Velocity distribution of helium and neon atoms released from graphite and tungsten limiters in TEXTOR

The release mechanisms of noble gases from plasma-facing components were observed spectroscopically in the TEXTOR plasma boundary by determining the velocity distribution from Doppler broadening. For the first time, three different mechanisms for helium and neon release from graphite and tungsten limiters could be distinguished quantitatively in a tokamak: thermal desorption, ion-induced desorption and particle reflection. Under the assumption that the thermal desorption follows a Maxwellian velocity distribution, the ion-induced desorption can be expressed by a Thompson velocity distribution. Calculating the particle reflection by the Monte-Carlo code TRIM, these processes could be separated in the measured velocity distribution.