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Enhanced volume production of negative ions in the post discharge of a multicusp hydrogen discharge

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3 Author(s)
Hopkins, M.B. ; Laboratoire de Physique des Milieux Ionisés, Laboratoire du C.N.R.S., Ecole Polytechnique, 91128 Palaiseau Cedex, France ; Bacal, M. ; Graham, W.G.

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In this paper we demonstrate a new concept in the production of negative hydrogen ions in a low‐pressure multicusp discharge. The discharge voltage is modulated to produce a non‐Maxwellian, hot‐electron plasma during the current pulse, followed by a cool Maxwellian electron plasma in the post discharge. This procedure, of separating in time the required hot and cold electron plasmas required for volume H- production, is called a temporal filter. The time evolution of the electron energy distribution function is measured using the time‐resolved second derivative of a Langmuir probe characteristic. Time‐resolved measurements of the negative ion density are made using laser photodetachment. The measurements show that the negative ion density in the center of the source, at a gas pressure of 0.07 Pa, increases by a factor of 2 when the discharge is switched off. At this low pressure the average H- beam current extracted from the source, when operated with a discharge current of 1 A in the pulse modulated mode exceeds the H- beam current from a 5 A continuously operated source. The increase in efficiency of the pulsed source is explained in terms of a two‐step H- production mechanism.

Published in:

Journal of Applied Physics  (Volume:70 ,  Issue: 4 )

Date of Publication:

Aug 1991

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