RU2610148C1 - Vaccum-insulated tandem accelerator - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: invention relates to acceleration engineering and can be used to obtain charged particle beams for ion implantation, neutron capture cancer therapy or for detecting explosive and narcotic substances, as well as calibration of detectors of weakly interacting dark matter particles and other applications. In the disclosed tandem accelerator, between the low-energy negative hydrogen ion channel and the accelerator there is a metal ring, a cooled metal diaphragm, coated on the accelerator side by a mesh at negative potential, a vacuum pump, and at the output of the accelerator, the surface of the vacuum tank is covered by a mesh at negative potential.

EFFECT: reduced parasitic streams of charged particles in the acceleration channel, improved resistance of the accelerator to full voltage breakdown and high proton beam current.

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Description

The invention relates to the technique of accelerators and can be used to create accelerators of charged particles, as well as devices based on them. Such devices can be used for research in the physics of atomic and nuclear collisions, in the semiconductor industry for ion implantation, in medicine for neutron and neutron capture therapy for cancer, and in security systems for the detection of explosive and narcotic substances.

A charged particle accelerator is a device for producing high-energy charged particles. The accelerator operation is based on the interaction of charged particles with electric and magnetic fields. Structurally, accelerators can be divided into two large groups. These are linear accelerators, where the particle beam passes accelerating gaps once, and cyclic accelerators, in which the beam moves along closed curves, passing accelerating gaps many times.

In the simplest linear accelerator, charged particles are accelerated by a constant electric field and move linearly in the accelerator tube, to the ends of which a high voltage is applied. Structurally, the accelerating tube is made in the form of alternating ring insulators and electrodes. A vacuum is created inside the accelerating tube, the area outside is filled, if necessary, with insulating gas under pressure, usually with SF6 gas. The voltage on the intermediate electrodes is fed through a resistive divider. The ion source is placed under the high voltage potential.

A recharged particle accelerator (tandem) was proposed in the middle of the 20th century. It consists of two accelerating tubes and a peeling (rechargeable) target between them, which is at high potential. An accelerated negative hydrogen ion, when interacting with a stripping target, turns into a positive one, and then accelerates in the second accelerator tube with the same potential. The use of the same accelerating voltage twice allows one to double the final energy of singly charged particles and several times multiply charged ones. An important advantage of tandem accelerators is the placement of an ion source under a grounded potential.

The tandem accelerators in accelerator mass spectrometry and ion implantation complexes with a characteristic beam current of less than 1 mA (milliamperes) are most widely used. Typically, such tandem accelerators use a gas stripping target made in the form of a tube with a gas inlet in the middle.

The interaction of an accelerated beam of charged particles with residual gas in a vacuum volume leads to the appearance of electrons, ions and ultraviolet radiation, which reduce the high-voltage strength of accelerating vacuum gaps and vacuum surfaces of the insulating rings of the accelerating tube.

To preserve the high voltage strength of accelerating vacuum gaps with increasing beam current, various solutions are proposed, including a tandem accelerator with vacuum insulation.

The first tandem accelerator with vacuum insulation was proposed in 1994 [G. Proudfoot, R. McAdams, A.J.T. Holmes. A new design of a compact high current ion accelerator. Nuclear Instruments and Methods in Physics Research B 89 (1994) 1-7]. The proposal is that the entire tandem accelerator is placed in a vacuum volume. Those. vacuum is created not only inside the accelerator tube, but also in the area outside, which in traditional devices is filled with SF6 gas. In the description by them of the executed patent US 5293134 dated 03/08/1994 it is emphasized that this proposal allows you to get rid of the use of potentially dangerous and expensive SF6 gas.

Later in 1998 [V. Bayanov et al. Accelerator based neutron source for the neutron-capture and fast neutron therapy at hospital. Nuclear Instruments and Methods in Physics Research A 413 / 2-3 (1998) 397-426] also proposed a tandem accelerator with vacuum insulation, fundamentally different from that described above. The main idea was not to get rid of the use of SF6 gas, but to take the insulator away from the beam of charged particles to maintain the high voltage strength of the vacuum gap. In such a vacuum-insulated tandem accelerator, accelerator tubes as such are absent. The potential distribution is set by embedded electrodes forming a multilayer structure, mounted on the vacuum part of a single sectioned bushing insulator filled with SF6 gas. The insulator is out of line of sight from the beam propagation region. Coaxial tubes are located inside the bushing, which transmit the potential to the high-voltage electrode from the high voltage source and to the intermediate electrodes from the ohmic divider installed on the gas part of the bushing. The high voltage source and the gas part of the bushing are placed in a single volume filled with gas under pressure.

