Versatile Experiment Spherical Torus Research Articles

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.

Coupling study of fast wave near the lower hybrid frequency range in VEST

The fast wave branch in lower hybrid resonance frequency range, especially higher than 2ωlh, has been proposed for the central or off-axis electron heating and current drive in higher density plasmas than the slow wave scheme. With a higher cutoff density for launching, efficient coupling between the antenna and plasma would be a priority issue for feasibility. The fast wave coupling characteristics with the wave frequency, gap size, and electron density profile of the Versatile Experiment Spherical Torus (VEST) device are investigated using a commercial full wave FEM solver, COMSOL. Maximum coupling between combline antenna and plasma is expected to be at ∼500 MHz with n∥∼4.5. The coupled power ranges from 90% to 60% in the gap size between 0.5 cm and 1.5 cm. The relative power fraction of the fast wave is larger than 80% at these conditions. The propagation and coupling power of the fast wave is crucially dependent on the plasma density window by launching and confluence densities. Initial experimental result with low power shows that measured coupling efficiency starts to increase as electron density in front of antenna attains the level of cutoff density for the fast wave propagation. It varies from 30% to 90% with the edge density evolution, which is consistent with the coupling simulation using the measured edge density profile. Coupling simulation verified in this study will make it possible to predict and analyze the coupling characteristics of future lower hybrid fast wave experiments.

Design features and commissioning of pulse arc plasma source and power supply system for the VEST neutral beam injection system

A short-pulse, high-power filament arc ion source is developed for the neutral beam injection (NBI) system of the Versatile Experiment Spherical Torus (VEST) device. The 0.6-MW NBI system is designed to deliver positive hydrogen ion beams at 20 keV for a pulse length >10 ms. In addition, the power supply system for the VEST NBI system is designed to change the beam injection energy during the pulse under VEST's initial target plasma parameters to increase beam–plasma coupling efficiency. To satisfy these requirements, a large-area, high-power arc plasma source with a pulse power system based on a Marx generator utilizing high-energy capacitors and a solid-state switching system is developed and successfully commissioned. In this paper, design features of the plasma source and the power supply system for the VEST NBI system is described and the arc plasma discharge characteristics are presented.

Traveling-wave antenna for the current drive using the fast wave in the lower hybrid-frequency range in the Versatile Experiment Spherical Torus (VEST)

An antenna using the fast wave in the lower hybrid-resonance frequency range is proposed for the significant attainment of the heating and current-drive efficiencies. For the operational stability regarding the change of the plasma load, the proposed antenna is designed as the combline-structure traveling-wave antenna. It consists of two tapered coaxial cables, two exciting straps of the folded-monopole structure, 12 radiating straps, 11 field isolators, and the Faraday shield. Regarding the vacuum load, the simulated and measured results show the wide impedance bandwidth (45 MHz, VSWR < 2) of the proposed antenna in conjunction with the traveling-wave-antenna structure and the folded-monopole exciting strap. Regarding the plasma load, the simulated results show that the proposed antenna provides the impedance bandwidth (35 MHz, VSWR < 2), and the fast wave is coupled with an input power from 51.5 − 90%. The measured and simulated coupling efficiencies are highly compatible in the Versatile Experiment Spherical Torus (VEST). According to the frequency-range changes, the simulated-variable range of the parallel refractive index (n||) is from 3 − 4.5 in the operational frequency band of 475–510 MHz.

Development of equilibrium fitting code using finite element method in versatile experiment spherical torus

An equilibrium fitting code using finite element method is developed for various plasma studies, especially on non-inductive start-up and ECH heating & current drive experiments in Versatile Experiment Spherical Torus. The magnetic diagnostics are applied to provide the basic equilibrium fitting constraints. As severe eddy currents are induced by not only the PF coils but also the plasma current itself, the eddy current component from the magnetic signals is dealt with the vessel wall model in the eddy current solver and carefully compensated, which is essential for accurate equilibrium fitting. The fast camera image analysis method is adopted and the limiter current monitoring system is developed for validating the results of the plasma position and shape from the code. The equilibrium fitting code is applied to the limited and diverted plasma configuration in VEST and the time evolution of the plasma position and shape and even averaged magnetohydrodynamic parameters such as the edge safety factor and the internal inductance are extracted. Additionally, the equilibrium fitting code is used to analyze the non-inductive plasma start-up experiment using the direct current helicity injection in VEST. Especially, it is the first time to apply the finite element method to direct current helicity injection experiment and ECH heated plasmas which have significant amount of plasma current flowing along the open field line.

