Experimental study on liquid metal free surface flow under magnetic and electric field for nuclear fusion

In the future application of nuclear fusion, the liquid metal flows are considered to be an attractive option of the first wall of the Tokamak which can effectively remove impurities and improve the confinement of plasma. Moreover, the flowing liquid metal can solve the problem of the corrosion of the solid first wall due to high thermal load and particle discharge. In the magnetic confinement fusion reactor, the liquid metal flow experiences strong magnetic and electric, fields from plasma. In the present paper, an experiment has been conducted to explore the influence of electric and magnetic fields on liquid metal flow. The direction of electric current is perpendicular to that of the magnetic field direction, and thus the Lorentz force is upward or downward. A laser profilometer (LP) based on the laser triangulation technique is used to measure the thickness of the liquid film of Galinstan. The phenomenon of the liquid column from the free surface is observed by the high-speed camera under various flow rates, intensities of magnetic field and electric field. Under a constant external magnetic field, the liquid column appears at the position of the incident current once the external current exceeds a critical value, which is inversely proportional to the magnetic field. The thickness of the flowing liquid film increases with the intensities of magnetic field, electric field, and Reynolds number. The thickness of the liquid film at the incident current position reaches a maximum value when the force is upward. The distribution of liquid metal in the channel presents a parabolic shape with high central and low marginal. Additionally, the splashing, i.e., the detachment of liquid metal is not observed in the present experiment, which suggests a higher critical current for splashing to occur.

Prevention of ECCD triggering explosive bursts in reversed magnetic shear tokamak plasmas for disruption avoidance

The explosive burst excited by neoclassical tearing mode (NTM) is one of the possible candidates of disruptive terminations in reversed magnetic shear (RMS) tokamak plasmas. For the purpose of disruption avoidance, numerical investigations have been implemented on the prevention of explosive burst triggered by the ill-advised application of electron cyclotron current drive (ECCD) in RMS configuration. Under the situation of controlling NTMs by ECCD in RMS tokamak plasmas, a threshold in EC driven current has been found. Below the threshold, not only are the NTM islands not effectively suppressed, but also a deleterious explosive burst could be triggered, which might contribute to the major disruption of tokamak plasmas. In order to prevent this ECCD triggering explosive burst, three control strategies have been attempted in this work and two of them have been recognized to be effective. One is to apply differential poloidal plasma rotation in the proximity of outer rational surface during the ECCD control process; The other is to apply two ECCDs to control NTM islands on both rational surfaces at the same time. In the former strategy, the threshold is diminished due to the modification of classical TM index. In the latter strategy, the prevention is accomplished as a consequence of the reduction of the coupling strength between the two rational surfaces via the stabilization of inner islands. Moreover, the physical mechanism behind the excitation of the explosive burst and the control processes by different control strategies have all been discussed in detail.

Investigations of plasma response associated with Resonant Magnetic Perturbation fields using perturbation method in KSTAR H-mode plasmas

The plasma response associated with the Resonant Magnetic Perturbation (RMP) field was investigated using the small edge perturbations induced by a modulated Supersonic Molecular Beam Injection (SMBI) in KSTAR. The modulated SMBI provides a time-varying perturbation of the plasma density source in the region just inside the last closed flux surface (LCFS) and a modulated flow damping rate. Radial propagation of the toroidal rotation perturbation induced by SMBI from the q=3 surface to the q=2 surface was observed. Theoretical analysis using the General Perturbed Equilibrium Code (GPEC) of the RMP intensity profiles of the RMP field is consistent with the phase profile of the toroidal rotation perturbation.

Numerical modeling of pedestal stability and broadband turbulence of wide-pedestal QH-mode plasmas on DIII-D

Wide pedestal Quiescent High confinement (QH) mode discovered on DIII-D in recent years is a stationary and quiescent H-mode with the pedestal width exceeding EPED prediction by at least 25%. Its characteristics, such as low rotation, high energy confinement and ELM-free operation, make it an attractive operation mode for future reactors. Linear and nonlinear simulations using BOUT++ reduced two fluid MHD model are carried out to investigate the bursty broadband turbulence often observed in the edge of wide-pedestal QH-mode plasmas. Two kinds of MHD-scale instabilities in different spatial locations within the pedestal were found in the simulations: one mild peeling-ballooning (PB) mode (γ_PB<0.04ω_A) located near the minimum in Er well propagating in ion diamagnetic drift direction; and one drift-Alfvén wave (DAW) locates at smaller radius compared to Er well propagating in the electron diamagnetic drift direction and unstable only when the parallel electron dynamics is included in the simulation. The coupling between drift wave and shear Alfvén wave provides a possible cause of the experimentally observed local profile flattening in the upper-pedestal. The rotation direction, mode location, as well as the wavenumber of these two modes from BOUT++ simulations agree reasonably well with the experimental measurements, while the lack of quantitatively agreement is likely due to the lack of trapped electron physics in current fluid model. This work presents improved physics understanding of the pedestal stability and turbulence dynamics for wide-pedestal QH-mode.

