HIGH-CURRENT ELECTROSTATIC ACCELERATOR-TANDEM
FOR THE NEUTRON THERAPY FACILITY
B. Bayanov1, V. Belov1, G. Dimov1, G. Derevyankin1, V. Dolgushin1, A. Dranichnikov1, V. Kononov2, G. Kraynov1, A. Krivenko1, N. Kuksanov1, V. Palchikov1, R. Salimov1, V. Savkin1, V. Shirokov1, G. Silvestrov1, I. Sorokin1, and S. Taskaev1
1Budker Inst. Nucl. Phys., Novosibirsk, Russia,
2Inst. Phys. and Power Engineering, Obninsk, Russia
ABSTRACT: Original 2.5 MeV, 50 mA proton tandem accelerator for the neutron therapy facility is described. The main idea of tandem usage is providing high rate acceleration of high current hydrogen negative ions by special geometry of potential electrodes with vacuum insulation and one strongly focusing lens. Pulse 1 MeV vacuum insulation tandem accelerator experimental results are presented. Steady-state 100 kW 1.25 MV sectioned rectifier is a high voltage source. The rectifier is a part of the industrial ELV-8 electron accelerator developed at BINP and widely used. Accelerating voltage is stabilized with accuracy of 0.1 %. Various charge-exchange targets are considered. Namely, targets are gas target with outward pumping, gas target with pumping inside of high-voltage electrode, and liquid lithium stream target. Problems of development of steady-state 50 – 100 mA source of hydrogen negative ions are discussed.
KEY WORDS: Tandem accelerator, Neutron therapy, Charge-exchange target
INTRODUCTION: Selecting a variant of accelerator for neutron source , it is desirable to provide the possibility of operation in two regimes in the near threshold region  allowing to use the source for irradiating in open geometry without external collimator and for production epithermal and fast neutrons at protons energy 2.5 MeV with moderators. But in spite of attractiveness and elegance of operation in the near threshold region such method of neutron-production demands high monochromaticity and stability of proton beam energy (0.1 %). In this project we offer to create a neutron source based on construction of vacuum insulation tandem accelerator (VITA) developed at BINP using the sectionalized rectifier from electron accelerator of ELV type as a powerful source of high voltage.
In the conventional scheme of the tandem, two accelerating columns based on ceramic tubes are connected by the high voltage parts with the charge-exchange target in between. The prospect of high current (a few tens milliamperes) accelerator design according to this scheme is limited by its two basic disadvantages — the necessity of pumping the gas of charge-exchange target through accelerating columns and an inevitable current emission of secondary electrons and ions from the high current beam passage region to the inner surface of ceramic insulators.
RESULTS AND DISCUSSION: There are no ceramic accelerating columns in the tandem proposed. In this scheme, to the cylindrical potential electrode with charge-exchange target, placed into vacuum tank, high voltage is applied through ceramic inputting insulator which can be arbitrarily remote from the accelerated beam passage region.
The high voltage electrode is surrounded by system of different potential shields providing the homogeneous distribution of the potential and preventing the full voltage effects. Coaxial round holes for the beam passage are in the walls of vacuum tank, potential electrode and in the shields. Since the thin-wall shields placed along the equipotential surfaces of the electrostatic field hardly contribute into focusing, the beam focusing in this system is only provided by two axisymmetric lenses — by the lens strongly focusing the low energy beam in the input hole of the grounded wall of vacuum tank and by the lens defocusing the already accelerated beam placed in the hole of potential electrode.
The charge-exchange target is a pipe with an inner hole of 12 mm diameter and 400 mm length. In the center of the pipe, the gas leaks at a rate providing the efficient density of the target 3´ 1016 mol cm–2 required for the charge-exchange. In the geometry under consideration at a given rate of gas leak, the pressure distribution in the whole volume, required for the high voltage strength of a gap, is provided at a pumping rate of ~10,000 l/s.
