MHD activity in CDX-U
Abstract
The CDX-U MHD spectrum can roughly be described as linear combinations of two extreme states: a rotating m=1/n=1 'snake' in the core coupled with a 2/1 island at the periphery, and a purely sawtoothing core with very low peripheral fluctuations. Overall confinement and profile shapes differ significantly between these states, with the (1/1, 2/1) dominated plasma (most frequent) having reduced core temperature and a tenous edge. The level of (1/1, 2/1) activity seems correlated with Zeff. Using rotational tomography and SVD techniques in conjunction with USXR imaging, we find the snake to be a large island-like structure having up to about a/3 and 1.5 radian radial and poloidal extent, respectively. Using fast multilayer mirror monochromators and a 'differential fluctuation spectroscopy' technique we estimate a strong impurity accumulation in the snake and a hollow impurity profile in the background plasma, suggesting a 'vacuum-cleaner' effect for the 1/1 island. The CDX-U snake seems in many respects similar to the pellet induced snake observed in large tokamaks. Reconstruction of the sawtooth emissivity indicates an evolution in good agreement with the Kadomtsev reconnection model. Interestingly however, at the sawtooth crash we do not observe any major perturbations in the spectroscopic signals outside q=1, while the emissivity gradient there is unusually sharp. Finally, the Internal Reconnection Event is described as reflected in the USXR and magnetic signals. Work supported by DoE grant No. DE-FG02-86ER52314ATDoE and DoE contract No. DE-AC02-76-CHO-3073.
- Publication:
-
APS Division of Plasma Physics Meeting Abstracts
- Pub Date:
- November 1998
- Bibcode:
- 1998APS..DPP.Q7Q23S