KSTAR Plasma Research Articles

Estimation of neutral beam injector power transferred to KSTAR plasmas

KSTAR shows the long pulse capability based on superconducting magnet and NBI plays a crucial role in sustaining the plasma via plasma heating and current driving. So, the accurate NBI power measurement transferred to KSTAR plasma is important for the analysis of plasma transport as well as plasma performance parameters. In a long pulse operation, the water coolant temperature is at the steady state condition in longer pulse more than 20s and the coupled neutral beam power to KSTAR plasma during the tokamak experiments is rechecked by using water flow calorimetric method after the experiment. The effect of beam duct scraper loss which was not considered at the neutral beam power calibration process is less than 5 % in terms of neutral beam power. However, in long pulse operation of NBI in KSTAR experiments, high strength of stray PF affects the beam path, the neutral beam power registered on mds + using beam current method is over estimated and it is calculated up to a few percent in terms of neutral beam power using calorimetric method. Therefore, it is necessary to consider the beam power variation by PF effect to interpret the plasma performance degradation in long pulse operation. Lastly. when the ion source tun on or turn off in condition other ion sources are operated, the beam transmission power is also affected because of sharing the beam box for ion sources so that the careful power estimation is necessary for such kind of beam power modulation experiments in KSTAR.

Criterion for long sustained highly peaked ion temperature in diverted configuration of KSTAR tokamak

In the context of KSTAR plasma research, the discovery of the fast ion regulated enhanced mode is noteworthy due to its remarkable ability to maintain ion temperatures exceeding up to 10 keV for a few tens of seconds, avoid impurity accumulation, and keep low loop voltages. This new plasma operating scenario is achieved in a diverted configuration plasma by avoiding the H-mode transitions with sufficient additional power for the transition. Keeping the density low is the primary method for the avoidance. Additionally, adjustments to other parameters (plasma shape, neutral beam injection, and toroidal magnetic field) associated with the H-mode threshold power are applied to inhibit the transition process. This paper includes an experimental analysis and discussion of these findings.

Formation of small-scale modes via ECCD injection into KSTAR plasma core

In KSTAR experiments exhibiting sawtooth instability, the formation of multiple flux tubes (MFTs) has been frequently observed when electron cyclotron resonance heating or a current drive is applied near the inversion radius of the sawtooth. On the global scale, these MFTs evolve into a single flux tube mode or dual modes. The modes are observed as multiple Fourier harmonics in the spectrogram. A comprehensive correlation analysis of 2D imaging diagnostic data reveals a notable energy transfer within structures of varying sizes during the global mode transition. Broadband fluctuations are enhanced, and energy transfer between Fourier harmonics occurs in the presence of MFTs. Cross-power spectrum in the presence of multiple Fourier harmonics aligns with power law of inverse cascade. This suggests that energy inverse cascade process can contribute to formation of MFTs.

KSTAR plasma commissioning to support the tokamak physics experiments

At the beginning of every KSTAR campaign, during the plasma commissioning period, a stable tokamak plasma operation condition is aimed to be established to support various physics experiments throughout the rest of the KSTAR campaign. To achieve this, considering the KSTAR's superconducting operation, the plasma startup recipe is confirmed, and every component of auxiliary devices for fuel, heating, and diagnostics must be checked. A robust plasma startup scenario has been secured at the beginning of every plasma commissioning period for all plasma experiments. Then, the commissioning work deals with the plasma shaping and the auxiliary devices for fueling and heating. As the last stage, the radial profile diagnostics are tested by moving the plasma boundary and varying the operating toroidal magnetic field for cross-validation in a dedicated session. This paper explains in detail the established routine process for plasma commissioning in the KSTAR campaign.

