Measurements of Magnetic Field Fluctuations in the Caltech Research Tokamak

Citation

Hedemann, Mark Allen (1982) Measurements of Magnetic Field Fluctuations in the Caltech Research Tokamak. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/z70q-f715. https://resolver.caltech.edu/CaltechETD:etd-09132006-153307

Abstract

An experimental investigation of magnetic field fluctuations in a research tokamak plasma has been performed. The fluctuations were measured with movable probes inserted directly into the plasma. Estimates of the fluctuating field strength, power spectral density, and correlation lengths have been made by calculations on the raw data. The fluctuations were found to be of comparable strength for the radial and poloidal components, while the toroidal component was found to be at least a factor of 5 weaker in strength. The fluctuating field strength showed no apparent dependence on plasma current and safety factor at the edge, but increased with electron density during gas puffing and as the minor radius of measurement was decreased.

The power spectral density indicated that the fluctuations could be divided into two frequency regions. The low frequency region (f < 100 kHz) was dominated by coherent MHD modes with correlation lengths on the order of the tokamak size. The high frequency (f > 100 kHz) region appeared to be turbulent in nature with short correlation lengths in the poloidal direction (< .01 m), but longer correlation lengths in the radial direction (> .04 m). The high frequency region showed a roll-off in frequency of δB_r of approximately f^-2.5.

The results are compared with theories of fluctuation sources and the effects of fluctuations on anomalous electron thermal transport. The best fit to the high frequency region spectral density is given by a statistical theory of fluctuations. The fluctuating field strength appears to be more than an order of magnitude too small to account for the anomalous electron thermal transport using stochastic field theory.

Thesis Files