Application of very high harmonic fast waves for off-axis current drive in the DIII-D and FNSF-AT tokamaks

Abstract

Fast waves at frequencies far above the ion cyclotron frequency and approaching the lower hybrid frequency (also called 'helicons' or ‘whistlers’) have application to off-axis current drive in tokamaks with high electron beta. The high frequency causes the whistler-like behaviour of the wave power nearly following field lines, but with a small radial component, so the waves spiral slowly towards the plasma centre. The high frequency also contributes to strong damping. Modelling predicts robust off-axis current drive with good efficiency compared to alternatives in high performance discharges in DIII-D and Fusion Nuclear Science Facility (FNSF) when the electron beta is above about 1.8%. Detailed analysis of ray behaviour shows that ray trajectories and damping are deterministic (that is, not strongly affected by plasma profiles or initial ray conditions), unlike the chaotic ray behaviour in lower frequency fast wave experiments. Current drive was found to not be sensitive to the launched value of the parallel index of refraction n‖, so wave accessibility issues can be reduced. Use of a travelling wave antenna provides a very narrow n‖spectrum, which also helps avoid accessibility problems.

Using AORSA to simulate helicon waves in DIII-D

“…The current profile peak is at ρ = .5 for both n || =3 and 4, similar to results in Ref. 2 where ray trajectories and damping do not strongly depend on initial ray conditions. Both GENRAY and AORSA calculations also show strong single pass absorption with greater than 99% of the power absorption to the 080011-2 electron species and insignificant power absorption to any ion species in both n || =3 and 4 cases.…”

“…Helicon waves have recently been computationally investigated as an option for driving efficient off-axis current drive in ITER and DEMO using the full wave simulation, PSTELION [1], and DIII-D and FNSF-AT using the ray tracing simulation, GENRAY [2]. For high beta and helicon frequencies > 20ω ci on the DIII-D tokamak, GENRAY simulations show helicon waves can drive significant off-axis current drive that is approximately two times more efficient than neutral beams current drive and four times more efficient than electron cyclotron current drive.…”

High Field Side Lower Hybrid Current Drive Simulations for Off- axis Current Drive in DIII-D

“…[5] A key ingredient is current profile optimization. [6] Given the existing and planned DIII-D off axis current drive actuators (neutral beam [7], electron cyclotron [8], and helicon [9]), the HFS LHCD is to drive current at normalized minor radius, , in the range of 0.6-0.8 with current density approaching 0.4 MA/m 2 for fields down to 1.6 T with central densities approaching 1x10 20 m -3 . The goal of this work is to identify a credible HFS LHCD scenario for DIII-D.…”

A current drive by using the fast wave in frequency range higher than two timeslower hybrid resonance frequency on tokamaks

“…And it is planned to confirm the theory in KSTAR and DIIID. [4][5][6] The current drive using fast wave more close to LHR in frequency range ω~ω lh was noted in 1980 and the fast wave coupling is analyzed, where it is insisted that the fast wave can be operated in more lower frequency than LHCD since it does not suffer thermal mode conversion, and accessible parallel refractive index N || is broadened with more viable RF system requirement. [7] The fast wave experiment in this frequency range had been tried in JIPPT-IIU, JFT-2M, and PLT.…”