Progress in the optimization of wall conditioning techniques for the reliable operation of KSTAR plasmas

Progress in the optimization of wall conditioning techniques for the reliable operation of KSTAR plasmas

Machine-learning Algorithms Based on Time-series Data for Real-time Classification of Nuclear Fusion Plasmas in KSTAR

A high confinement mode (H-mode)1) was selected as one of the basic operation scenarios of ITER.2) The H-mode is a phenomenon in which the plasma confinement performance suddenly increases because a transport barrier is formed near the plasma boundary. However, although there are advantages obtained in the H-mode, instabilities at the plasma boundary also appear, such as edge localized modes (ELMs). The problem with the ELMs is that the transport barrier periodically collapses (ELM crash). As a result, high temperature energy and particles are released, damaging the plasma-facing components. Therefore, various studies for controlling the ELMs are continuously conducted in various tokamak devices worldwide. The most effective method to prevent ELM crashes is resonant magnetic perturbation (RMP). Also, the most problematic phenomenon among various instability phenomena of plasma, including the ELMs, is plasma disruption.3) The disruption is a phenomenon in which the plasma inside the tokamak loses heat and electromagnetic energy very quickly and disappears. This sudden energy loss induces a considerable heat flux to the plasma-facing components. Furthermore, it applies a strong electromagnetic force to the surrounding conductor to damage the tokamak device, so it is an unstable phenomenon that must be resolved.

Design and experimental demonstration of feedback adaptive RMP ELM controller toward complete long pulse ELM suppression on KSTAR

Operation of a fusion power plant requires robust edge localized mode (ELM) suppression simultaneously with high plasma performance. In this paper, we describe a novel feedback adaptive resonant magnetic perturbation (RMP) ELM controller designed to address this problem by achieving optimized ELM suppression through the advanced application of 3D RMPs. From real-time Dα data, the controller is able to achieve robust ELM suppression while simultaneously minimizing the applied RMP in order to enhance plasma performance. In real-time, the instantaneous ELM-frequency is analyzed with an adaptive feedback algorithm to determine amplitudes and phases of RMP coil currents that will maximize plasma performance while maintaining ELM suppression. When applied through the KSTAR plasma control system in several experiments using n = 1 RMPs, robust ELM suppression is achieved and sustained in feedback while reducing the RMP strength to ∼65% of its initial value. Minimization of the RMP strength in this manner not only allows for operation of longer discharges due to a decrease in flux consumption but also allows for a strong recovery of up to ∼60% of βN throughout the ELM-free period.

Suppression of toroidal Alfvén eigenmodes by the electron cyclotron current drive in KSTAR plasmas

Advanced operation scenarios such as high poloidal beta (β P) or high q min are promising concepts to achieve the steady-state high-performance fusion plasmas. However, those scenarios are prone to substantial Alfvénic activity, causing fast-ion transport and losses. Recent experiments with the advanced operation scenario on KSTAR tokamak have shown that the electron cyclotron current drive (ECCD) is able to mitigate and suppress the beam-ion driven toroidal Alfvén eigenmodes (TAEs) for over several tens of global energy confinement time. Co-current directional intermediate off-axis ECCD lowers the central safety factor slightly and tilts the central q-profile shape so that the continuum damping in the core region increases. Besides, the rise of central plasma pressure and increased thermal-ion Landau damping contribute to TAE stabilization. While the TAEs are suppressed, neutron emission rate and total stored energy increase by approximately 45% and 25%, respectively. Fast-ion transport estimated by TRANSP calculations approaches the classical level during the TAE suppression period. Substantial reduction in fast-ion loss and neutron deficit is also observed. Enhancement of fast-ion confinement by suppressing the TAEs leads to an increase of non-inductive current fraction and will benefit the sustainment of the long-pulse high-performance discharges.

Dynamic simulation on flow characteristics of KSTAR PF magnet cryogenic network

During its nominal operation, strong current variation, more than 10 kA per second, can be imposed on the KSTAR PF (Poloidal Field) magnet system. Due to AC loss, transient massive backward flow can occur, especially at the cryogen inlets of each magnet. To protect cryogen circulator, several valves are activated to adjust or lower pressure below the circulator's operation limit. In this work, we discuss whether currently available thermo-hydraulic code, such as SUPERMAGNET, can describe this overall dynamic flow characteristics of KSTAR PF magnet cryogenic network. First, three major functions of the KSTAR PF magnet cryogenic system during plasma operation are classified and discussed. Cryogenic components mostly related with other functions, such as cool-down process, are removed from the simulation circuit to minimize computation time. SUPERMAGNET code was used with a slight modification in its cryogenic network simulation module, FLOWER. The codes for cryogenic valves are modified so that not only steady state but also general compressible flows can be described. A case study for an actual KSTAR plasma operation has been intensively carried out. It was shown that most relevant data, pressure, temperature and mass-flow, at the inlets and outlets of the magnet, are in agreement with simulation results.

