RU2420045C1 - Method for inductive ion acceleration - Google Patents

Abstract

FIELD: physics. ^ SUBSTANCE: particles are accelerated in a magnetic field increasing over time, in which the average value of magnetic flux density inside the equilibrium orbit Bav is much less than the magnetic flux density at the equilibrium orbit. B0 ("Bav<<B0"), In order to keep the radius of the equlibrium orbit constant during acceleration, special relationships between amplitude-time characteristics of magnetic flux density on the orbit and the induced acceleration voltage are satisfied. In order to realise strong focusing, magnetic field with a large alternating gradient is formed on the orbit. The absence of resonant accelerating systems and synchronisation of accelerating pulses with pulses of the time-of-flight beam system enables to accelerate particles in a wide range of particle energy (velocity) with constant equilibrium radius during acceleration. ^ EFFECT: reduced weight and size of the accelerator with a wide range of ion velocity during acceleration, reduced cost of the accelerator and wide range for regulating energy of the accelerated ions, which is necessary when using accelerators in medicine and scientific research. ^ 3 cl, 5 dwg

Description

The invention relates to accelerator technology and can be used to create induction cyclic ion accelerators with controlled kinetic energy in medicine and scientific research. One of the most important applications of ion beams with controlled kinetic energy in medicine is the treatment of cancer patients.

Known ion accelerators with adjustable kinetic energy, specially designed for medical purposes, are a compact superconducting cyclotron [1], which implements a method consisting in the fact that particles with a continuously growing equilibrium radius are accelerated in a constant magnetic field in time, and accelerating duants powered by high-frequency electric voltage, and a compact phasotron [2], in which the acceleration of particles is carried out in an increasing magnetic field in time in an equilibrium orbit of constant radius, and nsnaya frequency accelerating cavities varies according to the frequency of revolution of the particles. In [3, 4], a variant of the induction accelerator of charged particles of the betatron type was proposed.

The disadvantages of a compact superconducting cyclotron [1] are the large mass of magnetic material (350 tons) and the cost of the installation, in which the regulation of ion energy is carried out by slowing the speed of accelerated ions in a linear ion accelerator.

The disadvantages of the compact phasotron [2] are the large weight (40-50 tons) and the dependence of the frequency of the accelerating voltage on the speed of accelerated ions, which determines the time of flight of ions in orbit. The limited tuning range of the resonant frequency of the accelerating system of the phasotron requires limiting the speed range of the accelerated ions and the use of additional injectors with high beam injection energy.

Induction accelerators [3, 4] and the criteria proposed in these works have a small mass of magnetic material and do not have tunable resonant accelerating structures, but they are designed for induction acceleration of light particles, which quickly gain speed at first revolutions, and the main acceleration process at a particle velocity close to the speed of light.

As a prototype, select the method [4]. This method consists in forming a magnetic field increasing in time with an average value of magnetic induction much less than the magnetic field induction in orbit; form an alternating magnetic field gradient in orbit with a field index much larger than unity (strong focusing); charged particles are injected into it; accelerate charged particles by pulses of an induction electric field with a pulse duration much shorter than the rise time of the field in orbit, and a pulse repetition rate equal to the frequency of revolution of the particles in orbit, and accelerated particles are output.

The technical result of the invention is to reduce the weight and dimensions of the accelerator with a wide range of ion velocities during the acceleration process, reduce its cost and increase the range of regulation of the energy of accelerated ions, which is necessary when using accelerators in medicine and scientific research.

The method consists in forming a magnetic field increasing in time with an average value of magnetic induction much lower than the magnetic field in orbit; form an alternating magnetic field gradient in orbit with a field index much larger than unity (strong focusing); charged particles are injected into it; accelerate charged particles by pulses of an induction electric field with a pulse duration much shorter than the rise time of the field in orbit, with a pulse repetition rate equal to the frequency of revolution of ions in orbit, with a duration of the accelerating part of the induced pulses, constant during acceleration and equal to half the period of revolution of ions at the end acceleration cycle, and their amplitude is ΔU = L · R ₀ · dB / dt,

where L is the perimeter of the orbit taking into account the straight sections, R ₀ is the radius of the equilibrium orbit of ions, dB / dt is the rate of change of magnetic induction in orbit. The pulses of the accelerating induction electric field form the tips of the pulses with an inclined table to ensure the stability of the azimuthal motion. In the regime of single-turn particle extraction, the pulse front of the deflector of the output system is formed less than or equal to half the ion revolution period at the end of the acceleration cycle, and the pulse duration is greater than or equal to half this period.

