Direct measurements of safety factor profiles with motional Stark effect for KSTAR tokamak discharges with internal transport barriers

Investigation of multiple-ion-source neutral beam operation conditions compatible with motional Stark effect diagnostics

The motional Stark effect (MSE) diagnostic system at KSTAR (Korea Superconducting Tokamak Advanced Research) often suffers from the drawback of possible systematic uncertainties in measurements due to overlap of the MSE spectra generated from three different ion sources that constitute a single neutral beam injection system. In particular, one ion source injected in the most tangential direction always causes strong spectral overlaps which, therefore, imposes regulations and constraints on the energy combination among the ion sources. A Stokes-vector analysis has been performed to produce operation windows for the energy combination between the ion source used in the MSE measurement and the ion source with the largest tangential injection angle. The analysis includes various practical factors, such as the distortion of the transmission function of bandpass filters and pitch angle profiles collected from a vast amount of KSTAR discharges. The two-dimensional parameter space, or the contour plot, on the expected systematic offsets in the measured pitch angle has been generated from this analysis, which can serve as a quantitative guideline for operating the multiple-ion-source neutral beam heating system.

Considerations of the q-profile control in KSTAR for advanced tokamak operation scenarios

The q-profile control is essential for tokamaks exploring the advanced tokamak scenarios, which is expected to be able to provide a possible route toward a steady-state high performance operation in a fully non-inductive current drive state. This is because the pressure and current profiles must remain optimal for the scenario during the injection of large amounts of heating and current drive. Here, essential tools for the q-profile control are the motional Stark effect diagnostic for measuring the radial magnetic pitch angle profile and a state-of-the-art plasma control system. The pulse duration of the H-mode discharge at KSTAR has been extended year by year with improved control performance, and the experiment of internal transport barrier (ITB) formation in a weakly reversed q-profile with a marginal neutral beam injection majority heating successfully demonstrated that the ITB is an alternative candidate to achieve a high performance regime in KSTAR. These recent achievements are attributed to reliable profile measurement, which means that profile feedback control has become a necessary step to ensure a robust and reliable approach to advanced scenarios as the next step of research in KSTAR. In this paper, we discuss the technical and conceptual requirements for q-profile control according to the upgrade plan for heating and current drive systems in the coming years.

Effect of multi-ion-source injection on motional Stark effect diagnostic

Many tokamak devices utilize high-power neutral beams for various beam-based active spectroscopic diagnostics such as the motional Stark effect (MSE). For higher heating performance, it is customary for the neutral beam injection to be made with a multiple number of ion sources, which often makes unfavorable conditions for the active spectroscopic diagnostics. This is mainly because the atomic and molecular emissions taking place from the interactions with multiple beams, or from different flux surfaces, are collected through the front optics at the same time, resulting in systematic errors in the measured quantities. In this work, the effect of the multiple ion source injections on the pitch angle measurements by the MSE diagnostic is quantitatively studied based on both numerical modeling and measurements made from the plasma discharges for the Korea Superconducting Tokamak Advanced Research. The sensitivity of the pitch angle against various combinations of the acceleration voltages of the ion sources is evaluated, yielding the optimum configuration of the beam injection that can maximize the heating efficiency with an acceptable level of the systematic offset in the MSE measurements.