RU2558384C2 - Gas stripping target - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: invention relates to acceleration engineering and can be used in tandem charged particle accelerators for ion implantation, neutron capture therapy of cancer or detection of explosives and narcotic drugs. A gas stripping target is placed inside a high-voltage electrode (2), the target being in the form of a tube (4) with gas release in the middle through a gas inlet system (8). The stripping target is inclined relative to the charged particle acceleration axis; magnets (3) with a transverse magnetic field are placed before the input of the target and after the target. The transverse magnetic field deflects low-energy positive ions of the stripping gas flowing from the stripping target by an angle of the order of a unit into an absorber (5). The transverse magnetic field also deflects high-energy ions by a small angle. The gas stream flowing from the target is directed not towards the centre of the input and output diaphragms of the high-voltage electrode, but towards the inner wall of the high-voltage electrode.

EFFECT: stripping a beam of negative gas ions in a stripping tube and preventing acceleration of the formed positive gas ions of the stripping target and improved gas conditions in the acceleration channel.

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Description

The invention relates to nuclear physics and accelerator technology and can be applied in tandem charged particle accelerators, as well as in 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 capture therapy of cancer, and in security systems for the detection of explosive and narcotic substances.

The concept of a recharged particle accelerator (tandem) was proposed in the middle of the 20th century. It allowed to reduce the required voltage of the high-voltage generator and thereby reduce the size of the accelerator, using peeling (recharging) - a change in the sign of the particle charge during acceleration. In the process of stripping, a negative ion, when interacting with a special target, turns into a positive one, which makes it possible to use the same accelerating voltage twice, i.e. double the final energy of singly charged particles and several times multiply charged ones.

The stripping target is a tube filled with gas, a stream of steam or a film of solid.

Tandem accelerators in accelerator mass spectrometry (AMS) and ion implantation complexes with a characteristic beam current of less than 1 mA (milliamperes) are most widely used. Typically, tandem accelerators use a gas peeling (recharge) target made in the form of a tube with a gas inlet in the middle. Such a rechargeable gas target is described in USSR author's certificate No. 387541 of 06/21/1973. In patents US 5247263 from 09.21.1993, US 5293134 from 08.03.1994, JP 10223399 from 08.21.1998, KR 100166220 from 09.22.1998, US 6903336 from 07.06.2005, US 20060011866 from 01.19.2006, US 20130112869 from 05.09.2013 descriptions of similar gas stripping targets as components of tandem charged particle accelerators are given.

Neutron capture cancer therapy requires an ion beam current of more than 5 mA.

It was previously proposed to use a tandem accelerator with vacuum insulation of electrodes for these purposes [B. Bayanov et al. Accelerator based neutron source for the neutron-capture and fast neutron therapy at hospital. Nuclear Instr. and Methods in Physics Research A 413 / 2-3 (1998) 397-426]. Also, the principle of vacuum insulation is implemented in a design protected by patent US 5293134 from 03/08/1994.

There are no accelerator tubes in such an accelerator. The potential distribution is determined by embedded electrodes forming a multilayer structure, mounted on a single sectioned bushing. The insulator is out of line of sight from the beam propagation region. This design is compact and more reliable with respect to high-voltage breakdowns.

At a beam current of more than 5 mA, the solid film used as a peeling target quickly collapses. The best option for stripping a high-current beam of charged particles is a gas target. Since with increasing beam current it is necessary to increase the diameter of the stripping tube filled with gas through which the beam passes, the flow of gas flowing out of the tube increases significantly.

Gas falling into accelerating gaps reduces the high-voltage strength and reliability of the accelerator. In addition, it can lead to premature stripping of negative ions, and some of the ions at the output of the accelerator will have an energy lower than necessary.

To reduce the gas flow into the accelerating gaps, pumping is used with a turbomolecular pump, which is located next to the stripping tube. Similar designs of gas stripping targets are described in patents US 6069459 from 05/30/2000 and RF №2360315 from 05/28/2007.

