With the first injection of neutral beams into the National Spherical Torus Experiment (NSTX) [Ono, et al., Nucl. Fusion 40 (2000) p557] a broad spectrum of fluctuations consisting of nearly equally spaced peaks in the frequency range from about 0.2 to 1.2 times the ion cyclotron frequency was observed. The frequencies scale with toroidal field and plasma density consistently with Alfvén waves. From these and other observations, the modes have been identified as Compressional Alfvén Eigenmodes (CAE). It has also recently been found that the ratio of the measured ion and electron temperatures in NSTX during neutral beam heating is anomalously high [Bell, Bull. Am. Phys. Soc. 46, p206 (2001)]. To explain the anomaly in the ratio of ion to electron temperature, it has been suggested that the CAE, driven by the beam ions, stochastically heat the thermal ions [Gates, et al., PRL 87, p205003 (2001)]. In this paper it is shown through studies of the power balance that stochastic heating of the thermal ions by the observed CAE alone is not solely responsible for the anomaly in the ion to electron temperature ratio.

A new spectroscopic diagnostic on the National Spherical Torus Experiment (NSTX) [J. Spitzer, M. Ono, et al., Fusion Technology 30, 1337] measures the velocity distribution of ions in the plasma edge simultaneously along both poloidal and toroidal views. An anisotropic ion temperature is measured during high power High Harmonic Fast Wave (HHFW) rf heating in helium plasmas, with the poloidal ion temperature roughly twice the toroidal ion temperature. Moreover, the measured spectral distribution suggests that two populations of ions are present and have temperatures of typically 500 eV and 50 eV with rotation velocities of -50 km/s and -10 km/s, respectively (predominantly perpendicular to the local magnetic field). This bi-modal distribution is observed in both the toroidal and poloidal views (for both He + and C 2+ ions), and is well correlated with the period of rf power application to the plasma. The temperature of the hot component is observed to increase with the applied rf power, which was scanned between 0 and 4.3 MW. The 30 MHz HHFW launched by the NSTX antenna is expected and observed to heat core electrons, but plasma ions do not resonate with the launched wave, which is typically at > 10 th harmonic of the ion cyclotron frequency in the region of observation. A likely ion heating mechanism is parametric decay of the launched HHFW into an Ion Bernstein Wave (IBW). The presence of the IBW in NSTX plasmas during HHFW application has been directly confirmed with probe measurements. IBW heating occurs in the perpendicular ion distribution, consistent with the toroidal and poloidal observations. Calculations of IBW propagation indicate that multiple waves could be created in the parametric decay process, and that most of the IBW power would be absorbed in the outer 10 to 20 cm of the plasma, predominantly on fully stripped ions. These predictions are in qualitative agreement with the observations, and must be accounted for when calculating the energy budget of the plasma.

No abstract

No abstract

A collective scattering system has measured electron gyro-scale fluctuations in National Spher-