Spherical Tokamak QUEST Research Articles

Electron cyclotron current start-up using a retarding electric field in the QUEST spherical tokamak

Abstract The plasma current start-up experiment is conducted through electron cyclotron (EC) heating in the QUEST spherical tokamak. During the EC heating, the application of a toroidal electric field in the opposite direction to the plasma current effectively inhibits the growth of energetic electrons. Observations show rapid increases in plasma current and hard x-ray count immediately following the cancellation of the retarding electric field. When a compact tokamak configuration maintains equilibrium on the high field side, along with the retarding field, it leads to effective bulk electron heating. This heating achieved an electron temperature of T e ≈ 1 keV at electron density n e > 1.0 × 1018 m−3. Ray tracing of the EC wave verifies that more power absorption into plasma through a single-pass occurs around the second resonance layer with higher values of electron density and temperature.

Hydrogen removal by electron cyclotron wall conditioning with neon gas and its impact of tokamak plasma start-up on the QUEST spherical tokamak

Electron cyclotron wall conditioning with neon gas (Ne-ECWC) has been performed on the normal conducting spherial tokamak QUEST with metal walls under a trapped particle configuration with O-mode EC waves including X-mode polarization with a frequency of 8.2 GHz and an injection power of 16 kW. The Ne-ECWC removes hydrogen from the wall with small neon retention. The Ne-ECWC decreases hydrogen recycling at the following tokamak discharges, contributing to an improvement of the following tokamak plasma start-up: the plasma current increases and the start-up timing of the plasma current shifts forward. However, defects such as voids and bubbles are formed on tungsten surface exposed to the Ne-ECWC plasma.

Demonstration of transient CHI startup using a floating biased electrode configuration

Results from the successful solenoid-free plasma startup using the method of transient coaxial helicity injection (transient CHI) in the QUEST spherical tokamak (ST) are reported. Unlike previous applications of CHI on HIT-II and on NSTX which required two toroidal insulating breaks to the vacuum vessel, QUEST uses a first of its kind, floating single biased electrode configuration, which does not use such a vacuum break. Instead, the CHI electrode is simply insulated from the outer lower divertor plate support structure. This configuration is much more suitable for implementation in a fusion reactor than the previous configurations. Transient CHI generated toroidal currents of 135 kA were obtained. The toroidal current during the formation of a closed flux configuration was over 50 kA. These results bode well for the application of transient CHI in a new generation of compact high-field STs and tokamaks in which the space for the central solenoid is very restricted.

Effects of toroidally-distributed-divertor biasing on scrape-off-layer (SOL) current drive, divertor particle flux and fast electron confinement in the QUEST spherical tokamak

A novel divertor biasing by four biasing plates distributed toroidally (TDDB) on the upper divertor target plate is applied to low density tokamak plasmas started-up by 28 GHz 2nd harmonic electron cyclotron current drive (ECCD) in a quasi-double null configuration of the QUEST spherical tokamak (ST). In the ST plasmas of line averaged electron density < ne > ∼0.7–1 × 1018 m−3, about 20%–40% of the current Ibias driven by ∼85 V sawtooth bias voltage reaches the lower divertor plate along the biased scrape-off-layer (SOL) flux tube as the SOL current ISOL . The fact ISOL is noticeably lower than Ibias indicates an appreciable leakage of parallel current from the biased SOL flux tube. The leakage currents in the toroidal and radial directions are confirmed by detection of them using the unbiased plates in the TDDB experiments. From the ion saturation current density profile obtained by a divertor Langmuir probe array, the fall-off lengths of divertor particle flux are estimated together with the strike line position. Total particle flux to the upper divertor, evaluated by the integrated ion saturation current density profile is reduced by up to 45% during positive biasing of the TDDB, depending on the position of the strike line to the biased plate. In addition, the TDDB also induces a noticeable loss of fast electrons produced by ECCD, leading to an ∼2% reduction in the maximum toroidal current of the ST plasma compared to a shot without the TDDB. Reduction of the divertor particle flux and enhancement of the fast electron losses are thought to be dominantly caused by E ×B drift induced by the TDDB. In the present experimental conditions, the effects of magnetic perturbations produced by the SOL currents on the fast electron losses can be neglected because of a too small SOL current.

