High harmonic fast wave heating efficiency enhancement and current drive at longer wavelength on the National Spherical Torus Experiment

TLDR

High‑harmonic fast‑wave heating and current drive are being developed on NSTX to support startup and sustainment, but RF waves near the wall at low magnetic field and parallel wavenumber can increase power losses through parametric decay instability and sheath dissipation. The efficiency gain results from shifting the perpendicular fast‑wave cutoff density farther from the antenna and wall, suppressing surface‑wave fields, and from the higher coupling efficiency that permits localized current‑drive measurements agreeing with advanced RF simulations. Increasing the magnetic field from 4.5 to 5.5 kG and optimizing antenna phasing raised core heating efficiency from 44 % to 65 %, mainly by reducing wall‑related losses while leaving parametric‑decay‑instability losses unchanged.

Abstract

High harmonic fast wave heating and current drive (CD) are being developed on the National Spherical Torus Experiment [M. Ono et al., Nucl. Fusion 41, 1435 (2001)] for supporting startup and sustainment of the spherical torus plasma. Considerable enhancement of the core heating efficiency (η) from 44% to 65% has been obtained for CD phasing of the antenna (strap-to-strap ϕ=−90°, kϕ=−8m−1) by increasing the magnetic field from 4.5to5.5kG. This increase in efficiency is strongly correlated to moving the location of the onset density for perpendicular fast wave propagation (nonset∝B×k∥2∕ω) away from the antenna face and wall, and hence reducing the propagating surface wave fields. Radio frequency (RF) waves propagating close to the wall at lower B and k∥ can enhance power losses from both the parametric decay instability (PDI) and wave dissipation in sheaths and structures around the machine. The improved efficiency found here is attributed to a reduction in the latter, as PDI losses are little changed at the higher magnetic field. Under these conditions of higher coupling efficiency, initial measurements of localized CD effects have been made and compared with advanced RF code simulations.