Spherical Torus Plasmas Research Articles

Performance Assessment of Helicon Wave Heating and Current Drive in EXL-50 Spherical Torus Plasmas

Performance Assessment of Helicon Wave Heating and Current Drive in EXL-50 Spherical Torus Plasmas

Nonlocal Theory of Excitation of Electron Bernstein Waves by a Relativistic Electron Beam in Plasma with Loss-Cone Distribution of Electron

A nonlocal theory of excitation of electron Bernstein waves in a magnetized plasma slab by a relativistic electron beam with loss-cone distribution of electrons is presented. Kinetic theory is employed to obtain the susceptibility of plasma electrons whereas fluid theory is used to obtain the susceptibility of beam electrons. The mode structure and growth rate of the electron Bernstein wave are obtained. The growth rate in the case of Cerenkov and slow cyclotron interactions are proportional to $${{\omega }_{b0}}^{1/3}$$ and $${\omega }_{b0}$$ , respectively. For typical parameter, the normalized growth rate attains maximum value at $$b\approx 1.743$$ with different normalized beam velocities. These parameters control the growth rate due to nonlocality which rapidly decreases. The present theoretical scheme may be possible application in the field of heating of spherical torus plasmas.

Four-fluid axisymmetric plasma equilibrium model including relativistic electrons and computational method and results

A non-relativistic multi-fluid plasma axisymmetric equilibrium model [A. Ishida et al., “Three-fluid axisymmetric equilibrium model and application to spherical torus plasmas sustained by RF electron heating,” Plasma Fusion Res. 10, 1403084 (2015)] was developed recently to account for the presence of an energetic electron fluid in addition to thermal electron and ion fluids. The equilibrium formulation of a multi-fluid plasma with relativistic energetic electrons is developed and reported in this paper. Relativistic effects in a fluid model approximation can appear in two ways: due to a large macroscopic fluid velocity comparable to the speed of light and a large particle's microscopic random motion which becomes significant if the temperature becomes comparable to or larger than the electron rest mass energy. It is found that the axial component of relativistic generalized angular momentum can be used to describe relativistic axisymmetric equilibrium. The formulation is applied to a four-fluid plasma composed of a relativistic energetic electron fluid, a thermal electron fluid, and fluids of two thermal ion species (e.g., proton and boron ions). The four-fluid density expression which is consistent with the electrostatic potential is obtained and applied in the computation. An example equilibrium approximating a four-fluid plasma recently observed in a solenoid-free ECRH sustained spherical torus plasma [Y.-K. M. Peng et al., “Initial results of the EXL-50 experiment,” (to be published)] is calculated and presented. A second equilibrium that extends the energetic electron temperature of the first example to 679 keV is calculated revealing significant relativistic effects.

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

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.

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.

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.

Three-Fluid Axisymmetric Equilibrium Model and Application to Spherical Torus Plasmas Sustained by RF Electron Heating

Three-Fluid Axisymmetric Equilibrium Model and Application to Spherical Torus Plasmas Sustained by RF Electron Heating

Turbulent equipartition pinch of toroidal momentum in spherical torus

We present a new analytic expression for turbulent equipartition (TEP) pinch of toroidal angular momentum originating from magnetic field inhomogeneity of spherical torus (ST) plasmas. Starting from a conservative modern nonlinear gyrokinetic equation (Hahm et al 1988 Phys. Fluids 31 2670), we derive an expression for pinch to momentum diffusivity ratio without using a usual tokamak approximation of B ∝ 1/R which has been previously employed for TEP momentum pinch derivation in tokamaks (Hahm et al 2007 Phys. Plasmas 14 072302). Our new formula is evaluated for model equilibria of National Spherical Torus eXperiment (NSTX) (Ono et al 2001 Nucl. Fusion 41 1435) and Versatile Experiment Spherical Torus (VEST) (Chung et al 2013 Plasma Sci. Technol. 15 244) plasmas. Our result predicts stronger inward pinch for both cases, as compared to the prediction based on the tokamak formula.

Measured improvement of global magnetohydrodynamic mode stability at high-beta, and in reduced collisionality spherical torus plasmas

Global mode stability is studied in high-β National Spherical Torus Experiment (NSTX) plasmas to avoid disruptions. Dedicated experiments in NSTX using low frequency active magnetohydrodynamic spectroscopy of applied rotating n = 1 magnetic fields revealed key dependencies of stability on plasma parameters. Observations from previous NSTX resistive wall mode (RWM) active control experiments and the wider NSTX disruption database indicated that the highest βN plasmas were not the least stable. Significantly, here, stability was measured to increase at βN∕li higher than the point where disruptions were found. This favorable behavior is shown to correlate with kinetic stability rotational resonances, and an experimentally determined range of measured E × B frequency with improved stability is identified. Stable plasmas appear to benefit further from reduced collisionality, in agreement with expectation from kinetic RWM stabilization theory, but low collisionality plasmas are also susceptible to sudden instability when kinetic profiles change.

