A combined retarding field analyser and E×B probe for measurement of ion and electron energy distribution in tokamak edge plasmas

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Abstract
A new probe has been designed to measure the electron and ion energy distributions in a tokamak edge plasma; it may be operated in both the retarding field analyser and E*B modes. In this type of probe aperture design is important and must include detailed consideration of Debye length, selective transmission, heat flux and space charge effects. Results obtained in the retarding analyser ion mode show that the ion parallel velocity distribution function as measured by the probe is a displaced Maxwellian in accordance with theoretical prediction. The electron parallel velocity distribution on the other hand is seen to be cut off by the wall sheath. Ion temperatures obtained under similar conditions in the E*B mode are in agreement with values obtained by fitting theoretical distributions to retarding field ion mode characteristics. The results which are presented were obtained in the edge region of discharges with line average densities of 1019 m-3. In this particular case approximately equal ion and electron temperatures of order 15 eV were measured. There is experimental evidence that at the current levels used space charge effects are negligible.