Spherical Tokamak Plasmas Research Articles

Effects of reconnection downstream conditions on electron parallel acceleration during the merging start-up of a spherical tokamak

The axial merging method is one of the candidates to provide a center-solenoid-free start-up of high-beta spherical tokamak (ST) plasma. Two initially formed STs merge through magnetic reconnection in the presence of the guide (toroidal) magnetic field, which is perpendicular to the reconnection (poloidal) magnetic field. During ST merging start-up, electrons are effectively accelerated near the reconnection point where the reconnection electric field is almost parallel to the magnetic field. In order to evaluate the effectivity of this acceleration process on electron heating, the temporal and spatial distributions of generated energetic electrons are observed by a soft x-ray fast imaging system equipped on the UTST device. The energetic electrons were generated not only in the vicinity of the reconnection point but transiently in the inboard-side downstream region until the static electric field by charge separation grew to cancel the reconnection electric field component parallel to the magnetic field line. Adequate control of the downstream condition could enhance the generation of energetic electrons and provide a more effective conversion from the released magnetic energy to electron energy.

Magnetized Target Fusion with a Spherical Tokamak

With good confinement and relatively high beta (up to 40%), the spherical tokamak could be a suitable magnetized target. To illustrate this potential, the theoretical fusion yield from compressing a spherical tokamak adiabatically to fusion conditions is calculated. While it is found that the large amount of energy stored in the compressed toroidal field constrains the potential energy gain, a system can be designed to produce significant useful output by directly recovering a large fraction of the compression energy. With 80% energy recovery, an example system using 140 MJ of pressurized gas to compress the spherical tokamak plasma within a 2 m radius liquid metal flux conserver can produce a fusion yield of 140 MJ and a net output of 40 MJ per pulse. At ~ 1 Hz this would produce 40 MWelectric.

Kinetic simulations of X-B and O-X-B mode conversion and its deterioration at high input power

Spherical tokamak plasmas are typically overdense and thus inaccessible to externally-injected microwaves in the electron cyclotron range. The electrostatic electron Bernstein wave (EBW), however, provides a method to access the plasma core for heating and diagnostic purposes. Understanding the details of the coupling process to electromagnetic waves is thus important both for the interpretation of microwave diagnostic data and for assessing the feasibility of EBW heating and current drive. While the coupling is reasonably well–understood in the linear regime, nonlinear physics arising from high input power has not been previously quantified. To tackle this problem, we have performed one- and two-dimensional fully kinetic particle-in-cell simulations of the two possible coupling mechanisms, namely X-B and O-X-B mode conversion. We find that the ion dynamics has a profound effect on the field structure in the nonlinear regime, as high amplitude short-scale oscillations of the longitudinal electric field are excited in the region below the high-density cut-off prior to the arrival of the EBW. We identify this effect as the instability of the X wave with respect to resonant scattering into an EBW and a lower-hybrid wave. We calculate the instability rate analytically and find this basic theory to be in reasonable agreement with our simulation results.

Overview of spherical tokamak research in Japan

Nationally coordinated research on spherical tokamak is being conducted in Japan. Recent achievements include: (i) plasma current start-up and ramp-up without the use of the central solenoid by RF waves (in electron cyclotron and lower hybrid frequency ranges), (ii) plasma current start-up by AC Ohmic operation and by coaxial helicity injection, (iii) development of an advanced fuelling technique by compact toroid injection, (iv) ultra-long-pulse operation and particle control using a high temperature metal wall, (v) access to the ultra-high-β regime by high-power reconnection heating, and (vi) improvement of spherical tokamak plasma stability by externally applied helical field.

Nonlinear fishbone dynamics in spherical tokamaks

Linear and nonlinear kinetic-MHD hybrid simulations have been carried out to investigate linear stability and nonlinear dynamics of beam-driven fishbone instability in spherical tokamak plasmas. Realistic NSTX parameters with finite toroidal rotation were used. The results show that the fishbone is driven by both trapped and passing particles. The instability drive of passing particles is comparable to that of trapped particles in the linear regime. The effects of rotation are destabilizing and a new region of instability appears at higher qmin (>1.5) values, qmin being the minimum of safety factor profile. In the nonlinear regime, the mode saturates due to flattening of beam ion distribution, and this persists after initial saturation while mode frequency chirps down in such a way that the resonant trapped particles move out radially and keep in resonance with the mode. Correspondingly, the flattening region of beam ion distribution expands radially outward. A substantial fraction of initially non-resonant trapped particles become resonant around the time of mode saturation and keep in resonance with the mode as frequency chirps down. On the other hand, the fraction of resonant passing particles is significantly smaller than that of trapped particles. Our analysis shows that trapped particles provide the main drive to the mode in the nonlinear regime.

Plasma diagnostics in spherical tokamaks with silicon charged-particle detectors.