Kinetic equilibrium reconstruction and the impact on stability analysis of KSTAR plasmas

High fidelity kinetic plasma equilibrium reconstructions are an essential requirement for accurate stability and disruption prediction analyses to support continuous operation of high beta tokamak plasmas. The present kinetic equilibrium reconstructions of plasmas in the KSTAR device include plasma density and temperature profiles from Thomson scattering and ion temperature from charge exchange spectroscopy diagnostics, and allowance for fast particle pressure. In addition, up to 25 channels of motional Stark effect diagnostic data are used to constrain the magnetic field pitch angle profile in the plasma to produce a reliable computation of the safety factor, q, profile. H-mode plasmas exhibit clear pedestal characteristics in the reconstructed pressure profile compared to internal transport barrier or L-mode plasmas. The plasma configuration and vertical position of inner strike points are validated by CCD and infrared camera images. Ideal and resistive magnetohydrodynamic (MHD) stability analyses using the DCON and resistive DCON codes utilize these kinetic equilibrium reconstructions to compare to experimental plasma stability. Equilibria with sufficiently low convergence error can provide reliable computation of ideal and resistive MHD stability analysis.

Observation of ICRH effect on toroidal rotation for Ohmic and ECH plasmas in KSTAR

Toroidal rotation behaviors are investigated in KSTAR when ion cyclotron resonance heating (ICRH) is applied in Ohmic and electron cyclotron resonance heating (ECH) plasmas. The ICRH induces the core toroidal rotation to the co-current direction, and H-mode transition is achieved by ICRH in ECH plasmas. Distinctive behaviors of the toroidal rotation during L- to H-mode transition triggered by ICRH and influences of edge localized modes in toroidal rotation are discussed.

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

Resonant error fields with toroidal harmonics n = 1 and n = 2 in otherwise axisymmetric tokamak plasmas can drive disruptive locked modes when their strength exceeds a threshold value. This paper presents the sensitivity of error field threshold in global parameters such as line-averaged density and toroidal magnetic field as well as toroidal harmonics dependence in KSTAR L-mode plasmas. The error field threshold for mode-locking increases along with density up to a roll-over density, which coincides well with the linear ohmic confinement (LOC) to saturated ohmic confinement (SOC) density for the studied KSTAR plasmas. This non-monotonic density correlation of the error field threshold in LOC–SOC can be explained by a modification of existing theoretical scaling in SOC, while plasma rotation can also play an important role in setting the resonant error field threshold.

Power deposition of H minority heating with a new compact ICRF antenna in KSTAR

The new compact ICRH antenna (CIA) with low electric field and conjugate-T has been installed in KSTAR. It is essential for studying the effect on the KSTAR plasma of H minority heating as a main ICRH scheme. The TRANSP-TORIC code has been adopted for analyzing the ICRH effect. The calculation using TRANSP-TORIC has been conducted with an additional 1 MW of ICRH power to the KSTAR operation at B0 = 2.0 T. About 75% of the ICRH power has coupled to the H minority heating, and most of the power of H minority heating has contributed to the energy of D majority ions and electrons. The deposited profile of the minority heating power varies with the concentration rate of the H minority ions and shows the possibility of producing energetic particles which appears to the increasing energy of D majority ions. The power of the H minority heating has good coupling to the majority ions in the broad range of 5 < n// < 35, including n// of the newly installed CIA in KSTAR.

