Spherical Torus Research Articles

Prototype Coil Evaluation for NSTX-U Replacement Inner Poloidal Field Coils

The National Spherical Torus eXperiment Upgrade (NSTX-U) is an innovative magnetic fusion device constructed at the Princeton Plasma Physics Laboratory (PPPL). In 2016, because of the failure of the PF-1a upper divertor coil, which experienced a coolant blockage, the NSTX-U operation was suspended. A postmortem investigation indicated that an undetected gradual deterioration of coil inductance preceded the coolant blockage leading up to the operational suspension. The project team decided that all inner poiloidal field (PF) upper and lower coil pairs, denoted PF-1a, PF-1b, and PF-1c, shall be replaced with new coils of improved design and manufacture. The new prototype inner PF coils from four suppliers across the globe were evaluated at PPPL following a prototype technical evaluation procedure. Mechanical inspection and electrical testing were performed to qualify each supplier.This paper discusses the details of the mechanical and electrical tests and measurements performed on the complete coils. The test results were used to assess quality of turn-to-turn and turn-to-ground insulations of the prototype coils. Two prototype coils were power tested at PPPL for five pulses to reach its rated current and maximum temperature following the completion of low-power electrical testing. During pulses, the conductors experience a near adiabatic temperature rise and hoop stress. Between pulses, cold water enters the inlet, and a cooling wave propagates through the coils as slugs of cold water heat up to the conductor temperature and then pass through the coil to the outlet. Results show that full power testing did not change coil electrical characteristics. Each prototype coil was then sectioned into two halves to permit examination of the internal insulation, conductor spacing, and vacuum pressure impregnation quality. The high-voltage breakdown test of sectioned coils was performed to evaluate turn and ground insulation breakdown voltage. The estimate for the production coils is based largely on the experience learned from the prototype coil program. The first production coil will be delivered to PPPL for testing by January 2020.

Optimization of the snowflake divertor for power and particle exhaust on NSTX–U

Abstract In this paper, simple analytical modeling and numerical simulations performed with the multi-fluid edge transport code UEDGE are used to identify optimal snowflake divertor (SFD) configurations for heat flux mitigation and sufficient cryopumping performance on the National Spherical Torus eXperiment Upgrade (NSTX–U). A model is presented that describes the partitioning of sheath-limited SOL power and particle exhaust in the SFD as a result of diffusive transport to multiple activated strike points. The model is validated against UEDGE predictions and used to analyze a database of 70 SFD-minus equilibria. The optimal location for the entrance to a divertor cryopumping system on NSTX–U is computed for enabling sufficient pumping performance with acceptable power loading in a variety of SFD-minus configurations. UEDGE simulations of one promising equilibrium from the database indicate that a significant redistribution of power to the divertor legs occurs as a result of neutral particle removal near one of the SFD-minus strike points in the outboard scrape-off layer. It is concluded that pump placement at the optimal location is advantageous as the large number of compatible equilibria reduces the precision required of real-time SFD configuration control systems and enables acceptable divertor solutions even if UEDGE-predicted power redistribution slightly reduces the achievable pumping performance.

A scalable real-time framework for Thomson scattering analysis: Application to NSTX-U.

A detailed description of a prototype setup for real-time (RT) Thomson scattering (TS) analysis is presented and implemented in the multi-point Thomson scattering (MPTS) diagnostic system at the National Spherical Torus Experiment Upgrade (NSTX-U). The data acquisition hardware was upgraded with RT capable electronics (RT-analog digital converters and a RT server) that allow for fast digitization of the laser pulse signal of eight radial MPTS channels. In addition, a new TS spectrum analysis software for a rapid calculation of electron temperature (Te) and electron density (ne) was developed. Testing of the RT hardware and data analysis software was successfully completed and benchmarked against the standard, post-shot evaluation. Timing tests were performed showing that the end-to-end processing time was reproducibly below 17 ms for the duration of at least 5 s, meeting a 60 Hz deadline by the laser pulse repetition rate over the length of a NSTX-U discharge. The presented RT framework is designed to be scalable in system size, i.e., incorporation of additional radial channels by solely adding additional RT capable hardware. Furthermore, it is scalable in its operation duration and was continuously running for up to 30 min, making it an attractive solution for machines with long discharges such as advanced, non-inductive tokamaks or stellarators.

