Non-Maxwellian Electron Velocity Distributions Observed with Thomson Scattering in the Tortur Tokamak

The Thomson scattering spectrum represents the projection of the three-dimensional electron velocity distribution on the scattering vector. From this the local electron temperature and density can be derived. To determine the three-dimensional electron velocity distribution it is necessary to have several viewing directions, assuming axial symmetry in velocity space perpendicular to the magnetic field. Thomson scattering experiments in the TORTUR tokamak demonstrated the first experimental determination of f (v(perpendicular-to), v(parallel-to) from radially and tangentially observed spectra. The latter made it possible to determine the toroidal current density. The scattering spectra, averaged over 30 laser pulses observed in both the radial and the tangential direction, show clearly two symmetrical dips near the top of the Gaussian spectrum. The magnitude of the dips is relatively small, only 7% of the amplitude of the Gaussian. The partial electron density responsible for the non-thermal features was found to scale proportionally to the local density and to exhibit a specific development when the plasma is grossly distorted by fast current pulses or minor disruptions. The individual spectra show a spread in photon yield around the wavelength of these dips which is much larger than can be explained by photon statistics. If a spherically symmetric velocity distribution is assumed, the dips in the averaged spectrum (over 30 pulses) can only be explained by strong distortions of the electron velocity distribution (at least 50%). For the individual spectra, which show even larger distortions, the explanation cannot be found by considering only the velocity space. The observations can possibly be explained by strong spatial anisotropy, for example filamentation of the plasma current.