Evolution on Nonthermal Particle Distributions in Radio Frequency Heating of Fusion Plasmas Paul Bonoli, MIT

Plasma heating and pressure profile control by the use of radio waves in the ion cyclotron range of frequencies (ICRF) is an essential application of RF power in present day tokamaks and in burning plasmas such as ITER. These waves can generate energetic ion particle distributions known as "tails" as they damp on a minority ion species in plasmas. These fast ion tails heat bulk plasma ions via collisional slowing down and can also stabilize deleterious magnetohydrodynamic instabiliities such as sawteeth or neoclassical tearing modes (NTM's). The ICRF waves can also interact with fast ions present in the plasma from nuclear fusion processes (alpha particles) or from neutral beam injection. A predective description of these processes is therefore crucial in order to insure the success of ICRF applications in ITER. In the SciDAC Center for Simulation of Wave Plasma Interactions we have exploited terascale computing capability as well as applied mathemeatics expertise to self-consistently solve for the evolution of nonthermal ion tails using a fill-wave electromagnetic field solver (AORSA) and a bounce averged Fokker Planck code (CQL3D). The combined simulation model has shown excellent scalability up to the 10,000 processor level. The full-wave and Fokker Planck solvers work in an iterative loop where at each iteration the full-wave solver evaluates a new plasma response using the most recent nonthermal ion distribution computed by the Fokker Planck solver. The iteration is advance by evaluating a new quasilinear RF diffusion coefficient from the full-wave electric fields and then using that in the Fokker Planck solver. Recently we have worked with experimentalists at the Alcator C-Mod facility to use the predicted nonthermal ion distribution functions from the AORSA-CQL3D calculation in a synthetic diagnostic code for neutral particle analysis (NPA). The synthetic code predictions have been found to accurately reproduce the velocity space structure of energetic ion tail distributions during minority ICRF heating in C-Mod. This same simulation model has been used to study the interaction of ICRF waves with fusion generated alpha particles and to study the eveolution of a nonthermal tritium tail during second harmonic tritium heating in the ITER device. Most recently we have started to advance multiple ion species in the Fokker Planck calculation to study the simultaneous interaction of ICRF waves with fast neutral beam ions as well as with a thermal background ion species. Finally we have used full-wave ICRF fields from an electromagnetic field solver (TORIC) in a Monte Carlo orbit following code (ORBIT-RF) to confirm the validity of the quasilinear description for the wave-particle interaction in ICRF minority heating. It has been shown that phase space orbits of resonant ions are superadiabatic at very low electric field strengths but that the orbits become stochastic as the electric field is increased to values typical of present day ICRF heating experiments.