Development of a collective scattering system and its application to the measurement of multiscale fluctuations in KSTAR plasmas

A four-channel collective scattering system (CSS) has been developed to extend the current turbulence measurement capability (poloidal wavenumber 3 cm−1) beyond the ion gyroscale in the KSTAR tokamak. By utilizing a 300 GHz probe beam and four-channel detector array, the CSS can measure electron density fluctuations of four distinct poloidal wavenumbers from 14 to 26 cm−1 at a high sampling rate (typically 10 MS s−1). In between discharges the radial measurement location can be varied from the plasma center to the outer edge with the remote control of the optical system. In this paper, the details of the major components and laboratory evaluation of the optical system are described. Several initial measurements such as (1) the broadband turbulence reduction during L–H transition, (2) the broadband turbulence increase in H-mode when the amplitude of edge-localized mode crashes is reduced by increased density, (3) the amplification of a quasi-coherent mode by additional heating in high-performance H-mode, and (4) the appearance of high-frequency magnetohydro-dynamical modes during slow L–H transition, are briefly presented. The possibility of the CSS as a hybrid system (of a scattering system and fluctuation interferometer), which complicates the interpretation of the CSS data, is also discussed.

Low-risk beginning of the density control on the KSTAR plasmas

During the early campaigns of the KSTAR project, feedback control of plasma density has been successfully commissioned at the very first attempt by using a transfer function analysis. In order to minimize a chance of any risk happening, a stable and robust Ohmic discharge was chosen as a test-bed of 300 kA (Ip) circular limited plasma in 2.0T(Bt). Pre-programmed fueling modulation was carried out first by puffing the deuterium gas via a piezoelectric valve. Line-averaged plasma density was measured in real-time by a 280 GHz interferometer system. From those modulations, both the density decay time (τi*) and the external fueling efficiency (fex) were approximated in windows: 3.0–5.0 s and 10–20% respectively. By using the results, a simple plant model has been established based on a global particle balance equation. Then several transient responses such as rising time, settling time and overshoot ratio were estimated in a certain range depending on the windows of τi* and fex. It is found that τi* has little effect on those response characteristics while fex plays primary role together with magnitude of the proportional gain GP. This is due to predominance of valve response whose characteristic time τv was approximately 60 ms, which is much shorter than τi*. Considering the responses, GP for closed-loop control were set initially very low i.e. 2.5 being concerned on excessive fueling. It was followed by several stepwise increments to reduce steady-state error instead of using any integral gain GI to avoid any chance of instability and uncertainty. Similarly, the target density was also initially low and gradually growing. In this way the very first density feedback control was successfully completed although some of the transient responses were different from the anticipated results while the predicted steady-state error was in good agreement with the experimental undershoot. By investigating an arbitrary time delay τa, it is found that the digital-low-pass filter embedded in the plasma control system (PCS) of 50 ms also plays crucial role together with the τv. In such way, settling time τs and low overshoot case were well agreed but rising time tr and strong overshoot were not able to be found with those time delays. Considering well predicted steady-state error, with settling time ts, the transfer function analysis with the simple particle balance model is appropriate for relatively slow responses but not applicable for the faster responses and strong overshoot, which may not be linear-time-invariant (LTI) system anymore. More comprehensive physics-basis modeling will be necessary for more accurate prediction including fast responses and strong overshoot of feedback control of the density.

Advances and challenges in KSTAR plasma control toward long-pulse, high-performance experiments

An overview of advances and progress on the KSTAR plasma control improvement is given. The enhancement of the KSTAR plasma control system (PCS) continues in order to enable implementations of more sophisticated control algorithms and capabilities of integrated controls on magnets, gas, heating and off-normal event handling. Features and functionalities on the generic magnetic controls, kinetic controls and new fault scheme design are added for the operation space expansion, mainly regarding recent achievements of high plasma current up to 1.1 MA for 10 s, high-performance long pulse up to 90 s, and discharges relevant to ITER research. Technical challenges and prospective upgrades in the near future are described.

