A novel system for the direct measurement of the electron energy distribution function by Langmuir probe in ion sources

To realise a high yield H− (or D−) ion source capable of long pulse operation for the ITER neutral beam heating system, it is vital to understand and optimise the plasma parameters in the ion source. Calculating the electron energy distribution function (EEDF) in a non-Maxwellian plasma using conventional Langmuir probe numerical techniques is prone to error and generally produces poor results. Using a technique developed by Boyd and Twiddy [R.L.F. Boyd, N.D. Twiddy, Proc. R. Soc. A 53 (1959) 250] in the 1950s a new Langmuir probe system has been developed. This paper presents a detailed account of the experimental method, apparatus, software and analysis. The impact of the development is demonstrated by comparing data obtained from the ITER Negative Ion Beam Development Test Stand at CEA Cadarache using this method with similar data obtained with conventional methods.