Modeling lower hybrid current drive in presence of an extreme spectral gap

In very large aspect ratio tokamaks, the radio-frequency wave at the Lower Hybrid (LH) frequency propagation domain may be strongly bounded, thus preventing in principle the build-up of a fast electron tail to drive all the toroidal plasma current, the usual toroidal upshift being much too weak in this case to bridge the well-known "spectral gap". Nevertheless, even if these conditions are fulfilled, a full current drive regime may be achieved experimentally, and non-thermal electrons with kinetic energies well above the thermal level are observed by measuring bremsstrahlung in the hard x-ray range. This contradiction raises the question of the physical mechanisms to pull out a tail of fast electrons from the thermal bulk, highlighting the fact that toroidal refraction may not be the single one at play. This was suggested by the improved agreement between modeling and observations when using the heuristic tail spectral model [J. Decker et al., Phys. Plasmas 21 (2014) 092504] on many machines, considering that the LH power spectrum is already broad as the wave crosses the separatrix from the antenna. In this context, the possibility to eliminate the contribution of the toroidal refraction allows to clearly identify the ability of this model to reproduce alone experimental results. This has been carried out for TRIAM-1M [H. Zushi, et al., Nucl. Fusion 43 (2003) 1600] and WEST [C. Bourdelle, et al., Nucl. Fusion 55 (2015) 063017] tokamaks, both characterized by a very large aspect ratio R/a>5.5, using the ALOHA/C3PO/LUKE/R5-X2 chain of codes based on a first principles approach. In extreme spectral gap conditions, simulations show that it must be already filled at the separatrix to reproduce observations quantitatively and some important parametric dependencies. In the context of the edge spectral broadening here addressed, the LH wave absorption is strongly enhanced for the main lobes of the power spectrum at low parallel refractive indexes even when the plasma is cold. They can be therefore absorbed in an almost single pass regime, an important feature which restores the validity of the underlying WKB approximation in ray-tracing calculations, thus justifying its use. A comparison with a discharge corresponding to a marginal unbridgeable spectral for WEST is also shown.