Measurement of fast electron velocity distribution function in a reversed field pinch plasma

Fast electrons in the edge region of the reversed field pinch (RFP) device, TPE-1RM20 are studied using an electron energy analyzer (EEA) in the energy range below 1 keV. Results show that the fast electron velocity distribution function can be approximated either by a drift Maxwellian distribution function or two asymmetric half-Maxwellian distribution functions. The fast electron temperature is comparable to or slightly less than the central electron temperature depending on the fitting functions. The temperature obtained using the EEA is more than one order of magnitude lower than the characteristic fast electron energy estimated from soft X-ray emissions from the inserted materials at 2-8 keV. These observations indicate that fast electrons mainly have a Maxwellian distribution function with a temperature comparable to the central electron temperature and that the distribution function may have a higher energy tail in the keV region. (author)