Detection of charged fusion products, such as protons and tritons resulting from D(d, p) t reactions, can be used to determine the position and time dependent fusion reaction rate profile in spherical tokamak plasmas with neutral beam heating. We have developed a prototype instrument consisting of 6 ion-implanted-silicon surface barrier detectors combined with collimators in such a way that each detector can accept 3 MeV protons and 1 MeV tritons and thus provides a curved view across the plasma cross section. The combination of the results from all six detectors will provide information on the spatial distribution of the fusion reaction rate. The expected time resolution of about 1 ms makes it possible to study changes in the reaction rate due to slow variations in the neutral beam density profile, as well as rapid changes resulting from MHD instabilities. Details of the new instrument, its data acquisition system, simulation results, and electrical noise testing results are discussed in this paper. First experimental data are expected to be taken during the current experimental campaign at NSTX-U.

Distinct turbulence sources and confinement features in the spherical tokamak plasma regime

New turbulence contributions to plasma transport and confinement in the spherical tokamak (ST) regime are identified through nonlinear gyrokinetic simulations. The drift wave Kelvin–Helmholtz (KH) mode characterized by intrinsic mode asymmetry is shown to drive significant ion thermal transport in strongly rotating national spherical torus experiment (NSTX) L-modes. The long wavelength, quasi-coherent dissipative trapped electron mode (TEM) is destabilized in NSTX H-modes despite the presence of strong shear, providing a robust turbulence source dominant over collisionless TEM. Dissipative trapped electron mode (DTEM)-driven transport in the NSTX parametric regime is shown to increase with electron collision frequency, offering one possible source for the confinement scaling observed in experiments. There exists a turbulence-free regime in the collision-induced collisionless trapped electron mode to DTEM transition for ST plasmas. This predicts a natural access to a minimum transport state in the low collisionality regime that future advanced STs may cover.

Localized electron heating by strong guide-field magnetic reconnection

Localized electron heating of magnetic reconnection was studied under strong guide-field using two merging spherical tokamak plasmas in the University of Tokyo Spherical Tokamak experiment. Our new slide-type two-dimensional Thomson scattering system is documented for the first time the electron heating localized around the X-point. Shape of the high electron temperature area does not agree with that of energy dissipation term Et·jt. If we include a guide-field effect term Bt/(Bp+αBt) for Et·jt, the energy dissipation area becomes localized around the X-point, suggesting that the electrons are accelerated by the reconnection electric field parallel to the magnetic field and thermalized around the X-point.

Centre-solenoid-free merging start-up of spherical tokamak plasmas in UTST

A centre-solenoid-free merging start-up scheme for spherical tokamak plasmas was developed in a University of Tokyo spherical tokamak (UTST) experiment by using outer poloidal field coils. Torus breakdown was initiated at null points and two spherical tokamak plasmas with a total current up to 80 kA were generated inductively. Their merging process provided substantial ion and electron heating by magnetic reconnection. The obtained dependence of heating on plasma current suggests that high-temperature and high-current plasma suitable for neutral beam injection is attainable under the realistic conditions in the merging start-up method.

中性粒子ビーム入射に向けた合体生成球状トカマクプラズマの電子密度・温度の評価

The axial profile evolutions of electron density and temperature in spherical tokamak plasma formed by merging method were measured by Thomson scattering measurement system in UTST device. The peak electron density at X-point decayed just after merging, then it regrew up to 5 × 1019 m-3, which would provide high charge exchange efficiency for 25keV neutral beam injection.

First Measurement of Electron Temperature and Density Profiles for Spherical Tokamak Plasmas Sustained by Lower Hybrid Waves

First Measurement of Electron Temperature and Density Profiles for Spherical Tokamak Plasmas Sustained by Lower Hybrid Waves

The stabilizing effect of core pressure on the edge pedestal in MAST plasmas

The pedestal pressure measured in Mega Ampere Spherical Tokamak plasmas has been shown to increase as the global plasma pressure increases. By deliberately suppressing the transition into the high-confinement regime, the core plasma pressure was systematically altered at the time of the first edge localized mode. Stability analysis shows that the enhanced Shafranov shift at higher core pressure stabilizes the ballooning modes driven by the pedestal pressure gradient, consequently allowing the pedestal to reach higher pressures.

Direct X-B mode conversion for high-β national spherical torus experiment in nonlinear regime

Electron Bernstein wave (EBW) can be effective for heating and driving currents in spherical tokamak plasmas. Power can be coupled to EBW via mode conversion of the extraordinary (X) mode wave. The most common and successful approach to study the conditions for optimized mode conversion to EBW was evaluated analytically and numerically using a cold plasma model and an approximate kinetic model. The major drawback in using radio frequency waves was the lack of continuous wave sources at very high frequencies (above the electron plasma frequency), which has been addressed. A future milestone is to approach high power regime, where the nonlinear effects become significant, exceeding the limits of validity for present linear theory. Therefore, one appropriate tool would be particle in cell (PIC) simulation. The PIC method retains most of the nonlinear physics without approximations. In this work, we study the direct X-B mode conversion process stages using PIC method for incident wave frequency f0 = 15 GHz, and maximum amplitude E0 = 105 V/m in the national spherical torus experiment (NSTX). The modelling shows a considerable reduction in X-B mode conversion efficiency, Cmodelling = 0.43, due to the presence of nonlinearities. Comparison of system properties to the linear state reveals predominant nonlinear effects; EBW wavelength and group velocity in comparison with linear regime exhibit an increment around ∼36% and 17%, respectively.

