Current drive at plasma densities required for thermonuclear reactors

Abstract

Progress in thermonuclear fusion energy research based on deuterium plasmas magnetically confi ned in toroidal tokamak devices requires the development of effi cient current drive methods. Previous experiments have shown that plasma current can be driven effectively by externally launched radio frequency power coupled to lower hybrid plasma waves. However, at the high plasma densities required for fusion power plants, the coupled radio frequency power does not penetrate into the plasma core, possibly because of strong wave interactions with the plasma edge. Here we show experiments performed on FTU (Frascati Tokamak Upgrade) based on theoretical predictions that nonlinear interactions diminish when the peripheral plasma electron temperature is high, allowing signifi cant wave penetration at high density. The results show that the coupled radio frequency power can penetrate into high-density plasmas due to weaker plasma edge effects, thus extending the effective range of lower hybrid current drive towards the domain relevant for fusion reactors.

Lower hybrid current drive at high density in Alcator C-Mod

“…Prior investigation of LHCD at such high densities was conducted in circular, limited tokamaks such as Alcator C [24] and FTU [25,18,19]. Although the experiments on C-Mod show no dependence on elongation, the difference between limited and diverted plasmas is significant and this may explain the unexpectedly low density limit encountered for diverted discharges in C-Mod.…”

“…Results from the FTU LHCD experiment suggest that the LHCD density limit can be overcome by raising the temperature in the plasma edge (r/a > 0.8) [18,19]. The I-mode confinement regime on C-Mod consists of an H-mode like temperature pedestal at the plasma edge with an L-mode like density profile [20,21,22].…”

Studies of challenge in lower hybrid current drive capability at high density regime in experimental advanced superconducting tokamak

“…Reactor relevant high plasma density regimes have faced the LHCD tool with the challenge of effectively penetrating the main plasma without excessive power dissipation of the coupled radio frequency (RF) power at the edge. LHCD experiments on many tokamaks (Pericoli-Ridolfini, Giannone & Bartiromo 1994;Cesario et al 2010;Wallace et al 2010;Cesario et al 2011) have shown a decrease in non-thermal tails and a drop in bremsstrahlung emission much steeper than 1/n e at high densities, which is stronger than expected based on current models. Similar results (Ding et al 2013a have also been reported in EAST.…”

Overview of LH operation at JET

“…It is well known [3,4] that the spectral broadening of the launched LH (pump) wave at 3.7GHz is caused by scattering from low frequency density fluctuations at the plasma edge, while lower sideband appearance can be related to parametric decay of the pump wave into another LH wave (the lower sideband) and a low frequency Fig. 1(a), were analyzed versus the launched LH power, P LH , line averaged plasma density, n e , and magnetic field, B T .…”

“…Furthermore, a new LH-related diagnostic based on RF spectra measurements was setup, tested and validated during JET restart. Initial data have been collected and analyzed in order to address important issues regarding the impact of the Parametric Decay Instabilities (PDI) [3,4] on the CD efficiency at high densities. In addition a new RT/FP code was developed at JET in order to study the impact of the real plasma shape and poloidal variations in the launched spectrum on the LH wave power deposition and CD efficiency.…”