Poloidal inhomogenity of the particle fluctuation induced fluxes near the LCFS at lower hybrid heating and improved confinement transition at the FT-2 tokamak

Abstract

IntroductionThe problem of the anomalous high periphery thermal transport at tokamak plasmas and mechanism of their suppression, when a transport barrier is formed has been discussed in a number of recent publications [1,2]. This paper present our observations and conclusions about development of the transport process at the plasma periphery of the small tokamak FT-2 during additional Lower Hybrid Heating (LHH), when external (ETB) transport barrier followed by Internal (ITB) transport barrier is observed [3,4].The main part of the previous papers is concerned with plasma under q = 6 (R=0.55m, a=0.079m, I pl = 22kA and B t = 2.2T, P LHH = 90÷100kW), where the effective LHH and improved confinement transition are realised [3]. The L-H transition with ETB has been observed after RF pulse end. The RF pulse (∆t LH = 5ms) is applied at the 30 th ms of a ∆t pl = 50ms plasma shot. During additional LHH the ITB is formed spontaneously a few ms after the RF pulse start. The essential gradient rise of the ion temperature profiles and its flattening in the core center during LHH indicates that ITB is formed at 32 -33 ms in the r = 5 cm region. The transport barrier formation can be confirmed by time history of the ion temperature variation obtained by CX analyser and spectral diagnostic using Doppler broadening of the CIII line radiation and density profile changes [4].It should be stressed that in our papers effective LH heating and spontaneous ITB formation have been explained through careful numerical Monte Carlo modelling [5]. One deduced that the high energy ions are well confined in the case, when poloidal Mach number M p = ≥ 1, which has been realised in FT-2 experiment with plasma under q = 6. It was found the radial electric field provides a spontaneous transition to high negative value, if the local Mach number is about one. This is partially due to rotational runaway due to negative inertia [6], and partially due to finite orbit effects [7].The L-H transition with ETB has been observed after RF pulse end. Distinction of the mechanism which can trigger L -H transition is discussed in this paper. The paper deals with the new microturbulence and spectral experimental data and their analysis, which show, that the radial electric field E r generated at the LH heating (LHH) in the FT-2 is high enough to form the transport barriers.