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The target thickness of the neutraliser gas in neutral beam injectors is smaller than expected from gas flow calculations. This is explained by direct and indirect beam heating of the gas and a consequent flow acceleration. The role of the neutraliser plasma electrons in the gas heating scenario is reconsidered here. The particle and energy balances of the electrons are solved with allowance for ionisation and dissociation of the gas by the electrons. It is shown that the direct contribution of the electrons to heating is not dominant. Their indirect contribution via ion acceleration in the space charge sheath in front of the walls is usually stronger. Methods to increase the neutraliser target thickness include (1) structuring the neutraliser surface in order to reduce the reflection of medium energy particles and to increase the gas accommodation, (2) cooling down the neutraliser to LN(2-) temperature. The scaling of the target thickness with various parameters is investigated and compared with "cold" flow. Initially it was assumed that the neutraliser plasma electrons are hot and able to excite Balmer alpha radiation in collisions with H(2) molecules. The investigation shows that the electrons must be cold instead (Te < 5eV). An enhanced emission of the unshifted Balmer alpha line as measured on the IPP test bed is probably caused by dissociative recombination of H(2)+ and H(3)+ ions leading to excited hydrogen atoms.

Additional information

Authors: OTT W, Max-Planck-Institut für Plasmaphysik, Garching bei München (DE)
Bibliographic Reference: Report: IPP 4/237 EN (1989)
Availability: Available from Max-Planck-Institut für Plasmaphysik, 8046 Garching bei München (DE)
Record Number: 198910593 / Last updated on: 1994-12-01
Original language: en
Available languages: en
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