The idea of a tandem accelerator with vacuum insulation was developed in [P. Beasley, O. Heid, T. Hugles. A new life for high voltage electrostatic accelerators. Proc. of the International Particle Accelerator Conference, Kyoto, Japan (2010) 711-713]. They suggested using the natural capacitance between the electrodes. If you cut the embedded electrodes in half and connect them in the form of a ladder with diodes, then the Cockcroft-Walton cascade generator (voltage multiplier) is realized directly inside the vacuum volume of the accelerator. However, the authors of this proposal did not consider the technological aspects of the practical implementation of such a design.

As a prototype, the design of the tandem accelerator with vacuum insulation, described in [B. Bayanov et al. Accelerator based neutron source for the neutron-capture and fast neutron therapy at hospital. Nuclear Instruments and Methods in Physics Research A 413 / 2-3 (1998) 397-426].

A prototype diagram is shown in FIG. 1. In FIG. 1 are shown:

1 - source of negative hydrogen ions,

2 - tract transporting a beam of negative hydrogen ions of low energy,

3 - accelerator

4 - vacuum pump,

5 - intermediate electrodes,

6 - high voltage electrode,

7 - gas peeling target,

8 - bushing

9 - high voltage power source.

часть не показана) через проходной изолятор, в котором установлен омический делитель. Пучок отрицательных ионов водорода, создаваемый источником 1, транспортируется по тракту транспортировки пучка 2 до ускорителя 3, в котором ускоряется до энергии 1 МэВ. В газовой (аргоновой) обдирочной мишени 7, установленной внутри высоковольтного электрода 5, отрицательные ионы водорода превращаются в протоны, которые тем же потенциалом 1 MB доускоряются до энергии 2 МэВ. Стрелками на Фиг. 1 показаны направление движения отрицательных ионов водорода (Н^-) и протонов (p).The prototype consists of a source of negative hydrogen ions 1, a beam transport path 2 and an accelerator 3, in which there are a high-voltage 6 and intermediate electrodes 5, a gas stripping target 7, a vacuum pump for pumping gas 4, and a bushing 8 on which the electrodes are mounted. The potential for the high-voltage and five intermediate electrodes of the accelerator is supplied from the high-voltage voltage source 9 (

part not shown) through the bushing in which the ohmic divider is installed. The beam of negative hydrogen ions created by the source 1 is transported along the beam transport path 2 to the accelerator 3, in which it is accelerated to an energy of 1 MeV. In the gas (argon) stripping target 7, mounted inside the high-voltage electrode 5, negative hydrogen ions are converted into protons, which with the same potential of 1 MB are accelerated to an energy of 2 MeV. The arrows in FIG. 1 shows the direction of movement of negative hydrogen ions (H ^- ) and protons (p).

On the accelerator after reducing the dark current to an acceptable level [V.I. Aleinik, A.A. Ivanov, A.S. Kuznetsov et al. Dark currents of a tandem accelerator with vacuum insulation. Instruments and experimental technique. Volume 5, 2013, p. 5-13], optimizing the input of a beam of negative hydrogen ions into the accelerator [V.I. Aleinik, A.G. Bashkirtsev, A.S. Kuznetsov et al. Optimization of transportation of a beam of negative hydrogen ions to a tandem accelerator with vacuum insulation. Reports of the Academy of Sciences of the Higher School of the Russian Federation. No. 1 (20), 2013, pp. 47–55] and optimization of their stripping in a gas stripping target inside a high-voltage electrode [V.I. Aleinik, A.S. Kuznetsov, I.N. Sorokin et al. Calibration of a peeling target of a tandem accelerator with vacuum insulation. Scientific Bulletin of the Novosibirsk State Technical University. No. 1 (50), 2013, pp. 83–92], the proton beam current was increased from the initial values in the region of 140 μA [Kuznetsov AS, Malyshkin GN, Makarov AN et al. The first experiments on the registration of neutrons at an accelerating source for boron-neutron capture therapy. Letters in ЖТФ, 2009, Volume 35, Issue 8, pp. 1-6] up to 1.6 mA [D. Kasatov, A. Kuznetsov, A. Makarov et al. Proton beam of 2 MeV 1.6 mA on a tandem accelerator with vacuum insulation. Journal of Instrumentation 9 (2014) P12016], stable for more than an hour. When elucidating the reasons for the current limitation in the acceleration channel of negative hydrogen ions, a significant flux of accompanying electrons and a counter flow of positive ions generated in the accelerator channel and the stripping target were detected and measured [D.A. Kasatov, A.N. Makarov, S.Yu. Taskaev, I.M. Shaky. Registration of the current accompanying the ion beam in a tandem accelerator with vacuum insulation. Letters to the ZhTF 41 (2015) 74-80].

