ITG instabilities and transport studies in the Columbia Linear Machine

Abstract

Both the slab and toroidal branches of the ion temperature gradient (ITG) driven instabilities have been studied in the Columbia Linear Machine (CLM). The ITG instabilities are observed as m=2 modes when a sufficient parallel ion temperature gradient is reached by using either d.c. ion acceleration heating or rf‐transit time heating methods. The slab branch, m=2 mode, is identified as the ITG mode based on the four basic characteristics predicted by most theoretical work. The toroidal ITG mode is studied by starting with the slab ITG mode and gradually turning on the mirror current in CLM. The curvature effect is shown to further destabilize the mode and reduce the real frequency in the laboratory frame. The experimentally measured features in the presence of the ion temperature gradient and the mirror field also correspond closely to the theoretical predictions of a newly postulated hybrid ITG‐dissipative trapped electron drift mode. The anomalous local ion thermal conductivities due to the slab mode has been measured via a temperature relaxation method. Its average value of 0.5 m2/sec is about 3 orders of magnitude larger than the classical and 1 order of magnitude lower than the Bohm.