Supersonic Molecular Beam Injection Research Articles

A novel approach for the improvement of the sensitivity of the flow visualization system on supersonic molecular beam of tokamak fuelling

Supersonic molecular beam injection is a robust alternative to control the particles in fusion plasma devices. It is widely used in many experimental studies on the plasma physics. Direct visualization of beam makes it possible for precise optimization of the beam characteristics. However, a normal visualization system does not fully meet the requirements of the measurement of the beam distribution in the low gas density vacuum environment. This paper reports a newly designed multi-pass visualization system without changing the schlieren mirrors. This multi-pass system is designed to make the light passing the testing area several times, where the deflection angle of the light would simultaneously increase. Thus, the sensitivity of the schlieren image is enhanced. The testing result proves the improvement of the sensitivity, which makes it possible to optimized the beam structure in low gas density working conditions.

Flow visualization and density measurement of the supersonic molecular beam for fusion plasma fueling.

The supersonic molecular beam injection (SMBI) technique is widely used in magnetic fusion devices for plasma fueling and active control of particles. The beam flow formed by the SMBI system is directly visualized by a newly established schlieren diagnostic system on the SMBI testing platform. This schlieren system could provide a 2D density distribution of the beam with the sub-mm spatial resolution by detecting the change in the refractive index of the medium caused by the SMBI in the low-pressure vacuum chamber. The structure and the divergence angle of the supersonic molecular beam for fusion plasma fueling are first measured. Additionally, the density profile of the supersonic molecular beam is reconstructed from the measured light intensity by the schlieren system. The experimental comparison between the standard nozzle and the optimized nozzle of the SMBI system shows the great potential of the diagnostic system in optimizing the beam characteristics.

Implementation and data processing of a five-channel microwave interferometer with high temporal resolution and low noise on Sino-UNIted Spherical Tokamak.

A five-channel microwave interferometer with high temporal resolution and high phase resolution has been developed for measuring electron density profiles and fluctuations on the Sino-UNIted Spherical Tokamak. A correction algorithm, based on the low signal amplitude detection, is proposed to eliminate the fringe jump errors. The correction algorithm achieves an accuracy of 92%. The density profile is reconstructed through the Park-matrix method, with the geometry of magnetic surfaces calculated by the equilibrium fitting. The reconstructed density profiles for discharges with supersonic molecular beam injection are in good agreement with the results of another 94GHz single-channel horizontal interferometer and the Langmuir probes. The temporal resolution of the system is 0.5 µs and the line-integrated electron density resolution is 1 × 1015m-2, which benefits from the single sideband modulation technique. Therefore, the multichannel interferometer system is capable of studying the details of the high-frequency (up to 200 kHz) density fluctuation such as that in the minor disruption event.

Dynamics of weak-magnetic-shear-sustained internal transport barrier during supersonic molecular-beam injection in JT-60U

The characteristics of ion heat transport inside the internal transport barrier (ITB), which is sustained by weak magnetic shear, are investigated using a cold pulse induced by supersonic molecular-beam injection (SMBI) in JT-60U. It is known that cold-pulse propagation in ITBs usually significantly reduces the temperature and degrades the temperature gradient. When subsequent SMBIs are launched before the temperature has recovered, it is observed that the temperature gradient of the ITB is not monotonically decreased but alternately decreased and increased. Alternating decreasing and increasing phases of the ITB temperature gradient continue for about 1 s (∼6τ E), and the properties of the cold-pulse propagation and the flux-gradient relations differ according to the phase. The usual transient transport analysis is also provided by inducing a cold pulse in stationary ITBs. A rapid reduction and recovery of the temperature is observed inside the ITBs, and it is found that the ion-heat flux changes without a variation in the local ion-temperature gradient or a change to the other observable local parameters. The flux-gradient relations exhibit significant hysteresis in two ITB cases: (i) ITBs with strong electron-density gradients and (ii) ITBs with weak ion-temperature gradients. In addition, the range in which hysteresis appears is most likely to depend on the width of the ITB.

Runaway electron mitigation with supersonic molecular beam injection (SMBI) in ADITYA-U tokamak

The generation and subsequent loss of runaway electrons (REs) during the operation sequence in a tokamak is a potent threat to the plasma-facing components and the interface of actively cooled parts. Control and mitigation of REs are of prime importance to the safe operation and machine health of a fusion device. A supersonic molecular beam injection (SMBI) system has been installed in the ADITYA-U tokamak to explore the effects of the high Mach number molecular beam on the REs and ways to mitigate the REs. In the majority of discharges in which SMBI has been injected, a burst in hard x-rays has been observed accompanying the SMBI pulse, indicating significant RE loss. This is followed by a long RE-mitigated phase in the discharge. The most plausible explanation of the mitigation of REs is minor disruption caused by SMBI. This in turn triggers field line stochastization and subsequent rapid RE loss. Finally, this leads to reorganization of the flux surfaces, resulting in bigger islands with the potential of trapping any surviving RE fraction.