Sudden Change Events of Plasma Current during Electron-Cyclotron Current Start-Up on the QUEST Spherical Tokamak

Sudden Change Events of Plasma Current during Electron-Cyclotron Current Start-Up on the QUEST Spherical Tokamak

Circuit design for doubling the toroidal magnetic field on the QUEST spherical tokamak

Various scenarios of electron cyclotron heating can be explored in a spherical tokamak with a high toroidal field. On the QUEST spherical tokamak, doubling the toroidal field results in the emergence of the fundamental resonance layer for 28 GHz electron cyclotron waves. The second-harmonic resonance layer shifts toward the plasma central area. To increase the coil current from 50 to 100 kA, a current source is designed with 125 modules in parallel. The module power supply comprises a bank of lithium-ion capacitors and a DC–DC buck converter. In the bench test of a single module, the current was found to increase linearly and maintained the flat-top as programmed.

A versatile power supply system for the central solenoid of the QUEST spherical tokamak

A versatile power supply system for the central solenoid of the QUEST spherical tokamak

Development of Thomson Scattering Measurement System for Long Duration Discharges on the QUEST Spherical Tokamak

The Thomson scattering control system has been modified to measure the time evolution of the electron density and temperature profiles during long duration discharges in the QUEST spherical tokamak. The system consists of a signal generator and a control circuit. The former accepts a QUEST main trigger and provides multiple triggers, each of which starts a short-term (e.g. 15 s) measurement. The latter provides triggers to synchronize the oscilloscope and laser oscillation during a short-term measurement. The system was used in 1000 s long duration discharges in QUEST, and the temporal evolutions of density and temperature profiles were obtained successfully. It was found the profiles are stationary after about 300 s.

Initial testing of ohmic heating through double flux swing during electron cyclotron start-up in the QUEST spherical tokamak

A power-supply system was developed for Ohmic heating (OH) to double × 1018 the amount of change magnetic flux in the primary central solenoid (CS) on the QUEST spherical tokamak. Two power supplies are connected with stacks of insulated-gate bipolar transistors, and sequentially operated to generate positive and negative CS currents. This bipolar power-supply system is controlled via a field-programmable gate array, which guarantees the safety of the entire system operation. The new OH system, assisted by electron cyclotron heating, enables the stable generation of plasma currents exceeding 100 kA. Moreover, the achieved electron density over the wide range in the major radial direction exceeds the cut-off density for one of the high-power microwave sources in QUEST. This strategy yields target plasmas for future experiments with the electron Bernstein wave.

Conceptual design of a heavy ion beam probe for the QUEST spherical tokamak.

A heavy ion beam probe (HIBP) has been designed for the QUEST spherical tokamak to measure plasma turbulence and the profiles of electric potential profiles. Using a cesium ion beam with an energy of several 10keV, the observable region covers most of the upper half of the plasma. Although the probe beam is deflected by the poloidal magnetic field produced by plasma current and poloidal coil currents, it can be detected under plasma current up to 150kA by modifying the trajectories with two electrostatic sweepers. According to the numerical estimation of the intensity of the detected beam, sufficient signal intensity for measuring plasma turbulence can be obtained over almost the measurable area when the electron density is up to 1 × 1019 m-3, which is larger than the cut-off density of electron cyclotron heating in QUEST. The performance of the designed HIBP is sufficient to explore the mechanisms of heat and particle transport in magnetically confined plasmas, including the influence of plasma wall interactions, which is a goal of the QUEST project.

Effects of Toroidally Distributed Divertor Biasing on Scrape-Off-Layer Plasma in the QUEST Spherical Tokamak

Effects of Toroidally Distributed Divertor Biasing on Scrape-Off-Layer Plasma in the QUEST Spherical Tokamak

Designing an upgrade of ohmic heating system for the QUEST spherical tokamak

An upgraded Ohmic heating (OH) system with a bipolar circuit is designed to achieve plasma current over 100 kA ideally without the effect of eddy current in the QUEST spherical tokamak. IGBT (Insulated Gate Bipolar Transistor) stacks work as switches to change the polarity of OH primary current up to ±8 kA. A main target of the new OH is to generate over-dense plasmas for the 8.56 GHz electron cyclotron (EC) wave. Heating with electron Bernstein waves can be conducted in such over-dense plasmas. The effect of eddy current flowing in the vacuum vessel, which is generated by the current swing through the central solenoid, on null point is investigated by analysis of magnetic field and flux surfaces. In the initial phase, eddy current flowing through the vacuum chamber shifts the null point to the high magnetic field side where multiple EC resonance layers are located.

