Acceleration of electrons in the vicinity of a lower hybrid waveguide array

Abstract

The interaction of tokamak plasma edge electrons with the electric near field generated by a lower hybrid slow wave antenna is studied. Antenna field spectra of interest for current drive and/or plasma heating have lobes at high-n∥ values (n∥≳30) intense enough for resonant acceleration of the relatively cold (∼25 eV) edge electrons. For waveguide electric fields, typically around 3 kV/cm, the higher-order modes overlap in the phase-space [B. V. Chirikov, Phys. Rep. 52, 263 (1979)], so that electron global stochasticity is induced. For Tokamak de Varennes (TdeV) [Décoste et al., Phys. Plasmas 1, 1497 (1994)] conditions and for 90° waveguide phasing, the stochastic limit in the current drive direction is about 2 keV, determined by the last overlapping mode. The progress of electrons through accessible phase space is very efficient: the TdeV 32 waveguide array can accelerate the electrons to the possible limit. An area-preserving map is derived to study the electron dynamics. Surface-of-section plots fully confirm the resonant wave-particle nature of the interaction.

Fast electron flux driven by lower hybrid wave in the scrape-off layer

“…Theoretical studies have already been carried out to better predict and try to avoid generation of the hot spots when LHW is switched on. 8,14,15 Those studies suggest that the high parallel wave refractive index n jj (n jj > 20) components of the incident wave can couple with the plasmas closed to the LHW antenna through the mechanism of electron Landau damping. In Tore Supra, retarding field analyzers (RFA) have measured a large current density.…”

Radially scanned probe measurements in front of the CASTOR lower hybrid antenna

“…According to the theory estimates, the power losses could reach ten percents of the power launched and energy of accelerated particles several keV for LH power on the large tokamaks currently used. Theory suggests a mechanism of the particle acceleration by absorption of higher spatial harmonics in the antenna spectrum [2] and enhancement of this acceleration or even acceleration without presence of the high LHW harmonics by formation of random toroidal electric fields [3]. No experimental assessment of the relative magnitude of these two mechanisms has been done yet.…”

Interaction of lower hybrid waves with scrape-off-layer electrons

“…In parasitic absorption, the short-wavelength modes emitted by the grill are absorbed by electrons within a narrow distance in front of the launcher creating a population of fast electrons. The energetic electrons are a probable reason for the formation of the hot spots [6].…”