Versatile Experiment Spherical Torus Research Articles

Line integrated density measurements on the Versatile Experiment Spherical Torus (VEST) using frequency sweep interferometer

A frequency sweep interferometer (FSI) operating in the frequency range of 50–75 GHz is installed in the versatile experiment spherical torus spherical tokamak to measure the line integrated density (LID). FSI measures the time derivative of phase to calculate the group delay, which is proportional to the LID under the condition that the microwave frequency is much higher than the plasma frequency. Since the group delay is calculated from the time derivative of phase and the frequency sweep rate, FSI is very sensitive to the measurement noise. In the view point of signal processing, derivative exaggerates the measurement noise. Therefore, sophisticated techniques for phase measurement and frequency linearization are required to obtain meaningful results with FSI. The detailed techniques and the hardware setup are explained in the paper. The LID measured by FSI is benchmarked with the LID measured by a conventional 94 GHz heterodyne interferometer. The two measurements agree well. A conventional interferometer can no longer provide LID when severe phase errors occur. This is because phase errors propagate to subsequent measurements. However, FSI provides LID during the entire discharge time successfully regardless of frequent measurement failure because the LID is obtained in FSI from the time derivative of phase rather than the phase. In this sense, FSI is suitable as a diagnostics for steady state plasmas. The main cause for the phase errors is identified as the beam path displacement due to the refraction of the plasma.

Development of Penning ion gauge for in-situ measurement of neutral pressure in VEST

A Penning Ion Gauge (PIG) has been developed to measure the responsive temporal changes of neutral pressure inside the magnetic fusion devices, and successfully operated in Versatile Experiment Spherical Torus (VEST). To check its reliabilities during in-situ operation, feasibility tests are conducted by specifically investigating both the penning discharge mode transition and the effect of magnetic field alignment on the penning discharge. The mode transition of penning discharge is clearly observed, confirming the developed PIG is applicable to the operation regime of VEST. The PIG is also designed to measure discharge current stably during magnetic field angle variation of upto 15∘. In addition, it has been verified that the operation range of PIG can be expanded by increasing inverse aspect ratio of the gauge cell geometry. Finally, the developed PIG is installed in VEST, measuring a millisecond-order temporal changes in neutral pressure within a range of 3×10−5 mbar to 1×10−3 mbar, under the magnetic field angle variation of up to 15∘. Finally, it is successfully measured rapidly evolving neutral pressures resulting from the plasma wall interaction during ohmic discharges in VEST.

Electron Temperature Measurements Using a Two-Filter Soft X-ray Array in VEST.

A multichannel soft X-ray (SXR) array has been developed to measure the electron temperature in the Versatile Experiment Spherical Torus (VEST). To estimate electron temperature using the two-filter method applied to SXR intensity, we designed a pinhole camera that has two photodiode arrays with different metallic filters. We also adopted a filter wheel and tested various filter parameters to find the optimal filter set. Through tests, the combination of aluminum and beryllium was found to be the most suitable for the current experimental conditions in VEST. The filtered SXR signals were acquired with a low-noise preamplifier, exhibiting sufficient signal-to-noise ratios for electron temperature estimation based on the intensity ratio of two signals obtained with different filters. The estimated electron temperature from the developed two-filter SXR array showed reasonably matched levels and consistent trends with Thomson scattering measurements. Error contribution from impurity line emission is also discussed.

Design of a slotted waveguide antenna to launch lower hybrid fast wave in the KSTAR Tokamak

Lower hybrid fast wave (LHFW) has been proposed for heating and current drive in high density plasmas to overcome the density limit of conventional slow wave current drive (LHCD). The feasibility of the LHFW was tested through coupling experiments in the Versatile Experiment Spherical Torus (VEST) device. We design a 2.45 GHz waveguide antenna for the further investigation of the LHFW in the KSTAR tokamak. RF modelling of a slotted waveguide antenna is conducted in a vacuum and plasma medium using COMSOL in order to launch 2.45 GHz fast waves with n∥0 ∼ 2.87. A slotted waveguide antenna for the LHFW is designed according to simulation results which show field patterns for fast wave polarization, a n∥ spectrum and coupling efficiency between the antenna and plasmas.

