Supersonic Molecular Beam Injection Research Articles

Characteristics of divertor heat flux distribution with an island divertor configuration on the J-TEXT tokamak

On J-TEXT, the temporal evolution of heat flux distribution on the high-field side (HFS) divertor plate has been measured by an infrared (IR) camera during the plasma operation with an island divertor configuration. In experiments, the island divertor configuration is an edge magnetic island chain structure surrounded by stochastic layers, which can be induced by resonant magnetic perturbations (RMPs). The experimental results show that the heat flux distribution on the HFS target plate depends significantly on the edge magnetic topology. Furthermore, the impact of hydrogen fueling using supersonic molecular beam injection (SMBI) on the divertor heat flux distributions is studied on J-TEXT with an island divertor configuration. It has been observed that power detachment can be achieved when the radiation front approaches the last closed flux surface (LCFS) after each SMBI pulse. This result may provide a method of access for divertor detachment on a fusion device with a three-dimensional (3D) boundary magnetic structure.

Impurity convection reversal caused by edge localized turbulence for generating a stationary edge radiative layer in the HL-2A tokamak

Abstract Effect of impurity on edge turbulence was investigated on the HL-2A tokamak by using Neon supersonic molecular beam injection (SMBI). For the first time, a stationary radiative layer accompanied by edge localized turbulence lasting more than 100ms is observed at the edge region ρ=0.7-0.9. This duration is much longer than the SMBI pulse length (1.2ms), indicating that the impurity diffusion is on the whole cancelled out by the impurity convection without spreading of the localized turbulence during this period. This implies that the impurity convection is reversed from positive to negative impurity density gradient region. This reversal could be explained by the parallel impurity compression mechanism. The observed turbulence is a broadband electrostatic turbulence (25-70 kHz) propagating in the electron diamagnetic drift direction with normalized poloidal wavenumber of 0.1≤ kθρs≤0.4. It has been found that the excitation of the edge localized turbulence strongly depends on the local collisionality. This feature is qualitatively predicted by the spectral multiscale gyrokinetic turbulence simulation, which suggests that the turbulence could be a dissipative trapped electron mode (DTEM). This experiment demonstrates that mutual interaction between externally injected impurity ions and edge turbulence can generate a stationary edge radiative layer and avoid impurity core accumulation. These results could provide a potential approach for the formation of a stable impurity radiative layer by impurity injection in tokamaks.

First realization of LHW–plasma coupling feedback control for long-pulse operation in EAST

To sustain good lower hybrid wave (LHW)–plasma coupling for long-pulse plasma operation, for the first time, coupling feedback control is designed and realized in EAST using a proportion integration differentiation method by choosing the reflection coefficient (RC) of LHW power as the reference for gas-puffing feedback, and including one pulse test and multi-pulse experiments. Experiments show that such feedback control can work correctly and maintains good LHW–plasma coupling effectively for a long time, suggesting the possibility of feedback control application on LHW–plasma coupling in long-pulse plasma. Furthermore, during the feedback control process of multi-pulse supersonic molecular beam injection (SMBI), the stored energy changes from 29 kJ to 58 kJ, and the energy confinement factor (H 89) increases from 0.98 to 1.45, implying a positive effect of coupling feedback on plasma performance. Experiments between SMBI puffing and the gas puffing system, fed by a piezoelectric valve near the antenna, are further investigated, showing that the response time of the RC with SMBI is faster than that by the piezoelectric valve. In addition, SMBI puffing on the electron-drift side of the LHW antenna is a little quicker than that on the ion-drift side. Studies suggest that such feedback control is effective for long-pulse LHW–plasma coupling, and the gas puffing by SMBI on the electron-drift side of the LHW antenna could offer an effective way to sustain good LHW coupling in steady-state operation in the future. Further optimization will be continued at a later date.

Computational Study of the Supersonic Molecular Beam Injection in Thailand Tokamak-1 based on the 2D Fluid Model

Computational Study of the Supersonic Molecular Beam Injection in Thailand Tokamak-1 based on the 2D Fluid Model

Characteristics of edge temperature ring oscillation during the stationary improved confinement mode in EAST

The I-mode is a natural edge localized mode (ELM)-free regime with H-mode-like improved energy confinement and L-mode-like particle confinement, making it an attractive scenario for future tokamak-based fusion reactors. A kind of low-frequency oscillation has been widely observed, with a frequency between stationary zonal flow and geodesic-acoustic mode (GAM) zonal flow. In EAST, most stationary I-mode shots have such a mode, called edge temperature ring oscillation (ETRO). This mode probably plays an important role in development and maintenance of the I-mode , while investigations are needed to clarify the differences between ETRO and similar mode low-frequency oscillation in other devices, such as limit cycle oscillation (LCO). In this paper, the properties of ETRO are described in detail, including the structure of its magnetic components, its radial propagation characteristics, statistics of its central frequency, a linear analysis of the alternating transition turbulences and a comparison with GAM and LCO. Although some similarities can be found between ETRO and both GAM and LCO, the main features are not identical. ETRO is probably a novel type of finite frequency zonal flow or pressure gradient-induced drift that is unique to the I-mode. It is found that modest fueling can reduce ETRO intensity while maintaining I-mode confinement, suggesting that supersonic molecular beam injection could be used as an effective tool to control ETRO.

