Determination of electron density fluctuation correlation functions via beam emission spectroscopy

Measurement of electron density fluctuations in magnetically confined fusion plasmas by the beam emission spectroscopy (BES) method is discussed and an algorithm is given for the correct reconstruction of correlation functions of the electron density fluctuation from measured correlation functions of beam light fluctuations. The technique is generally applicable to all kinds of neutral beams, provided an appropriate model for beam light emission is available and the beam penetration time through the observation volume is shelter than the characteristic times of the plasma turbulence. Light from a 48 keV Lithium beam on W7-AS plasmas is analysed in detail. whereby electron density fluctuations in the scrape-off layer and in the edge plasma are analysed for a low-density electron cyclotron resonance heated (ECRH) heated discharge. Characteristic changes in the autocorrelation function are found at the last closed flux surface. Limitations of the technique are discussed.