Observation of MHD-correlated blobs during internal reconnection events in VEST

Internal reconnection events (IREs), one of the relaxation events driven by internal magnetohydrodynamic (MHD) instabilities in fusion plasmas, are accompanied by a strongly MHD-correlated blob at the edge in the Versatile Experiment Spherical Torus spherical tokamak. The MHD-correlated blob plays a significant role in the onset and the strength of IREs. Various techniques analyzing visible camera images show correlated waveforms between blobs and magnetic fluctuations, and they produce visualized images of corotating structures of the MHD modes and the MHD-correlated blobs. In the images, a phase drag in the rotations of the two structures initially appears and vanishes on the verge of IREs. IREs maintaining the phase drag, however, leads to a less violent impact in terms of current decrease and magnetic field bursting. In addition, the MHD-correlated blobs are followed by the increasing degree of nonlinear interactions between the internal MHD mode and high-frequency broadband fluctuations (>60 kHz) at the edge. These results suggest that boundary plasmas can impact internally driven relaxation events via MHD-correlated edge phenomena.

Development of a soft x-ray (SXR) array diagnostic system on versatile experiment spherical torus (VEST).

A new soft x-ray (SXR) array diagnostic system has been developed on versatile experiment spherical torus (VEST) for measurements of 2D SXR emissivity profile and identification of poloidal mode structure. Through tomographic inversion techniques, 2D SXR emissivity profile can be acquired from the line-integrated SXR data, which enables the visualization of mode structure of plasma instability, such as the magnetohydrodynamics mode. The SXR array diagnostic system consists of two 20-channel arrays positioned at the middle and the top on the same poloidal plane for horizontal and vertical lines of sight, respectively. Each array of the diagnostic system uses absolute extreme ultraviolet photodiode array as the detector. To apply appropriate filters (up to four) for different energy regimes without breaking the vacuum, a filter wheel and its rotatable vacuum feed-through are installed behind the pinhole. SXR data are acquired with a digitizer at the sampling rate of up to 125MHz. Finally, we discuss initial measurement data obtained from Ohmic plasma in VEST.

Identification of kink instability in 3D helical flux ropes at VEST

Local helicity injection (LHI) is a non-inductive startup and current drive method via Taylor relaxation for the spherical torus. In achieving Taylor relaxation, it has been suggested that kink instability in 3D helical flux ropes plays an important role. However, the role and occurrence of kink instability during LHI have yet to be validated. Experimentally, determining the kink mode in a flux rope relies on measuring internal information using a probe. However, for LHI, the 3D geometry complicates this measurement process. Here, we propose a new approach for determining the kink modes of 3D helical flux ropes without any internal probe measurements. It is confirmed by this approach that flux ropes exhibit two different kink modes. With increasing plasma current in the flux ropes, a transition from the coherent internal kink mode to the external kink mode is observed. Kink mode properties such as rotating frequency calculated from the kink theory agree well with the magnetic signature driven by the kink mode. During the LHI experiment in the versatile experiment spherical torus, three distinguishable phases are confirmed by the approach, consistent with NIMROD simulation. Before driving the toroidal plasma current, the external kink mode is observed for 3D helical flux ropes. As the toroidal plasma current increases, the external kink mode disappears while generating broadband internal modes instead of coherent internal kink of flux ropes. Decoupling between the toroidal plasma and flux rope results in both decay of toroidal plasma current and re-appearance of the external kink mode in the flux ropes.

Acceleration of ion rotation during internal reconnection events in the versatile experiment spherical torus (VEST)

Acceleration of ion rotation is observed during internal reconnection events (IREs) in the versatile experiment spherical torus. Two IRE discharges with opposite torques, i.e. acceleration or deceleration of ohmic plasmas with intrinsic, counter-I p rotation, can be generated using different wall conditionings method. When an IRE occurs, acceleration and deceleration of impurity ion rotation as well as well-known ion heating are observed globally via ion Doppler spectroscopy with multiple channels including a single channel with a high temporal resolution of 0.2 ms. Interestingly, ion heating is observed earlier than the ion rotation acceleration, indicating that this phenomenon has a different mechanism from usual magnetic reconnection. We present several possible mechanisms for the rotation acceleration. For several reasons, the ions are thought to be accelerated by a neoclassical toroidal viscosity (NTV) torque rather than mechanisms such as the reconnection outflow and toroidal electric field induced by the current profile change. A simple 0D momentum balance model with NTV torque in the 1/ν collisionality regime agrees well with the experimental results. Furthermore, a positive correlation between ion temperature and plasma rotation is identified, whereby the NTV torque S NTV and ion temperature T i are related via .