Progress of Experimental Studies in the HL-2A Tokamak

During the last several years, the HL-2A experiment has made significant progress in the following areas: (1) lower-hybrid wave (LHW) heating and current drive, (2) plasma confinement and turbulent transport, (3) magnetohydrodynamic (MHD) instabilities and energetic particle physics and (4) H-mode and edge localized mode (ELM) control. The results show that the LHW system working in the co-current mode can reach higher driving efficiency and full non-inductive lower-hybrid current drive (LHCD) has been achieved. The intrinsic poloidal torque characterized by the divergence of the residual stress is deduced from synthesis for the first time. The dynamics of spectral symmetry breaking in drift wave turbulence is in good agreement with the development of the poloidal torque to drive the edge poloidal flow. The influence of the cross-phase dynamics on turbulent stress was also investigated. The ion internal transport barrier has been observed in the NBI-heated plasma, and inside the barrier the ion thermal transport is reduced to the neoclassical level. Besides, micro-turbulence is modulated by the rotation frequency of the magnetic island, and this modulation effect is related to a critical island width. Strong E × B shear is found at the island boundary. Three kinds of axisymmetric modes, beta-induced Alfven eigenmode (BAE), toroidal Alfven eigenmode (TAE) and the ellipticity-induced Alfven eigenmode (EAE), are found to be driven unstable by nonlinear mode coupling between Alfven eigenmodes and tearing mode which is well explained by the nonlinear gyrokinetic theory. The fishbone and tearing modes were actively controlled by the electron cyclotron resonance heating (ECRH). The dynamics of the edge plasma flows and turbulence during the L–I–H transition have been dedicatedly investigated. The geodesic acoustic mode (GAM) and limit cycle oscillation (LCO) coexist for a short time and disappear in the H-mode plasma with the increasing of E × B shear flow before the I–H transition, which plays an important role in the turbulence suppression. Different techniques, such as LHW, ECRH, resonant magnetic perturbation (RMP), and impurity seeding by the laser blow-off (LBO) and supersonic molecular beam injection (SMBI), have been successfully applied to control the large ELMs. It has been found that pedestal turbulence enhancement might be responsible for the observed mitigation effect.

Overview of the Recent Study on ELM Mitigation Physics with Different External Actuators on HL-2A Tokamak

Mitigation of Edge localized mode(ELM) has been achieved with different external actuators such as lower hybrid wave (LHW), mixture supersonic molecular beam injection(SMBI), and laser blow-off(LBO) impurity seeding on HL-2A. During these experiments, the pedestal turbulence enhancement is commonly observed, which is found closely related to ELM mitigation. The turbulence enhancement is caused by the externally driven the velocity shear rate without change of the turbulence correlation length, but correlated to its radial wavenumber spectral shift. A common plausible mechanism for the ELM mitigation with different external source input seems to be involved. A modified theoretical model based on the turbulence radial wavenumber spectral shift is used and successfully interprets the experimental observations. The simulation suggests that a critical growth rate of the most unstable mode, also identified by the experimental results, survives in the competition of the velocity shear rate, enhancing the turbulence intensity. An example of the LHW case is used and good agreements have been found between the experimental results and simulation results.

Numerical simulation of impact of supersonic molecular beam injection on edge localized modes

A three-field model with the impact of supersonic molecular beam injection (SMBI) based on the BOUT++ code is built to simulate edge localized modes (ELMs). Different parameters of SMBI are explored to find an optimal SMBI scenario for ELM mitigation. The linear simulations show that the growth rate of peeling-ballooning mode is reduced by SMBI. The reduction amplitude of the growth rate is increased when the amplitude or width of SMBI is increased, and when SMBI is deposited at the top, bottom and middle of the pedestal, the reduction amplitude increases successively. The nonlinear simulations show that the ELM size is reduced by SMBI. The reduction amplitude of the ELM size is increased when the amplitude or width of SMBI is increased, and when SMBI is deposited at the bottom, top and middle of the pedestal, the reduction amplitude increases successively. Surface-averaged pressure profiles and filamentary structures are analyzed when the ELMs erupt. Deep deposition of SMBI such as at the top and middle of the pedestal reduces the inward collapse amplitude of the pressure profiles, which can improve the confinement efficiency during ELMs. Shallow deposition of SMBI such as at the middle and bottom of the pedestal reduces the outer extent of the filamentary structures, which can slow down the erosion of plasma-facing components caused by ELMs. In conclusion, shallow deposition of SMBI with sufficient amplitude and width can meet the needs of ELM mitigation.

