RU2360315C2 - Compression gas target - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: present group of inventions pertain to physics and acceleration techniques and can be used in accelerator - tandems for cancer neutron capture therapy or for detecting explosive and narcotic substances. The compression gas target for peeling a beam of negative ions has a peeling tube, a gas source and a turbo-molecular pump. The housing of the turbo-molecular pump is completely located in a vacuum, next to the peeling tube. The accelerator-tandem with vacuum insulation comprises the said compression gas target.

EFFECT: peeling a beam of negative ions with current more than 5mA and increased compactness of the device, which allows for using it in an accelerator-tandem with vacuum insulation.

3 cl, 3 dwg

Description

The invention relates to nuclear physics and accelerator technology and can be applied in charged particle accelerators - tandems, as well as in devices based on them. Such devices can be used in medicine for neutron capture cancer therapy or in security systems for the detection of explosive and narcotic substances.

The concept of a recharged particle accelerator (tandem) was proposed in the mid-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 the particles.

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

The tandem accelerators in accelerator mass spectrometry (AMS) complexes or in the semiconductor industry for ion implantation are most widely used. However, in such complexes, a significant ion beam current is not required; usually it does not exceed 1 mA (milliamps).

For neutron capture cancer therapy or for the detection of explosives and narcotic substances, an ion beam current of more than 5 mA is required.

Earlier it was proposed to use a tandem accelerator with vacuum insulation [B.F. 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]. The principle of vacuum insulation is also implemented in a design protected by Pat. US 5293134, H05H 5/06, March 8, 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.

The best option for stripping the beam is a gas target. At a beam current of more than 5 mA, the solid film quickly collapses. However, with increasing beam current, the required diameter of the gas-stripping tube through which the beam passes increases. 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 flow of gas into the accelerating gaps, pumping is used with a turbomolecular pump, the outlet of which is connected to the stripping tube. Most of the gas circulates through the pump and stripping tube. The remainder flows into the accelerating gaps through openings in the pumped-out volume for beam entry and exit. This part of the gas flow into the stripping tube is made up from a gas source.

The casing of the turbomolecular pump is surrounded by air or other gas from the outside, which fills part of the high-voltage electrode (terminal), which provides cooling of the pump casing and ease of operation. In such gas targets, several turbomolecular pumps can be used.

Similar gas target designs have been used previously and are described in Pat. US 6903336, 7 H01J 37/08, 04/17/2004, 6069459, H05H 5/02, 05/30/2002.

As a prototype, the design of the gas target was selected, which provides good gas pumping when using several pumps and pumping stages [JP 9027400, G21K 1/14, Н05Н 5/06, 01/28/1997].

However, these structures cannot be used in a tandem accelerator with vacuum insulation. They are not compact and require placement of the housing of the turbomolecular pump outside the vacuum volume. The high-voltage accelerator electrode, a tandem with vacuum insulation in which the target is located, does not border the external atmosphere and has a small volume due to the general compactness of the accelerator.

The invention is directed to the creation of a device that strips a beam of negative ions with a current of more than 5 mA, compact and mostly placed in a vacuum, which allows its use in a tandem accelerator with vacuum insulation. The device should not reduce the high-voltage strength of the accelerator and lead to premature peeling of negative ions.

To solve the problem in a known device containing a stripping tube, a gas source and a turbomolecular pump, the housing of the turbomolecular pump is completely located in vacuum near the stripping tube and is cooled by a dielectric fluid, for example transformer oil.

After acceleration by the first stage of the accelerator, the negative ion beam is stripped off in a stripping tube, into which gas from a gas source and from a turbomolecular pump enter. After exiting the compression gas target, the beam is accelerated by the second stage of the tandem accelerator.

The turbomolecular pump pumps out most of the gas leaving the stripping tube, compresses it and directs it back to the stripping tube, recirculating the gas. Thanks to the use of a turbomolecular pump, the gas flow into the accelerating gaps is reduced by several times, which prevents the high-voltage strength of the accelerator from decreasing and the premature grinding of negative ions. The housing of the turbomolecular pump is located in close proximity to the stripping tube, in a vacuum, which ensures the compactness of the device at the required pumping speed. In this case, the target requires a small area for the output of communications from the vacuum part of the accelerator through a sectioned bushing.

The part of the gas stream that the turbomolecular pump does not pump out leaves the target through the holes for the entrance of the negative ion beam and the exit of the positive ion beam. This part of the flow is made up by a gas source, which may not be in vacuum, in the immediate vicinity of the stripping tube, and it can be connected to it by a thin tube for supplying gas.

The power supply and control system of the pump can also be located outside the vacuum volume and connected to the pump through a sectioned bushing.

