Lower hybrid current drive at high densities of ITER

As widely documented by available experimental and modelling data [1,2], a broadening in the lower hybrid (LH) wave frequency and n_// (parallel wavenumber) spectrum is produced by parametric instability (PI) that can affect the propagation and deposition of the LH power launched in tokamak plasmas. The electronic dynamics at the plasma edge mainly determines the occurrence of the ion-sound quasimode-driven PI, whose growth rate is intrinsically high at the radial layers close to the antenna-plasma interface, as its maximum occurs in the same conditions necessary for performing the antenna coupling (i.e.: ω_pe⁄ω_0 ≥1 at the plasma edge). The radial extent in the plasma of the region with high growth rate determines the spectral broadening size. Radially deeper is this region, smaller are the convective losses and consequently bigger is the spectral broadening. The present work shows that considering a case of relatively high densities (n_e) and low electron temperatures (T_e) of the scrape-off layer (SOL), the LH spectral broadening should be strong enough to produce the radiofrequency (RF) power deposition at the very plasma periphery of ITER (International Thermonuclear Experiment Reactor). Conversely, operating with slightly low ne and high T_e in the SOL, the radial extent of the PI region results reduced and, consequently, a useful LHCD can be performed in the plasma bulk.