Investigation of Runaway Electron Eeam in EAST

It is the first time to observe the synchrotron radiation emitted by runaway electrons with a visible CMOS camera in tokamak. Some information about runaway electron has been obtained in the EAST tokamak, which is the first full superconducting tokamak with non-circular cross-section in the world. Both the energy and pitch angle of runaway electrons have been derived from the measurement of synchrotron radiation. The synchrotron radiation of runaway electrons was observed in the center region in most cases. The normal synchrotron radiation pattern is ellipse with an inclination angle to the equator, which is consistent with the numerical calculation of runaway electron synchrotron radiation. In few cases, a stable shell shape of synchrotron radiation can be observed. The mechanism for the formation of the shell is unclear. Observations of a rapid changes in synchrotron spot and intensity are also presentd. In these rapid changing cases, the ECE signal has corresponding response, which are belonged to fast pitch angle scattering events (FPAS). The FPAS events can appear both in current flap-top phase or current decay phase. 1. Introuduction Electrons in a tokamak with energies higher than some critical energy are continuously accelerated by the toroidal electric field, i.e., they run away [1-3]. The energy that a runaway electron reaches can be limited by the synchrotron radiation, the drift orbit shift, the flux swing, the magnetic fluctuations, the interaction with magnetic field ripple and waves [4-10]. Recently, bremsstrahlung radiation has been considered as mechanism to limit the maximum energy that runaway electrons can gained for high enough values of the effective charge and electron density [11, 12]. In large tokamaks, runaway electrons confined for a sufficiently long time can gain enough energy to cause serious damage to the first wall structures. The effect of runaway electrons when they impinge on the first wall is strongly dependent on the energy gained in the toroidal electric field. Therefore, it is of interest to investigate mechanisms that might help to control the energy that the runaway electrons can achieve. The energy limit of the runaway and the mechanisms for the energy limit have been predicted by some theories [4-9, 11-12]. In EAST, the runaway electron beams been observed with a visible CMOS camera. The experimental derived maximum energy of runaway electrons can reach 45 MeV. Two type of synchrotron radiation patterns of runaway beams have been observed in EAST. After the description of the experimental setup in section 2, the experimental results are presented in section 3. Finally, a summary is presented in section 4. EXW/P7-24