As a result of the studies, it was found that the main disadvantage of the prototype is that parasitic currents, significant in magnitude, flow in the accelerator channel, which lead to uncontrolled charge transfer, to a change in the potentials of the electrodes and, as a result, to the loss of high-voltage strength of vacuum accelerating gaps, manifested in the form of high voltage breakdowns. This circumstance limits the proton beam current.

The invention is directed to suppressing parasitic flows of charged particles in a tandem accelerator with vacuum insulation.

To solve the problem in a known device containing a source of negative hydrogen ions, a path for transporting a beam of negative hydrogen ions of low energy and a tandem accelerator with vacuum insulation, additional elements are installed (Fig. 2).

In FIG. 2 are shown:

1 - source of negative hydrogen ions,

2 - tract transporting a beam of negative hydrogen ions of low energy,

3 - accelerator

4 - vacuum pump,

5 - intermediate electrodes,

6 - high voltage electrode,

7 - gas peeling target,

8 - bushing

9 - high voltage power source,

10 - vacuum pump

11 - a metal ring (electrode),

12 - cooled metal diaphragm with a mesh,

13 is the grid.

At the entrance to the accelerator 3, a cooled metal diaphragm 12 is installed with an aperture - the diameter of the transverse size of the beam, in the mount of which it is possible to move it to center relative to the axis of the beam. The diaphragm separates the path for transporting a beam of negative hydrogen ions from the accelerating channel. The installation of the diaphragm can significantly reduce the flow of gas and ultraviolet radiation from a source of negative hydrogen ions into the accelerator channel. The surface of the cooled diaphragm 12 from the side of the accelerator is covered with a grid of metal wire, the supply of a negative potential to which should lead to the blocking of the secondary electrons that arise when the walls of the vacuum chamber are irradiated with positive ions. At the output of the accelerator, the surface of the vacuum tank is also covered with a grid 13, to which a negative potential is supplied for locking secondary electrons.

A vacuum pump 10 with a high gas evacuation rate is installed on the flange of the vacuum volume at the inlet of the accelerator, which will improve the vacuum conditions both in the beam transport path and in the accelerator channel.

Between the exit of the beam transport path 2 and the cooled diaphragm 12, a metal ring (electrode) 11 is installed, the supply of a negative potential to which was supposed to block the flow of electrons accompanying the beam of negative hydrogen ions.

The supply of a negative potential to the grid significantly reduced the dose rate of bremsstrahlung caused by the absorption of electrons accelerated to 1 MeV in the metal [I. Shchudlo, D. Kasatov, A. Makarov, S. Taskaev. Measurement of the spatial distribution of gamma radiation at tandem accelerator with vacuum insulation. Proceedings of RUPAC 2014, Obninsk, Russia, October 6-10, 2014, p. 116-117]. From the measured current-voltage characteristics of the end detector (disk with a grid) at the entrance to the accelerator, it was determined that the secondary electron emission coefficient under the action of the generated positive ions is of the order of 10. Such a high value of the secondary electron emission coefficient is characteristic of many-electron ions and atoms with an energy of more than 100 keV [PP Rakhimov, O.V. Kozinsky. Proceedings of the USSR Academy of Sciences, series: Physics, Volume 26, 1962, p. 1398].

As a result of the installation of a diaphragm, a cryogenic pump, a ring and grids, a significant reduction in the parasitic flows of charged particles was achieved. In particular, the flux of electrons accelerated to full voltage was reduced by about 20 times to a value of the order of 0.5% of the ion beam current.

The suppression of parasitic flows of charged particles in the accelerator made it possible to improve the stability of the accelerator to breakdowns at full voltage and to significantly increase the proton beam current from 1.6 mA to 5 mA. In FIG. Figure 3 shows the current waveforms and proton beam energy obtained in one of the approaches. The measured proton beam current for one hour exceeds 5 mA, has an average value of 5.120 ± 0.060 mA, maximum 5.327 mA. At a proton beam current of 3 mA and 4 mA, an operating mode was implemented for one hour with a current stability of 0.5% and without breakdowns at high voltage. In FIG. Figure 3 shows that at a current of more than 5 mA, two breakdowns occurred, after which the current is restored to its previous parameters in 35 seconds, which is acceptable.

Claims (1)

A vacuum-insulated tandem accelerator containing a source of negative hydrogen ions, a low energy negative hydrogen ion beam transport channel and a tandem type ion accelerator, characterized in that a metal ring, a cooled metal diaphragm are placed between the low energy hydrogen ion beam transport path and the accelerator, a vacuum pump coated on the side of the accelerator, a vacuum pump, and at the outlet of the accelerator the surface of the vacuum tank is covered on an insulated mesh d.

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