Characterization of hydrogen plasma in a permanent ring magnet based helicon plasma source for negative ion source research

Abstract: HELicon Experiment for Negative ion source (HELEN-I) with single driver is developed with a focus on the production of negative hydrogen ions. In the Helicon wave heated plasmas, very high plasma densities ($\sim10^{19} m^{-3}$) can be attained with electron temperatures as low as $\sim$1 eV in the downstream region. These conditions favor the production of negative hydrogen ions. In HELEN-I device at IPR, helicon plasma is produced using Hydrogen gas in a diverging magnetic field, created by a permanent ring magnet. RF Power ($P_{RF}$) of 800-1000W at 13.56 MHz frequency is applied to a Nagoya-III antenna to excite m = 1 helicon mode in the plasma. The plasma is confined by a multi-cusp field configuration in the expansion chamber. The transition from inductively coupled mode to Helicon mode is observed near $P_{RF}$ 700W with plasma density $\sim 10^{18} m^{-3}$ and electron temperature $\sim$ 5 eV in the driver and $\sim$ 1eV in the expansion volume. Line integrated negative hydrogen ion density is measured in the expansion chamber by employing an Optical Emission Spectroscopy (OES) diagnostic technique using $H_\alpha/H_\beta$ ratio and Laser photo-detachment based Cavity Ring Down spectroscopic (CRDS) diagnostic technique. The measured value of negative hydrogen ion density is in the order of $10^{16} m^{-3}$ at 6 mTorr pressure and does not vary significantly with power in the helicon mode, pressure and downstream axial magnetic field variation. The negative ion density measurements are compared with theoretically estimated values calculated using particle balance method considering different reaction rates responsible for negative hydrogen ion creation and destruction. It is to be noted that at present Caesium (Cs) is not injected in the plasma discharge to enhance $H^-$ ion density.