Distinctive features of the claimed method are as follows.

The duration of the accelerating part of the induced pulses is formed constant during the acceleration process and equal to half the ion revolution period at the end of the acceleration cycle. This allows us to ensure the azimuthal ("phase") stability of the accelerated particle beam and increase the range of regulation of the energy of accelerated ions.

The amplitude of the induced accelerating pulses is formed equal to ΔU = L · R ₀ · dB / dt, where L is the perimeter of the orbit taking into account rectilinear sections, R ₀ is the radius of the equilibrium orbit of ions, dB / dt is the rate of change of magnetic induction in orbit, which allows the process accelerations to keep the radius of the equilibrium electron orbit constant and to reduce the weight and dimensions of the accelerator with a wide range of ion velocities during acceleration.

To enhance the azimuthal stability of the accelerated particle beam, the apex of the accelerating pulses of the induced voltage is formed inhomogeneous.

In the regime of single-turn output of accelerated particles, the duration of the pulse front of the deflector of the output system is formed for less than half the period of revolution of the particles at the end of the acceleration cycle, and the duration of this pulse is longer than half of this period.

The technical result of the invention using this method, namely, reducing the weight and dimensions of the accelerator with a wide range of ion velocities in the acceleration process, reducing its cost and increasing the range of regulation of accelerated ion energy, which is necessary when using accelerators in medicine and scientific research, is achieved by that the combination of all the essential features of the formula allows us to accelerate ions in the range of relative velocities of 0.01≤β = v / c≤1.0 in an induction accelerator with a rigid focus with a constant radius of the equilibrium orbit, which eliminates the need for additional injector accelerators and many times reduces the volume and weight of the magnetic system and lowers the cost of the accelerator.

In addition, the use of an induction acceleration method to accelerate ions in an increasing magnetic field with an average magnetic field much smaller than the magnetic field in orbit, and with an alternating magnetic field gradient in orbit (strong focusing) can further increase the beam intensity and reduce the weight and cost of the accelerator ions.

The list of drawings.

Figure 1. Accelerator circuit.

Figure 2. Scheme of C-shaped cores with alternating gradient.

Figure 3. The voltage waveform of the power source (a) and the magnetic induction waveform in equilibrium orbit (b).

Figure 4. Scheme of O-shaped cores of the induction accelerating section.

Figure 5. The amplitude-time characteristics of magnetic induction in equilibrium orbit (a) and accelerating voltage on the turns of O-shaped cores (inductors) (b).

Figure 1 shows a diagram of a device that implements the proposed method, where

1) C-shaped electromagnets, 2) O-shaped cores of an induction accelerating system, 3) an injection and extraction system for an ion beam.

The method works as follows. The beam of charged particles (ions) is brought into equilibrium orbit, the beam particles are held in equilibrium by the magnetic field of C-shaped electromagnets (1), accelerated by the electric field induced by O-shaped ferromagnetic cores (inductors) (2) and, when the required energy is reached, they are removed from accelerator.

To implement the proposed method of acceleration, an increasing magnetic field in time in orbit is formed by C-shaped electromagnets with high field gradients n >> 1 and n << - 1. The combination of two types of C-shaped cores allows for rigid focusing of ions. Figure 2 presents a diagram of the C-shaped cores of electromagnets with an alternating gradient. The electromagnets are fed by a rectangular voltage wave V ₀ (figa). The magnetic induction waveform in orbit is shown in FIG.

