IOP PUBLISHING and INTERNATIONAL ATOMIC ENERGY AGENCY NUCLEAR FUSION Nucl. Fusion 47 (2007) 1318–1325 doi:10.1088/0029-5515/47/9/032 Current profile control and optimization under dominant electron heating in HL-2A Q.D. Gao 1 , R.V. Budny 2 , Y.M. Jiao 1 and K. Indireshkumar 2 1 Southwestern Institute of Physics, P O Box 432, Chengdu 610041, People’s Republic of China 2 Princeton Plasma Physics Laboratory, Princeton University, Princeton, NJ 08543, USA E-mail: qgao@swip.ac.cn Received 10 January 2007, accepted for publication 17 July 2007 Published 30 August 2007 Online at stacks.iop.org/NF/47/1318 Abstract The establishment of the current profile as in the hybrid scenario is studied under the condition of dominant electron heating in HL-2A. The scenarios with injecting lower hybrid (LH) and electron cyclotron (EC) waves are under numerical study. Carefully adjusting the position of non-inductive current driven by two groups of gyrotron, an optimized q -profile was obtained with q a = 3.78 and a weak shear region extending to ρ ∼ 0.45 (where ρ is the square-root of toroidal flux normalized to its value at the plasma boundary) in low-density discharges of ¯ n e = 1.0 × 10 19 m −3 . When 0.5 MW LH power in the current drive mode and 0.95 MW EC power mainly for plasma heating are used to control the current profile, a hybrid discharge scenario with a weak magnetic shear region extended to ρ = 0.6 and q a = 3.21 is established by controlling the EC absorption position. The mechanism of the LH wave absorption in the HL-2A plasma causes interplay of the distribution of the LH driven current with the modification of the plasma configuration, which constitutes non-linearity in the LH wave deposition. Due to the non-linearity the LH wave deposition position changes spontaneously or oscillates. The oscillatory behaviour caused by the non-linear effect of the LH wave deposition is analysed. PACS numbers: 52.55.Fa, 52.50.Sw, 52.55.Wq (Some figures in this article are in colour only in the electronic version) 1. Introduction Control and optimization of the plasma current profile is a key point in enhancing plasma performance. Although several tools have been identified to modify transport directly, the effect of the current profile on transport is large and remains an important transport control feature. In tokamak experiments it is demonstrated that the configuration with a flat q-profile in the central plasma region is beneficial to improving plasma confinement. Discharges with an internal transport barrier (ITB) have been established with optimized magnetic shear (OS) in JET [1] and ASDEX Upgrade [2] and the developed ITB improved central plasma confinement. Recently, so-called hybrid scenarios characterized by a current density profile, enclosing a large volume of low magnetic shear with q 0 near 1, have achieved improved confinement and higher beta limits [3–7]. Combining high fusion gain and steady state operation, the ELMy H-mode has been the reference regime for the ITER design so far. However, this performance is limited at moderate plasma pressure (typically for β N < 2) by the triggering of neoclassical tearing modes (NTMs) driven by the sawtooth m = 1, n = 1 activity. The hybrid mode has successfully eliminated the deleterious effect of sawteeth and reduced the NTMs triggering by establishing a current profile in the stationary state with q above unity. In many experiments these hybrid discharges have produced high fusion figures of merit β N H 89 /q 2 95 ∼ 0.4–0.6 (where β N is the normalized plasma pressure, H 89 the confinement factor in terms of ITER- 89 scaling) in stationary conditions at reduced plasma current. Fusion performance at this lower current is maintained by operation at β N up to 3. The enhanced performance relative to the conventional ELMy H-mode scenario offers the potential of achieving similar values of Q fus at lower plasma current, thereby increasing the duration over which such performance can be maintained and reducing the risk of damage due to disruptions. Various improved confinement scenarios have been established by tailoring current profile with (radio-frequency waves) RF-only schemes. In FTU, plasmas with a large central volume of improved confinement are obtained with 0029-5515/07/091318+08$30.00 © 2007 IAEA, Vienna Printed in the UK 1318