Neoclassical tearing mode control using vertical shifts on MAST

Triggered vertical shifts of the MAST spherical tokamak plasma have been found to stabilize 2/1 neoclassical tearing modes (NTMs) for a number of MAST shots, without impacting on core confinement. This stabilization is a result of favourable modifications of the density, temperature and pressure profiles at the location of an NTM by means of a brief transition from high (H) to low (L) confinement mode. Using this method, the high confinement phase can typically be recovered, and the NTM removed, within 20 ms of onset.

Bean-Shaped Spherical Tokamak Equilibrium With Ergodic Limiter

Equilibrium simulations of a spherical tokamak (ST) with a bean-shaped plasma in rail limiter geometry are presented for the first time. Preliminary results in a not fully self-consistent scenario show this equilibrium is possible and, compared with the more usual ST rail limiter D-shaped geometry, retain consistently the same kinetic storage energy. The related parameters such as volume-averaged beta increase by 17% due to pure geometric effects, i.e., the increase of plasma major radius (10%), which then increases the aspect ratio (1.4-1.6) due to the simultaneous reduction of plasma minor radius (9%), while the elongation increases (1.2-1.3). The plasma volume is virtually the same but the plasma triangularity increases substantially (0.32-0.45) thus favoring beta limits because of shaping. These results have been obtained after a careful pressure tailoring by fixing the plasma outboard within a 3% error. This unique bean-shaped equilibrium can be attained simultaneously (but independently) setting an edge localized ergodization as observed via a Poincaré mapping, using a low level of nonresonant perturbation to the plasma current ( ~ 2%). This combination can theoretically lead to a higher beta limit ST plasma with different thermal power loads distribution, which is an interesting area of study for larger devices.

Basic Study of Two‐Dimensional Thomson Scattering Measurement by Multiple Reflections and Time‐of‐Flight of Laser Beam

SUMMARYA novel two‐dimensional Thomson scattering (TS) measurement system has been developed by adopting the time‐of‐flight (TOF) difference in the signals scattered from three parallel laser beams formed by multiple reflections. New ideas are: (1) a YAG laser beam is reflected by 6 mirrors so that it covers the whole r–z plane of a spherical Tokamak (ST) plasma; (2) the observation position is determined by the time delay of the scattered light, which helps to significantly reduce the necessary numbers of spectrometers and detectors. Using a TS‐4 torus plasma merging device. we successfully measured the electron temperature at nine points, demonstrating the basic theory of this novel TS system.

Ion Heating Characteristics of Merging Spherical Tokamak Plasmas using the Improved Doppler Tomography Spectroscopy System

Axial scan of 2D Doppler tomography measurement made clear detailed 2D profiles of ion temperature during and after merging of two spherical tokamak (ST) plasmas. The magnetic reconnection heats plasma ions in the downstream, causing preferential ion heating around a specific flux surface. The merging STs were found to form the hollow ion temperature profile inside the produced ST.

Plasma Density Suppression by Electron Cyclotron Wave in Lower Hybrid Wave Driven TST-2 Spherical Tokamak Plasma

For successful plasma current (Ip) ramp-up by the lower hybrid wave (LHW), the plasma density must be kept at a low level. In the TST-2 Ip ramp-up experiment using the LHW, the application of the electron cyclotron wave (ECW) was found to reduce the plasma density by about 10%. This allowed for further Ip ramp-up later in the discharge.

Development of a Local Current Diagnostic using a Small Rogowski Coil for a Spherical Tokamak Plasma in TST-2

Development of a Local Current Diagnostic using a Small Rogowski Coil for a Spherical Tokamak Plasma in TST-2

Two-fluid simulations of driven reconnection in the mega-ampere spherical tokamak

In the merging-compression method of plasma start-up, two flux-ropes with parallel toroidal current are formed around in-vessel poloidal field coils, before merging to form a spherical tokamak plasma. This start-up method, used in the Mega-Ampere Spherical Tokamak (MAST), is studied as a high Lundquist number and low plasma-beta magnetic reconnection experiment. In this paper, 2D fluid simulations are presented of this merging process in order to understand the underlying physics, and better interpret the experimental data. These simulations examine the individual and combined effects of tight-aspect ratio geometry and two-fluid physics on the merging. The ideal self-driven flux-rope dynamics are coupled to the diffusion layer physics, resulting in a large range of phenomena. For resistive MHD simulations, the flux-ropes enter the sloshing regime for normalised resistivity η≲10−5. In Hall-MHD, three regimes are found for the qualitative behaviour of the current sheet, depending on the ratio of the current sheet width to the ion-sound radius. These are a stable collisional regime, an open X-point regime, and an intermediate regime that is highly unstable to tearing-type instabilities. In toroidal axisymmetric geometry, the final state after merging is a MAST-like spherical tokamak with nested flux-surfaces. It is also shown that the evolution of simulated 1D radial density profiles closely resembles the Thomson scattering electron density measurements in MAST. An intuitive explanation for the origin of the measured density structures is proposed, based upon the results of the toroidal Hall-MHD simulations.