Speaker
Description
High power microwaves are commonly applied in magnetically confined fusion devices for electron cyclotron (EC) resonance heating (ECRH) and current drive (ECCD). Whilst ECRH and ECCD have several favorable qualities in a future fusion power plant, ECCD is generally not as efficient at driving current as neutral beam injection (NBI) or lower hybrid current drive. Older microwave experiments indicate that electron Bernstein waves (EBWs) may rival the current drive efficiency of NBI[1]. However, the EBWs are excited from X-mode at the upper hybrid (UH) layer, which is also associated with several nonlinear effects. At high power densities, nonlinear wave interactions may degrade the performance of EBW based current drive and heating.
Two gyrotrons are being installed at the Mega Amp Spherical Tokamak Upgrade (MAST-U) to investigate the viability of EBW operation. If high power EBWs are found to drive current efficiently, the future UK Spherical Tokamak for Energy Production (STEP) demonstration power plant is expected to use EBWs to achieve better performance[2]. In preparation of EBW experiments at MAST-U, fully kinetic particle-in-cell simulations have indicated that nonlinear effects may strongly interfere with the linear excitation of EBWs at the UH layer[3]. Whilst there has previously been a focus on the parametric decay instability and stochastic electron heating, the same simulations have indicated that another effect known as nonlinear Landau damping (NLD)[4] may strongly suppress the desired EBWs. NLD is a nonlinear wave particle interaction, and in a magnetized fusion plasma with an UH frequency greater than twice the EC frequency, NLD can cause the desired EBWs to interact with the gyrating electrons to generate a strong EBW signal downshifted by the EC frequency. Such a wave would not have an EC harmonic inside the plasma in MAST-U and it is not clear how this would affect the microwave power deposition profile. The impact of NLD is found to vary strongly with the plasma parameters.
References
[1] V. Shevchenko, et al., Nucl. Fusion 50, 022004 (2010)
[2] S. Freethy, et al., EPJ Web of Conferences 277, 04001 (2023)
[3] M. G. Senstius, et al., EPJ Web of Conferences 277, 01009 (2023)
[4] M. Porkolab, et al., The Physics of Fluids 15, 283 (1972)
