Electron Density Measurements in Reduced-Scale Inductively Coupled Plasma Ion Source for the DIII-D NBI System

The Large, Uniform Plasma for Ionizing Neutrals (LUPIN) is a reduced-scale inductively coupled plasma positive ion source for the DIII-D neutral beam injection system. LUPIN has been designed and constructed to study positive ion source physics in order to optimize designs prior to construction of a full-scale ion source. In this work we present electron density, n_e, measurements via a microwave resonator hairpin probe in pure helium and hydrogen plasmas operated between 10-100 mTorr pressure and 150-1000 W delivered radio-frequency (RF) power at 2 MHz. For both main ion species, the largest n_e values were observed at the highest gas pressures, achieving n_e on the order of 10¹⁷ m^-3. He plasmas exhibited significantly larger n_e relative to H₂ plasmas at a given pressure and a clear linear dependence of electron density upon the delivered RF power, which is consistent with analytical models. In H₂ plasmas non-linear increases in n_e were observed between 200-600 W delivered power, which was attributed to a transition from capacitive coupling to inductive coupling modes. This was confirmed by calculating collisional skin depths which approached chamber dimensions in the non-linear region, which is a characteristic feature of capacitive-inductive mode transitions. Experimental measurements were compared with simple 0-D plasma models and complex Hybrid Plasma Equipment Model (HPEM) simulations; 100 mTorr He plasma experimental measurements demonstrated excellent agreement with HPEM simulations. Measurements and simulations will be utilized to further validate our plasma models and to extrapolate LUPIN plasma behavior at high power to inform design choices for a full-power, full-scale ion source for DIII-D.

This work is supported by the U.S DOE under awards DE-SC0024523 and DE-FC02-04ER54698.