Novel 2.5 MeV, 40 mA proton tandem accelerator for the neutron therapy facility [1] 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.

A set of experiments on study of high voltage durability of vacuum gap with large square electrodes were finished on available pulse tandem-accelerator. The results allowed to determine high voltage and energetic parameters of 2.5 MeV accelerator.

2.5 MeV tandem accelerator is under construction now in a 3-layered protected bunker with necessary infrastructure. Mechanic and mounting works at sectionized rectifier were finished at its working place, it was started-up and operating voltage of 1.25 MV was obtained. Design drawings for high voltage through-pass insulator and draft for accelerator were prepared. Vacuum tank for accelerator was manufactured. Design of high voltage electrodes of rectifier and tandem was finished.

At test desk available, dc H- ion beam of 9 mA was obtained with negative ion source having Penning geometry electrodes. Under 5 mA the normalized emittance 0.3 p mm mrad obtained meets the requirements.

Simulation of transport of a dense beam is carried out taking account of space charge and emittance of the beam. Focusing optic system was to be optimized for transporting 10 mA hydrogen ion beam without significant increase of the beam emittance. Two ways of beam injection to accelerator were studied, “strict” the by use of beam overfocusing, and "soft" introduction without beam overfocusing, with increased first gap and more fluent increase of electric field tension in tandem. Negligible changes of entrance socket allow to use the same path with the same lenses in both cases. This allows to test both regimes of beam entrance into accelerator experimentally and to choose the best one.

An analysis of different types of charge-exchange target had been made. A gas target was chosen for use. Following gas charge-exchange targets were assigned to be used: i) argon gas target with outer pumping; ii) argon gas target with recycling turbo-molecular pumping inside the high voltage electrode; iii) gas target with gas freezing on the nitrogen trap inside the high voltage electrode. Charge-exchange target, cryogen pump-out system, system for transporting target gas, liquid and gas nitrogen under the high voltage electrode potential were manufactured.

Experimental results on pulse 0.6 MeV vacuum insulation tandem accelerator are presented. Design of 2 MeV tandem accelerator is presented and discussed.

Neutron therapy; tandem-accelerator; charge-exchange target

Sergey Taskaev, Dr., Budker Institute of Nuclear Physics, 11 Lavrentiev ave., 6300090 Novosibirsk, Russia, Tel: 7 3832 394121, Fax: 7 3832 342163, e-mail: taskaev@inp.nsk.su