To provide such rate of pumping out, a cryogenic pump is used placed directly inside the potential electrode. The pump is a toroidal volume of ~ 12 liters placed in the upper part of the electrode above the tube of the charge-exchange target. It can be filled with liquid nitrogen. The lower part of the torus with a disc covering the central hole is the surface of pumping out. It is constructed as copper rings that are in close heat contact with the pump body made of stainless steel. A number of copper plates increasing the surface of pumping out to ~ 1 m2 is soldered to the rings. Three coaxial insulating tubes (ceramics, polyethylene) 12, 20, and 50 mm diameter are placed in the center of the rectifier to provide liquid nitrogen supply into the pump that is under high voltage. The outer ceramic tube separates the rectifier volume filled with SF6 and the inner volume of the tube filled with nitrogen. Liquid nitrogen is supplied to toroidal volume through the inner tubes thermoinsulated in this way. CO2 is supposed to be used as charge-exchange gas, it condenses at –56 °C and is highly heat-conducting in solid state.
The most important component of the accelerator is the high voltage inputting insulator through which the potential is transferred into the vacuum cavity from the tank filled with SF6 gas with the transformer of the ELV-type industrial accelerator being the powerful source of high voltage. The potential electrode is placed on the insulator end on the metal flange which is vacuum tightened to the end of ceramic tube by the tightening rod passing along its axis connected to the high voltage source. On the opposite side of the input (placed in SF6) the rod is tightened to the second end flange supported by the nonvacuum part of insulator made of glass rings separated by metal rings for the potential distribution. The voltage is applied to these rings from the resistive-capacitive divider providing the homogeneous distribution of the potential along the insulator length. Inside the insulator around the tightening rod thin wall pipes of various lengths are concentrically located to connect the respective rings of different potential on the vacuum ceramic part of the insulator and its lower part placed in SF6 gas.
Specific features of the tandem are high rate of acceleration and the only strongly focusing lens of the tandem input hole where the particle energy is still low. The second (defocusing) lens in the potential electrode input hole is weak since here the beam has already high energy. Focal distance of the focusing lens increases if the hole in the thick wall has not round but cone shape. Diameter of all the holes are selected to be 50 mm, the input hole on the output shifts to the cone with resultant of 45° and a base of 110 mm. As a result, the lens focus is placed at a distance of 80 mm from the tank's inner surface. The crossover for the converging beam transported from the outer source of negative ions should be located here. Fig. 2 shows the characteristic particle trajectories and envelopes of 50 mA laminar flow and nonlaminar 50 mA beam in the tandem.
Fig. 1. High-current electrostatic accelerator-tandem. 1 — H– ion source, 2 — tandem-accelerator, 3 — nitrogen trap, 4 — charge-exchange target, 5 — high-voltage source.
Fig. 2. a — The characteristic particle trajectories in the tandem without regard for space charge and the envelope of 50 mA laminar flow (the upper curve is in the range of negative z); b — The envelope of nonlaminar 50 mA beam. A diagram of longitudinal electric field is also shown. Note that longitudinal and transverse scales differ. The electrode surfaces are shown in the area r ł 2.5 cm.
For a tandem proton accelerator for neutron generation, intended reliably to work in hospitals, it is necessary to create a steady-state reliable source of negative hydrogen ions with a long operation lifetime. The source should permit to obtain a H- ion beam with a relatively small emittance and a steady-state ion current of some tens milliamperes, as appear, not more than 40 mA. To satisfy these requirements a surface-plasma source with a cesiated emitter of H - ions was chosen. A vacuum chamber connecting the H - ion source to the tandem accelerator is shown in Fig. 1. The chamber is the two-volume cylindrical vacuum chamber with a horizontal axis being in line with an axis of the tandem accelerating column. The H - ion beam with the current of 40 mA has a large enough space charge density and requires a strong enough focusing almost continuous along a transport path from the ion source to the tandem accelerator.
The prototype of such a tandem with vacuum insulation at an energy of 1 MeV is used successfully as an injector of the proton synchrotron. This 1 MeV tandem is supposed to be used for experiments necessary for final design of the high-current electrostatic accelerator-tandem.