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One of the parts is the target, which converts negative hydrogen ions accelerated to half of the total energy into protons. The target in solid, liquid or gaseous state should satisfy the following requirements:
1. It should provide 98 - 99 % stripping of hydrogen negative ions beam with energy from 0.96 to 1.25 MeV into protons.
2. It should not lead to proton spectrum widening more than by 2 keV.
3. The vacuum deterioration in accelerating gap should not cause neither decrease of electrical strength of vacuum gap nor the potential redistribution on the electrodes of accelerating gap by the additional current on the electrodes. It is considered [I.Slivkov. Electrical insulation and vacuum discharge. M.: Atomizdat, 1972], that 10^-5 Torr pressure ensures the high-voltage strength of accelerating gaps, but 10^-4 Torr pressure degrades the tandem operation reliability because of the breakdowns of high-voltage vacuum gaps.
4. The numerical simulations of cold H^- beam transportation show that the use of magnetic or electrostatic decelerating lens before the tandem entrance allows transporting 25 mA beam 6 mm in diameter through 400 mm length in the target region. Therefore the aperture of stripping target should be more than 6 mm.

It has been defined that gas stripping target, used for stripping of H^- ions accelerated to half of total energy, is the most preferable for the electrostatic tandem-accelerator with vacuum insulation on energy of protons up to 2.5 MeV and with dc current up to 40 mA. Three variants of gas stripping target were chosen for further development and realization:
1. Argon gas target with external pumping. This target is attractive by the lack of dissociation effects, but requires providing the beam passage less than 10 mm in diameter on 400 mm length.
2. Argon gas target with turbo-molecular pump in immediate vicinity and with recirculation. It allows weakening the requirements on beam transportation and improving gas conditions.
3. Target with cryogenic nitrogen pump in immediate vicinity. It also allows weakening the requirements on beam transportation, but its use is accompanied by many effects which require experimental testing. Cryogenic nitrogen pump was manufactured for experiments (see photo).

In details:

G. Derevyankin, G. Dimov, V. Dolgushin, A. Dranichnikov, G. Krainov, A. Krivenko, V. Palchikov, M. Petrichenkov, E. Pokhlebenin, R. Salimov, V. Shirokov, G. Silvestrov, and S. Taskaev. Charge-exchange target for 40 mA 2 MeV tandem accelerator. Preprint BINP 2001-23. Novosibirsk. 2001. [.html file] [.pdf file]