Particles moving in equilibrium orbit are accelerated by an electric field induced by O-shaped ferromagnetic cores (inductors), which are located in straight sections of the accelerator. Figure 4 shows a diagram of the O-shaped cores of an induction accelerating system. In order for the radius of the equilibrium orbit to remain constant during acceleration, it is necessary to fulfill the condition of constancy in the process of acceleration of the relation

P (t) / qB (t) = const,

where P (t) is the ion momentum, B (t) is the magnetic field induction in orbit, q is the ion charge. This condition is fulfilled when the total voltage induced by O-shaped cores is equal to:

V _accele = R ₀ · L · dB / dt,

where R ₀ is the radius of the equilibrium orbit, L is the total perimeter of the orbit, taking into account the straight sections.

The amplitude-time characteristics of magnetic induction in orbit and accelerating voltage on the turns of O-shaped cores (inductors) are shown in Fig.5. The pulse duration of the accelerating voltage is 1/2 of the period of revolution of the ions at the final stage of acceleration. This duration remains constant throughout the acceleration process. The positive part of the pulse accelerates the ions, and the negative part magnetizes the O-shaped cores and returns them to their original state. The short pulse duration provides a small volume and weight of O-shaped cores.

The repetition period of accelerating pulses depends on the speed of the particles and should correspond to the period of revolution of the particles during acceleration

τ ₀ = L / v ₀ , τ _min = L / V _max

v ₀ is the initial ion velocity, V _max is the velocity corresponding to the final ion energy, τ _min is the pulse repetition period at the end of the acceleration process, τ ₀ is the pulse repetition period at the beginning of the acceleration process, L is the total perimeter of the ion orbit taking into account straight sections.

The starting moment of each subsequent accelerating pulse is determined by the signals of the ion beam monitoring system.

The slope of the peak of the accelerating pulses provides azimuthal ("phase") stability of the launch of ions. The magnitude of the slope of the peak of the pulses is determined by calculation and depends on the total perimeter of the orbit and the type of accelerated ions.

The formation of a magnetic field in the gaps of C-shaped electromagnets with a large radial gradient and the focusing of ions by an alternating field gradient (strong focusing) makes it possible to increase the number of ions accelerated in one acceleration cycle. A large indicator of the magnetic field reduces the amplitude of betatron oscillations and allows you to reduce the size of the vacuum chamber, to reduce the size, weight and cost of C-shaped magnets.

The short duration of the accelerating pulses of the induction field can significantly reduce the cross section of O-shaped cores, their weight and cost.

The synchronization of accelerating pulses with the pulses of the system for measuring the time of flight of an ion beam makes it possible to accelerate ions in a large range of energies (ion velocities) at a constant equilibrium radius and to abandon injector accelerators.

The energy of accelerated ions is regulated by changing the pulse duration of a rectangular voltage wave (Fig.3a), which feeds C-shaped electromagnets, the duration of which determines the amplitude of the leading magnetic field in orbit B _max (Fig.3b) and, accordingly, the final ion energy

E _{ion (max)} = q · V _max R ₀ · c ² / V _max ,

where q is the ion charge, R ₀ is the radius of the equilibrium orbit, c is the speed of light, v is the speed of ions.

As an example, we consider the main parameters of a proton accelerator with an energy of 200 MeV. At an energy of 200 MeV, the relativistic factor of protons is γ = 1.2, and their relative velocity is β = 0.553. When the amplitude of the magnetic induction B _max = 1 T, the radius of the equilibrium orbit R ₀ is:

R ₀ = P _max / qB _max = Mc ² βγ / eB _max · c = 2.2 m,

where P _max is the proton momentum, q is the proton charge, M is the proton mass, s is the speed of light, β = v / s is the relative speed of the protons, B _max is the maximum amplitude of the magnetic induction of the field.

If the total perimeter of the orbit, taking into account the straight sections, is 15 m, then the period of revolution of the accelerated protons will be τ _min = 90 · 10 ^-9 s. To implement the proposed method of acceleration, the duration of the accelerating pulses should be equal to τ _accele = 45 · 10 ^-9 s.

Losses during high-frequency magnetization reversal of O-shaped cores will be small if their maximum induction does not exceed ΔВ = 0.1 T. With a total core cross-section S = 20-400 cm, the value of the accelerating voltage will be:

V _USAC = ΔV · S / τ = _{the Start-88} kV 4.4kV

To implement such an accelerating system, 10 ferrite rings 120 × 80 × 10 mm in size (S = 20 cm) or 20 rings 240 × 80 × 25 mm in size (S = 400 cm), the primary turns of which are excited by pulses with an amplitude of about 450 V ( S = 20 cm) or 4500 V (S = 400 cm).