Development of dual-path multi-pass Thomson scattering system in GAMMA 10/PDX

A dual-path Thomson scattering (DPTS) system was developed to measure electron temperature and density in a GAMMA 10/PDX central cell (CC) and an end cell (EC). The DPTS comprises CC Thomson scattering system (CC-TS) and an EC Thomson scattering system (EC-TS). In the CC-TS, we developed a multipass system to improve the signal intensities and time resolution. In the EC-TS, a double-pass system was developed as a preliminary step in developing a multipass system. In a previous EC-TS, the optical components for the double-pass configuration were placed in a vacuum vessel. To reduce the influence of stray light and facilitate alignment, a hole for the double-pass laser path was made on the partition wall in the vacuum chamber such that the probe laser could be extracted into the atmosphere and the double-path configuration could be easily adjusted. We applied DPTS to detached plasma experiments and successfully measured the core and edge electron temperatures and densities with the detached plasma experiments with the additional application of supersonic molecular beam injection and central electron cyclotron heating in the core plasma to simultaneously introduce intermittent particle flux.

Experimental study on the characteristics of supersonic molecular beam injection

Experimental study on the characteristics of supersonic molecular beam injection

An innovative approach to the improved radiating divertor concept by supersonic molecular beam injection on HL-2A

The ‘puff-and-pump’ radiating divertor condition is a promising approach for the reduction of excessive thermal power loading on the divertor targets. The divertor supersonic molecular beam injection (SMBI) system installed on HL-2A strongly enhances the capacity of the divertor heat load control. This system has a faster time response than the divertor gas puffing (GP) system. Divertor heat load control capacity has been compared between these two techniques by experiments on HL-2A and simulations. Experimental results suggest that the divertor SMBI system reduces the divertor heat flux peak to a lower level with less time delay than that for the divertor GP system, owing to a higher injection rate and particle velocity. Simulations by SOLPS demonstrate that a high injection rate shortens the function time which is defined by the time interval between the arrival of the particles at the divertor plasma edge and the reduction of divertor electron temperature. High velocity of SMBI injected neutral gas particle also shortens the particle flight time to arrive at the edge. Less flight time and function time lead to a faster response of the divertor SMBI system compared to the divertor GP system. In addition, simulations by EMC3-EIRENE also suggest that, compared to the GP system, stronger heat load reduction of the SMBI system can also be caused by deeper deposition of gas source owing to its higher particle velocity.

Particle pump-out induced by trapped electron mode turbulence in electron cyclotron heated plasmas on XuanLong-50 spherical torus

Particle pump-out effects induced by low-frequency (<200 kHz) density fluctuations were observed in solely electron cyclotron wave (ECW)-heated plasmas on the spherical torus XuanLong-50 (EXL-50) without a central solenoid. The intensity of the relative density fluctuations increases with increasing ECW power and decays when the ECW is turned off while sustaining the plasma current. The electron densities are maintained relatively high and steady when the density fluctuations are completely absent, indicating that the outward transport of electrons is dominated by the particle pump-out effect of the ECW. The density fluctuations are modulated by a supersonic molecular beam injection pulse and the modulation amplitude decreases with increasing electron density at the same ECW injection power and decreasing ECW power at the same electron density, respectively. Analysis revealed that a critical value of electron temperature gradient (ETG) triggers the density fluctuations, and the intensity of the relative density fluctuations is positively correlated with the ETG and approximately inversely proportional to the effective collision frequency. With plasma parameters similar to those of EXL-50 experiments, the HD7 code simulations demonstrate that trapped electron mode (TEM) turbulence can be excited by ETG higher than the critical value observed in the experiment. In addition, the dependence of the mode growth rate (supposed to be proportional to the saturation level of fluctuations in quasi-linear theory) and the measured intensity of the density fluctuations is comparable. The simulated outward particle flux integrated over the poloidal wave number spectrum is significant and proportional to ETG. These observations demonstrate that the density fluctuation is TEM turbulence, which is driven by ETG and induces particle pump-out when the electron density/effective electron collision frequency is low. The potential relevance of this work with the controls of plasma profiles, impurities, helium ash, and heat transport in future reactors of similar low effective collision frequency is also discussed.

