TJ-II:Electron Cyclotron Emission

Diagram showing the position of the ECE antenna-mirror system at TJ-II.

Electron temperature profiles are measured at TJ-II by means of a 16 channel heterodyne radiometer, covering the frequency range 50–60 GHz, corresponding to the second harmonic of electron cyclotron emission (ECE) in X-mode polarization at a magnetic field of 0.95 T on the plasma axis. The measurements are performed from the low field side (LFS) in the horizontal midplane (sector C4). Each frequency corresponds to a different value of the major radius R. The system is operated close to the strong ECR heating source (f_ECRH = 53.2 GHz). To protect the radiometer against stray radiation from the gyrotron, the radiometer band is split into two parts. The second harmonic emission above and below 53.2 GHz are measured separately by means of microwave couplers in the signal path. The system is calibrated absolutely by comparing room temperature with liquid nitrogen temperature. The emission can be simulated by the TRECE ray tracing code. The typical sampling rate is 100 kHz. The signals are called 'ECE1' ... 'ECE16' in the TJ-II database. The measured ECE amplitude is proportional to the electron temperature, provided certain conditions are met.