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 ≈ 1.5) at the Princeton Plasma Physics Laboratory. We use a spectroscopic technique combining fast, multichordal measurements of the ultrasoft x-ray line emission from intrinsic C and O. The results show that in a few cases substantial (nearly 50%) increases in the core electron temperature can be obtained with only about 100 kW of radio-frequency power, compared to ≈ 150 kW ohmic input. Modifications of the core magnetohydrodynamic activity consistent 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 the heating occurring outside r/a ≈ 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 effect recently described for high-Z impurities in large tokamaks.