Electron heating of over-dense plasma with dual-frequency electron cyclotron waves in fully non-inductive plasma ramp-up on the QUEST spherical tokamak

A 28 GHz system with a high-power gyrotron tube has been used for the QUEST spherical tokamak to form an over-dense plasma for electron Bernstein wave (EBW) heating and current drive with an 8.2 GHz-wave. Non-inductive high-density plasma ramp-up experiments with dual-frequency (dual-f ) electron cyclotron (EC) (8.2 GHz and 28 GHz) waves were conducted. A spontaneous density jump (SDJ) to an over-dense state was first observed as a bifurcation phenomenon in the dual-f wave experiment. The over-dense plasma on the 8.2 GHz wave was non-inductively ramped up to 25 kA, and was maintained for 0.4 s under stable plasma equilibrium after two such jumps in one shot. Heating to mildly energetic electrons and bulk electrons was observed even in the over-dense region. The electrostatic EBW heating effect on the mildly energetic electrons in the over-dense region is assessed following a dispersion analysis of the 8.2 GHz wave. The bulk electron heating effect observed is explained as heat exchange from mildly energetic electrons heated by the electrostatic EBW. Remarkably, a high hard-x-ray-radiation temperature of ∼500 keV was also observed in tangential viewing for current-carrying electrons in the over-dense core region. Synergetic heating from the overlap of different 28 GHz EC harmonic resonances as well as higher harmonic heating is discussed for maintaining the highly energetic electrons in the over-dense core region. In addition, the SDJ process and mechanism are considered based on the discussion of the electron heating effects with the 8.2 GHz wave.

Prototype of a Quasi-Optical Launcher System of a 4 mm Round-Trip Interferometer for the QUEST Spherical Tokamak Experiments

Prototype of a Quasi-Optical Launcher System of a 4 mm Round-Trip Interferometer for the QUEST Spherical Tokamak Experiments

High voltage electrical system of 8.56 GHz CW klystron for electron cyclotron heating on QUEST spherical tokamak

Abstract A high voltage DC power supply for the cathode of the 8.56 GHz CW klystron has been set up for electron cyclotron heating (ECH) in steady state tokamak operation on QUEST spherical tokamak. The power supply is equipped with an IGBT array and a reactor for fast shutoff of the voltage in 10 μs, where the influx of electric energy at the short circuit is limited to 5 J. AC switches also have been installed in the three-phase power lines. High voltage relays are useful to save electric energy consumption. Fast three-phase AC switching by IGBT-stack is applicable to reduce the electric load of the components of the klystron power supply.

Quasi-optical polarizer system for ECHCD experiments in the QUEST

Abstract A new transmission line from 28 GHz gyrotron to QUEST spherical tokamak (ST) has been developed for the highly efficient electron cyclotron heating and current drive (ECHCD) experiments. The ECHCD effect depends on the incident mode or polarization states of the injected beam. Although two corrugated-polarizer miter bends were developed and assembled to transmission line in the QUEST, arcing events were frequently detected at the polarizer miter bend when the high-power millimeter-wave were transmitted. A new quasi-optical (QO) concept for the polarizer system composed of two corrugated plate and a QO mirror was proposed to avoid the arcing at polarizer. The QO mirror was designed for coupling of input and output HE11 modes at the polarizer, its coupling property is discussed with Kirchhoff integral. Polarization-setting performance of the developed QO polarizer was examined in a low-power test system, and is discussed with grating-polarizer calculation.

