Studies of electron heat pulse propagation at the T-10 tokamak with gyrotrons at two different frequencies**This paper is based on a report done at Strong Microwaves in Plasmas Int. Workshop (Nyznii Novgorod, Russia, August 2002) H-19.

Abstract

The internal transport barrier (ITB) was found by means of heat pulse propagation (HPP) and cold pulse propagation (CPP) analysis in target sawteeth-free plasma. The target sawteeth-free plasma was created either by high-field-side off-axis ECRH (140 GHz) or by low-field-side off-axis ECRH (130 GHz) at normalized minor radius r/alimiter ≈ ± (0.45). Outward HPP with dynamic electron heat diffusivity (at 0.2 < r/a < 0.37 and R/LTe = R∇Te/Te up to 23, where Te is the electron temperature and R is the major radius) was created by switching on the on-axis ECRH (130 or 140 GHz) imposed on the background, which was created by off-axis ECRH. Inward CPP was created by turning off the off-axis ECRH. The central Te responds with significant ∼ 20 ms delay. Slow inward CPP with R/LTe rising up to 17 on the cold wave front, can be described well by at r/a < 0.3 in a time interval limited by the appearance of the first sawteeth oscillation. Off-axis ECRH forms the region of the improved transport in low-shear zone with q slightly above 1. Later, q reduces due to the current redistribution during CPP (HPP). The improved transport is able to survive under R/LTe up to 23 (10 is Ohmic value of R/LTe). In the context of a ‘critical gradient’ model, the Ohmic value of R/LTe lies above the ‘critical’ one because enhanced HPP (induced by sawteeth or on-axis ECRH) is observed at all tokamaks at Ohmic background. Somehow, off-axis ECRH and HPP (CPP) processes increase the ‘critical’ value of R/LTe over 2.3 times. The combination of large gradients with low transport means that ITB is present during ∼ 20 ms time interval of CPP (HPP). The analysis of HPP (CPP) at both sides of plasma column simultaneously confirms the validity of the results while also showing a light asymmetry of values.