Experiment of Double-pulsing Helicity Injection on Helicity Injected Spherical Torus Plasmas

Coaxial Helicity Injection (CHI) is one of useful methods for start-up to spherical torus (ST) and spheromak plasmas. The new approach of CHI so called the Multi-pulsing CHI operation has been proposed for the purpose of achieving the quasi-steady-state plasma and demonstrated in the sustained spheromak physics experiment (SSPX) gun-spheromak device for the first time. We conducted the initial experiment with the first application of double-pulsing CHI for the ST configurations. The experimental results showed flux/current amplification and sustainment of the plasmas. We observed repeating the driven phase and the decay phase from change of profile of force-free parameter λ. By the double-pulsing CHI operation, the sustainment time had increased up to about 8 ms which was longer than that in the single discharge. We investigated dependency of plasma current, density and the current density profile on the external toroidal field strength.

Fast ion confinement and stability in a neutral beam injected reversed field pinch

The behavior of energetic ions is fundamentally important in the study of fusion plasmas. While well-studied in tokamak, spherical torus, and stellarator plasmas, relatively little is known in reversed field pinch plasmas about the dynamics of fast ions and the effects they cause as a large population. These studies are now underway in the Madison Symmetric Torus with an intense 25 keV, 1 MW hydrogen neutral beam injector (NBI). Measurements of the time-resolved fast ion distribution via a high energy neutral particle analyzer, as well as beam-target neutron flux (when NBI fuel is doped with 3–5% D2) both demonstrate that at low concentration the fast ion population is consistent with classical slowing of the fast ions, negligible cross-field transport, and charge exchange as the dominant ion loss mechanism. A significant population of fast ions develops; simulations predict a super-Alfvénic ion density of up to 25% of the electron density with both a significant velocity space gradient and a sharp radial density gradient. There are several effects on the background plasma including enhanced toroidal rotation, electron heating, and an altered current density profile. The abundant fast particles affect the plasma stability. Fast ions at the island of the core-most resonant tearing mode have a stabilizing effect, and up to 60% reduction in the magnetic fluctuation amplitude is observed during NBI. The sharp reduction in amplitude, however, has little effect on the underlying magnetic island structure. Simultaneously, beam driven instabilities are observed as repetitive ∼50 μs bursts which coincide with fast particle redistribution; data indicate a saturated core fast ion density well below purely classical predictions.

Observation of Direct Ion Heating in Double-Pulsing CHI in Helicity Injected Spherical Torus Plasmas

Anomalous ion heating has been observed for the first time during flux/current amplification by doublepulsing coaxial helicity injection (CHI) in the helicity injected spherical torus (HIST) device. Doppler ion temperature increases significantly near the separatrix on the inboard side during the second CHI pulse, whereas electron temperature remains constant. The experimental results indicate that selective ion heating may be associated with viscous damping of poloidal flows driven by the CHI pulse.

Reduced model simulations of the scrape-off-layer heat-flux width and comparison with experiment

Reduced model simulations of turbulence in the edge and scrape-off-layer (SOL) region of a spherical torus or tokamak plasma are employed to address the physics of the scrape-off-layer heat-flux width. The simulation model is an electrostatic two-dimensional fluid turbulence model, applied in the plane perpendicular to the magnetic field at the outboard midplane of the torus. The model contains curvature-driven-interchange modes, sheath losses, and both perpendicular turbulent diffusive and convective (blob) transport. These transport processes compete with classical parallel transport to set the SOL width. Midplane SOL profiles of density, temperature, and parallel heat flux are obtained from the simulation and compared with experimental results from the National Spherical Torus Experiment [S. M. Kaye et al., Phys. Plasmas 8, 1977 (2001)] to study the scaling of the heat-flux width with power and plasma current. It is concluded that midplane turbulence is the main contributor to the SOL heat-flux width for the low power H-mode discharges studied, while additional physics is required to fully explain the plasma current scaling of the SOL heat-flux width observed experimentally in higher power discharges. Intermittent separatrix-spanning convective cells are found to be the main mechanism that sets the near-SOL width in the simulations. The roles of sheared flows and blob trapping versus emission are discussed.

Two-fluid low-collisionality equilibrium model and application to spherical torus plasmas

A two-fluid equilibrium model with low-collisionality is developed including a new flow-singularity condition. This description is applied to the rapidly rotating, high-performance National Spherical Torus eXperiment (NSTX) [Peng et al., Plasma Phys. Controlled Fusion 47, B263 (2005)]. The model replicates the primary features of an example equilibrium, such as the profiles of electron and ion temperatures, density, and toroidal flow of an example equilibrium. This is the first full-two-fluid computation of two-dimensional equilibrium with rapid ion flow near the thermal speed. In consequence several important features emerge: (1) the ion toroidal current exceeds the toroidal plasma current as a result of electron rotational flow reversal; (2) the ion flow in the core region is roughly parallel to the magnetic field, i.e., the ion fluid in the core is nearly force free; (3) E+ui×B differs considerably from zero, so that the E×B drift fails to describe the ion and electron flows perpendicular to the magnetic field. Simpler models of equilibrium, such as the static equilibrium or the one-fluid flowing model, cannot replicate these properties. These results suggest that the rapidly rotating, high-performance NSTX plasma may represent a new parameter regime of fusion plasmas.

