ECH effects on toroidal rotation: KSTAR experiments, intrinsic torque modelling and gyrokinetic stability analyses

Abstract

Torodial rotation profiles have been investigated in KSTAR H-mode plasma using combined auxiliary heating by co-neutral beam injection (NBI) and electron cyclotron resonance heating (ECH). The ion temperature and toroidal rotation are measured with x-ray imaging crystal spectroscopy (XICS) and charge exchange recombination spectroscopy (CES).H-mode plasma is achieved using co-current 1.3MW NBI, and a 0.35 MW ECH pulse is added to the flattop of H-mode. The core rotation profiles, which are centrally peaked in the pure NBI heating phase, flatten when ECH is injected, while the edge pedestal is unchanged.Dramatic decreases in the core toroidal rotation values (V tor /V tor ~ -30%) are observed when on-axis ECH is added to H-mode. The experimental data shows that the decrease of core rotation velocity and its gradient are correlated with the increase of core electron temperature and its gradient, and also with the likely steepening of the density gradient. We thus explore the viability of a hypothesized ITG→TEM transition as the explanation of the observed counter-current flow induced by ECH. However, the results of linear microstability analyses using inferred profiles suggest that the TEM is excited only in the deep core, so the viability of the hypothesized explanation is not yet clear.PACS numbers: 52.55Hc, 52.55Fa, 52.50.Sw, 52.50.Gj IntroductionFlow and velocity shear are very important for stabilizing micro-and macro-instabilities in tokamak plasmas. Neutral beam injection (NBI) is generally used as the external momentum input source to produce and control plasma rotation in present day tokamaks. For ITER and future reactors, the input torque from NBI will be very low or nonexistent and cannot produce the needed rotation. As a result, there is a need to develop alternative or complementary methods for driving plasma rotation. Significant intrinsic rotation (without external momentum input) has been observed on many tokamaks [1], which suggests that it may be possible to reap the benefits of such self-generated flows in ITER and reactors.Since the discovery of intrinsic rotation in the 1990s [2, 3], plasma rotation without external torque has been a topic of intense interest. While significant progress has been made, the driving mechanism is not fully understood. Theoretical models have been proposed to explain intrinsic rotation as due to a turbulence driven intrinsic torque [4][5][6][7]. Intrinsic torque is dynamic and variable -due to evolving turbulent Reynolds stresses. Momentum transport bifurcations and reversals have been observed in several experiments [8,9]. The effects on toroidal rotation of ECH have being observed previously in CHS [10], DIII-D [11], JT-60U[12] and AUG [13,14]. The counter-current rotation increment or flattening of co-current rotation profile was confirmed in these ECH experiments. The proposed explanations vary considerably. The KSTAR results in this paper, though partly similar to some previous experiments, will be combined with gyrokinetic stability analyses to elucidate poss...

Dynamics of intrinsic axial flows in unsheared, uniform magnetic fields

“…Moreover, the dynamical symmetry breaking mechanism is also relevant to intrinsic rotation of electron cyclotron heated (ECH) heated plasmas. 28 This mechanism can enhance the effect of flow drive induced by ECH. For example, the ECH injection can induce a residual stress, P Res via conventional symmetry breaking mechanisms, and the flow gradient is thus enhanced by the negative viscosity increment resulting from dynamical symmetry breaking, i.e., hv k i 0 $ P Res =ðv / À jv Res / jÞ.…”

“…In addition, as the dynamical symmetry breaking uses a simple model of electron drift wave turbulence, this mechanism can be used to understand the intrinsic rotation in burning plasmas where the turbulence is the CTEM turbulence, 26,27 and to address the effect of electron cyclotron resonance heating (ECRH) on toroidal rotation. 28,29 …”

Discriminating the trapped electron modes contribution in density fluctuation spectra

“…ECRH has been widely used to destabilize TEM in core tokamak plasmas [9,10,43,53]. Here, its effect on Nickel transport, linear gyro-kinetic simulations and reflectometry fluctuation spectra are investigated in Tore Supra (R = 2.4 m, a = 0.72 m, circular plasma).…”

Main-Ion Intrinsic Toroidal Rotation Profile Driven by Residual Stress Torque from Ion Temperature Gradient Turbulence in the DIII-D Tokamak

“…Variations of the rotation gradient with changes of the local density gradient have been also been observed [12] indicating the need for local profile investigation to organize the observations. Investigations into the type of linearly unstable turbulence between ion and electron modes that correlate with the observations have been explored [12][13][14], indicating that other processes beyond the dominant turbulence mode, such as the collisionality scaling of non-Maxwellian equilibria [15], are required to explain the wide range of observed phenomenology. The results in this Letter with global nonlinear gyrokinetic simulations using experimental profiles show that the intrinsic torque from ITG has the correct spatial location, structure and magnitude to produce the experimentally observed main-ion intrinsic rotation profiles under the conditions reported in the following section.…”