Study on Intrinsic Rotation in KSTAR Ohmic Plasmas

Abstract

Plasma rotation is known to affect tokamak plasma performance and stability. The intrinsic rotation study is one of the important issues of tokamak plasma because it is expected that the plasma rotation by external torque sources is small in large devices such as ITER unlike the present tokamaks. Ohmic plasma could be the most suitable plasma for studying the physical mechanism of the intrinsic rotation generation and the experiments for the intrinsic rotation in Ohmic plasma has been conducted in various tokamaks. KSTAR, a domestic tokamak, had also been conducting experimental studies on the intrinsic rotation in Ohmic plasmas, but previous studies only focused on the core intrinsic rotation using XICS measurement (V_ϕ (0) based analysis on the intrinsic rotation). In this research, the experiment was designed to measure the profiles of the intrinsic rotation, and the profile-based analysis was performed to investigate the experimental characteristics of the intrinsic rotation in KSTAR Ohmic plasmas. Based on this database and experimental characteristics, a comparative analysis with various theoretical models for the intrinsic rotation generation was carried out. Then, the integrated modelling was conducted to predict the intrinsic rotation profiles using the selected model in KSTAR Ohmic plasmas and compare the intrinsic rotation profiles in experiments. In order to measure the intrinsic rotation profiles in KSTAR, neutral beam injection is inevitably required. At this time, neutral beam injection not only disturb the plasma properties but also cause deformation of the plasma rotation due to external torques, so it is of the highest priority to minimize the external perturbation and to measure the intrinsic rotation in pure Ohmic plasma as much as possible. For this purpose, the optimal operating conditions of the neutral beam injection for measuring the intrinsic rotation profiles in KSTAR Ohmic plasma were experimentally derived. The optimal operating conditions depend on the plasma condition and purpose

in KSTAR Ohmic plasma with a plasma current of 400-600 kA, it is confirmed that 20 ms * 2 Hz or less of beam injection duration and frequency should be at least satisfied. As a result of the investigation of the intrinsic rotation profiles measured under various operating and plasma conditions, it is confirmed that V_ϕ (0) based analysis could be limited to capture the intrinsic rotation characteristics. To sufficiently cover the intrinsic rotation characteristics using V_ϕ (0) based analysis, two conditions are required. One is that the intrinsic rotation near the plasma boundary must be negligible, and the other is that the intrinsic rotation profile must be a simple curve. It has been confirmed that the intrinsic rotation near the plasma boundary depends on various operating and plasma conditions in KSTAR Ohmic plasma, and that even under the same operating conditions, it depends on when the experiment is performed. This means that the intrinsic rotation near the plasma boundary is sensitive to not only the operating conditions but also wall conditions. Furthermore, the change of the intrinsic rotation near the plasma boundary in KSTAR Ohmic plasma is up to 20 km/s. This is a level that cannot be ignored when compared with the typical variation of V_ϕ (0) in KSTAR Ohmic plasma. The other thing is that the intrinsic rotation profile does not have a simple curved shape and the toroidal rotation gradient could be varied significantly depending on the radial location. It means that the intrinsic rotation profile must be considered in order to understand the actual intrinsic rotation characteristics. In consideration of these characteristics, we adopted the profile-based analysis and investigated the intrinsic rotation characteristics in KSTAR Ohmic plasma based on about 300 databases accumulated until 2016 KSTAR campaign. First, we have examined the typical feature of the intrinsic rotation profiles. It is observed that the intrinsic rotation profiles in KSTAR Ohmic plasma could have a feature divided into four regions. The innermost region near the center of the plasma has a somewhat flat or slightly bulging shape, which is consistent with the inversion radius of sawtooth activity. Next, the region with a steep slope, called gradient region, is observed, and it usually has a positive gradient in KSTAR Ohmic plasmas. This region is strongly associated with the change in V_ϕ (0) as a result of the large change in slope with the increase of plasma density. The next is the anchor point region. Here, the profile seems again flattened and appears to act as the boundary region of the gradient region. This region is far from the actual plasma boundary, and is distinct from the actual plasma boundary region. The last part is the area between the anchor point region and the actual plasma boundary, and there are various shapes and values of the intrinsic rotation according to the plasma operating conditions. By analyzing the intrinsic rotation profiles for various operating conditions, it can be confirmed that the intrinsic rotation evolves from nearly flat to hollow shapes with the increase of the plasma density in KSTAR Ohmic plasmas and the feature of the intrinsic rotation is closely related to q_95. Here, the evolution of the intrinsic rotation mainly results from the change of the normalized rotation gradient (u') and its variation is only observed in the gradient region with the increase of plasma density while no significant change is observed in the anchor point region. Note that, the gradient region corresponds to the region with a low q value and a weak magnetic shear, and an anchor point region with a high q value and a strong magnetic shear. The change of u' in the gradient region could be approximated by the ratio of the residual stress (the intrinsic rotation generation mechanism) and the momentum diffusivity. Here, it is expected that both are related to the turbulence characteristics such as ITG / TEM, so that the correlation of u' with the plasma properties such as R/L_(T_i ),R/L_(T_e ),T_e/T_i,ν_eff which effect on the turbulence characteristics was investigated. The analysis represents that the most important correlative properties are ν_eff and T_e/T_i. Interestingly, it was derived from the time-varying transport simulation that the change of u' in the gradient region would be closely related to the significant change of the momentum diffusivity rather than the change of the residual stress. This is in agreement with experimental observations that u' always have a negative value even with a much lower collisionality with the ECH injection and the linear/nonlinear gyrokinetic simulations. Using the KSTAR Ohmic plasma database, we have evaluated which theoretical model can well predict the intrinsic rotation characteristics. As a result of comparing and analyzing the neoclassical transport model and various turbulent transport models, it was found that the "finite ballooning tilting" model could well capture the intrinsic rotation characteristics observed in experiments. Although an arbitrary (but within a range of possible levels) tilting angle was used, the similar magnitude of u' in experiments could be represented, and even under fixed tilting angles, the model seems to predict similar ν_eff dependence in both gradient region and anchor point region. The difference between the gradient region and the anchor point region was found to be related to the magnetic shear and the q value. The predicted u' represents the similar ν_eff dependence of u' observed in the gradient region for the weak magnetic shear and the low q value case while it becomes smaller and the tendency is changed for the strong magnetic shear and the high q value case. This is consistent with experimental observations where the gradient region and the anchor point region are related to the q value and the magnetic shear value. Finally, in order to calculate the intrinsic rotation profiles in KSTAR Ohmic plasma by applying the "finite ballooning tilting" model, an integrated modelling was performed by combining the GKW code and the transport code ASTRA. Although the level of the intrinsic rotation from the modelling was predicted to be smaller than the intrinsic rotation in experiments, the change of u' with the increase of the collisionality and the profile feature which consists of the gradient region and the anchor point region seem to be well predicted. This shows that the finite ballooning tilting model has a high potential as the intrinsic rotation generation mechanism in KSTAR Ohmic plasmas and the integrated modelling for the intrinsic rotation prediction would be utilized for various applications.