical generatrix of the resonator body;

α is the opening angle of the inserted conductor in the shape of an arc in Option 1 and Option 4;

r is the average radius of the circle of the inserted conductor in the form of a part of a torus in Option 2 and Option 4;

L _c is the length of the inserted conductor, along its contour, from one attachment point with the resonator body to another, for all options.

In option 1 in Fig. 7, the inserted conductor is formed in an arcuate shape with an arc opening angle α and consists of two straight segments and a part of a toroid. In option 2, the introduced conductor is formed in the form of a part of a toroid with an average radius r and centered on the cylindrical surface of the resonator. In option 3, the inserted conductor is formed in the form of a straight cylinder segment. In option 4, as in option 1, the inserted conductor is formed from two straight sections and a part of a toroid centered on the axis of the resonator.

Physically important parameters are the length of the inserted conductor, Lc, along its contour, from one attachment point with the resonator body to another, and the area of the closed loop S formed by the inserted conductor and part of the cylindrical generatrix of the resonator body.

The lowest-frequency TE111 oscillation has an antinode of the magnetic field component Hz in the plane of the rods, which is also the plane of symmetry of the resonator. The conductor for the field of this oscillation, located in a plane perpendicular to the axis of the resonator, is short-circuiting. Strong surface currents are induced on the surface of the conductor, suppressing the field of the initial TE111 oscillation in the circuit formed by the conductor and the cylindrical wall of the resonator, as shown in Fig. 8. As a result, the oscillation frequency of TE111 in the resonator with an introduced conductor increases. The efficiency, or speed, of increasing frequency with increasing conductor length Lc and the corresponding increase in loop area S, is determined by the proportion of the magnetic flux of the TE111 oscillation blocked by the loop. To increase this efficiency, it is necessary to take into account the structure of the magnetic field of the lowest TE111 vibration. The distribution of the H _z component is described in cylindrical coordinates by the relation:

where the angle φ is measured from the plane of symmetry of the rods. The maximum value of H _z is achieved on the cylindrical wall of the resonator at r=R, z=L/2, φ=0,(π). Therefore, the contour of the inserted conductor must be symmetrical relative to the plane of the rods, as shown in Fig. 7 and the conductor should be at an equal distance from the end walls of the resonator, as shown in Fig. 6. The choice of the contour shape is carried out taking into account numerical modeling of field distributions and the frequency of the lowest TE111 vibration.

The simulation results show that the greatest efficiency, i.e. the rate of increase in the frequency of the TE111 mode with increasing conductor length Lc is provided by the straight conductor shown in Option 3 in Fig. 6.

Oscillations of the TM01n type, where n is the number of field variations along the z axis, belong to the same family with the operating mode TM010. The frequencies of the TM01n modes can be qualitatively estimated by the relation:

At the energy of accelerated hydrogen ions ~ 200 MeV, β ~ 0.56, the TM0111 type vibration also significantly approaches the operating mode in frequency, which limits the possible number of periods of the SDTL resonator to N<6.

For vibrations of the working family - type TM01n, the introduced conductor is a resonant element - a half-wave vibrator with grounded ends. It implements a TEM-type oscillation with an antinode of the electric field in the middle of the conductor. The frequency of this oscillation depends on the length of the conductor:

Therefore, the nature of the influence of the introduced conductor on the frequencies of oscillations of the TM01n type depends on the ratio of the frequency of the TEM oscillations developing on the conductor with the frequencies of the TM01n modes in the resonator. Of practical interest is the interaction of the TEM mode with three oscillations - the operating mode TM010 and the nearest resonator modes TM011 and TM012.

