Studies on Hydrogen Plasma in an ECR Based Large Volume Chamber

Abstract

In the present work, hydrogen plasma was investigated in a large volume plasma chamber (dia: 1.0 m and height: 1.0 m). Plasma was produced in an indigenously made compact size ECR plasma source1, 2, (dia: 0.09 m, length: 0.11 m) located coaxially within an NdFeB ring magnet assembly, placed at the center of the top dome of the plasma chamber. The magnetic field inside the source generates the ECR zone to permit absorption of the incoming microwave at 2.45 GHz for plasma heating, and extends as a diverging field into the large chamber. Plasma from the source flows along magnetic field to form a beam–like column near the source. Langmuir probe measurements along the chamber axis ($z$) at pressures 1–3 mTorr and input power 400 W show that the plasma density ($n_{e}$) decays rapidly up to $z=20\ \text{cm}$ from the source mouth ($\mathrm{z}=0$) followed by a slow decay thereafter towards the bottom of the chamber. The initial decay profile tracks the axial magnetic field profile closely confirming the $n/B$ scaling obeyed by hydrogen plasma as observed earlier in a smaller vessel3. Radial measurements taken at different $z$ planes show good radial uniformity away from the source. At $z=70\ \text{cm}, n_{e}\approx 3.5-4.0\times 10^{10}\ \text{cm}^{-3}$ and $T_{e}\approx 1-2\ \text{eV}$ over 40 cm radius at ≈ 3 mTorr pressure and 400 W power. Studies showed increase in $n_{e}$ with the pressure, although increasing the microwave power did not alter the density or temperature significantly. The good radial uniformity and the low $T_{e}$ could be useful for extraction of H− beams. Results from these experiments will be presented in this manuscript.