First observation of ELM pacing with vertical jogs in a spherical torus

Experiments in a number of conventional aspect ratio tokamaks have been successful in pacing edge localized modes (ELMs) by rapid vertical jogging of the plasma. This paper demonstrates the first pacing of ELMs in a spherical torus plasma. Applied 30 Hz vertical jogs synchronized the ELMs with the upward motion of the plasma. 45 Hz jogs also lead to an increase in the ELM frequency, though the synchronization of the ELMs and jogs was unclear. A reduction in the ELM energy was observed at the higher driven ELM frequencies.

High harmonic fast wave heating efficiency enhancement and current drive at longer wavelength on the National Spherical Torus Experiment

High harmonic fast wave heating and current drive (CD) are being developed on the National Spherical Torus Experiment [M. Ono et al., Nucl. Fusion 41, 1435 (2001)] for supporting startup and sustainment of the spherical torus plasma. Considerable enhancement of the core heating efficiency (η) from 44% to 65% has been obtained for CD phasing of the antenna (strap-to-strap ϕ=−90°, kϕ=−8m−1) by increasing the magnetic field from 4.5to5.5kG. This increase in efficiency is strongly correlated to moving the location of the onset density for perpendicular fast wave propagation (nonset∝B×k∥2∕ω) away from the antenna face and wall, and hence reducing the propagating surface wave fields. Radio frequency (RF) waves propagating close to the wall at lower B and k∥ can enhance power losses from both the parametric decay instability (PDI) and wave dissipation in sheaths and structures around the machine. The improved efficiency found here is attributed to a reduction in the latter, as PDI losses are little changed at the higher magnetic field. Under these conditions of higher coupling efficiency, initial measurements of localized CD effects have been made and compared with advanced RF code simulations.

Magnetic probe response function calibrations for plasma equilibrium reconstructions of CDX-U

A novel response function calibration technique has been developed to account for time-dependent nonaxisymmetric eddy currents near magnetic sensors in toroidal magnetic confinement devices. The response function technique provides a means to cross calibrate against all available external field coil systems to calculate the absolute sensitivity of each magnetic field sensor, even when induced eddy currents are present in the vacuum vessel wall. The response function information derived in the calibration process can be used in equilibrium reconstructions to separate plasma signals from signals due to externally produced eddy currents at magnetic field sensor locations, without invoking localized wall current distribution details. The response function technique was used for the first ever equilibrium reconstructions of spherical torus plasmas, when applied to the Current Drive Experiment-Upgrade (CDX-U) device. In conjunction with the equilibrium and stability code (ESC), equilibria were obtained for recent CDX-U experiments with lithium plasma-facing components. A description of the CDX-U magnetic sensor configuration and the response function calibration technique will be presented along with examples of resulting plasma equilibrium for CDX-U lithium wall operations.

Influence of vacuum toroidal field on two-fluid flowing equilibria of helicity-driven spherical torus plasmas

Two-fluid flowing equilibrium configurations of a helicity-driven spherical torus (HD-ST) in the realistic confinement region, including a flux conserver and a coaxial helicity source, are numerically determined by means of the combination of the finite difference and the boundary element methods. It is found from the numerical results that electron fluid near the central conductor is tied to a vacuum toroidal field and ion fluid is not. The magnetic configurations change from the high-q HD-ST (safety factor, q>1) with a paramagnetic toroidal field and low-β (volume average β value, ⟨β⟩≈2%) through the helicity-driven spheromak and reversed-field pinch to the ultra-low-q HD-ST (0<q<1) with a diamagnetic toroidal field and high-β (⟨β⟩≈18%) as the vacuum toroidal field at the inner edge regions decreases and reverses the sign. The two-fluid effects are more significant in this equilibrium transition when the ion diamagnetic drift has the same direction as the E×B one.

Computation of two-fluid flowing equilibrium of helicity-injected spherical torus plasma

A two-fluid flowing equilibrium of a helicity-injected spherical torus (HI-ST) plasma in the more realistic region, including a flux conserver and a coaxial helicity source, is numerically determined by using the combination of the finite difference and the boundary element methods. It is found from the numerical results that magnetic configurations change from the high-q HI-ST (safety factor q>1) with paramagnetic toroidal field and low-beta (volume average beta value, langbetarangap2%) through the helicity-injected spheromak and reversed-field pinch to the ultralow-q HI-ST (0 ap18%) as the external toroidal field at the inner edge regions decreases and reverses the sign. The effects of the ion flow on the MHD equilibrium configurations are discussed