A two colour interferometer for PROTO-SPHERA experiment

PROTO-SPHERA (Spherical Plasma for Helicity Relaxation Assessment) is a new concept of torus that aims to produce a spherical torus with closed flux surfaces and a force-free Screw Pinch with open flux surfaces, fed by electrodes. In order to verify the stability of the configuration and the technical components of the PROTO-SPHERA initial arc, the initial Phase-1 experiment was built, and its plasma, in the year 2108, has reached both the total current (10 kA) and total duration (1 s) targets.A two colour interferometer is one of the diagnostic system proposed to provide PROTO-SPHERA with the measurement of its electron density, which is currently of the order of ∼1020 m−3. It will allow to obtain line density, necessary to characterize this uncommon plasma, as integrate measurement along the equatorial plane. The design and the interferometer characteristics are described in this paper.

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.

Computation of two-fluid flowing equilibrium of spherical torus plasma using multi-grid method

Computation of two-fluid flowing equilibrium of spherical torus plasma using multi-grid method

Resolving interactions between ion-cyclotron range of frequencies heating and the scrape-off layer plasma in EAST using divertor probes**This article is dedicated to the memory of Joel C Hosea.

Waves in the ion-cyclotron range of frequencies (ICRF) used for tokamak heating are known to interact strongly with the scrape-off layer (SOL) plasma, potentially limiting ICRF performance. We present the effect of ICRF heating on triple-probe signals in the Experimental Advanced Superconducting Tokamak (EAST) experiment, an advanced superconducting tokamak with two ICRF antennas and 12 MW of ICRF source power in support of high-power and long-pulse operation. Applying time-resolved magnetic mapping to the divertor probe arrays enables detailed sensing of the spatial structure of the SOL modifications. We observe a sharp transition in floating potential from negative to positive over a length scale of a centimeter at a major radius just inboard of the ICRF antenna. The largest changes in floating potential are observed not for probes connecting to the active ICRF antennas but rather to the main limiter or limiters for inactive lower hybrid launchers. Ion saturation current can be greatly reduced for probes mapping just in front of an antenna. Where relevant, comparison with results from the National Spherical Torus Experiment are made. This work will provide insight into sputtering, impurity production and antenna coupling as higher ICRF power levels are attempted on EAST.

TRANSP-based closed-loop simulations of current profile optimal regulation in NSTX-Upgrade

Active control of the toroidal current density profile is critical for the upgraded National Spherical Torus eXperiment device (NSTX-U) to maintain operation at the desired high-performance, MHD-stable, plasma regime. Initial efforts towards current density profile control have led to the development of a control-oriented, physics-based, plasma-response model, which combines the magnetic diffusion equation with empirical correlations for the kinetic profiles and the non-inductive current sources. The developed control-oriented model has been successfully tailored to the NSTX-U geometry and actuators. Moreover, a series of efforts have been made towards the design of model-based controllers, including a linear-quadratic-integral optimal control strategy that can regulate the current density profile around a prescribed target profile while rejecting disturbances. In this work, the tracking performance of the proposed current-profile optimal controller is tested in numerical simulations based on the physics-oriented code TRANSP. These high-fidelity closed-loop simulations, which are a critical step before experimental implementation and testing, are enabled by a flexible framework recently developed to perform feedback control design and simulation in TRANSP.

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.

Design and simulation of the snowflake divertor control for NSTX–U

This paper presents the development of a physics-based multiple-input-multiple-output algorithm for real-time feedback control of snowflake divertor (SFD) configurations on the National Spherical Torus eXperiment Upgrade (NSTX–U). A model of the SFD configuration response to applied voltages on the divertor control coils is first derived and then used, in conjunction with multivariable control synthesis techniques, to design an optimal state feedback controller for the configuration. To demonstrate the capabilities of the controller, a nonlinear simulator for axisymmetric shape control was developed for NSTX–U which simultaneously evolves the currents in poloidal field coils based upon a set of feedback-computed voltage commands, calculates the induced currents in passive conducting structures, and updates the plasma equilibrium by solving the free-boundary Grad–Shafranov problem. Closed-loop simulations demonstrate that the algorithm enables controlled operations in a variety of SFD configurations and provides capabilities for accurate tracking of time-dependent target trajectories for the divertor geometry. In particular, simulation results suggest that a time-varying controller which can properly account for the evolving SFD dynamical response is not only desirable but necessary for achieving acceptable control performance. The algorithm presented in this paper has been implemented in the NSTX–U Plasma Control System in preparation for future control and divertor physics experiments.

