Spectroscopic evidence for high harmonic fast wave heating in the CDX-U spherical torus

We present evidence for high harmonic fast wave (HHFW) heating in the core of the CDX-U spherical torus (aspect ratio R/a approximate to 1.5) at the P rinceton Plasma Physics Laboratory. We use a spectroscopic technique combin ing fast, multichordal measurements of the ultrasoft x-ray line emission fr om intrinsic C and O. The results show that in a few cases substantial (nea rly 50%) increases in the core electron temperature can be obtained with on ly about 100 kW of radio-frequency power, compared to approximate to 150 kW ohmic input. Modifications of the core magnetohydrodynamic activity consis tent with the estimated T-e increase are also observed. The majority of the discharges exhibit only modest core T-e increases however, with most of th e heating occurring outside r/a approximate to 0.5. A proposed explanation for this effect consistent with the experimental observations is based on a neoclassical transport instability for low-Z impurities, similar to an eff ect recently described for high-Z impurities in large tokamaks.