Experimental study of perturbative particle transport in the HL-2A tokamak

Perturbative particle transport experiment has been performed in the HL-2A tokamak by using supersonic molecular beam injection (SMBI) as an external particle source. The spatiotemporal evolution of edge density perturbation is traced and the particle source and the flux-gradient relation are obtained experimentally. The flux-gradient relation is found to be far from the diffusive model and three different transport processes are revealed, including pinch-dominant process, diffusion-pinch process and intermittent decays.

Stimulated effect of SMBI on low-to-high confinement transition of tokamak plasmas

The impact of supersonic molecular beam injection (SMBI) fuelling on the plasma turbulence and shear flows, which form a self-consistent state, has been studied in the HL-2A tokamak. The SMBI flattens the shear layer by the collisional flow damping and increases the turbulence intensity. It is found that the SMBI can enhance the non-linear regulation dynamics and acts as an external tool for turbulence quench via stimulating two dynamic processes. The SMBI-stimulated non-linear interactions facilitate the L-H transition at the marginal heating power for accessing the H-mode. The effective reduction of the H-mode power threshold by SMBI is demonstrated. The result suggests that the SMBI could be a general tool for realization of controllable L-H transition via reducing the H-mode power threshold in future devices.

A reduced model for edge localized mode control by supersonic molecular beam injection and pellet injection

We develop a diffusive, bistable, tri-unstable cellular automata (CA) model to study the dynamics of H-mode pedestal with edge localized modes (ELMs) and their control by supersonic molecular beam injection (SMBI) and pellet injection (PI). It is shown that the new CA model can reproduce the key features of H-mode pedestals with various types of ELM, including Type-I ELM. SMBI and PI are modeled as additional grain injections into pedestal with varying degrees of injected materials and profiles. It is found that H-mode pedestal responds to SMBI differently depending on the baseline fueling. If the baseline fueling is large enough to allow Type-I ELM, SMBI enhances large transport avalanches caused by ballooning instabilities. These avalanches prevent the total pedestal current from reaching the boundary for peeling instability. On the other hand, if the baseline fueling is low to avoid Type-I ELM, SMBI enhances small scale avalanches, which prevent the pedestal from growing to profiles globally vulnerable to ballooning instabilities. These imply that SMBI can mitigate different types of ELM by converting them to more benign types. From CA modeling of pellet injection, it is shown that Type-I ELM can be triggered by pellet injection with sufficient strength and depth. Scanning the frequency of pellet injection, it is found that a maximum efficiency of pellet pacing is achieved when the injection frequency is approximately ten times the natural frequency of Type-I ELM.

Numerical Study of Supersonic Molecular Beam Injection System in Thailand Tokamak I

Thailand Institute of Nuclear Technology (TINT) is planning to develop Thailand Tokamak I from HT-6M tokamak. In the first phase of operation, the device will be equipped with two fueling systems: gas puffing (GP) and supersonic molecular beam injection (SMBI). Since the SMBI system has never been used experimentally with this device, this work, therefore, numerically studies the penetration features of SMBI in the plasma of Thailand Tokamak I, based on the nominal parameters of HT-6M tokamak. The interaction of the supersonic molecules with the plasma has been computed by BOUT++ code which solves six-field fluid model of SMBI model in the radial direction. The preliminary results demonstrate that the SMBI is an efficient method for fueling the plasma and the beam can be delivered to the center of the plasma core.

Influence of injection parameters on fueling efficiency of supersonic molecular beam injection into turbulent fusion plasmas

The fueling efficiency of supersonic molecular beam injection (SMBI) depends on injection parameters. In this article, we report on simulations of SMBI into turbulent edge fusion plasmas. Density continuity equations, which describe the dynamics of molecules and Franck-Condon atoms, are coupled with the plasma equations in the hot-edge-sol-electrostatic (HESEL) drift-fluid model. Numerical simulations, with parameters relevant to experimental conditions, are performed in order to investigate the dependency of the fueling efficiency of SMBI on the molecular beam density, velocity, and beam width. In all conditions, the efficiency grows with the increase in the molecular beam velocity. The variation in efficiency with respect to the beam density depends on the plasma pressure and the ratio of the density source, produced by ionization of neutrals, to the plasma density source provided by core fueling. The effect of the molecular beam width is pronounced only if the beam density is high and the plasma, into which the beam is injected, is relatively dense and hot. The influence of plasma fluctuations on SMBI is weak; however, a strong neutral injection leads to suppression of a turbulence transport due to relaxation of pressure gradient profiles.

