Dynamics of pedestal profiles in ELMy H-mode plasmas in TCV at different collisionalities

摘要

The dynamics of the profiles of electron- temperature, density and pressure during H-mode phases in the TCV tokamak has been investigated for cycles with ELMs of type I and type III, characterized by different regimes of collisionality (v~* = 10~(-14)Z_(eff)R_on_eT_e~(-2)) of the plasma near the edge. Quasi-stationary H-mode phases with ELMs of type-Ill at v~* = 0.75 were already obtained in TCV with ohmic heating only, whereas additional power from electron cyclotron heating (ECH) was necessary to reach a regime at lower collisionality (v~* = 0.40) with large ELMs of type I. The main plasma parameters like current, line-averaged density and shaping were identical for both series of shots analysed in this study (I_p: 370kA,B_T: l-A3T,q_(95): 2.3,neL: 3.2 3.6. 10~(19)m~(_2),k:1.7,δ: 0.45) and a plasma configuration with single-null X- point and ion (B x ??B) drift in the favourable direction was chosen. For ECH at the 3~(rd) harmonic, 2 gyrotrons operating at 118GHz injected a total power of 1MW launched in vertical direction from the top of the TCV vessel. The profiles were measured using the Thomson scattering system on TCV, equipped with channels of high,spatial resolution to resolve the expected gradients in the so-called pedestal region. With an effective repetition rate of 60Hz set by the Nd:YAG lasers, this diagnostic does not seem suitable to follow the time evolution during ELM cycles with a period in the range of 5 to 25ms. However, in cases with extended and quasi-stationary H-mode phases and for series of reproducible shots, the method of random sampling and coherent averaging can be used to reconstruct a typical ELM cycle with an effective time resolution on the ms scale. For this analysis, the profiles measured on a vertical chord were projected onto flux surfaces and then onto radial coordinates in the equatorial plane. After fitting by an analytical function (tanh) the profiles were characterised by a set of parameters to describe their time evolution [1]. Other diagnostics, like ECE and Lithium ion beams, have been used to measure edge profiles [2]. A major advantage of using Thomson scattering relies in the fact that electron temperature and density are obtained simultaneously and at the same locations. Therefore, this technique has also been applied on other machines [3].