Lower hybrid current drive at high density in the multi-pass regime

Abstract

Assessing the performance of lower hybrid current drive (LHCD) at high density is critical for developing non-inductive current drive systems on future steady-state experiments. Excellent LHCD efficiency has been observed during fully non-inductive operation (η = 2.0 − 2.5 × 10 19 AW −1 m −2 atn e = 0.5 × 10 20 m −3 ) on Alcator C-Mod [I. H. Hutchinson, et al, Physics of Plasmas, 1, 1511] under conditions (n e , magnetic field and topology, LHCD frequency) relevant to ITER [S. Shiraiwa, et al, Nuclear Fusion, 51, 103024 (2011)]. To extend these results to advanced tokamak regimes with higher bootstrap current fractions on C-Mod, it is necessary to increasen e to 1.0 − 1.5 × 10 20 m −3 . However, the number of current-carrying, non-thermal electrons generated by LHCD drops sharply in diverted configurations at densities that are well below the density limit previously observed on limited tokamaks. In these cases, changes in scrape off layer (SOL) ionization and density profiles are observed during LHCD, indicating that significant power is transferred from the LH waves to the SOL. Fokker-Planck simulations of these discharges utilizing ray tracing and full wave propagation codes indicate that LH waves in the high density, multi-pass absorption regime linger in the plasma edge and SOL region, where absorption near or outside the LCFS results in the loss of current drive efficiency. Modeling predicts that non-thermal emission increases with stronger single-pass absorption. Experimental data show that increasing T e in high density LH discharges results in higher non-thermal electron emission, as predicted by the models.

Characterization of onset of parametric decay instability of lower hybrid waves

“…LH launcher on Alcator C-Mod bootstrap current fraction ( 50 %), which is critical for achieving steady state tokamak operation. Nonetheless, high density LHCD experiments on Alcator C-Mod have revealed a loss of current drive efficiency [3,4]. In low density plasmas, fully non-inductive plasma discharges [5] were demonstrated with engineering efficiency η ≡ n 20 IR/P ≈ 0.25 A/mW 2 , consistent with expected efficiency based on theory.…”

20 years of research on the Alcator C-Mod tokamak

“…358,359 Figure 59 compares experimental measurements of the hard x-rays produced by fast electrons and the prediction of a ray tracing calculation that neglects propagation and absorption processes in the plasma edge. The changes in the measured ionization rates and profiles in the SOL suggest that wave-plasma interactions in the edge are significant.…”

Impact of SOL plasma profiles on lower hybrid current drive: Experimental evidence, mitigation and modeling approaches

“…GENRAY/CQL3D prediction of HXR emission intensity when 2D realistic SOL is used (blue). Other simulation data are based on References [5] and [8].…”

“…Experimentally, the loss of current drive is observed most clearly in a rapid decrease of non-thermal hard X-ray emission as the density is increased. Since the standard LHCD modeling based on the ray-tracing/Fokker-Planck code was found to overpredict the current drive, we sought for physics processes to add in order to reproduce this experimental trend [4,5]. We first extended the computational domain to include the SOL plasma.…”