Nonlinear electromagnetic stabilization of ITG microturbulence by ICRF-driven fast ions in ASDEX Upgrade

Introduction Electromagnetic waves in the range of Ion Cyclotron Resonance Frequencies (ICRF) have many applications in fusion devices. While their use for external heating of magnetically confined fusion plasmas is well established, their effects on the enhancement of the plasma confinement by microturbulence stabilization have only been recently discovered [1,2]. For this effect a key parameter of merit is ?=-q2?R?P/P where R is the tokamak major radius, ? is the plasma beta, q is the safety factor and P is the plasma pressure. By increasing a local plasma pressure gradient and/or beta by ICRF-accelerated resonant ions, we can decrease turbulent transport driven by microinstabilities. We investigate the impact of ICRF-accelerated fast ions in the stabilization of microturbulence in two ASDEX Upgrade H-mode discharges [3]. In these discharges, in addition to 4.5 MW of deuterium NBI, 3.5 MW of ICRF power was applied at a frequency of 30 MHz tuned to a centrally located 3He minority resonance. The location of the 3He minority resonance was varied by about 10 cm by changing the toroidal magnetic field from 2.8 T in discharge 31562 to 3 T in discharge 31563. The plasma current was 0.6 MA and the main ion species was deuterium. An increase of up to 80% in the central ion temperature was measured, from 3 keV to 5.5 keV, as compared to the reference discharge 31555 with NBI heating only (c.f. Fig. 1). The normalized logarithmic ion temperature gradient, R/LTi, reached a high value of about 20, corresponding to a radial gradient of the Ti profile of about 50 keV/m. The 3He ion density is below 5% of the electron density in all discharges. Thus, the possible effect of main ion dilution [4] on microturbulence stabilization is not expected to be significant.