Nonlinear ICRF-plasma interactions

Abstract

Abstract. The well developed linear theory of ICRF (including FW, HHFW and IBW) interactions with plasma has enjoyed considerable success in describing antenna coupling and wave propagation, and provides a well-known framework for calculating power absorption, current drive, etc. In some situations, less well studied nonlinear effects are of interest, such as flow drive, ponderomotive forces, rf sheaths, parametric decay and related interactions with the edge plasma. Standard ICRF codes have begun to integrate this physics to achieve improved modeling capabilities. This paper concentrates on basic rf-plasma-interaction physics with illustrative applications to tokamaks. For FW antennas, the parallel electric field near launching structures is known to drive rf-sheaths which can give rise to convective cells, interaction with plasma "blobs", impurity production, and edge power dissipation. In addition to sheaths, IBW waves in the edge plasma are subject to strong ponderomotive effects and parametric decay. In the core plasma, slow waves can sometimes induce nonlinear effects. Mechanisms by which these waves can influence the radial electric field and its shear are summarized, and related to the general (reactive-ponderomotive and dissipative) force on a plasma from rf waves. It is argued that there are significant opportunities now for new predictive capabilities by advances in integrated simulation.

Spectral effects on fast wave core heating and current drive

“…Parametric decay instabilities may account for some of the losses, but as discussed above, are not large enough to account for the observed degradation of the core heating efficiency on NSTX, so that a number of processes may be contributing to the net loss. Sheath rectification losses in particular are associated with hot spots on antennas and nearby limiters and in general are exacerbated when the current straps are not aligned with the equilibrium magnetic field [16,17,18]. Visible light pictures, taken with a fast camera at the end of the 2008 experimental campaign, of a few NSTX H-mode discharges with and without HHFW Figure 5 Visible light camera picture showing toroidally localized HHFW interaction with the divertor in an NSTX H-mode plasma (I p = 1 MA, P NB = 2 MW, B T = 0.55 T, P rf ~ 1.8 MW, -90˚ phasing).…”

ICRF operation with improved antennas in ASDEX Upgrade with W wall

“…This approach is used rather than a reduction of the integrated E || on a family of magnetic field lines, because of the known complexity of the antenna-plasma-wall interactions [5,6]. Recently, the popular criterion of reduction of the integrated E || on the "long" field lines (the open field lines which pass in front of an antenna) has been questioned by the experimental observations in Tore Supra and in Alcator C-Mod.…”

N=2 ICRH of H majority plasmas in JET-ILW

“…As increased ICRH power yields larger impurity levels and higher radiation, and given the fact that these may affect the ICRF heating efficiency itself, it is critical to understand the causal link between these phenomena. The standard interpretation is that the electric fields close to the antenna non-resonantly accelerate ions in the SOL via RF sheath rectification effects and give rise to enhanced first wall sputtering [9]. Alternatively, sputtering can be caused by ICRH induced fast particles that are not confined in the bulk plasma and bombard the wall.…”