Evaluation of ambipolar potential barrier in the gas dynamic trap by Doppler spectroscopy

The recently developed Doppler spectroscopy diagnostic has been used to evaluate the height of the ambipolar potential barrier forming in the gas dynamic trap (GDT) plasma between the central cell and the region with a large magnetic expansion ratio beyond the mirror. The diagnostic technique based on the gas jet charge exchange target, allowed to measure the potential profile along the line of sight covering the radial range from the axis to the limiter. The on-axis potential drop was found to be 2.6÷3.1 in units of the central plane electron temperature, which supports the existing theoretical understanding of suppression of electron thermal conductivity in the GDT expander.

Analysis of hydrogen fueling, recycling, and confinement at Wendelstein 7-X via a single-reservoir particle balance

A single-reservoir particle balance for the main plasma species hydrogen has been established for Wendelstein 7-X (W7-X). This has enabled the quantitative characterization of the particle sources in the standard island divertor configuration for the first time. Findings from attached scenarios with two different island sizes with a boronized wall and turbo molecular pumping are presented. Fueling efficiencies, particle flows and source locations were measured and used to infer the total particle confinement time $\tau_{\rm{p}}$. Perturbative gas injection experiments served to measure the effective particle confinement time $\tau_{\rm{p}}^*$. Combining both confinement times provides access to the global recycling coefficient $\bar{R}$. Hydrogen particle inventories have been addressed and the knowledge of particle sources and sinks reveals the core fueling distribution and provides insight into the capability of the magnetic islands to control exhaust features. Measurements of hydrogen fueling efficiencies were sensitive to the precise fueling location and measured between 12~\% and 31~\% with the recycling fueling at the strike line modeled at only 6~\%, due to much higher densities. 15~\% of the total \SI{5.2E+22}{a/s} recycling flow ionizes far away from the recycling surfaces in the main chamber. It was shown that 60~\% of recycled particles ionize above the horizontal and 18~\% above the vertical divertor target, while the remainder of the recycling flow ionizes above the baffle (7~\%). Combining these source terms with their individual fueling efficiencies resolves the core fueling distribution. Due to the higher fueling efficiency in the main chamber, up to 51~\% of the total \SI{5.1E+21}{1/s} core fueling particles are entering the confined plasma from the main chamber. $\tau_{\rm{p}}$ values in the range of 260 ms were extracted for these discharges. Together with $\tau_{\rm{p}}$, the global recycling coefficient $\bar{R}$ was resolved for every $\tau_{\rm{p}}^*$ measurement and a typical value close to unity was obtained. An increase of the island size, resulted in no change of $\tau_{\rm{p}}$, but doubled $\tau_{\rm{p}}^*$, indicating the feasibility of the control coils as an actuator to control exhaust features without affecting core confinement properties.

DC performance and AC loss of sub-size MgB2 CICC conductor for fusion magnet application

Given the low price and relatively high transition temperature (39 K) of MgB2 conductor, MgB2-based superconductors are a potential candidate for the lower field fusion coils, such as Poloidal Field (PF) coils, Correction Coils (CC) and Feeders. However, to date, the application of MgB2 is limited to demonstrators in a low magnetic field of up to 5 T and at temperatures of up to 10 to 20 K, relying on cryogen-free, helium gas or liquid hydrogen cooling, which significantly reduce the cost of cryogenic systems. To demonstrate the feasibility and performance verification of large size MgB2 PF conductors based on ITER and CFETR requirements, a 4th-stage subsize MgB2 Cable-In-Conduit Conductor (CICC) cable sample is made at the Institute of Plasma Physics, Chinese Academy of Sciences (ASIPP). The CICC contains 96 in-situ MgB2 superconducting wires, manufactured by Western Superconducting Technology Ltd. (WST) and 48 copper wires. The critical current of the sub-size cables and MgB2 witness wires are examined with different background magnetic fields at 4.2 K. In addition, the AC loss is measured utilizing magnetization and calorimetric methods. To further clarify the influence of electromagnetic force on the AC loss performance, the cable sample is pressed transversely at room temperature and then inserted into a dipole magnet for AC loss measurement at 4.2 K. The critical current at 4.2 K of the subsize MgB2 CICC cable shows 20% degradation compared to the witness wires at 2 T background magnetic field. However, no further critical current degradation is visible during ramping up and down the magnetic field. The coupling loss time constant for 1 T background magnetic field amounts to 480 ms. No significant effect of the applied transverse stress on the coupling loss is observed between 0 and 10 MPa.