In FIG. Figure 9 shows, in units of the operating frequency of the resonator, graphs of the calculated dependences of the oscillation frequencies in the SDTL resonator, N=5, W=50 MeV, on the relative length Lc/λ of the conductor shown in Fig. 6, option 3. Physical explanation of the behavior shown in Fig. 9 graphs are fully consistent with the general theoretical principles [V.B. Steinshleger, Phenomena of interaction of waves in electromagnetic resonators, Moscow, State Publishing House of the Defense Industry, 1955, Chapter 2. Inhomogeneities in resonators and coincidence phenomena]. With a short conductor length L _c <0.4λ, the TEM oscillation frequency on the conductor is significantly higher than the oscillation frequencies of TM010, TM011 and TM012 in the resonator and no interaction is observed. At 0.4λ<L _c <0.5λ, the interaction of modes is manifested, which is expressed in a decrease in the oscillation frequencies originating from the TM011 and TM012 modes, the field distribution of which now has a TEM oscillation component on the conductor. The dimensions of the interaction region are determined by the oscillation coupling coefficient k _c , which can be defined as the integral of the overlap of oscillation fields over the volume of the resonator V:

where μ ₀ and ε ₀ are the magnetic and dielectric constants of vacuum, W _TM01n and W _TEM are the stored vibration energies. From the symmetry properties of the distributions of oscillation fields it follows that for a conductor placed in the middle of the resonator and in a plane perpendicular to the axis of the resonator, the connection between TEM oscillations on the conductor and TM010 in the resonator is minimal, and the connection between TEM oscillations and TM011 is maximum.

When oscillations interact, the field of the resulting oscillation contains components of both resonator modes and conductor modes. With a continuous increase in Lc/λ, the field of one oscillation is gradually replaced by the field of another. At L _c >0.51λ, the frequency of oscillations originating from the TM010 mode in the resonator quickly decreases, which is reflected by the behavior of the blue line in Fig. 9, and this oscillation is already a TEM mode on the introduced conductor. And the distribution of the field of the operating oscillation TM010 has a mode that was originally the TM011 mode of the resonator and was transformed in the process of interaction of oscillations.

With an increase in Lc/λ, the oscillation frequency TE111, which was initially lower than the operating frequency, continuously increases and the increasing beneficial effect of exceeding the frequency f _TE111 >f _TV010 appears.

Shown in FIG. 9, the behavior of the dependences of oscillation frequencies on the length of the conductor Lc is characteristic and for conductors of other shapes the differences are contained in the rate of increase in the frequency of the TE111 mode with increasing Lc and the magnitude of the coupling coefficient of the TM01n modes of the resonator and the TEM mode on the conductor.

There are two areas of application of the beneficial effect without compromising the performance of the resonator.

The region of pre-resonance interaction between TM01n vibrations and TEM vibrations at 0.4λ<L _c <0.495λ. In this region, the TE111 oscillation frequency is already higher than the operating frequency and increases with increasing Lcλ. But at the same time, the frequency of the interacting oscillation TM011+TEM decreases. Therefore, by increasing Lc/λ, this region is limited by the equality of approaching frequencies. Moreover, the additional losses of RF power dissipated on the conductor do not exceed 0.4% of the power of RF losses in the resonator.

In the post-resonance region L _c >0.52λ, the frequency of TE111 oscillation continues to increase, while the frequency of the TEM mode quickly decreases. For optimal frequency separation of the operating vibration with all others, this region is limited from above by L _c ~0.595λ, which gives equality of frequencies of the TE111 vibration and the composite vibration TM011+TEM+TM010, which replaces the TM011 mode in the original resonator. In this case, the frequency separation of the operating mode with the nearest higher ones is better than that of the original resonator Lc/λ. But the radius of the resonator at the operating vibration TM010 R~0.61λ, taking into account the capacitive load by the drift tubes. The placement of a conductor of large length L _c ~0.595λ leads to a conflict with the placement of drift tubes or the location of the conductor in close proximity to the resonator aperture - the area where the beam passes. Only the conductors of option 3 shown in FIG. 7, with L _c <0.55λ, or option 4, with L _c <0.58λ.