Commissioning and initial heating results of the neutral beam injection system in Versatile Experiment Spherical Torus

The initial neutral beam heating experiment in Versatile Experiment Spherical Torus (VEST) has been conducted via neutral beam characterizations such as neutralization efficiency, beam fraction, and shine-through loss. For the neutralization efficiency and shine-through loss estimation, a secondary electron emission (SEE) detector, which can be used in rapidly changing plasma, has been developed. The measurement results of the monatomic fraction by Doppler shift spectroscopy and the neutralization efficiency by SEE detector are 65%, and 70%, respectively. The shine-through loss measurement shows that the injection of a 12 keV neutral beam into a typical ohmic discharge of VEST results in a large amount of shine-through loss. To reduce the shine-through loss, a new target plasma discharge (0.17 T, 120 kA, 13 ms) with increased plasma density of ~2×1019m−3 is developed by providing additional gas injection. The electron-heating effect has been successfully observed for this target plasma with a 200 kW neutral beam injection at the beam energy of 12 keV. The electron temperature increases from 20 to 68 eV without significant density change when the neutral beam is injected.

Development of a hard X-ray detector for measuring continuous spectra generated by runaway electrons in VEST

A hard X-ray measurement system is developed to observe the activities of runaway electrons in versatile experiment spherical torus (VEST) at Seoul National University. The detection system provides information on the generation of runaway electrons by detecting hard X-rays from its bremsstrahlung radiation. To acquire such information, the X-ray spectrum at several tens to hundreds of keV at the maximum is reconstructed using signals from four thin scintillation detectors with different energy sensitivities, and the spectrum is converted to the original X-ray energy. The unfolding process is implemented by an artificial neural network model trained using a measured X-ray spectrum generated by an X-ray tube source at 50 to 320 keV. The system would reveal the generation mechanism of runaway electrons during plasma discharges with various MHD activities in VEST to mitigate and suppress runaway electrons.

Radial profile measurement with an improved 1 kHz Thomson scattering system on Versatile Experiment Spherical Torus.

A Thomson scattering (TS) system has been utilized to measure the electron temperature and density of the core region of Versatile Experiment Spherical Torus (VEST). Recently, the laser injection system is successfully upgraded adopting the burst laser with the repetition rate of 1 kHz and the energy of 2J. Furthermore, improved collection optics with additional polychromators and a 32-channel fast digitizer are prepared to observe the fast time evolution of radial profiles. This improvement is essential to study fast phenomena such as internal reconnection event (IRE). We increase the TS signal and reduce the stray light by introducing new filters having better optical properties such as high optical density at 1064nm, transmission, and reflectance. Moreover, the optimum reverse bias voltages are newly set to make the system operational independent of the background radiation. As a result, 1 kHz radial profiles of the core electron temperature and density are measured for the first time, showing characteristics of IREs in VEST.

Development of Thomson scattering system on VEST

A Thomson scattering (TS) system has been developed on Versatile Experiment Spherical Torus (VEST) to measure the electron temperature and the density. In VEST, it is quite challenging to measure TS signals because the energy of the utilized pulsed laser and the target electron density are low, 0.85 J and ∼5 × 1018 m−3, respectively. Furthermore, the difficulties caused by filter specifications have to be resolved because of the use of commercial interference filters with broad bandwidth and poor blocking at the laser wavelength. Therefore, we especially optimize the placement order of the filter sets based on the evaluation of the relative errors to overcome the poor spectral resolution. In addition, the amount of the stray light that penetrating filters is reduced by vacuum components such as baffles, louvres, and a viewing dump. With the careful installation and the optimization of the system, we can develop an efficient Thomson scattering system. Recently, TS signals have been successfully measured from the ohmic plasmas, and the electron temperature and the density are determined by analyzing the signals with the Bayesian method.

