Abstract
With a spheroidal electron cyclotron resonance (ECR) surface completely enclosed in a vacuum vessel in a simple cusp field, we can produce ECR plasmas with a peak electron density up to ne≃5×1011 cm-3 at a low Ar pressure of p≃1.5×10-5 Torr by injection of 2.45 GHz microwave power of Pin=150–400 W. The degree of ionization reaches over 50% at this local density peak point. Such a high degree of ionization may be due to the efficient ionization by suprathermal tail electrons (W>30 eV), which are observed in single probe characteristics. On the other hand, at a relatively high pressure of p≃1.5×10-4 Torr, a flat density profile with density nearly equal to the plasma cutoff density (7×1010 cm-3) is obtained inside the ECR spheroid.
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