Diagnostic development for current density profile control at KSTAR

Observation of a new type of self-generated current in magnetized plasmas

A tokamak, a torus-shaped nuclear fusion device, needs an electric current in the plasma to produce magnetic field in the poloidal direction for confining fusion plasmas. Plasma current is conventionally generated by electromagnetic induction. However, for a steady-state fusion reactor, minimizing the inductive current is essential to extend the tokamak operating duration. Several non-inductive current drive schemes have been developed for steady-state operations such as radio-frequency waves and neutral beams. However, commercial reactors require minimal use of these external sources to maximize the fusion gain, Q, the ratio of the fusion power to the external power. Apart from these external current drives, a self-generated current, so-called bootstrap current, was predicted theoretically and demonstrated experimentally. Here, we reveal another self-generated current that can exist in a tokamak and this has not yet been discussed by present theories. We report conclusive experimental evidence of this self-generated current observed in the KSTAR tokamak.

Fusion devices like tokamaks require plasma current to generate magnetic field for plasma confinement. Here the authors report an observation of a self-generated anomalous current that contributes up to 30% of the total current in the fusion plasma at KSTAR.

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.

Application of motional Stark effect in situ background correction to a superconducting tokamak

A polychrometer-type motional Stark effect (MSE) diagnostic technique, originally developed for the Alcator C-Mod tokamak, has been extended and applied to the Korea Superconducting Advanced Tokamak Research (KSTAR) device, the long-pulse superconducting tokamak, for the first time. It demonstrates a successful in situ subtraction of the polarized reflections off the vacuum vessel wall, sometimes up to half the total signal in some sightlines. To avoid the secondary neutral beam emission that may contaminate conventional beam-into-gas calibrations, a new approach, where the beam-into-gas measurements are made at various torus pressures with fixed vacuum fields, has been devised, which is possible with the stable superconducting coil systems of KSTAR. The validity of this new calibration scheme has been checked via plasma jog experiments. The experimental evidence of the polarized background light and the necessity of its correction in the MSE measurements made in KSTAR are presented as well.

Atomic Data for Calculation of the Intensities of Stark Components of Excited Hydrogen Atoms in Fusion Plasmas

Motional Stark effect (MSE) spectroscopy represents a unique diagnostic tool capable of determining the magnitude of the magnetic field and its direction in the core of fusion plasmas. The primary excitation channel for fast hydrogen atoms in injected neutral beams, with energy in the range of 25–1000 keV, is due to collisions with protons and impurity ions (e.g., He 2 + and heavier impurities). As a result of such excitation, at the particle density of 10 13 –10 14 cm − 3 , the line intensities of the Stark multiplets do not follow statistical expectations (i.e., the populations of fine-structure levels within the same principal quantum number n are not proportional to their statistical weights). Hence, any realistic modeling of MSE spectra has to include the relevant collisional atomic data. In this paper we provide a general expression for the excitation cross sections in parabolic states within n = 3 for an arbitrary orientation between the direction of the motion-induced electric field and the proton-atom collisional axis. The calculations make use of the density matrix obtained with the atomic orbital close coupling method and the method can be applied to other collisional systems (e.g., He 2 + , Be 4 + , C 6 + , etc.). The resulting cross sections are given as simple fits that can be directly applied to spectral modeling. For illustration we note that the asymmetry detected in the first classical cathode ray experiments between the red- and blue-shifted spectral components can be quantitatively studied using the proposed approach.

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.

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

The safety factor profile evolutions have been measured from the plasma discharges with the external current drive mechanism such as the multi-ion-source neutral beam injection for the Korea Superconducting Tokamak Advanced Research (KSTAR) for the first time. This measurement has been possible by the newly installed motional Stark effect (MSE) diagnostic system that utilizes the polarized Balmer-alpha emission from the energetic neutral deuterium atoms induced by the Stark effect under the Lorentz electric field. The 25-channel KSTAR MSE diagnostic is based on the conventional photoelastic modulator approach with the spatial and temporal resolutions less than 2 cm (for the most of the channels except 2 to 3 channels inside the magnetic axis) and about 10 ms, respectively. The strong Faraday rotation imposed on the optical elements in the diagnostic system is calibrated out from a separate and well-designed polarization measurement procedure using an in-vessel reference polarizer during the toroidal-field ramp-up phase before the plasma experiment starts. The combination of the non-inductive current drive during the ramp-up and shape control enables the formation of the internal transport barrier where the pitch angle profiles indicate flat or slightly hollow profiles in the safety factor.

Sensitivity of magnetic field-line pitch angle measurements to sawtooth events in tokamaks

The sensitivity of the pitch angle profiles measured by the motional Stark effect (MSE) diagnostic to the evolution of the safety factor, q, profiles during the tokamak sawtooth events has been investigated for Korea Superconducting Tokamak Advanced Research (KSTAR). An analytic relation between the tokamak pitch angle, γ, and q estimates that Δγ ∼ 0.1° is required for detecting Δq ∼ 0.05 near the magnetic axis (not at the magnetic axis, though). The pitch angle becomes less sensitive to the same Δq for the middle and outer regions of the plasma (Δγ ∼ 0.5°). At the magnetic axis, it is not straightforward to directly relate the γ sensitivity to Δq since the gradient of γ(R), where R is the major radius of the tokamak, is involved. Many of the MSE data obtained from the 2015 KSTAR campaign, when calibrated carefully, can meet these requirements with the time integration down to 10 ms. The analysis with the measured data shows that the pitch angle profiles and their gradients near the magnetic axis can resolve the change of the q profiles including the central safety factor, q₀, during the sawtooth events.