Measurement of the correlation spectrum of electrostatic potential fluctuations in an ECRH Helimak plasma

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 novel toroidal configuration which the authors have termed the Helimak. The configuration consists of a toroidal field Bφ of approximately 800 gauss and a vertical field Bz of typically 10 gauss, produced by a Helmholtz coil. Te is approximately 10 eV. The density exhibited a peaked profile having nmax ∼2 x 1010 cm-3 and a density gradient scale length of ∼10 cm. The fluctuation experiments were conducted using a mobile vertical array of eight Langmuir probes. At major radii outside the density peak, the vertical correlation lengths λc perpendicular of fluctuations were found to be on the order of 5-10 cm for fluctuation frequencies below 3-8 kHz, and on the order of 1-2 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 which scales linearly with vertical field. This observation indicates that these fluctuations have a correlation length on the order of 2πRo ∼600 cm in the direction parallel to the helical magnetic field lines