Efficiency of LH current drive in tokamaks featuring an internal transport barrier

Abstract

In this paper, we study the effects of the occurrence of radial transport of particles in a tokamak on the efficiency of the current drive by lower hybrid (LH) waves, in the presence of an internal transport barrier. The results are obtained by numerical solution of the Fokker-Planck equation which rules the evolution of the electron distribution function. We assume that the radial transport of particles can be due to magnetic or electrostatic fluctuations. In both cases the efficiency of the current drive is shown to increase with the increase of the fluctuations that originate the transport. The dependence of the current drive efficiency on the depth and position of the barrier is also investigated.

Efficiency of LH+EC current drive in tokamaks featuring an internal transport barrier

“…where D LH is the diffusion coefficient in momentum space, and parallel means the direction parallel to the equilibrium magnetic field. The explicit expression for the coefficient D LH can be found in the literature and will not be repeated here, for the sake of economy of space [8][9][10]. The important point is that the LH diffusion coefficient depends on the quantity S(s, τ, N ), which represents the spectrum of the energy flux for LH waves.…”

“…These limits of the LH spectrum in N -space are obtained from a well known propagation model which assumes several back and forth trips of the LH waves before complete absorption due to Landau damping [8,11,12]. The use of this propagation model is one of the main differences between the investigation developed in the present paper and the approach adopted in Ref.…”

“…For the numerical solution, we have assumed that the term due to electrostatic fluctuations goes smoothly to zero for |u | well below the lower limit of the range of parallel velocities resonant with the LH and EC waves, therefore avoiding the divergence at |u | = 0 which appears in Eq. (8). The quantities b ≡ (B/B 0 ) and e ≡ (E/cB 0 ) give, respectively, the magnetic and electrostatic perturbation level of turbulence, normalized to the magnitude of the local ambient magnetic field, B 0 (s).…”

“…[7] the propagation of LH waves was described by a cruder model, if compared with the model employed in the present investigation and also employed in Ref. [8], which considered the case of current generation by LH waves alone. According to the present more detailed model of LH propagation, the LH waves do not arrive to regions of the tokamak in which they could be present with the previous model.…”

“…According to the present more detailed model of LH propagation, the LH waves do not arrive to regions of the tokamak in which they could be present with the previous model. Significant differences occur in the profiles of LH power absorption and generated current, and therefore the effects of transport and of the presence of the ITB may be significantly modified, as demonstrated in the case of current drive with LH waves [8]. Another difference between the approach of Ref.…”