Advances in modeling of lower hybrid current drive

Abstract

First principle modeling of the lower hybrid (LH) current drive in tokamak plasmas is a longstanding activity, which is gradually gaining in accuracy thanks to quantitative comparisons with experimental observations. The ability to reproduce simulatenously the plasma current and the non-thermal bremsstrahlung radial profiles in the hard x-ray (HXR) photon energy range represents in this context a significant achievement. Though subject to limitations, ray tracing calculations are commonly used for describing wave propagation in conjunction with Fokker-Planck codes, as it can capture prominent features of the LH wave dynamics in a tokamak plasma-like toroidal refraction. This tool has been validated on several machines when the full absorption of the LH wave requires the transfer of a small fraction of power from the main lobes of the launched power spectrum to a tail at a higher parallel refractive index. Conversely, standard modeling based on toroidal refraction only becomes more challenging when the spectral gap is large, except if other physical mechanisms may dominate to bridge it, like parametric instabilities, as suggested for JET LH discharges (Cesario et al 2004 Phys. Rev. Lett. 92 175002), or fast fluctuations of the launched power spectrum or 'tail' LH model, as shown for Tore Supra (Decker et al 2014 Phys. Plasma 21 092504). The applicability of the heuristic 'tail' LH model is investigated for a broader range of plasma parameters as compared to the Tore Supra study and with different LH wave characteristics. Discrepancies and agreements between simulations and experiments depending upon the different models used are discussed. The existence of a 'tail' in the launched power spectrum significantly improves the agreement between modeling and experiments in plasma conditions for which the spectral gap is large in EAST and Alcator C-Mod tokamaks. For the Alcator C-Mod tokamak, the experimental evolution of the HXR profiles with density suggests that this model is valid up to a line-averaged density of × + n 1.0 10 e 20 m −3 , a statement that is confirmed by simulations of the HXR scaling law with density. While simulations with GENRAY/CQL3D codes have ascribed the fast decrease of the HXR emission with density to parasitic absorption in the scrape-off layer by collisional damping, an alternative interpretetation based on an enhanced refraction as the LH wave propagates in the vicinity of the X-point is provided by C3PO/LUKE codes. The consequences for the predictions of LH current in ITER are discussed.

Current Challenges in the First Principle Quantitative Modelling of the Lower Hybrid Current Drive in Tokamaks

“…1, the general scheme for first principle modeling consists to couple different models describing successively wave propagation from the antenna in the Scrape-Off Layer (SOL), while wave propagation and absorption of the LH wave inside the separatrix are determined by solving numerically Maxwell and Boltzman equations in the geometrical optics and Fokker-Planck approximations respectively [2]. The standard tools used for this purpose are C3PO/LUKE or GENRAYCQL3D raytracing/Fokker-Planck codes [4,5,6,7].…”

“…For C3PO/LUKE, each lobe in the power spectrum is considered locally as a plane wave, even for the tail LH model, with a spectral width given by the Fourier transform of the initial wave electric field pattern. With this approach, all rays are well separated from each other in phase space, a condition for a correct calculation the LH diffusion coefficient by simply adding all ray contributions [2,16].…”

“…However, combined with the plasma current, it cannot constrain unambiguously simulations as illustrated in Fig.4 for a set of EAST tokamak discharges. Only quantitative measurements that combine simultaneously dynamics in momentum and configuration spaces like HXR are more selective between models [2]. Time modulation of the LH power or the antenna phasing may represent also a challenging constraint for first principle modeling.…”

“…In this context, first principle modeling has started long time ago and is still active after more than 30 years of research. Thanks to the challenges concerning ITER physics performances by appropriate current profile tailoring, an active international modeling activity has been carried out during the last five years, involving quantitative comparisons between simulations and experiments for several tokamaks (Tore Supra, Alcator C-Mod, EAST) [2,3]. In this paper, a review of main achievements is presented, which highlights the state of the art in this field of physics.…”

Assessment of quasi-linear effect of RF power spectrum for enabling lower hybrid current drive in reactor plasmas

“…case of linear wave approximation, under higher values of higher p RF or smaller Δn // , for given reactor plasma parameters [7,8]. Differently from other numerical approaches [19,23], the LH star tool, in order to assess the j LH profiles, treats the intrinsically LHCD single pass regime of reactor in simpler way because not affected by complicated architectures seeking to consider multi-pass regime, conceived with the aim of considering the case of LHCD experiments in experiments performed so far in less warm plasmas. Consequenlty, by using antenna spectra sufficiently narrow and however compatible with constraints imposed by QL theory (trapping effect) it is possible enabling the LHCD effect and shaping the profile, spanning over the whole outer radial half of plasma of DEMO [7,8].…”