Electron Velocity Distributions Measured with Soft-X-Ray PHA at RTP

A soft x-ray pulse height analysis (PHA) system is begin used at the Rijhuizen Tokamak Project to study the electron velocity distribution. A liquid nitrogen cooled Si(Li) detector is used to view the plasma along a tangential line of sight. A gas cell in combination with Al foils is used for filtering. The data-acquisition system is set up in such a way that 16 subsequent spectra, of 1ms-1 s duration and in the energy range 4-30 keV, can be measured during a single tokamak discharge. The PHA system has been used extensively for measuring the electron temperature, the effective charge number Z(eff), and the species and concentrations of high-Z impurities in the plasma. Special attention was devoted to study the effect on the electron velocity distribution of electron cyclotron resonance heating (ECRH) power (60 GHz, up to 180 kW) launched in the ordinary mode from the low-field side. During application of ECRH the temperature deduced from PHA always exceeds the value from Thomson scattering. This deviation can be explained by the presence of nonthermal components in the electron velocity distribution. Finally, the effect of boronization of the vacuum vessel on the impurity concentration was studied.