Minor disruptions triggered by supersonic molecular beam injection on J-TEXT tokamak

Disruption mitigation experiments with supersonic molecular beam injection of argon gas on J-TEXT were conducted to explore the evolution process during multistage minor disruption. The corresponding numerical simulation was performed using a 3D magnetohydrodynamic (MHD) code, namely, NIMROD (Sovinec et al 2004 J. Comput. Phys. 195 355). The experimental and numerical simulation results are shown in the following. The central plasma temperature decreases to less than tens of electovolts after a relatively long period of multistage thermal collapse. Different MHD instability modes appear when the impurity cold front propagates toward the q = 2 surface. During the burst of MHD activity and central plasma thermal collapse, the heat flux is transferred from the core region to the plasma boundary. Further numerical simulation results show that the MHD process is accompanied by a significant increase in the core-oriented spread of impurities through the q = 2 surface. During the transition stage from the minor disruption to the major disruption, the high order modes play an important role and the magnitude of the n = 3 growth rate is even larger than that of the n = 1 or n = 2 mode. What seems to separate major from minor disruptions is that in a minor disruption, a single MHD crash is not sufficient to cause a full radiative collapse.

Mechanism of the active divertor flux control by the supersonic-molecular-beam-injection with lower hybrid wave-induced magnetic perturbations on the EAST tokamak

The redistribution of the divertor flux caused by the synergy of the supersonic-molecular-beam-injection (SMBI) and the magnetic perturbations induced by lower hybrid waves (LHWs), has been observed on the Experimental Advanced Superconducting Tokamak (EAST) (Li et al 2013 Nature Phys. 9 817). To reveal the physical mechanism, simulations with good agreements to the experimental findings are first performed by utilizing a self-consistent fluid 3D edge plasma Monte-Carlo code coupled to a kinetic neutral particle transport code. The ions and electrons originating from the ionization of injected neutral particles in the plasma edge flow along the magnetic flux tube towards to the divertor, thus directly increasing the divertor flux on the split strike lines in the footprint. Combining this with the multi-lobe structure of the edge magnetic topology, actively controlling the divertor flux can be realized by adjusting the SMBI position or the phase of the magnetic perturbations.

Simulation of helium supersonic molecular beam injection in tokamak plasma**Project supported by the Chunhui Program of the Ministry of Education of China (Grant No. Z2017091), the Sichuan Provincial Science Foundation for Distinguished Young Leaders of Disciplines in Science and Technology, China (Grant Nos. 2019JDJQ0051 and 2019JDJQ0050), the National Natural Science Foundation of China (Grant Nos. 11575055 and 11605143), the Fund

To study helium (He) supersonic molecular beam injection (SMBI) into H-mode tokamak plasma, a simplified multicomponent-plasma model under the assumption of quasi-neutral condition is developed and implemented in the frame of BOUT ++. The simulation results show that He species propagate inwards after He SMBI, and are deposited at the bottom of the pedestal due to intensive ionization and weak spreading speed. It is found that almost all injected helium particles strip off all the bounded electrons. He species interact intensively with background plasma along the injection path during He SMBI, making deuterium ion density profile drop at the He-deposited location and resulting in a large electron temperature decreasing, but deuterium ion temperature decreasing a little at the top of the pedestal.

Simulation of supersonic molecular beam injection fueling into H-mode plasmas on EAST using BOUT++

Transport dynamic 2D simulations for supersonic molecular beam injection (SMBI) fueling into H-mode deuterium plasmas on the Experimental Advanced Superconducting Tokamak (EAST) is first simulated using a seven-field two-fluid model in the BOUT++ framework. The SMB is assumed to be injected into plasma from the midplane at the low field side with a fixed width and constant molecular flux. The different densities and injection velocities of SMBI are investigated within the upper single-null geometry of EAST. The simulations indicate that the SMBI has a self-shielding effect on molecules' inward transport into the plasma, and the deposition of SMBI leads to a large increase in plasma density and decrease in plasma temperature. There is a velocity threshold for SMB penetrating the pedestal and depositing at the top of density pedestal. However, the deposition point would be back toward the plasma boundary after SMB arriving at the deepest penetration position. Comparing the different molecular injection velocities and densities, the outcomes demonstrate that the depth of deposition is closely related to the injection velocity rather than the molecular density. The simulated results show good agreement with the EAST experiments by comparing the electron density profiles obtained by simulation and experiment, respectively. These results will be helpful for guiding future experiments, as well as the design of the SMBI system.

