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

Abstract

Abstract.The Lower Hybrid (LH) wave is widely used in existing tokamaks for tailoring current density profile or extending pulse duration to steady-state regimes. Its high efficiency makes it particularly attractive for a fusion reactor, leading to consider it for this purpose in ITER tokamak. Nevertheless, if basics of the LH wave in tokamak plasma are well known, quantitative modeling of experimental observations based on first principles remains a highly challenging exercise, despite considerable numerical efforts achieved so far. In this context, a rigorous methodology must be carried out in the simulations to identify the minimum number of physical mechanisms that must be considered to reproduce experimental shot to shot observations and also scalings (density, power spectrum). Based on recent simulations carried out for EAST, Alcator C-Mod and Tore Supra tokamaks, the state of the art in LH modeling is reviewed. The capability of fast electron bremsstrahlung, internal inductance l i and LH driven current at zero loop voltage to constrain all together LH simulations is discussed, as well as the needs of further improvements (diagnostics, codes, LH model), for robust interpretative and predictive simulations.

Lower Hybrid Current Drive in High Aspect Ratio Tokamaks

“…As shown in Fig. 9, the current efficiency ⌘ drops down by increasing D 0 from 0 to 1.0 m 2 s 1 , and the value that matches experimental observations corresponds to D 0 = 0.12 m 2 s 1 , very close to levels found in other LH current drive simulations [2,18,19]. Even small, the impact of anomalous radial transport is quite large on the fastest electrons in the LH-driven non-thermal tail thanks to the v k dependence.…”

“…The key diagnostic of this study is the FEB, which is known to give detailed insights on the dynamics of the LH wave and the resonant fast electrons in the plasma [2,19,51].…”

“…With the TSM that can be interpreted as a fluctuating power spectrum on a time scale shorter than fast electron collisional slowing-down, the FEB profile at high photon energies can be well reproduced by RTFP simulations but also variations of the FEB characteristics with N k0 . This successful achievement has been extended to the modeling of the LH current drive in elongated diverted plasmas for ALCATOR C-Mod and EAST tokamaks, using the same simulation parameters for the TSM, in particular the fraction of the LH power transferred from the main lobe to the tail [2,19]. A possible explanation of this spectral broadening in the scrape-off layer may be fast longitudinal plasma density fluctuations in front of the antenna that diffract the LH wave [21].…”

“…The Lower Hybrid (LH) wave has received a considerable interest during the last decades owing to its attractive property of damping efficiently at high parallel (to B) phase velocities relative to the electron thermal speed [1,2]. As expected from theory, a high figure of merit of the current drive efficiency has been achieved, while the control of the current density profile has been successfully demonstrated in several machines [3][4][5][6][7].…”

“…Since N k varies during its propagation in the plasma as a consequence of the poloidal non-uniformity of the magnetic field, this mechanism has been considered as universal for toroidal magnetic configurations like tokamaks, to fill the spectral gap [10,11]. Consequently, combined ray-tracing (RT) and Fokker-Planck (FP) calculations have almost become a standard for solving the spectral gap problem and simulate LH current drive experiments, even if full-wave calculations have been successfully performed more recently, removing the well-known limitations of the WKB approximations at some specific locations of the ray trajectories [1,2,[12][13][14]. Nevertheless, advanced modeling techniques have never been conclusive, and despite their intrinsic limitations, RTFP calculations remain widely used for LH current drive calculations.…”