As a prototype, the design of the gas target was chosen, which provides good gas pumping and is described in the patent of the Russian Federation No. 2360315 dated 05/28/2007.

However, this design does not allow to solve the problem associated with ionization of the stripping target gas by a high-energy ion beam. Part of the positively charged ions of the stripping gas penetrates the accelerating path on both sides of the stripping target and is accelerated by the electric field, up to the full potential of the high-voltage electrode. The formation of a beam of accelerated stripping gas ions leads to the following problems. Firstly, part of the power of the high-voltage power supply is spent on accelerating this beam. Secondly, the uneven hit of the accelerated beam of stripping gas ions on the intermediate electrodes of the accelerator path leads to a redistribution of their potentials and to a change in the conditions of acceleration and focusing of the injected beam of charged particles. Thirdly, the hit of an accelerated strip of stripping gas on the electrodes can lead to their melting and deformation.

The invention is directed to a device that provides a stripping of a beam of negative ions with a current of more than 5 mA and eliminating the formation of an accelerated beam of ions of stripping gas.

To solve this problem, in a known device containing a peeling tube located inside the high-voltage electrode (terminal), with a gas supply system and a gas source, magnets are placed in the space between the peeling tube and the diaphragms of the high-voltage electrode, and the peeling tube is tilted (rotated).

Propagation of a high-energy ion beam in a stripping target leads to partial ionization of the stripping gas. A part of the generated positively charged stripping gas ions leaves the stripping tube and propagates in the direction of the diaphragm of the high-voltage electrode. If positively charged stripping gas ions pass through the diaphragm, they enter the accelerator path and are accelerated by the action of an electric field. The invention is aimed at deflecting the flow of positively charged stripping gas ions inside the high-voltage electrode so as to prevent them from passing through the diaphragm of the high-voltage electrode. Such a deviation of the ion flux can be achieved using electric and magnetic fields, in particular, a transverse magnetic field.

, где r берется в см, B - магнитное поле (Гаусс), E - энергия иона (эВ), Z - заряд иона, µ - отношение массы иона к массе протона. В магнитном поле 400 Гс ларморовский радиус положительно заряженного иона аргона как одного из применяемых газов обдирочной мишени с энергией 10 эВ будет равен 5 см. Такая величина ларморовского радиуса приемлема для сильноточного ускорителя-тандема [V. Aleynik, et al. BINP accelerator based epithermal neutron source. Applied Radiation and Isotopes, 69 (2011) 1635-1638], в котором обдирочная трубка длиной 40 см имеет отверстие диаметром 16 мм, а расстояние между торцом обдирочной трубки и диафрагмой равно 10 см. Таким образом, если на выходе из обдирочной трубки в области 5 см создать поперечное магнитное поле величиной 400 Гс, то можно отклонить поток вытекающих положительных ионов обдирочного газа и исключить их ускорение в ускорительном канале.As a beam of high-energy ions propagates, a weakly ionized plasma is formed in the stripping target. Since electrons are more mobile than ions, to preserve quasineutrality, the plasma acquires a positive potential of the order of 10 V. Under the action of this potential, positively charged ions exit the stripping tube with a characteristic energy of 10 eV. If a transverse magnetic field is created at the exit of the stripping tube, then the ions begin to move around the circle and deviate. The Larmor radius of the ion r is determined by the following practical expression: $$r=102\frac{\sqrt{\mu E}}{ZB}$$

, where r is taken in cm, B is the magnetic field (Gauss), E is the ion energy (eV), Z is the ion charge, μ is the ratio of the mass of the ion to the mass of the proton. In a magnetic field of 400 Gs, the Larmor radius of a positively charged argon ion as one of the used gases of a stripping target with an energy of 10 eV will be 5 cm. This value of the Larmor radius is acceptable for a high-current tandem accelerator [V. Aleynik, et al. BINP accelerator based epithermal neutron source. Applied Radiation and Isotopes, 69 (2011) 1635-1638], in which the peeling tube 40 cm long has an opening with a diameter of 16 mm and the distance between the peeling tube end and the diaphragm is 10 cm. Thus, if at the outlet of the peeling tube in the region 5 cm to create a transverse magnetic field of 400 Gs, it is possible to reject the flow of effluent positive ions of stripping gas and exclude their acceleration in the accelerating channel.

