Numerical experiments of island stabilization by RF heating with stiff temperature profile

Abstract

The nature of turbulent transport in tokamak plasmas results in temperature profiles that are called resilient or stiff, and the stabilization of magnetic islands by a localized heat source is expected to be extremely sensitive to the stiffness strength. Theoretical expectations are verified with nonlinear simulations, showing a good agreement and confirming the enhanced stabilization efficiency due to large profile stiffness when the power used for the control is small compared with the heating power producing the equilibrium profiles. Heat sources that are present in the island region before the RF heating is applied contribute to reduce the island size, but at the same time, they severely damp the control capability.

Non-linear simulations of neoclassical tearing mode control by externally driven RF current and heating, with application to ITER

“…We document this degradation in section 6, but we acknowledge that this issue is not fully understood for the moment. The heating contribution is found to follow theoretical predictions, including the background plasma heating effect already studied in the context of classical tearing modes [17]. In conditions comparable to the ITER ones, where the ratio between the applied RF power and the power injected inside the resonant surface ranges between 10 and 20%, RF heating is expected to contribute to 20% of the stabilization efficiency in the case of continuous application, a number that appears to be relatively large, but the reason comes from the degraded contribution of the current drive.…”

“…The implication is that the contribution of power density to the island stabilization can be much larger than anticipated with a non-stiff transport model when the RF power is moderate compared to the power flowing through the island position [31]. But on the other hand, even a limited background heating, present before the RF system is used, can damp the control capability by localized heating [17]. The RF heating contribution is therefore very much case dependent, and one of the goals of the present study is to evaluate it in an ITERlike configuration.…”

“…In tokamaks, the use of RF waves at the electron cyclotron (EC) frequency has proved to be very efficient in this respect because of the narrow deposition width that can be obtained, as demonstrated in a large number of experiments [5], and verified in numerical simulations [6][7][8][9][10][11]. Although the best results are obtained when the RF beam is toroidally oriented to drive a current, the favorable effect of pure heating has also been demonstrated exper imentally [12] and numerically [13][14][15][16][17].…”

“…A criterion on the required RF current drive and power needed for full stabilization is derived and illustrated for the ITER situation. In a second part, we present full MHD non linear simulations using the XTOR code [36] where a stiff transport model [17] and RF-heating and -current drive are implemented [10,37]. The MHD model that is considered includes ion and electron diamagnetic rotations as well as an ad-hoc bootstrap current, and we consider a simplified ITER-like configuration (section 3) where the (3,2) NTM saturates at about 10% of the minor plasma radius due to the bootstrap drive (section 4).…”

Analysis of fast turbulent reconnection with self-consistent determination of turbulence timescale

“…The turbulent electromotive force v × b as well as ε represents the feedback of MHD turbulence in the mean induction equation. As a first step, the turbulent stress-tensor v ⊗ v − b ⊗ b is neglected in the momentum equation (34) and only the turbulent electromotive force relates turbulence to the mean fields.…”