Effect of cusp magnetic field on positive ion flux in the LUPIN RF ICP source

Large Uniform Plasma for Ionizing Neutrals (LUPIN) is a radio-frequency (RF) inductively coupled plasma (ICP) positive hydrogen ion source prototype developed at NC State University. The goal of the project is to design an improved positive hydrogen ion source for the neutral beam injection (NBI) systems currently present at the DIII-D National Fusion Facility. The present LUPIN device operates without built-in permanent magnets, relying solely on RF currents for plasma confinement and ion production. However, permanent magnets at the backplate may shield that surface from ion bombardment and sputtering or improve power coupling by reducing the ion flux to the backplate. The cusp magnetic field can also alter electron temperature, ionization rates, thereby affecting plasma density, species composition, and the ion flux reaching the extraction grid. Introducing magnets in the LUPIN design will reveal how cusp magnetic field, beyond guarding against sputtering, can shape ion generation and transport in the full-scale NBI system. The Hybrid Plasma Equipment Model has been used to study the effects of increasing the magnitude of the magnetic field produced by these magnets. Initial parametric studies on a hydrogen RF ICP at 10 kW of deposited power, at 25 mTorr and an RF frequency of 2 MHz show that increasing the magnetic-field strength initially diminishes ion flux, but the trend reverses once the field becomes sufficiently strong.

This Work is supported by US DOE under DE-SC0024523 and DE-FC02-04ER54698.