Calibration of Thomson scattering system on VEST

The Thomson scattering system has been recently installed on Versatile Experiment Spherical Torus (VEST) to measure the electron temperature and the density of the core plasmas. Since the calibration of the system is required for the accurate measurement of these parameters, a polychromator and the system efficiency are calibrated. The bias voltage of the detector is optimized and the relative responsivity of the polychromator is measured to analyse the spectral broadening. The tendency of decreasing responsivity because of the ambient temperature change is addressed together. The efficiencies of the alignments using HeNe laser and Nd:YAG laser are compared. After the alignment using Rayleigh scattering, it is improved ∼ 7 times while the peak signal of the stray light is decreased. To evaluate the efficiencies of the alignment using HeNe laser, it is compared with the efficiency of the fine alignment by Rayleigh scattering. After absolute calibration is done, the Thomson scattering signal is estimated theoretically. The Bayesian analysis is tried using the synthetic data, and the results show that the input temperature and the density are inside the contour of the 90% confident level. The calibrated Thomson scattering system will provide the meaningful information of the core plasma of the VEST.

Development of pulse power supply system based on Marx generator for the arc ion source of the VEST NBI

A short-pulse and high power supply system is developed for the pulse arc ion source of the neutral beam injector (NBI) of the Versatile Experiment Spherical Torus (VEST). The NBI system, with the maximum ion beam power of 20keV, 0.8MW and pulse length of 10ms, was designed for the auxiliary plasma heating and current drive source. However, the beam injection energy needs to be lower than 20keV under the initial target plasma parameters of VEST due to the beam-plasma coupling efficiency. For this reason, modulation of the beam energy in pulse duration is important to increase efficiency of the VEST NBI system. To satisfy these requirements, a pulse power system based on Marx generator is developed by utilizing high-energy capacitors and a solid-state switching system for the ion beam extraction and arc power supply. In addition, a battery-based power supply system is developed for the filament heating power supply which is highly cost-effective. In this paper, circuit structure and features of the power supply system of the ion source for the VEST NBI is described and the results of the discharges test are presented.

Development of diverted plasma discharge and plan for advanced divertor study in VEST

Operational scenario for diverted plasmas has been developed in Versatile Experiment Spherical Torus (VEST) not only for improving plasma performance but also for preparing upcoming experiments on advanced divertor concepts such as snowflake and x-point divertor. Diverted discharge scenario is designed using an in-house simulation code and experimentally realized by adjusting turn-on time of two partial solenoid (PF2) coils located at both ends of the center stack. With the diverted scenario, we achieve the improvement in plasma duration by ∼20% compared to the limited plasma operation, mainly due to the mitigation of impurity influxes from the center stack made of tungsten. Indeed, the impurity O-I line is observed to be decreased to 50% of the limited plasma operation. Fast camera images and equilibrium reconstruction based on magnetic diagnostics clearly shows the successful generation of diverted plasmas as well. This paper presents the first results on the diverted plasma operation in VEST and future plan for the experiments on the advanced divertor concept.

Design of combined system of helium charge exchange spectroscopy and hydrogen beam emission spectroscopy in VEST

A 468.6-nm HeII charge exchange spectroscopy (CES) system combined with 656.3-nm H beam emission spectroscopy (BES) system is designed to measure helium impurity density as well as toroidal rotation and ion temperature in Versatile Experiment Spherical Torus (VEST). A dichroic beam splitter is used to measure CES and BES spectra simultaneously with sharing the same observation and transmission components which include the line of sight, lenses, and fiber bundles. In the combined system, we employ the transmission gratings with high diffraction efficiency for CES and BES in order to obtain high throughput and spectral resolution. Spectra simulation results show that the signal to noise ratio (SNR) of this diagnostic system is no less than 10 with the injection of a diagnostic neutral hydrogen (beam energy and current are 40keV and 0.4A, respectively).

Efficient pre-ionization by direct X-B mode conversion in VEST

Pre-ionization experiments with pure toroidal field have been carried out in VEST (Versatile Experiment Spherical Torus) to investigate the feasibility of direct XB mode conversion from perpendicular LFS (Low Field Side) injection for efficient pre-ionization. Pre-ionization plasmas are studied by measuring the electron density and temperature profiles with respect to microwave power and toroidal field strength, and 2D full wave cold plasma simulation using the COMSOL Multiphysics is performed for the comparison. It is experimentally figured out that exceeding the threshold microwave power (&amp;gt;3 kW), the parametric decay and localized collisional heating is observed near the UHR (Upper Hybrid Resonance), and the efficient XB mode conversion can be achieved in both short density scale length (Ln) and magnetic scale length (LB) region positioned at outboard and inboard sides, respectively. From the 2D full wave simulations, the reflection and tunneling of X-wave near the R-cutoff layer according to the measured electron density profiles are analyzed with electric field polarization and power flow. Threshold electric field and wave power density for parametric decay are evaluated at least more than 4.8 × 104 V/m and 100 W/cm2, respectively. This study shows that efficient pre-ionization schemes using direct XB mode conversion can be realized by considering the key factors such as Ln, LB, and transmitted wave power at the UHR. Application to Ohmic start-up experiment is carried out to confirm the effect of the pre-ionization schemes on tokamak plasma start-up in VEST.

