Oblique electron cyclotron emission for electron distribution studies (invited)

Abstract

Electron cyclotron emission (ECE) at an oblique angle to the magnetic field provides a means of probing the electron distribution function both in energy and physical space through changes in and constraints on the relativistic electron cyclotron resonance condition. Diagnostics based on this Doppler shifted resonance are able to study a variety of electron distributions through changes in the location of the resonance in physical or energy space accomplished by changes in the viewing angle and frequency, and the magnetic field. For the case of observation across a changing magnetic field, such as across the tokamak midplane, the constraint on the resonance condition for real solutions to the dispersion relation can constrain the physical location of optically thin emission. A new Oblique ECE diagnostic was installed and operated on the PBX-M tokamak for the study of energetic electrons during lower hybrid current drive. It has a view 33° with respect to perpendicular in the tokamak midplane, receives second harmonic X-mode emission, and is constrained to receive single pass emission by SiC viewing dumps on the tokamak walls. Spatial localization of optically thin emission from superthermal electrons (50–100 keV) was obtained by observation of emission upshifted from a thermal cyclotron harmonic. The localized measurements of the electron energy distribution and the superthermal density profile made by this diagnostic demonstrate its potential to study the spatial transport of energetic electrons on fast magnetohydrodynamic time scales or anomalous diffusion time scales. Oblique ECE can also be used to study electron distributions that may have a slight deviation from a Maxwellian by localizing the emission in energy space. In “near-Maxwellian” plasmas, the Doppler broadening of the resonance means that observed oblique ECE from optically thick harmonics will originate from electrons with energies near or slightly above the thermal energy [(1–2)vth for example, depending on the observation angle and spatial gradients]. This is different from observed perpendicular ECE from optically thick harmonics that originate from electrons below the thermal energy. Experiments are presently being performed on the Tore Supra tokamak with two oblique views of ECE in equal and opposite toroidal directions (approximately 25° with respect to perpendicular) using two Michelson interferometers capable of observing second and third harmonic emission in X mode.