In a vacuum, cooling the pump casing is difficult, so coolant is needed. The coolant supply system can be located outside the tandem accelerator, at zero potential. Then between the coolant supply system and the pump there will be a high voltage set by the accelerator. In this case, the coolant must be dielectric. Two tubes through which the liquid will be supplied through the accelerator must be partially dielectric. Directly through the bushing into the vacuum, the coolant can be piped from any vacuum-tight material, such as stainless steel.

Thus, the main part of a compact compression gas target can be placed in a vacuum, in a high-voltage electrode of a tandem accelerator with vacuum insulation. Thanks to the use of a turbomolecular pump, the gas flow into the accelerating gaps is reduced by several times, which prevents the high-voltage strength of the accelerator from decreasing and the premature grinding of negative ions.

The invention is illustrated in Fig.1-3.

Figure 1 shows a diagram of the vacuum part of a compression gas target, which shows:

1 - peeling tube;

2 - casing of a turbomolecular pump;

3 - a tube connecting a turbomolecular pump and a stripping tube;

4 - a) tubes for supplying coolant;

b) power and control cable for the turbomolecular pump;

5 - high voltage electrode;

6 - sectioned bushing;

7 - hole for the entrance of the beam of negative ions.

Figure 2 shows a diagram of the vacuum part of a compression gas target with an internal volume and vacuum resistance, where in addition to the above positions shown:

8 - vacuum resistance;

9 - internal volume.

Figure 3 shows a diagram of a tandem accelerator with vacuum insulation and an integrated compression gas target, where in addition to the above positions are shown:

10 - gas source;

11 - power supply and control unit for a turbomolecular pump;

12 - coolant supply system;

13 - gas part of the accelerator;

14 - high voltage accelerator power supply;

15 - place of entry of the beam of negative ions into the accelerator;

16 - exit point of the beam of accelerated positive ions from the accelerator;

17 - direction of the external vacuum pumping of the accelerator.

The device is part of a tandem accelerator and is optimized for placement in a tandem accelerator with vacuum insulation (Figure 3).

A compression gas target consists of a stripping tube 1, a turbomolecular pump 2, a gas source 10, and components ensuring their joint operation 11 (Figs. 1, 2, 3).

The peeling tube 1 is located in the high-voltage electrode 5 of the tandem accelerator (Figs. 1, 2, 3) coaxially with holes 7, 15, 16 (Figs. 1, 3) through which the ion beam passes. A typical turbomolecular pump consists of a housing 2 (Fig. 1, 2, 3), in which the turbine is located, and a power and control unit 11 (Fig. 3). The housing of the turbomolecular pump 2 is located in the immediate vicinity of the stripping tube 1, in the high-voltage electrode 5, in vacuum. The output flange of the housing of the turbomolecular pump 2 is hermetically connected to the stripping tube 1 using the tube 3 (Fig.1, 2). The power supply and control unit 11 is located in the gas part of the accelerator 13 (Figure 3). The turbomolecular pump can be powered from a high-voltage accelerator power supply 14 either through a high-voltage transformer or from an electric generator, the rotation of which can be transmitted by a dielectric rod, belt transmission or gas flow to a gas turbine (not shown).

The power supply and control unit of the turbomolecular pump 11 is connected to the housing 2 by electric wires 4b (Figs. 1, 2) through the bushing 6.

The coolant for cooling the housing 2 of the turbomolecular pump must be dielectric, for example, transformer oil can be used. In the housing of the turbomolecular pump 2, the coolant can pass through standard channels designed for water cooling. The coolant supply system 12 may be located outside the tandem accelerator, at zero potential (Figure 3). It may consist of a compressor and a heat exchanger (not shown). Two tubes (one for the fluid inlet, the other for the outlet), through which the fluid will be supplied through the accelerator, must be dielectric in the area of the high-voltage power supply unit of the accelerator 14 (Figure 3). Directly through the bushing insulator 6, the cooling liquid can be supplied through tubes 4a from any vacuum-tight material, for example stainless steel (Figs. 1, 2).

The gas source 10 may consist of a gas cylinder and a controlled valve, which are shown in FIG. 3. A gas cylinder is placed in the gas part of the accelerator 13 (Figure 3). At an accelerator voltage of less than 1 MB, a gas cylinder can be placed outside the accelerator, with gas supplied through a dielectric tube (not shown). The controlled valve may be electromagnetic, piezoelectric or pneumatic. Additionally, the gas source may be equipped with a pressure reducer, a filter for gas purification, a leak and a buffer volume, not shown.

A simple version of a gas source provides only on-off gas flow at a given flow rate.

In other embodiments, the gas source may control gas flow. If during the pulse opening of the valve the buffer volume is not filled to the maximum pressure, by adjusting the duration and pulse repetition rate, the gas flow from the gas source can be regulated.

In another case, the controlled valve may be a mass flow controller that regulates the gas flow taking into account the feedback with the built-in gas flow meter.

The controlled valve can be equipped with a separate power and control unit, not shown.

