Production efficiency of H− ions from a magnetized sheet plasma source

Abstract

The production efficiency of H⁻ ions extracted from a magnetized sheet plasma source is calculated using the second derivative of I-V curves obtained by a single Langmuir probe. Comparative results are presented for three different velocity distributions of particles namely: Maxwellian, Druyvesteyn and that of Amemiya. The discharge conditions where the addition of 10% argon raised the negative-ion current extracted by a factor of 9 were used. Probe measurements were taken at different distances from the core of the sheet plasma to its periphery. The discharge current was varied as well. Results indicate that the optimum value of production efficiency for the source is at 5 cm from the core of the sheet plasma and at discharge current of 1.5 A. On the average, the Druyvesteyn case leads to a production efficiency of 26.89% compared to the Maxwellian case at 34.32% and the Amemiya case at 27.42%. The estimated production efficiency using a mass analyzer calibrated with its collection factor is 28.8%, suggesting the energy distribution for this source is Druyvesteyn-like.