Production of Hydrogen Plasma by Compact ECR Source for Efficient Volume Generation of H− Ions

Hydrogen plasma produced inside a compact ECR plasma source (CEPS)¹ was allowed to expand into a large expansion chamber (dia. ≈ 50 cm, length ≈ 75 cm). Plasma parameters were measured using an on-axis Langmuir Probe. The CEPS uses NdFeB ring magnets with a complex field structure that has a magnetic mirror trap near the ECR zone followed by an on-axis magnetic null within the source region, and a monotonically decreasing field in the expansion chamber. Typically, at ≈ 600 W microwave power (2.45 GHz), 0.3–2 mTorr hydrogen pressure, the plasma density $n\approx 10^{10}\ \text{cm}^{-3}$ and electron temperature $T_{\mathrm{e}}\approx 25\ \ \text{eV}$ in the expansion chamber near the source, while further downstream $n$ and $T_{\mathrm{e}}$ are reduced to ≈ 10⁹ cm⁻³ and 5 eV, respectively². However, for 3–10 mTorr the plasma contains two electron populations - a high density, cold, bulk population and a low-density, warm population ($n_{w}/n\approx 0.01;\ T_{w}/T_{e}\approx 10$). Another feature observed at the higher pressures is that $T_{\mathrm{e}}$ cools to less than 1 eV about ≈ 30 cm from the source mouth. Such low $T_{\mathrm{e}}$ bulk electrons are important for generation of H⁻ ions by the volume production mode, which may be preferred over the caesiated grid method due to maintenance issues associated with the latter³. An interesting feature of the plasma is that the normalized plasma density and magnetic field profiles overlap fairly accurately indicating very little cross-field transport of the hydrogen plasma. In general, one finds from the experiments that the nature of the CEPS magnetic field is such as to promote efficient electron heating and high $T_{\mathrm{e}}$, which are not desirable for volume production of H⁻ ions. To achieve the latter, another field configuration that eliminates the adverse effects of the magnetic mirror and the null has been set up and results from the latter configuration will be presented at the conference.