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• Upgrade of Thomson scattering system on VEST has been performed. • The laser input power is increased up to 2.8 times by injection system improvement. • Optical fiber bundle replaces a single core fiber to increase the signal level 2.1 times. • Oscilloscope envelope mode is utilized to measure periodic large data for each burst. • Time resolution is improved to 1 ms with new burst laser and fast DAQ systems. 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 10–200 eV and 5 × 10 18 m −3 , respectively which are extrapolated from the data of triple Langmuir probes measuring the edge plasma parameters at R = 0.75 m 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. [ABSTRACT FROM AUTHOR]

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 0.1 T with 0.2 s flat-top is applied on magnetic axis during the discharge. With the assist of ECH (Electron Cyclotron Heating) pre-ionization (6 kW, 2.45 GHz), a plasma current of ∼70 kA has been generated with the pulse duration of ∼10 ms. The elongation and edge safety factor are estimated to be 1.6 and 3.7 respectively. [ABSTRACT FROM AUTHOR]

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 1.2 kV, injection current of 0.95 kA has been obtained with the injection voltage of 410 V for about 5 ms. Initial helicity injection experiments have been conducted under various toroidal and poloidal field strengths and a toroidal plasma current of up to 3.8 kA 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. [ABSTRACT FROM AUTHOR]

Abstract: A magnetic diagnostic system was designed, fabricated, and calibrated for measuring main parameters of the initial-phase-discharges in Versatile Experiment Spherical Torus (VEST) recently constructed at Seoul National University. Inner and outer Rogowski coils, flux loops, magnetic field probes, and an internal magnetic probe array were designed to have a desired frequency range with a reliable sensitivity. The sensors were carefully shielded to minimize damages from plasmas and microwaves with thermal and electromagnetic protections. The calibration with a known magnetic field generated by a Helmholtz coil or in-situ calibration was performed for the fabricated sensors. The prepared magnetic diagnostics have been successfully used to measure various parameters such as plasma current, loop voltages, and magnetic field structure inside the vacuum vessel for initial plasma experiments in VEST. [Copyright &y& Elsevier]

Abstract: A control and data acquisition system for VEST (Versatile Experiment Spherical Torus) at Seoul National University (SNU) has been developed to enable remote operation from a central control room. The control and data acquisition system consists of three subsystems; a main control and data acquisition system that triggers each device at the preprogrammed timing and collects various diagnostic signals during discharges, a monitoring system that watches and logs the device status continuously, and a data storage and distribution system that stores collected data and provides data access layer via Ethernet. The system is designed to be cost-effective, extensible and easy to develop by using well-established standard technologies and solutions. Combining broad accessibility with modern information technology, alarm signal can be sent immediately to the registered cell phones when the abnormal status of devices is found, and the web data distribution system enables data access from almost everywhere using smart phones or tablet computers. Since December 2011, VEST is operational and the control and data acquisition system has been successfully used for remote operation of VEST. [Copyright &y& Elsevier]

Abstract: Initial plasma start-up experiments based on ohmic discharge using partial solenoid coils located at both vertical ends of a center stack have been carried out in Versatile Experiment Spherical Torus (VEST) at Seoul National University. Ohmic discharges with the help of microwave pre-ionization have been performed according to the pre-programed start-up scenario which was experimentally verified by a series of vacuum field measurements using an internal magnetic probe array. A plasma current of around 0.4kA has been achieved by ohmic discharge using partial solenoid coils, under the toroidal magnetic field of 0.1T. The vacuum field calculation and fast camera image have revealed that the small plasma current even with significant amount of loop voltage up to 9.7V is attributed to the imbalance of poloidal field for equilibrium. Modification of the start-up scenario and upgrade of power supplies are proposed to be carried out in order to achieve higher plasma current in the future experiments. [Copyright &y& Elsevier]

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 × 10 19 m − 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. [ABSTRACT FROM AUTHOR]