Publisher's Note: "Electron density profile measurements from hydrogen line intensity ratio method in Versatile Experiment Spherical Torus" [Rev. Sci. Instrum. 87, 11E540 (2016)

Publisher's Note: "Electron density profile measurements from hydrogen line intensity ratio method in Versatile Experiment Spherical Torus" [Rev. Sci. Instrum. 87, 11E540 (2016)

Efficient ECH-assisted plasma start-up using trapped particle configuration in the versatile experiment spherical torus

An efficient and robust ECH (electron cyclotron heating)-assisted plasma start-up scheme with a low loop voltage and low volt-second consumption utilizing the trapped particle configuration (TPC) has been developed in the versatile experiment spherical torus (VEST). The TPC is a mirror-like magnetic field configuration providing a vertical magnetic field in the same direction as the equilibrium field. It significantly enhances ECH pre-ionization with enhanced particle confinement due to its mirror effect, and intrinsically provides an equilibrium field with a stable decay index enabling prompt plasma current initiation. Consequently, the formation of TPC before the onset of the loop voltage allows the plasma to start up with a lower loop voltage and lower volt-second consumption as well as a wider operation range in terms of ECH pre-ionization power and H2 filling pressure. The TPC can improve the widely-used field null configuration significantly for more efficient start-up when ECH pre-ionization is used. This can then be utilized in superconducting tokamaks requiring a low loop voltage start-up, such as ITER, or in spherical tori with limited volt-seconds. The TPC can be particularly useful in superconducting tokamaks with a limited current slew-rate of superconducting PF coils, as it can save volt-second consumption before plasma current initiation by providing prompt initiation with an intrinsic stable equilibrium field.

Electron density profile measurements from hydrogen line intensity ratio method in Versatile Experiment Spherical Torus.

Electron density profiles of versatile experiment spherical torus plasmas are measured by using a hydrogen line intensity ratio method. A fast-frame visible camera with appropriate bandpass filters is used to detect images of Balmer line intensities. The unique optical system makes it possible to take images of Hα and Hβ radiation simultaneously, with only one camera. The frame rate is 1000 fps and the spatial resolution of the system is about 0.5 cm. One-dimensional local emissivity profiles have been obtained from the toroidal line of sight with viewing dumps. An initial result for the electron density profile is presented and is in reasonable agreement with values measured by a triple Langmuir probe.

Improvement of neutral beam injection heating efficiency with magnetic field well structures in a tokamak with a low magnetic field

Magnetic well structures are introduced as an effective means to reduce the prompt loss of fast ions, the so-called first orbit loss from neutral beam injection (NBI), which is beneficial to tokamaks with a low magnetic field strength such as small spherical torus devices. It is found by single-particle analysis that this additional field structure can modify the gradient of the magnetic field to reduce the shift of the guiding center trajectory of the fast ion. This result is verified by a numerical calculation of following the fast ion’s trajectory. We apply this concept to the Versatile Experiment Spherical Torus [], where NBI is under design for the purpose of achieving high-performance plasma, to evaluate the effect of the magnetic well structure on NBI efficiency. A 1D NBI analysis code and the NUBEAM code are employed for detailed NBI calculations. The simulation results show that the orbit loss can be reduced by 70%–80%, thereby improving the beam efficiency twofold compared with the reference case without the well structure. The well-shaped magnetic field structure in the low-field side can significantly decrease orbit loss by broadening the non-orbit loss region and widening the range of the velocity direction, thus improving the heating efficiency. It is found that this magnetic well can also improve orbit loss during the slowing down process.