Influence of low-Z impurity on the stabilization of m/n = 2/1 tearing/locked modes in EAST

The impact of the low-Z impurity concentration on the modes stabilization has been investigated in the EAST tokamak. Series of tearing modes (TMs) with multiple helicities are excited by the concentration of low-Z (carbon) impurity, and the dominant mode structure is featured by m/n = 2/1 magnetic islands that propagate in electron diamagnetic drift direction (m and n are poloidal and toroidal mode numbers respectively). The m/n = 2/1 locked modes (LMs) can be formed by the redistribution of low-Z impurity concentration, which is unlocked spontaneously for the decreasing of impurity concentration, where the width of magnetic islands can reach w ≅ 5 cm (w/a ≅ 0.1, a is minor radius). The increasing of electromagnetic brake torque is the primary reason for the mode locking, and the 'O'-point of m/n = 2/1 magnetic islands is locked by the tungsten protector limiter (toroidal position: -0.4π ≦ φ ≦ -0.3π) with separation of Δφ ≅ 0. The 3D asymmetric structure of m/n = 2/1 magnetic islands is formed for the interaction with the tungsten protector limiter, and the electromagnetic interaction decreases dramatically for the separation of Δφ ≧ 0.2π. The mechanisms for the mode excitation and locking can be illustrated by the "hysteresis effect" between the low-Z impurity concentration and the width of m/n = 2/1 magnetic islands, namely the growth of magnetic islands is modulated by the low-Z impurity concentration, and the rotation velocity is decelerated accordingly. However, the intrinsic mechanism for the unlocking of m/n = 2/1 LMs is complicated by considering the concentration of the low-Z impurity, and the possible unlocking mechanism is discussed. Therefore, understanding of the relationship between the impurities and magnetic islands is more important for optimizing the control techniques (RMP→LMs, ECRH→NTM, impurity seeding→major collapse, et al).

Physics design point of high-field stellarator reactors

The ongoing development of electromagnets based on High Temperature Superconductors has led to the conceptual exploration of high-magnetic-field fusion reactors of the tokamak type, operating at on-axis fields above 10 T. In this work we explore the consequences of the potential future availability of high-field three-dimensional electromagnets on the physics design point of a stellarator reactor. We find that, when an increase in the magnetic field strength $B$ is used to maximally reduce the device linear size $R\sim B^{-4/3}$ (with otherwise fixed magnetic geometry), the physics design point is largely independent of the chosen field strength/device size. A similar degree of optimization is to be imposed on the magnetohydrodynamic, transport and fast ion confinement properties of the magnetic configuration of that family of reactor design points. Additionally, we show that the family shares an invariant operation map of fusion power output as a function of the auxiliary power and relative density variation. The effects of magnetic field over-engineering and the $R(B)$ scaling of design points with constant neutron wall loading are also inspected. In this study we use geometric parameters characteristic of the \emph{helias} reactor, but most results apply to other stellarator configurations.

Simulation of convective transport during frequency chirping of a TAE using the MEGA code

We present a procedure to examine energetic particle phase-space during long range frequency chirping phenomena in tokamak plasmas. To apply the proposed method, we have performed self-consistent simulations using the MEGA code and analyzed the simulation data. We demonstrate a travelling wave in phase-space and that there exist specific slices of phase-space on which the resonant particles lie throughout the wave evolution. For non-linear evolution of an n=6 toroidicity-induced Alfven eigenmode (TAE), our results reveal the formation of coherent phase-space structures (holes/clumps) after coarse-graining of the distribution function. These structures cause a convective transport in phase-space which implies a radial drift of the resonant particles. We also demonstrate that the rate of frequency chirping increases with the TAE damping rate. Our observations of the TAE behaviour and the corresponding phase-space dynamics are consistent with the Berk-Breizman (BB) theory.