The control signals of a turbomolecular pump or a controlled valve can be transmitted to the tandem accelerator via a fiber optic cable, not shown.

In a simple embodiment, the stripping tube 1 and the housing of the turbomolecular pump 2 are located directly in the high-voltage electrode 5 of the tandem accelerator (Figure 1). In this case, the high-voltage electrode 5 is sealed, except for the holes for the entrance and exit of the beam. To reduce the effect on the high-voltage strength of the accelerator, radiation and charged particles generated during the interaction of the beam with the target gas, an internal volume 9 can be added to the high-voltage electrode 5 (Figure 2). The internal volume 9 is sealed, except for the openings for the entrance and exit of the beam, and is separated from the high-voltage electrode by a distance of at least two diameters of the stripping tube 1. In this case, the high-voltage electrode 5 is not sealed, but has openings or shutters for external pumping 17, like the others electrodes of a tandem accelerator with vacuum insulation.

To increase the pumping efficiency and further reduce the gas flow into the accelerating accelerator gaps, vacuum resistances 8 (Fig. 2) can be used, which are tubes coaxial with the peeling tube 1 and hermetically fixed to the high-voltage electrode 5 or to the internal volume 9, in the last two versions , respectively.

The device operates as follows.

The beam created by the source of negative ions (not shown) is accelerated by the first stage of the accelerator. Then, through the hole for the entrance of the beam of negative ions 7, the beam enters the high-voltage electrode 5, where it is peeled in the stripping tube 1. Peeling occurs when the beam interacts with the gas. The flow of gas into the stripping tube 1 should be sufficient so that almost the entire beam is peeled. After exiting the stripping tube 1, the beam is accelerated by the second stage of the tandem accelerator. A gas flow sufficient for stripping the beam will be large in the case of a compact accelerator with a current of more than 5 mA, since the stripping tube 1 must be less than 1 meter long and an inner diameter of about 1 centimeter. Such a flow should be pumped out before entering the high-voltage accelerating gaps.

The gas leaving the stripping tube 1 enters the high-voltage electrode 5 (FIG. 1) or into the internal volume 9 (FIG. 2). Then, most of the gas stream is pumped out by the turbine in the housing of the turbomolecular pump 2, and the rest of the stream exits through openings 7 for the entrance and exit of the beam into the accelerating gaps. From the accelerating gaps, the gas is pumped out by the external vacuum pumping of the accelerator 17. In the second embodiment (FIG. 2), the part of the gas flow that leaves the internal volume 9 enters the high-voltage electrode 5, and from there into the accelerating gaps of the accelerator.

A turbomolecular pump 2 with a pumping speed of several hundred liters per second will reduce the gas flow into the accelerating gaps by several times if the diameter of the holes for the inlet 7 and the beam exit does not exceed 3 cm. The conductivity of the holes, and therefore the gas flow into the accelerating gaps, will decrease when adding vacuum resistance 8 (Figure 2). It is necessary to select the mentioned values so that the gas flow into the accelerating gaps does not lead to a decrease in the high-voltage strength of the accelerator and premature grinding of negative ions. The required speed of rotation of the turbine of the turbomolecular pump 2 is set by the power supply and control unit 11, possibly taking into account its temperature.

The gas pumped out by the turbomolecular pump 2 is compressed and through the tube 3 it flows back to the stripping tube 1. Also, the gas flow from the gas source 10 enters into the stripping tube 1, which is equal in magnitude to the accelerating gaps. When turning on the accelerator, it is advisable to reduce the gas flow from the gas source 10 in order to reduce the gas flow to the accelerating gaps, and then gradually increase to a stationary value. This will compensate for increased gas emission from the electrodes at the beginning of the accelerator.

During operation, the housing of the turbomolecular pump 2 heats up. In vacuum, the cooling of the pump casing is difficult, therefore, through the tubes 4a, a dielectric coolant is continuously supplied to it by the coolant supply system 12.

An advantage of the invention is that the housing of the turbomolecular pump is completely located in vacuum, next to the stripping tube. In this case, the compression gas target requires a small area for the output of communications from the vacuum part of the accelerator, through a sectioned bushing, which allows its use in a tandem accelerator with vacuum insulation. The device does not reduce the high-voltage strength of the accelerator and does not lead to premature grinding of negative ions. Despite being located in a vacuum, the turbomolecular pump housing does not overheat due to cooling with dielectric fluid.

Claims (3)

1. A compression gas target for stripping a beam of negative ions, containing a stripping tube, a gas source and a turbomolecular pump, characterized in that the housing of the turbomolecular pump is completely located in vacuum, next to the stripping tube. 2. The compression gas target according to claim 1, characterized in that the housing of the turbomolecular pump is cooled by a dielectric fluid. 3. The tandem accelerator with vacuum insulation, characterized in that it includes a compression gas target according to claim 1.

Images