Analysis of MHD stability and active mode control on KSTAR for high confinement, disruption-free plasma

Long-pulse plasma operation at high normalized beta, βN, above the n = 1 ideal MHD no-wall stability limit in KSTAR is presently limited by tearing instabilities rather than resistive wall modes. H-mode plasma operation during the recent KSTAR device campaign produced discharges having strong m/n = 2/1 tearing instabilities at βN lower than the ideal MHD no-wall beta limit. The unstable tearing mode consequently reduced plasma confinement and toroidal plasma rotation significantly. The experiment confirmed that an extended duration of electron cyclotron heating (ECH) at the initial phase of the discharge plays a critical role in mode destabilization. To study destabilizing mechanisms that affect the mode growth, the stability of the observed tearing modes from plasmas with significantly different βN is computed by using the resistive DCON code and the M3D-C1 code employing different physics. The computed tearing stability index, Δ′, differs between the mode that is destabilized by the early ECH at lower βN, and the mode that is destabilized at higher βN with observed mode triggering activity. Equilibrium reconstructions that include constraints from internal profile diagnostics are used as input for reliable computation of stability. The modified Rutherford equation (MRE) describing the evolution of the neoclassical tearing mode (NTM) island width has been constructed for KSTAR plasmas by using plasma parameters computed by the TRANSP code. In preparation for long-pulse plasma operation at higher beta utilizing increased plasma heating power, a resistive wall mode (RWM) active feedback control algorithm that includes magnetic sensor compensation of the prompt applied field and the field from the induced current on the passive conductors has been completed and enabled on KSTAR. Use of multiple toroidal sensor arrays is enabled for increased control performance by including the effect of varied mode helicities in the outboard region where the mode measurement is made. This analysis on beta-limiting instabilities and active mode control provides the required foundation for high confinement plasma operation on KSTAR without disruption.

Real-time classification of L-H transition and ELM in KSTAR

In order to automate the control of ELM and plasma density, an algorithm for classification of L-H transition and the first ELM burst in real-time has been developed with KSTAR campaign data. We applied a Long Short-term Memory (LSTM) which is a special kind of recurrent neural networks for storing long time-series data to classify the L-H transition phenomena and the first ELM burst. For training the LSTM classifier, we used Dα and line-averaged electron density diagnostic data that indicate the characteristic patterns of the L-H transition. After the training, we tested two versions for real-time experimental data and post-processed data. In addition, through the results related to the long intermediate states, we examined whether the statistical approach can reflect physical aspect of the KSTAR plasmas.

Development of a compact ICRF antenna for high-power and long-pulse plasma heating in the KSTAR

For the high-power and long-pulse ion cyclotron range of frequencies (ICRF) heating of the KSTAR plasma, we developed the compact ICRF antenna (CIA). The target injection power of CIA is 2 MW for 300 s. In order to continue injecting the power into plasma even if drastic instantaneous changes occur in the plasma condition, such as ELM events, we adopted the internal conjugate-T method for the load resilience. Between antenna heads and the junction point, impedance transformers were inserted to satisfy the condition of conjugate-T in a limited space keeping the electric field on the transformer low enough. To reduce the risk of water leakage into the vacuum chamber, only the backsides of antenna heads are water-cooled in the in-vessel region.

Local profiles of line emission of impurity ions in rotating fusion plasmas

We analyze the poloidal distribution of impurity ions, such as C V, C VI and O VIII, in fast rotating KSTAR plasmas using the impurity emission lines in the 1–7 nm wavelength range obtained from the space-resolved extreme ultra violet spectrometer. This is aided particularly by a new tomographic reconstruction technique with limited viewing angles. The vacuum and extreme ultraviolet spectrometer system at ITER and its prototypes installed at KSTAR are one of the main diagnostic systems to monitor the impurity species in fusion plasmas; however, they have an insufficient field of view to utilize the conventional reconstruction method. The new algorithm developed in this work considers a curvilinear grid of pixels mapped onto the magnetic equilibrium and a weight factor considering toroidal rotational effect on impurity species. Validation tests with poloidally asymmetric images of synthetic plasma demonstrate that this method is highly useful for emission diagnostics that have a narrow field of view. The experimental results of the spatially resolved spectra from KSTAR plasma clearly show that even the light impurities such as carbon and oxygen are significantly affected by the toroidal rotation. Accordingly, the method is expected to be utilized for estimating the poloidal density profile of each charge state of an impurity species, which will play a key role in impurity transport studies.