Measurement of the correlation spectrum of electrostatic potential fluctuations in a toroidal ECRH plasma

Abstract

Measurements of the length and time scales of turbulent potential fluctuations were carried out on the MIT Versatile Toroidal Facility (VTF). Plasmas were produced by ECRF heating in a HELIMAK configuration. The HELIMAK toroidal field Bφ was 800 gauss, vertical field Bz typically 10 gauss, and Te≊10 eV. The density exhibited is a peaked profile having nmax≊2×1010 cm−3. The fluctuation experiments were conducted using a moveable vertical array of eight Langmuir probes. At major radii outside the density peak, the correlation lengths of fluctuations were found to be on the order of 5–10 cm for fluctuation frequencies below 5–8 kHz, and on the order of 1 cm at higher frequencies. At major radii on the inner slope of the density peak, a new feature appears in the spatial coherence function consisting of a second peak at a probe separation of 5 cm. This observation suggests that the fluctuations have a toroidal extent greater than 2πR0.