Modelling of RF current drive in the presence of radial diffusion

Abstract

The kinetic theory of radiofrequency current drive in to-plasmas is investigated. The problem of the impact of anomalous uansport on the driven current profile and efficiency is considered in detail. b o n g the possible candidates for explaining anomalous transport in tokamaks, magnetic turbulence is known to have a saong influence on the dynamics of snperthemd elecaons and is assumed here to be the basic mechanism responsible for radial diffusion of therf-driven current. The 3-dimensional kinetic equation in the presence of rf heating and magnetic hubulence is studied. Its properties are f i s t investigated by a non-local response function technique. Both the adjoint formalism and the Langevin equations methcd are extended to the case of radially diffusing electrons.The full kinetic equation is numerically solved by means of a 3-D Fokker-Planck code. Applications to lower-hybrid current drive are presented and several kinetic effects are discussed.

Comparison of magnetic island stabilization strategies from magneto-hydrodynamic simulations

“…However, it has been observed experimentally Harvey et al (2002); Coda et al (2003); Kirov et al (2002) that the deposition can actually be larger than expected from ECCD/ECRH modeling. This can be due to cross-field diffusion of the fast electrons density (constituting the EC current), for instance because of turbulence on the deposition location, leading to a broadening of the driven current density deposition and therefore to a reduction in the maximum driven current density Giruzzi and Fidone (1989); Giruzzi (1993); Coda et al (2006); Bertelli and Westerhof (2009) ;Bertelli et al (2010). It has also been shown that the electron density fluctuations near the plasma edge, or along the path of the RF-waves, might lead to a broadening of the current deposition as well as a fluctuating power deposition profile Tsironis et al (2009) ;Peysson et al (2011); Decker, J. et al (2012).…”

Synergy of Electron-Cyclotron and Lower-Hybrid Current Drive in Steady-State Plasmas

“…2(a) have been performed, using a 3D relativistic Fokker-Planck code [22], and focusing the comparison on the synergy factor F syn . The measured temperature and density profiles as well as other experimental parameters are used as an input to the Fokker-Planck code.…”

Measurement of the Time Constants of Fast Electron Distributions in the Tore Supra Tokamak

“…The identification of the physical phenomena corresponding to each time scale implies a comparison with detailed theoretical models, in this case with the 3D kinetic theory of the tail formation and relaxation in the presence of Coulomb collisions, radial diffusion, rf waves, and dc electric field. This has been performed using a sophisticated computational model including a 3D time-dependent Fokker-Planck code (for the evaluation of the electron distribution function), coupled to a radiation code for the evaluation of the corresponding ECE and ECA outputs in a realistic geometry [6]. This comparison demonstrates that in a large machine the relaxation of fast electrons is dominated by Coulomb collisions, in contrast with small tokamaks, where radial diffusion plays a comparable or even dominant role [7].…”