Fast electron transport in lower-hybrid current drive

Abstract

The quasilinear Fokker–Planck formulation is generalized for lower-hybrid current drive to include the wave-induced radial transport of fast electrons. Toroidal ray tracing shows that the wave fields in the plasma develop a large poloidal component associated with the upshift in k∥ and the filling of the ‘‘spectral gap.’’ These fields lead to an enhanced radial E×B drift of resonant electrons. Two types of radial flows are obtained: an outward convective flow driven by the asymmetry in the poloidal wave spectrum, and a diffusive flow proportional to the width of the poloidal spectrum. Numerical results relevant to Alcator C [Phys. Rev. Lett. 53, 450 (1984)] and JT-60 [Nucl. Fusion 29, 1815 (1989)] are presented; they show that the radial convection velocity has a broad maximum of nearly 1 m/sec and is independent of the amplitude of fields. In both cases, the radial diffusion is found to be highly localized near the magnetic axis. For JT-60, the peak of the diffusion profile can be quite large, nearly 1 m2/sec.

Ray-tracing and Fokker–Planck modelling of the effect of plasma current on the propagation and absorption of lower hybrid waves

“…This suggests that some physics must be added to the classical RT/FP approach to account for the experimental phenomenology. There are several candidate broadening mechanisms, which may account for such a reduced sensitivity of the LH current profile with respect to the plasma profiles, at least qualitatively: radial transport of fast electrons [44,54], induced either by turbulence or by the LH waves themselves [55], scattering of the waves by density fluctuations [8,13], broadening of the launched n spectrum by parametric instabilities [56], wave diffraction [26,27] and breaking up of the quasilinear approximation [57]. Most of these effects are however very difficult to assess quantitatively by measurements.…”

Transport of fast electrons during LHCD in TS, JET, and ASDEX

“…In this code, the I-D Fokker-Planck equation which usually incorporates the radial transport losses in a Krook-type term, is replaced by a 2-D Fokker-Planck equation (1-D in parallel momenmm and 1-D in space). The wave-induced radial diffusion is neglected [29]. If radial transportis dominant, a decoupling between theLHpowerdeposition andtheLHdrivencurrent density profiles is predicted, as shown in figure 5a for a low density JET discharge (n&=l.4xlO+1gm-3).…”

Modelling of RF current drive in the presence of radial diffusion

“…Both the rf and the turbulence terms are evaluated in the framework of the quasilinear theory: this can be done by using a Hamiltonian formalism in actionangle variables (see, e.g., Kaufman 1972). This formalism naturally yields, together with the familiar quasilinear difhion terms, cross-terms such as wave-induced radial diffusion and pinch (Antonsen and Yosbioka, 1986;Kupfer and Bers, 1991) and ambipolar elecuic fields ensuring charge neudity (Molvig et al, 1978).…”

“…1992). Other kinetic effects, such as wave-induced transport, have been investigated, since they could, in principle, lead to a degradation of the current drive efficiency (Kupfer and Bers, 1991). Their impact has been found negligible on both the global efficiency and the cmrentprofile, for typicd parameters of LH and FW current drive in present-day large machines and in a reactor.…”