Penetration of lower hybrid current drive waves in tokamaks

Abstract

Lower hybrid (LH) ray propagation in toroidal plasma is shown to be controlled by combination of the azimuthal spectrum launched by the antenna, the poloidal variation of the magnetic field, and the scattering of the waves by the drift wave fluctuations. The width of the poloidal and radial radio frequency wave spectrum increases rapidly as the rays penetrate into higher density and scatter from the drift waves. The electron temperature gradient (ETG) spectrum is particularly effective in scattering the LH waves due to its comparable wavelengths and phase velocities. ETG turbulence is also driven by the radial gradient of the electron current profile giving rise to an anomalous viscosity spreading the LH driven plasma currents. The LH wave scattering is derived from a Fokker-Planck equation for the distribution of the ray trajectories with diffusivities derived from the drift wave fluctuations. The condition for chaotic diffusion for the rays is derived. The evolution of the poloidal and radial mode number spectrum of the lower hybrid waves are both on the antenna spectrum and the spectrum of the drift waves. Antennas launching higher poloidal mode number spectra drive off-axis current density profiles producing negative central shear [RS] plasmas with improved thermal confinement from ETG transport. Core plasma current drive requires antennas with low azimuthal mode spectra peaked at m = 0 azimuthal mode numbers.

Pushing Particles with Waves: Current Drive and <i>α</i>-Channeling

“…There have been a number of studies to optimize LHCD in and of itself, while avoiding the α-particles [64][65][66][67][68][69][70][71][72][73][74][75][76]. However, little effort was made to utilize the α-particle energy, even though, historically, the α-channeling paradigm was born out of the worry that lower hybrid waves would be damped by the α-particles.…”

The alpha channeling effect

“…It is worth noting that, while there have been a number of studies to optimize LHCD [47,48,49,50,51,52,53,54,55,56,57,58,59], the joint optimization of LHCD and α-channeling was considered only recently [60], where it was pointed out that launching the LH wave from the tokamak high-field side facilitated the joint optimization. This study was stimulated by the recent proposal using high-field launch so that the LH wave would penetrate more deeply the plasma core, with the waveguide better protected from the plasma [61,62].…”

RF wave propagation and scattering in turbulent tokamak plasmas

“…The ETG turbulence is largely responsible for the anomalously short energy confinement time τ E in tokamaks. Horton et al (2013) give the Nyquist stability analysis for the LHCD plasma using the kinetic response function with the three-temperature LHCD electron phase space density. The drift wave electron kinetic response function P LHCD e is given by…”

“…For a typical nonlinear ETG spectrum δ n e (q) n e 2 = δ n 2 e q 2n 0 n 2 e q 2n+2 (8) with 1 ≤ n < 2 the ray diffusivity reduces to…”