Additional RF power losses dissipated on a conductor of this length do not exceed 0.6% of the RF power losses in the resonator.

In the pre-resonant interaction region, the positive effect of increasing the frequency of the TE111 mode while simultaneously improving the frequency separation of the working mode with the composite vibration TM011 + TEM is achieved by introducing two conductors located at distances L/3 from the end walls of the resonator, also located in planes perpendicular to the axis of the resonator. The placement of the conductors is illustrated in Fig. 10, where:

1 - drift tubes;

2 - rods for fastening drift tubes;

3 - resonator body;

4 - inserted conductors;

5 - places where conductors are attached to the resonator body.

The components of the oscillation fields TM01n and TE111 in the resonator are distributed as follows:

where ν ₀₁ is the first root of the equation J ₀ (x)=0 for the Bessel function J ₀ . When placing conductors at distances z=L/3 and z=2L/3, the total blocking effect of two conductors for the Hz component of the TE111 oscillation increases by times, and the coupling coefficient of the TEM conductor modes with the TM010 and TM011 modes of the resonator practically does not increase.

Similar to Fig. 9, but for two conductors, at distances z=L/3 and z=2L/3, graphs of the calculated dependences of oscillation frequencies in the SDTL resonator, N=5, W=50 MeV on the relative length Lc/λ of the conductors, FIG. 6, option 3, are shown in Fig. eleven.

In the pre-resonant interaction region, two introduced conductors provide approximately times greater separation in operating oscillation frequency with other modes. The optimal value of the conductor lengths decreases by approximately 0.02λ and the additional losses of RF power dissipated on the conductors do not exceed 0.8% of the RF power losses in the resonator.

In the post-resonance region of interaction of two conductors at a frequency below the operating one, there are already two TEM-type oscillations on the introduced conductors. At the operating frequency with the field structure TM0101, there is an oscillation converted from the TM012 mode as a result of the interaction of oscillations. And the TM011 mode, which is the closest in frequency to the top, is formed from the converted TM013 mode. The positive effect of increasing the frequency of the TE111 mode is realized. Also, when the length of the introduced conductors is reduced to Lc~0.56λ, a greater frequency separation of the operating mode with the nearest ones is realized than in the original resonator. Additional RF power losses dissipated on the conductors increase to 2.0% of the RF power losses in the resonator. In general, the use of two introduced conductors in the post-resonance interaction region gives a positive effect, but no advantages have been identified compared to the use of one conductor in this area.

Claims (3)

1. An accelerating resonator for accelerating hydrogen ions in the energy range from 4 MeV to 200 MeV, operating on the TM010 oscillation, including a housing, drift tubes without focusing elements and rods for attaching the drift tubes to the resonator body, characterized in that in order to reduce the influence of oscillation TE111 on the performance characteristics of the resonator by increasing the frequency, a conductor is inserted into the middle of the resonator in a plane perpendicular to the axis of the resonator, its ends fixed to the resonator body, symmetrical relative to the plane of the rods and the length of the conductor is selected in the range from 0.42λ to 0.495λ, where λ is the operating wavelength fluctuations. 2. An accelerating resonator for accelerating hydrogen ions in the energy range from 4 MeV to 200 MeV, according to claim 1, characterized in that in order to increase the frequency separation between the operating vibration and the TE111 vibration into the resonator in planes perpendicular to the axis of the resonator located at a distance of one one-third of the length of the resonator, two conductors are inserted, the ends fixed to the resonator body and symmetrical relative to the plane of the rods, and the length of the conductors is selected in the range from 0.40λ to 0.47λ. 3. Accelerating resonator for accelerating hydrogen ions in the energy range from 4 MeV to 200 MeV, according to paragraphs. 1 and 2, characterized in that in order to increase the separation in frequency of the operating oscillation with other oscillations in the resonator that are closest in frequency, the length of the conductor or conductors is selected in the range from 0.53λ to 0.59λ.