In the transverse magnetic field created, not only the outgoing stripping gas ions will be deflected, but also high-energy ions, both injected negatively charged at the entrance to the stripping target and positively charged at the exit from it. The Larmor radius of 1 MeV of a hydrogen ion in a magnetic field of 400 G is 250 cm, and at a distance of 5 cm the beam will deviate by an angle of about 2 degrees. For optimal passage of the high-energy beam through the stripping tube, the latter should be tilted to the appropriate angle.

The invention is illustrated in Figure 1.

Figure 1 shows a diagram of an inclined peeling target with a magnetic field located inside the high-voltage electrode of the tandem accelerator.

The diagram shows:

1 - input diaphragm of a high voltage electrode;

2 - high voltage electrode;

3 - magnet;

4 - gas stripping tube;

5 - ion absorber;

6 - output diaphragm of a high voltage electrode;

7 - sectioned bushing;

8 - gas supply system.

Figure 1 also schematically shows the direction of propagation of a beam of high-energy ions (negative hydrogen ions H ^- and protons p), the deviation of the flow of positive ions of gas stripping target (Ar ⁺ ) and the direction of the magnetic field (B). Figure 1 does not show a gas source - it is located in the high voltage terminal of the high voltage source.

The device operates as follows.

A beam of charged particles created by a source of negative ions (not shown) is accelerated by the first stage of the tandem accelerator. Through the input diaphragm of the high-voltage electrode 1, a beam of high-energy negative ions enters the high-voltage electrode 2, where it is deflected by a magnet 3 by a small angle, peeled off in a gas stripping tube 4 and converted into a beam of positive high-energy ions. The beam of high-energy positive ions emerging from the gas stripping tube is deflected by a magnet 3 by a small angle and exits the high-voltage electrode 2 through the output diaphragm of the high-voltage electrode 6, after which it is accelerated by the second stage of the tandem accelerator.

Propagation of a beam of high-energy charged particles through a gas of a stripping target leads to partial ionization of the gas and the formation of positive ions with low energy. The positive ions of the stripping gas emerging from the gas stripping tube 3 are deflected by the magnet 3 by an angle of about 90 ° and enter the ion absorber 5, where they are neutralized by interaction with the absorber wall.

Thus, the inevitably generated positive stripping gas ions do not penetrate the accelerator channel and do not form a spurious beam of accelerated stripping gas ions, which prevents the generation of a beam of high-energy charged particles with a large current. The negative influence of the parasitic beam of accelerated stripping-gas ions could be manifested in the use of additional accelerator power, in a change in the potential of the intermediate electrodes, which would entail a change in the conditions of transport and acceleration of the charged particle beam, as well as in the deformation and melting of the electrodes.

The forced inclination of the gas stripping target also leads to an additional positive effect. The gas flowing out of the stripping tube propagates mainly along the axis, and the slope of the stripping tube will reduce the gas flow into the accelerator path, since the maximum distribution of the effluent gas will not be directed to the center of the diaphragms of the high voltage electrode, but higher or lower to the wall of the high voltage electrode. The gas remaining inside the high voltage electrode can be pumped by a high capacity pump.

Claims (1)

A stripping gas target for stripping a beam of negative ions, containing a stripping tube, a gas supply system and a gas source, characterized in that before entering the target and after leaving it there are magnets creating a transverse magnetic field, and the target is inclined relative to the axis of the accelerating path of the charged beam particles.