Scaling Laws for Inductively Coupled Positive Ion Sources

Inductively coupled plasma (ICP) sources promise higher reliability, and greater total beam power in neutral beam injectors (NBIs), but the geometric scaling laws in these sources remain poorly studied. Geometric scaling can modify ion source dynamics because ion transport paths lengthen with linear dimension, surface-to-volume ratio decreases, wall recombination changes, and the skin depth of the RF field that controls the radial deposition of power changes. Therefore, a computational study of geometric and power‐density scaling in an RF ICP positive-ion source will be performed to quantify how ion production, and transport efficiencies depend on size, shape, and operating conditions. The Hybrid Plasma Equipment Model, incorporating kinetic electrons, fluid ions, and a 144-reaction hydrogen chemistry set is employed to study these ion sources at steady state. A cylindrical chamber with initial radius of 10 cm and height of 20 cm will be uniformly magnified up to a factor of three while maintaining aspect ratios. Deposited power density will be swept from 2 to 6 MW/cm³, and neutral pressures from 10 to 40 mTorr. Additionally, a race-track-shaped chamber measuring 40 cm in length, with a turn diameter of 28 cm at the curved ends, & height of 30 cm will be analyzed for scaling factors at identical power densities and compared with the cylindrical shaped design. For each geometry and operating point, production efficiency, defined as the ratio of total volume production of positive ions to absorbed RF power, escape probability, the fraction of generated positive ions reaching the extraction plane without recombination, and axial ion flux uniformity across the extraction grid will be recorded. The scaling-law correlations will provide a framework for selecting operating points during the planned upgrade from the small-scale prototype to the full-scale NBI ion source at DIII-D.