[en] 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. Conventional Langmuir probe analysis is prone to error as low pressure plasmas are generally non-Maxwellian. In non-Maxwellian plasmas, the electron temperature is thought of as an effective electron temperature corresponding to a mean electron energy determined from the integrals of the electron energy distribution function (EEDF). Druyvesteyn showed that the EEDF may be found from the second derivative of the current-voltage characteristic. However determination with a conventional Langmuir Probe is noisy and generally yields poor results. In this paper we present an alternative method first used by Boyd and Twiddy. By superimposing a modulated AC voltage on the DC probe voltage the second derivative may be obtained from a direct measurement of the resulting current at the modulation frequency. This method has been adapted to take advantage of current technologies and computer power. The hardware comprises of a high speed data acquisition PC running a bipolar power supply. The output from this power supply is connected to the Langmuir probe and the current collected by the probe is sent to a dual channel instrumentation amplifier returning both signals to the PC. The software provides outputs, data acquisition and analysis. The dual channel amplifier separates the DC and AC component of the probe current and the AC component is analysed using a software Fast Fourier Transform (FFT) analyser. A calibration system is currently under development comprising of a plug-in to the main software suite and a programmable load which takes the place of the probe and plasma. The software provides a waveform to the programmable load which allows the user to adjust specific parameters to get the optimum signal for the smallest SNR. The paper presents a detailed account of the experimental method, analysis, apparatus, software and the calibration test system used. 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. (author)