A comprehensive study on rotation reversal in KSTAR: experimental observations and modelling

Abstract

Dedicated experiments have been performed in KSTAR Ohmic plasmas to investigate the detailed physics of the rotation reversal phenomena. Here we adapt the more general definition of rotation reversal, a large change of the intrinsic toroidal rotation gradient produced by minor changes in the control parameters (Camenen et al 2017 Plasma Phys. Control. Fusion 59 034001), which is commonly observed in KSTAR regardless of the operating conditions. The two main phenomenological features of the rotation reversal are the normalized toroidal rotation gradient () change in the gradient region and the existence of an anchor point. For the KSTAR Ohmic plasma database including the experiment results up to the 2016 experimental campaign, both features were investigated. First, the observations show that the locations of the gradient and the anchor point region are dependent on . Second, a strong dependence of on is clearly observed in the gradient region, whereas the dependence on , , and is unclear considering the usual variation of the normalized gradient length in KSTAR. The experimental observations were compared against several theoretical models. The rotation reversal might not occur due to the transition of the dominant turbulence from the trapped electron mode to the ion temperature gradient mode or the neoclassical equilibrium effect in KSTAR. Instead, it seems that the profile shearing effects associated with a finite ballooning tilting well reproduce the experimental observations of both the gradient region and the anchor point; the difference seems to be related to the magnetic shear and the value. Further analysis implies that the increase of in the gradient region with the increase of the collisionality would occur when the reduction of the momentum diffusivity is comparatively larger than the reduction of the residual stress. It is supported by the perturbative analysis of the experiments and the nonlinear gyrokinetic simulations. The absence of the sign change of even when a much lower collisionality is produced by additional electron cyclotron heating brings further experimental support to this interpretation.

Understanding LOC/SOC phenomenology in tokamaks

“…These QC modes will be discussed in detail in the next section. Similar microwave spectral characteristics have been observed on TEXTOR [74,79,43], T-10 [75], C-Mod [34], Tore Supra [40,41,43,80], JET [43] and KSTAR [66,82]. All of the turbulence measurements mentioned above were of density fluctuations.…”

“…8. This velocity profile behavior is widely observed in rotation reversals [55,63,64,34,36,38,39,44,65,45,66,46,47,56]. Further evidence that the non-diffusive temperature inversion cut-off and rotation reversals are connected comes from inspection of the electron temperature profiles in the bottom frame of Fig.…”

“…Over the years numerous gyro-kinetic simulations have been performed with an eye towards understanding the LOC/SOC transition [25,30,50,33,36,37,86,77,39,62,40,41,42,43,80,87,45,66,53,82,54]. An early example from DIII-D [30] is presented in Fig.…”

“…The final piece of the LOC/SOC puzzle comes from observed changes in turbulence across the transition. There are numerous cases of this from a wide variety of diagnostics [71,72,73,24,25,30,74,75,50,33,34,35,37,76,77,40,78,79,41,43,80,81,66,82], including laser scattering, microwave reflectometry, probes and correlation electron cyclotron emission (CECE). An early example using laser scattering is shown in Fig.…”

Parametric dependencies of locked mode thresholds in KSTAR L-mode plasmas

“…Intrinsic rotation reversal [28,29] is one example that shows bifurcated rotation change by the density variation. A connection to rotation reversal and the threshold density of the LOC-to-SOC transition has been discussed in previous studies including KSTAR [29,30,[36][37][38], and it indicates a connection of the error field threshold to rotation reversal considering the rotation dependence of the error field threshold. Unfortunately, the absence of rotation profile measurement without NBI limits the identification of rotation reversal in our ohmic database shown in figure 5.…”