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.

Free Vibration Analysis of Functionally Graded Spherical Torus Structure with Uniform Variable Thickness along Axial Direction

Based on the Ritz method, this paper focused on the free vibration of functionally graded (FG) spherical torus with uniform variable thickness along axial direction under different boundary conditions. The first‐order shear deformation theory (FSDT) is employed to formulate the analytical model. The method involves partitioning of the spherical torus structure into proper shell segments in order to satisfy the computing requirement of high‐order vibration responses according to the domain decomposition method. The two adjacent segments are connected by using the penalty method, where penalty parameters are defined by the artificial springs; the continuity condition and different boundary conditions can be obtained by assigning the appropriate values of springs. The displacement functions’ components are double mixed series, in which Fourier series and unified Jacobi polynomials, respectively, represent displacement function along circumferential direction and axial direction. Then the Ritz method is used to obtain final solutions. The numerical results obtained by the proposed method show great agreement with previously published literatures and those from the finite element program ABAQUS. The effects of boundary conditions and geometric parameters on the vibration responses of the structure are also presented. The most novelty of this paper is to generalize the selection of admissible displacement functions by using Jacobi polynomial.

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.

Full-f gyrokinetic simulation of turbulence in a helical open-field-line plasma

Curvature-driven turbulence in a helical open-field-line plasma is investigated using electrostatic five-dimensional gyrokinetic continuum simulations in an all-bad-curvature helical-slab geometry. Parameters for a National Spherical Torus Experiment scrape-off-layer plasma are used in the model. The formation and convective radial transport of plasma blobs is observed, and it is shown that the radial particle-transport levels are several times higher than diffusive Bohm-transport estimates. By reducing the strength of the poloidal magnetic field, the profile of the heat flux to the divertor plate is observed to broaden.

Effect of boronization on plasma-facing graphite surfaces and its correlation with the plasma behavior in NSTX-U

Boronization is a Plasma Facing Component (PFC) conditioning technique widely used in tokamak machines. The National Spherical Torus Experiment-Upgrade (NSTX-U) applied this conditioning, using a plasma glow with a deuterated Trimethyl-boron (d-TMB) and He mixture. The use of boronization during the campaign improved the plasma performance, allowing longer plasma discharges and H-mode access. The chemical state of an ATJ graphite sample, used as a proxy for the NSTX-U PFCs, was monitored in-situ using the Materials Analysis Particle Probe (MAPP) diagnostic and X-ray Photoelectron Spectroscopy (XPS). The XPS data showed a progressive rise (from < 5% to 23%) in the oxygen concentration of the boronized ATJ sample as the D+ fluence increased. Filterscopes were used to measure the light emitted by oxygen impurities in the plasma near the surface of the PFC. An increase in the registered magnitude of the OII line, normalized to the Dγ intensity, was observed as the concentration of O on the ATJ surface increased. The plasma performance was found to be strongly correlated to oxygen impurity concentrations at the plasma edge and on the PFC surface, as measured by the discharge length and access to the H-mode regime. In this work, we present a quantitative analysis of the evolution of the chemistry of the ATJ surface, and the oxygen presence in the plasma-material interface, and report relevant plasma parameters observed during the same period of time.

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.

Integrated plasma facing component calorimetry for measurement of shot integrated deposited energy in the NSTX-U.

The upgrade to the National Spherical Torus eXperiment (NSTX-U) [J. Menard et al., Nucl. Fusion 52, 083015 (2012)] increases the injected neutral beam power up to 12 MW and the plasma current up to Ip = 2 MA for plasma durations up to 5 s. The graphite plasma facing components have been re-designed to handle greater heat and energy fluxes than were seen in NSTX using a castellated design. We present the experimental testing and validation of a castellated graphite target, similar to the prototype tile design, instrumented with thermocouples at various depths in the castellation. During testing, incident heat flux is provided by a programmed electron beam system and surface temperatures are measured via infrared thermography directly viewing the target surface. It was found that the thermocouple response scaled linearly with the measured surface temperature rise regardless of thermocouple depth in the castellation. A sensitivity of 14.3 °C/kJ of deposited energy was found when treating individual castellations as a semi-infinite solid.