Stable operation of future fusion reactors might largely rely on the attainable rotation levels for suppressing instabilities and turbulence transport 1. For reliable prediction of possible intrinsic torque, characteristics of intrinsic torque in current tokamak plasmas can provide valuable reference 2. Direct measurements of the intrinsic torque profile in L-mode and H-mode plasmas on the EAST tokamak have been performed using the available balanced neutral beam injection (NBI) 3. Co- and Counter-current neutral beams are modulated to balance the intrinsic and externally injected torque, which result in the rotation close to zero profile. The experimental results show that the intrinsic torque derived from momentum balance equations is found to be in the co-current direction, peaked in the plasma edge and negligibly small in the core. The results were compared with those obtained on the DIII-D tokamak and it was found to follow the same scaling law against the edge pressure gradient derived on the DIII-D tokamak 2. Newly measured intrinsic torque results for extended plasma parameters are reported.
Figure 1 shows the neutral beam injection system of EAST, which consists two nearly identical neutral beam injectors designed to tangentially injecting high-energy beams up to 80keV/4MW. Both co- and ctr- NBI systes have two positive ion sources, which can be combined to inject various beam power and torques for obtain nearly zero rotation profile to measure intrinsic torque. The residual unbalanced external torque balances in intrinsic torque, which can be determined from momentum balance equation .
Figure 2 plots an experiment showing that slowly modulated co- and ctr- NBI power are added to the plasma so as to vary the plasma rotation in such a fashion that both the core rotation and rotation profiles reverse and keep a flat profile. Electron temperature and density profiles are shown to remain unchanged during the beam modulation. Also noted that, the normalized plasma pressure is also unchanged to minimize the effect of confinement change on the plasma rotation, since the confinement is closely related to plasma rotation as suggested by Rice scaling.
Figure 3 (a) shows the obtained rotation profile during the beam modulation, in which the rotation profiles were modulated to transiently reverses the direction and relatively flat to suggest the momentum flux contribution can be ignored to use Eq. (1) for calculating the intrinsic torque. As shown in Figure 3(b), using the NBI deposited torque computed by NUBEAM, intrinsic torque is determined. The results show that the intrinsic torque is co-current and peaked towards the plasma edge, which is consistent with the theory that the edge pressure gradient is a major driving force. The shape of intrinsic torque profile measured on EAST is very similar to the intrinsic torque profile on DIII-D 1 . The intrinsic torque profiles of these two devices are both in the co-current direction, peaked towards the edge and relatively small in the core, and of an order of magnitude different. The edge torque is strongly correlated with the edge pressure gradient as seen on both devices, which can assist the prediction of rotation level on future deviced like ITER or CFETR .
References: 1. Y. Liu et al Nucl. Fusion 44 (2004) 232; 2. W.M. Solomon et al, Nucl Fusion 51 (2011) 073010; 3. C.D. Hu et al, Plasma Sci. Technol. 17 (2015) 817; . W.M. Solomon et al, Phys. Plasma 17 (2010) 056108; . C. Chrystal et al., Phys. Plasma 24 (2017) 056113.
|Affiliation||Institute of Plasma Physics, Chinese Academy of Sciences|
|Country or International Organization||China|