Transition into the improved core confinement mode as a possible mechanism for additional electron heating observed in t
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Transition into the Improved Core Confinement Mode as a Possible Mechanism for Additional Electron Heating Observed in the Lower Hybrid Current Drive Experiments at the FT-2 Tokamak S. I. Lashkula, *, A. B. Altukhova, A. D. Gurchenkoa, E. Z. Gusakova, V. V. Dyachenkoa, L. A. Esipova, M. A. Irzaka, M. Yu. Kantora, D. V. Kouprienkoa, A. A. Perevalovb, A. N. Savelieva, A. Yu. Stepanova, and S. V. Shatalinb a Ioffe
b
Institute, St. Petersburg, 194021 Russia St. Petersburg Politechnical University, St. Petersburg, 195251 Russia *e-mail: [email protected] Received July 21, 2016; in final form, January 16, 2017
Abstract—In experiments on lower hybrid current drive (LHCD) carried out at the FT-2 tokamak, a substantial increase in the central electron temperature Te(r = 0 cm) from 550 to 700 eV was observed. A complex simulation procedure is used to explain a fairly high LHCD efficiency and the observed additional heating, which can be attributed to a transition into the improved core confinement (ICC) mode. For numerical simulations, data obtained in experiments with deuterium plasma at ne = 1.6 × 1019 m–3 were used. Simulations by the GRILL3D, FRTC, and ASTRA codes have shown that the increase in the density and central temperature is apparently caused by a significant suppression of heat transport in the electron component. The mechanism for transition into the improved confinement mode at r < 3 cm can be associated with the broadening of the plasma current channel due to the lower hybrid drive of the current carried by superthermal and runaway electrons. In this case, the magnetic shear s = (r/q)(dq/dr) in the axial region of the plasma column almost vanishes during the RF pulse. In this study, the effect of lower hybrid waves on the plasma parameters, resulting in a transition into the ICC mode, is considered. New experimental and calculated data are presented that evidence in favor of such a transition. Special attention is paid to the existence of a threshold for the transition into the ICC mode in deuterium plasma. DOI: 10.1134/S1063780X1707008X
1. INTRODUCTION The development of methods for noninductive drive of a quasi-stationary plasma current in a tokamak is one of the most challenging problems on the way to creating a fusion reactor [1]. Lower hybrid current drive (LHCD) is an efficient method for sustaining the plasma current and modifying the plasma current profile in order to decrease the magnetic shear s = (r/q)(dq/dr) in the energy deposition region. This enables one to locally reduce the electron heat transport. The mechanism for transport suppression is associated with the formation of a configuration with a negative magnetic shear [2, 3]. In this work, we present results of experimental studies of these effects. The experiments were carried out at the FT-2 tokamak with a major radius of R0 = 0.55 m and minor radius of a = 0.08 m, which is specified by a poloidal diaphragm [4]. The main parameters of the facility are as follows: the toroidal magnetic field is BT ≤ 3 T, the plasma curr
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