Upgrade of Thomson scattering system on VEST

A Thomson scattering diagnostic system was developed and successfully measured electron temperature and density of core plasma (R = 0.35 m) in Versatile Experiment Spherical Torus (VEST). However, the current system is able to measure only a single time point in a shot. Therefore, upgrade of the Thomson scattering system has been prepared and is in progress to obtain the time evolution. The upgrade is focused on three parts. First, the laser injection system is improved by introducing a burst mode laser with the energy of 2 J and repetition rate of 1 kHz for 10 ms. Furthermore, the laser injection optics is improved to minimize the optical loss. Second, minimum etendue limit of the collection optics is increased by employing a fiber bundle of three single-core fibers. Finally, a data acquisition system is prepared to record the burst laser signals for high sampling rate. Oscilloscope envelope mode is adopted and tested for measuring ten burst laser signal with 1.25 GS/s. Through the upgrade, the time evolution of electron temperature and density is expected to be measured in a single shot with enhanced signal-to-noise ratio.

RF conditioning for the improvement of lower hybrid fast wave comb-line antenna performance on VEST

RF conditioning is conducted to improve coupling and current drive performance of the comb-line antenna which is to excite fast wave in lower hybrid resonance frequency range in Versatile Experiment Spherical Torus (VEST). It has been observed that considerable outgassing, power dissipation and high reflection take place while diminishing transmitted power with poor reproducibility without the RF conditioning process which cleanses antenna surface by RF surface heating. Outgassing source during the RF conditioning is identified mainly as water vapor. A new dimensionless figure-of-merit to evaluate degree of the RF conditioning is defined with dissipated power and outgassing rate by analyzing experimentally observed RF conditioning data. Optimum performance of the antenna with low figure-of-merit (5 × 10−9) is achieved after conditioning for 3 days a week (∼40 shots) at less than 10 kW of input power. Coefficients of transmission, reflection, and dissipation reach to about 80%, 2%, and 18%, respectively indicating almost doubled antenna performance. Introduced new figure-of-merit provides a quantitative basis for antenna conditioning in order to improve antenna performance.

Modified propagation path and expanded coupling regime of lower hybrid fast wave by n∥-upshift via wave scattering in VEST

Experimental investigation of lower hybrid fast wave (LHFW) coupling and propagation is carried out by wave measurements utilizing magnetic probes in Versatile Experiment Spherical Torus. Coupling experiments are conducted in marginal (ne≥nconfluence) and prohibited (ne≫nconfluence) regimes for fast wave coupling. In the marginal regime of Ip = 25 kA, about two folds of n∥-upshift are observed. The propagation path of the fast wave is largely modified compared to that of full wave simulation, and more fast waves are detected inside the last-closed-flux surface despite the unsatisfied accessibility condition. Since spectral broadening is measured to be narrow and linearly proportional to coupled wave power with a high density fluctuation of about 20% in the frequency range (ω0/ωlh∼15), it is thought to be crucially correlated with wave scattering rather than parametric decay instabilities. Consequently, the spectral broadening implicates that n∥-upshift can take place via a wave scattering mechanism. In the prohibited regime of Ip = 100 kA, more enhanced coupling of up to 90% is observed with similar density fluctuation to that of the marginal regime, which is explained with intensified wave scattering due to the high dielectric constant of spherical torus plasmas. It is concluded that practical windows for propagation and coupling of LHFW are widened with the aid of n∥-upshift via wave scattering even though the accessibility condition is not satisfied right in front of the antenna.

SA79GENETIC ANALYSES ON INTERMEDIATE PHENOTYPES OF POST-TRAUMATIC STRESS DISORDER IN A DUTCH MILITARY COHORT

Wall conditioning and impurity monitoring systems are developed in Versatile Experiment Spherical Torus (VEST). As a wall conditioning system, a baking system covering the vacuum vessel wall partially and a glow discharge cleaning (GDC) system using two electrodes with dc and 50 kHz power supplies are installed. The GDC system operates with hydrogen and helium gases for both chemical and physical desorption. The impurity monitoring system with residual gas analyzer (RGA), operating at <10−5 Torr with a differential pumping system, is installed along with the optical emission spectroscopy (OES) system to monitor the hydrogen and impurity radiation lines. Effects of these wall conditioning techniques are investigated with the impurity monitoring system for ohmic discharges of VEST. The partial baking and He GDC show limited effects on plasma characteristics but sufficient H2 GDC above 4 h enables the longer plasma current duration up to ∼15 ms within 3 h from the end of treatment.