Conceptual design of NBI beamline for VEST plasma heating

A 10ms-pulsed NBI (Neutral Beam Injection) system for VEST (Versatile Experiment Spherical Torus) plasma heating is designed to provide a beam power of more than 0.6MW with 20keVH° neutrals. The VEST NBI injector is composed of 2 sets of 20keV/25A magnetic cusp type bucket ion source, neutralizer ducts, residual ion dump, NB vessel with a cryopump, and rotating calorimeter. The position and size of these beamline components are roughly determined with geometric calculation of beam power transmission: the overall length of beamline is 1.8m, spacing between ground grid and neutralizer of 1.0m, and position of residual ion dump is 0.8m downstream from the neutralizer exit. Optimizing the gas feed between ion source and neutralizer can leads to a neutralization efficiency of ∼76%. Finally, the beam power of maximum 0.6 MW can be delivered to the VEST plasma with a beam loss of 10%.

Heating and current drive by fast wave in lower hybrid range of frequency on Versatile Experiment Spherical Torus

An efficient heating and current drive scheme in central or off-axis region is required to realize steady state operation of tokamak fusion reactor. And the fast wave in lower hybrid resonance range of frequency could be a candidate for such an efficient scheme in high density and high temperature plasmas. Its propagation and absorption characteristics including current drive and coupling efficiency are analyzed for Versatile Experiment Spherical Torus and it is shown that it is possible to drive current with considerable current drive efficiency in central region. The RF system for the fast wave experiment including klystron, transmission systems, inter-digital antenna, and RF diagnostics are given as well in this paper.

Development of wall conditioning and impurity monitoring systems in Versatile Experiment Spherical Torus (VEST)

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 50kHz 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 <10−5Torr 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 4h enables the longer plasma current duration up to ∼15ms within 3h from the end of treatment.

Design of a Thomson scattering diagnostic system for VEST

A Thomson scattering diagnostic system is designed for Versatile Experiment Spherical Torus (VEST) to measure the spatial profiles of the electron temperature. The system is carefully designed to collect a sufficient number of photoelectrons and to reduce the noise sources, since relatively low electron densities and temperatures are expected in VEST due to the limited power capacity at present. The target electron temperature and the density are 10–200eV and 5×1018m−3, respectively which are extrapolated from the data of triple Langmuir probes measuring the edge plasma parameters at R=0.75m by assuming a parabolic density profile. The collecting optics is designed to have a wide-view angle and low cost by using a commercial photographic lens of low f-number and high transmittance optical fiber bundle. The bandwidths of the interference filters in the polychromator are designed for reliable measurements within the target electron temperature range. As a photo detector which is coupled with the filters, an avalanche photodiode (APD) with a low dark current and an adequate quantum efficiency near the laser wavelength is selected for the high signal-to-noise ratio. The number of photons transferred to the polychromator and the number of photoelectrons in the APD are calculated. At the commissioning phase, an oscilloscope with a high sampling rate will be adopted to check the necessity of the noise reduction by multi-shot signal accumulation.

Plasma start-up design and first plasma experiment in VEST

First plasma start-up experiment in Versatile Experiment Spherical Torus (VEST) has been successfully carried out with conventional start-up scheme using a central solenoid. A start-up design code has been developed for the development of start-up scenarios in VEST, which can predict evolutions of vacuum field structure and plasma current with given operation parameters from the PF coil power supply circuit considering the eddy currents. Start-up scenarios are successfully developed with the code, generating field null region in the inboard side at the onset of loop voltage in order to maximize the connection length, and then providing the required poloidal magnetic field for stable equilibrium as the plasma current evolves. Hydrogen is used as a working gas and the toroidal field of 0.1T with 0.2s flat-top is applied on magnetic axis during the discharge. With the assist of ECH (Electron Cyclotron Heating) pre-ionization (6kW, 2.45GHz), a plasma current of ∼70kA has been generated with the pulse duration of ∼10ms. The elongation and edge safety factor are estimated to be 1.6 and 3.7 respectively.

Design and development of the helicity injection system in Versatile Experiment Spherical Torus

A helicity injection system for the Versatile Experiment Spherical Torus (VEST) has been successfully developed and commissioned. A high current electron gun utilizing hollow cathode and washer stacks has been designed and constructed with a single pulse power system that can provide voltages for both arc discharge and extraction sequentially. Tests for electron gun operation with the single pulse power system have been conducted under various toroidal and poloidal field strengths. The estimated plasma impedance, depending on the injection magnetic field structure, can be utilized for the optimal gun operation by impedance matching between the pulse power system and plasma. With the charging voltage of 1.2kV, injection current of 0.95kA has been obtained with the injection voltage of 410V for about 5ms. Initial helicity injection experiments have been conducted under various toroidal and poloidal field strengths and a toroidal plasma current of up to 3.8kA is observed with the current multiplication larger than the geometric stacking ratio, confirming the possibility of relaxation into tokamak-like plasma with closed flux formation.