Diagnosis of mildly relativistic electron velocity distributions by electron cyclotron emission in the Alcator C tokamak

Electron cyclotron emission from nonthermal electron distributions has been measured with a vertical view and control of multiple reflections. The observed intensities at the first few harmonics provide information that can be used to deduce the electron distribution function. Although harmonic overlap restricts the detail that can be obtained when the distribution is very energetic, a fitting procedure enables useful results to be obtained. The distribution functions during Ohmic runaway and lower-hybrid current drive and heating are presented. The current-drive distributions are not inconsistent with theoretical Fokker–Planck calculations, giving perpendicular and parallel ‘‘temperatures’’ around 60 keV and 200 keV, respectively. The runaway cases have quite similar distributions.