Beam extraction characteristics of a low-energy, high-current pulsed ion source of a versatile experiment spherical torus neutral beam injector

A neutral beam (NB) ion source aimed to inject a 0.5-MW NB has been developed for Versatile Experiment Spherical Torus (VEST) plasma heating. Beam energy is generally limited to less than ∼20 keV to reduce particle orbit loss and beam shine-through. To meet this power demand, high-current ion beam extraction is important to achieve the required performance of the ion source. In this study, a 0.7-MW ion source was designed, fabricated, assembled, and optimized at the NB test stand in Korea Atomic Energy Research Institute (KAERI) considering the beam power losses in the beamline components. The ion source comprises a multicusped-bucket-type plasma generator and a multislot-triode-type ion accelerator with an extraction area of 12 cm × 46 cm. The characteristics of arc discharge and beam extraction were investigated using hydrogen gas to obtain the optimum operating parameters of the arc power, filament voltage, gas pressure, extraction voltage, and suppression voltage. The ion source was then conditioned and performance tests were primarily completed. Results of the experiments show that the ion source can extract sufficient ion beam power that is than 0.75 MW (50 A/15 keV) with 100-kW arc power. The arc efficiency and beam perveance were estimated to be 0.6 A/kW and 20–25 μP, respectively. Moreover, the design goal of delivering a 0.5-MW NB into the VEST plasma will be achievable based on the optimum operating conditions. This ion source has been installed in the VEST NB injection system at Seoul National University and its commissioning is ongoing.

Effect of boronization in VEST: Achieving 0.1 MA discharge

Boronization procedure for Versatile Experiment Spherical Torus (VEST) is developed in order to reduce the oxygen impurity during the Ohmic plasma discharge. Carborane (C2B10H12) powder has been chosen for a safe operation. During the helium glow discharge, the sublimated carborane is diffused into the vacuum vessel, which results in thin boron-containing coatings on the vacuum vessel wall. After the boronization, qualitative changes in visible light intensities of oxygen and boron lines are observed indicating that the water concentration is reduced globally. The quality of discharges is greatly improved: a discharge with less radiative loss is achieved, which leads to a record shot of 0.1 MA plasma current and a nearly doubled pulse length.

Simple and accurate method of diamagnetic flux measurement in Versatile Experimental Spherical Torus (VEST).

Diamagnetic flux is measured accurately in the Versatile Experiment Spherical Torus by simply measuring the change in the toroidal field (TF) coil current without additional poloidal loops. Stray couplings mainly with the plasma current (since poloidal field coils are aligned well to the TF coils) are compensated for, resulting in the minimum measurable flux of ±0.2 mWb determined mainly by the finite sensitivity of the TF coil current sensor, implying that the accuracy of this simple method can be improved by measuring the TF coil current change with a higher sensitivity. The poloidal beta is derived from the measured diamagnetic flux with the consideration of the low aspect ratio geometry. The poloidal beta and the plasma stored energy derived from the measurement are in good agreement with those from the equilibrium reconstruction, and the energy confinement time derived from the measurement is consistent with the L mode scaling.

Design of an imaging Fabry-Pérot interferometer for the VEST edge plasma temperature measurement.

A new technique for gas temperature determination in fusion plasmas based on time- and space-resolved imaging interferometry is presented. The proposed imaging interferometer includes a multi-channel optical fiber bundle, a pair of flat mirrors, collimating lenses, and a fast camera for registration of interferometry fringes. The technique is applied for ion temperature measurements in the Versatile Experiment Spherical Torus (VEST) tokamak located at the Seoul National University. The multi-channel operation regime of the interferometer is studied by simulations. The first experimental two-channel results obtained using the Hα emission line from the VEST plasma edge are presented and analyzed. The mentioned apparatus is suggested to be especially useful for the multi-point time-resolved characterization of the non-repetitive processes in low-temperature regions of fusion plasmas.