We present evidence for high harmonic fast wave (HHFW) heating in the core of the CDX-U spherical torus (aspect ratio R/a1.5) 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 Te increase are also observed. The majority of the discharges exhibit only modest core Te increases however, with most of the heating occurring outside r/a0.5. 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.