Edge Ion Heating by Launched High Harmonic Fast Waves in NSTX

Abstract

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.

Progress towards steady state at low aspect ratio on the National Spherical Torus Experiment (NSTX)

“…The improved coupling was observed at higher values of TF B t = 0.55 T. The higher field is thought to have reduced the parasitic coupling of the launched RF power to the parametric decay instability, which had been observed to interfere with the coupling at lower TF [28]. This is an important step towards using the HHFW as a tool for driving current during the startup phase of the NSTX plasma to aid plasma current ramp-up.…”

Progress towards steady state on NSTX

“…Work is ongoing to develop a better understanding of the physics that controls HHFW ion damping, but initial indications are that the fast ion heating is due to direct wave damping [15], whereas the edge bulk heating is due to parametric decay (i.e. a non-linear three-wave coupling which conserves momentum and energy) into an ion Bernstein mode and an ion quasi-mode [16].…”

Status and Plans for the National Spherical Torus Experimental Research Facility

“…Using ERD, an strong wave-particle interaction was observed which produces significant (~ 400 eV) edge perpendicular ion heating and edge rotation during application of high harmonic fast wave (HHFW) power. (18) The ERD was also used to measure the highly important plasma spin up behavior in the H-mode pedestal region prior to the L-H transition.…”