From the condition of constant equilibrium radius of the orbit we find:

dB / dt = V _max / T = V _accele / LR ₀

Rise time of the induction of the leading magnetic field:

T = LR ₀ B _max / V _accele = 7.4 · 10 ^-3 -3.7 · 10 ^-4 s

If necessary, the energy of accelerated particles can be reduced by reducing the rise time (pulse duration) of the magnetic field induction, T.

If the kinetic energy of proton injection is 50 kV, then the relative velocity and the initial repetition period of the accelerating pulses of the induction field will be equal to: β ₀ = 0.01; τ ₀ = 5 · 10 ^-6 . Thus, the repetition period of accelerating pulses should vary from 5 μs to 90 ns. In this case, the duration of the pulses and their amplitude should remain constant during acceleration.

The starting moment of each subsequent accelerating pulse is determined by the signals of the proton beam flight time monitoring system.

Of all the options for pulsed power systems, a transistor option is most preferred. Transistor converters DC voltage to AC voltage have high reliability, durability and efficiency In [5], the implementation of a 1.6 MW converter with a switching frequency of 150 kHz was reported. Transistor switches are available with a switching frequency of up to 30 MHz and a power of up to 1.8 kW [6]. The rapid progress in the field of semiconductor switches allows us to hope that powerful power systems for induction particle accelerators with a high repetition rate of cycles will be developed in the near future.

Literature

1. Luchiano Colabetto et al, A Novel Supercoducting Cyclotron for Therapy and Radioisotope Production, Nuclear Instrument and Method in Physics Research, A 562 (2006) p.p. 1009-1012.

2. V.E. Balakin et al, "TRAPP-Facility for Proton Therapy of Cancer", EPAC, Rome, 1988, v. 2, p. 1505.

3. G.V. Dolbilov, “The Compact Iduction Circular Accelerator for Radiation Technologies”, Proceedings of APAC 2007, Raja Ramanna Center for Advanced Technology (RRCAT), Indore, India, p. P. 628-629.

4. G.V.Dolbilov, “Method of induction acceleration of charged particles”, Patent for invention No. 2359434, July 05, 2007

5. H. Matthes, R. Jurgens, “1.6 MW 150 kHz Series Resonant Circuit Converter incorporating IGBT Device for welding Applikation”, International Heating Seminar, Padova, p.25-31.

6. Hammad Abo Zied, Peter Mutschler, Guido Bachmann, “A Modulator IGBT Converter System for High Frequency Induction Heating Application”, PCIM 2002, 14-16.05, Nurenberg.

Claims (3)

1. The method of induction acceleration of charged particles, which consists in the fact that they form an increasing magnetic field in time with an average value, much smaller than the magnetic field in orbit; form an alternating magnetic field gradient in orbit with a field index much larger than unity (strong focus); charged particles are injected into it; accelerate charged particles by pulses of an induction electric field with a pulse duration much shorter than the rise time of the field in orbit and a pulse repetition rate equal to the revolution frequency of ions in orbit, and accelerated particles are output, characterized in that the constancy condition is imposed on the duration of the accelerating part of the pulses of the induction electric field the duration of the pulses during acceleration, while the duration of the accelerating pulses is equal to half the period of revolution of the ions at the end of the cycle is accelerated I, and their amplitude is equal to _{the Start} V = L · R ₀ · dB / dt,
where L is the perimeter of the orbit taking into account the straight sections, R ₀ is the radius of the equilibrium orbit of the ions, dB / dt is the rate of change of magnetic induction in orbit. 2. The method according to claim 1, characterized in that to ensure the stability of the azimuthal movement of ions ("phase" stability), the top table of the accelerating pulse has a slope corresponding to the specific conditions of the azimuthal movement of ions. 3. The method according to claim 2, characterized in that in order to provide single-turn ion output, the pulse front of the deflector of the output system is formed to be less than or equal to half the ion revolution period at the end of the acceleration cycle, while the pulse duration is greater than or equal to half this period.

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