Manifestations of the MHD and Kinetic Dynamo Through Soft X-Rays
Abstract
The underlying mechanisms that produce and sustain the reversed toroidal field in RFP's are investigated by analyzing 2D x-ray emissivity reconstructions and by correlating the evolution of the hot electron properties to the reversed toroidal magnetic field. Reconnection of emissivity surfaces as seen in soft x-ray (SXR) reconstructions occur near the predicted resonant surfaces of the m = 1, n = -5,-6,-7 resistive tearing modes. Two distinct rates of reversed magnetic field generation are observed. First, in the MHD relaxation phase a sudden increase in B _{rm t}(a) is detected. This event coincides with a large increase in the edge hot electron current density. The second mode of flux generation is observed to have a slower rate and occurs during the diffusion phase. A variation of the edge hot electron current density by a factor of four produced only a small change in the measured B_{rm t}(a), implying that the contribution of the hot electrons to the dynamo during the diffusion phase is small. ~ T_{e,hot}/ <T_{rm e,hot}> was measured to be approximately 60%, which is much larger than the corresponding quantity for the bulk component which is about 30%. Scaling of the magnetic Reynolds number with the diffusion and MHD relaxation time, tau_{rm MHD} indicated that the tau _{rm MHD} does not have a strong dependence on the Spitzer resistivity whereas the diffusion time does depend on the classical resistivity. SXR emission-mode analysis during the transition from a rotating to a locked plasma shows a decrease in the m = 1 Fourier Bessel component of the emissivity. This is due to the flattening of the emissivity profile as seen in the SXR reconstructions. Finally, mode analysis of reconstructed emissivity profiles reveals the radial location of a localized disturbance, propagating in the ion diamagnetic drift direction, located close to the m = 1 rational surfaces.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 1991
- Bibcode:
- 1991PhDT.......217C
- Keywords:
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- REVERSED FIELD PINCH;
- Physics: Fluid and Plasma