Measurement of the electron energy distribution function by a Langmuir probe in an ITER-like hydrogen negative ion source

Determining d2I/dV2 from a traditional Langmuir probe trace using numerical techniques is inherently noisy and generally yields poor results. We have developed a Langmuir probe system based on a method first used in the 1950s by Boyd and Twiddy (1959 Proc. R. Soc. A 53 250). The system measures the 2nd derivative directly. This paper presents an account of the experimental method, apparatus and software used along with some preliminary results from the KAMABOKO III source including a comparison with conventional probe methods.