28-GHz ECHCD system with beam focusing launcher on the QUEST spherical tokamak

Abstract New polarizer and launcher systems on a 28-GHz electron-cyclotron (EC) heating and current drive (HCD) system have been developed for non-inductive second-harmonic EC-plasma-current ramp-up in the QUEST spherical tokamak. A launcher system with two quasi-optical mirrors providing beam steering capability was designed to focus the incident beam to a small-sized waist of ∼0.05 m at the second-harmonic EC resonance layer. A relatively large focusing mirror was designed based on a Kirchhoff integral code developed to derive wave solutions. The focusing property of the launched beam was first confirmed with a 3-dimensional (3D) electromagnetic-wave simulator. Focusing characteristics were also checked at low-power test facilities, together with the steering capability. The performance of this launcher system was demonstrated to work as designed, and assembled in the QUEST device. The system was applied to the non-inductive second-harmonic EC plasma ramp-up experiments with no optimization required regarding the incident polarization. The results obtained for the non-inductive plasma ramp-up are also presented.

Effect of magnetic shear on edge turbulence in SOL-like open field line configuration in QUEST

Intensity fluctuations are investigated using the fast camera imaging technique in the slab annular plasma as a function of magnetic shear and connection length in the spherical tokamak QUEST. Note that here QUEST is operated as a simple magnetized torus with a tight aspect ratio. Slab annular plasmas feature open magnetic field lines and can mimic the tokamak edge-scrape off layer (SOL)-like plasma attributes reasonably well. Three magnetic shear regimes are realized using three poloidal magnetic field (PF) coil pairs. A whole range of connection lengths (∼∞ ≥ Lc ≥ 5.5 m) is scanned by varying the PF strength for a given toroidal field for each magnetic shear regime. For the first time a systematic study of the effect of magnetic shear and field line pitch together on edge-SOL-like plasma fluctuations is being reported. Slab plasmas with intermediate magnetic shear are observed to be more susceptible to generate distinct blobs when Lc is reduced by increasing the PF strength. A distinct coherent mode appears only at the lowest magnetic shear slab featuring a deep potential well. Such mode is not apparent at other magnetic shear cases even at the same Lc. Finally, with a combination of PF coil pairs, both the features of intermediate and low magnetic shear slabs are shown to be realizable simultaneously. Significantly stronger blobs are observed with such combination of PF mirror ratios in the presence of a coherent mode. This study may provide better insight into the effect of magnetic configuration in the tokamak edge and SOL turbulence and can help in searching for better tools to control cross-field convective intermittent transport in tokamaks.

Investigation of hydrogen recycling in long-duration discharges and its modification with a hot wall in the spherical tokamak QUEST

Fully non-inductive plasma maintenance was achieved by a microwave of 8.2 GHz and 40 kW for more than 1 h 55 min with a well-controlled plasma-facing wall (PFW) temperature of 393 K, using a hot wall in the middle-sized spherical tokamak QUEST, until the discharge was finally terminated by the uncontrollability of the density. The PFW was composed of atmospheric plasma-sprayed tungsten and stainless steel. The hot wall plays an essential role in reducing the amount of wall-stored hydrogen and facilitates hydrogen recycling. The behaviour of fuel hydrogen in the PFW was investigated by monitoring the injection and evacuation of hydrogen into and from the plasma-producing vessel. A fuel particle balance equation based on the presence of a hydrogen transport barrier between the deposited layer and the substrate was applied to the long-duration discharges. It was found that the model could readily predict the observed behaviour in which a higher wall temperature likely gives rise to faster wall saturation.

Fully non-inductive second harmonic electron cyclotron plasma ramp-up in the QUEST spherical tokamak

Fully non-inductive second (2nd) harmonic electron cyclotron (EC) plasma current ramp-up was demonstrated with a newlly developed 28 GHz system in the QUEST spherical tokamak. A high plasma current of 54 kA was non-inductively ramped up and sustained stably for 0.9 s with a 270 kW 28 GHz wave. A higher plasma current of 66 kA was also non-inductively achieved with a slow ramp-up of the vertical field. We have achieved a significantly higher plasma current than those achieved previously with the 2nd harmonic EC waves. This fully non-inductive 2nd harmonic EC plasma ramp-up method might be useful for future burning plasma devices and fusion reactors, in particular for operations at half magnetic field with the same EC heating equipment.