Development and applications of schlieren system for measuring characteristics of supersonic molecular beam

Supersonic molecular beam injection (SMBI) is an effective fueling method for the magnetic fusion plasmas. The fueling method was first proposed in the HL-1 tokamak, and now has been applied to several tokamaks and stellarators. Pulsed molecular beam passes from a Laval type nozzle and has a high instantaneous intensity, high directionality and deep deposition in the plasma. The fueling efficiency is higher than the gas puffing efficiency. In addition, it is widely used for controlling plasma density and investigating plasma physics. To further improve the fueling capability in future fusion devices, it is highly desirable to optimize the characteristic of the SMB and further investigate the interactions between the molecular beam and the plasma. In this paper, a schlieren diagnostic system is developed to measure the parameters of molecular beam, and the testing application is performed. The schlieren system, which is based on the schlieren photography, is designed with the zigzag optical path and equipped on the SMBI testing platform to measure the characteristics of the supersonic molecular beam. In order to verify the effectiveness of the system, a series of tests is carried out with different nozzle shapes under atmospheric and vacuum conditions. The beam profiles of CO<sub>2</sub> and D<sub>2</sub> under different background pressures are obtained. The testing results indicate that the directionality of the integrated Laval nozzle is much better than that of the pinhole nozzle. The schlieren system provides a testing tool for optimizing the supersonic molecular beam.

Density compensation and stored energy recovery in resonant magnetic perturbation suppressed edge-localized mode H-mode plasmas using pellet fueling on EAST

The fueling of plasmas using frozen pellets in steady-state quiescent H-mode plasmas with edge-localized modes (ELMs) suppressed by external magnetic perturbations (RMPs) was successfully demonstrated on the EAST tokamak in plasmas with similar shapes to those in ITER with q95 ~ 3.8, and triangularity δ ~ 0.45. During the fully suppressed-ELM phase using RMPs, the plasmas were refueled by deuterium pellets with a frequency of 5 Hz from ~40 cm above the mid-plane on the low field side. The results show that the pellet fueling would partially compensate density, which decreased due to the effect of pump-out induced by RMPs. It was interesting to discover that the stored energy of the plasma could be raised to the value from before RMP application using pellet injection, which is rarely reported in other tokamaks. As for ELMs, they reappeared with a size smaller than the ELMs before RMP application accompanied with a magnetic coherent mode termination, however, the ELMs were still mitigated to a certain extent. Meanwhile, a similar experiment using supersonic molecular beam injection fueling was also carried out, which confirmed that pellet fueling is better for raising plasma stored energy. This investigation on the synergistic effect of pellet fueling and RMPs would be useful for simultaneous control of plasma density and ELMs in high performance H-mode plasmas in future devices, such as ITER.

Overview of HL-2A recent experiments

The mission of HL-2A is to explore the key physical topics relevant to ITER and to the advanced tokamak operation (e.g. the operation of future HL-2M), such as the access of H-mode, energetic particle physics, edge-localized mode (ELM) mitigation/suppression and disruption mitigation. Since the 2016 Fusion Energy Conference, the HL-2A team has focused on the investigations on the following areas: (i) pedestal dynamics and L–H transition, (ii) techniques of ELM control, (iii) turbulence and transport, (iv) energetic particle physics. The HL-2A results demonstrated that the increase of mean shear flow plays a key role in triggering L–I and I–H transitions, while the change of flow is mainly induced by the ion pressure gradient. Both mitigation and suppression of ELMs were realized by laser blow-off seeded impurity (Al, Fe, W). The 30% Ne mixture supersonic molecular beam injection seeding also robustly induced ELM mitigation. The ELMs were mitigated by low-hybrid current drive. The stabilization of m/n = 1/1 ion fishbone activities by electron cyclotron resonance heating was found on the HL-2A. A new m/n = 2/1 ion fishbone activity was observed recently, and the modelling indicated that passing fast ions dominantly contribute to the driving of 2/1 fishbone. The non-linear coupling between the toroidal Alfven eigenmode and tearing mode (TM) led to the generation of a high frequency mode with the toroidal mode number n = 0. The turbulence was modulated by TM when the island width exceeds a threshold and the modulation is localized merely in the inner area of the islands. Meanwhile, turbulence radial spread took place across the island region.