A coupled NS-DSMC method applied to supersonic molecular beam and experimental validation

In this paper, experimental and numerical studies are conducted on supersonic molecular beam injection (SMBI) for fusion devices based on steady-state assumption. To explore the characteristics of SMBI from continuous to rarefied flow, the flow information is predicted by solving the Navier-Stokes (NS) equation based CFD method and direct simulation Monte Carlo (DSMC) method. To verify the coupled NS-DSMC method, a flow measurement apparatus platform is designed and constructed. A moveable full-range vacuum gauge with Pitot probe in the vacuum chamber is employed to measure the spatial distribution of the beam pressure signal. The simulation results are converted into Pitot pressure, corrected for rarefaction effect and compared with the experimental data. It is observed that the experimental results are consistent with the simulation results. This study provides technical support for the accurate prediction of SMBI flow characteristics and optimization of the SMBI systems.

Coherent mode induced by supersonic molecular beam injection in EAST Ohmic plasmas

A coherent mode (CM) induced by the supersonic molecular beam injection (SMBI) was observed in Ohmic plasmas of EAST tokamak. The CM existed in the spectra of the electron temperature fluctuation and density fluctuation, but not in the spectrum of Mirnov fluctuation. The radial location of the CM was about 10 cm inside the separatrix (normalized minor radius ). In most cases, the CM has multiple discrete frequencies between with an interval of ∼1 kHz. Occasionally, there is only one dominant frequency. The poloidal scale of the CM (poloidal wavenumber , poloidal mode number ) is between that of typical ion turbulence and macroscopic magnetohydrodynamic activities. Statistical analysis results suggest that the onset of the CM requires a high temperature gradient and a relatively low density gradient. Furthermore, the transport analysis shows that the SMBI-caused perturbation propagates inward faster in plasmas with stronger CM, which implies that the CM might be correlated to a high local transport level. However, the impact of the CM on macroscopic plasma parameters including stored energy, energy confinement time, and SMBI-caused mean density fluctuation is too weak to be found in comparison with the random fluctuation and the measurement errors, which suggests that the CM does not have a clear influence on the global confinement.

The versatile fueling systems of WEST

New fueling systems have been designed for WEST, a full tungsten wall tokamak with divertor configuration. They consist of standard gas injection, supersonic molecular beam injection and pellets injection systems, each being able to fuel continuously the plasma discharge up to a duration of 1000s. A massive gas injection system is also implemented to mitigate the impact of disruptions and runaway electrons. The standard gas system consists of 4 independent lines allowing the injection of up to 4 different gas species during a plasma discharge through 11 injections areas, 4 of them being located in the divertors and in the private flux region. The pellet injection system is able to inject up to 10 pellets per second through 4 tracks: one on the low field side, and the other 3 on the high field side. The supersonic molecular beam injection system consists of 3 injectors located close to the midplane, two on the high field side and one on the low field side. On trigger, the massive gas injection system can deliver in a single puff up 800 Pa.m3 of noble gas in order to stop the plasma discharge. All the fueling systems have been characterized and can be used routinely on plasma.

Preliminary study of supersonic molecular beam injection in Thailand Tokamak-1 using 3D fluid model

Successful operation of a tokamak relies on effective and appropriate methods of plasma fueling. The development plan of Thailand Tokamak-1 (TT-1) will employ the supersonic molecular beam injection (SMBI) which delivers the fueling gas more effectively and deeper than the gas puffing method. In this work, we study the effect of SMBI on the plasma transport of TT-1 plasma by using 3D fluid simulation. The model includes the continuity equation, energy balance equation, momentum equation, continuity of fuel equation, and momentum of fuel. BOUT++ is then used to solve these equations by a finite difference method with the field-aligned coordinates in the edge region. The simulation shows that when hydrogen fuel gas is by the SMBI method from the low-field side into the plasma with a speed of 1000 m/s, the electron density in the edge locally rises as a result of the dissociation and ionization. The ion and electron temperatures then drop. After that, the density spread over the whole plasma volume within about 10 ms. When the injection speed increases, it results in a deeper penetration length of the fuel deposition.

Overview of runaway current suppression and dissipation on J-TEXT tokamak

The main works on disruption mitigation including suppression and mitigation of runaway current on the J-TEXT tokamak are summarized in this paper. Two strategies for the mitigation of runaway electron (RE) beams are applied in experiments. The first strategy enables the REs to be completely suppressed by means of supersonic molecular beam injection and resonant magnetic perturbation which can enhance RE loss, magnetic energy transfer which can reduce the electric field, and secondary massive gas injection (MGI) which can increase the collisional damping. For the second strategy, the runaway current is allowed to form but should be dissipated or soft landed within tolerance. It is observed that the runaway current can be significantly dissipated by MGI, and the dissipation rate increases with the injected impurity particle number and eventually stabilizes at 28 MA s−1. The dissipation rate of the runaway current can be up to 3 MA s−1 by ohmic field. Shattered pellet injection has been chosen as the main disruption mitigation method, which has the capability of injecting material deeper into the plasma for higher density assimilation when compared to MGI. Moreover, simulation works show that the RE seeds in the plasma are strongly influenced under different phases and sizes of 2/1 mode locked islands during thermal quench. The robust runaway suppression and runaway current dissipation provide an important insight on the disruption mitigation for future large tokamaks.

Studies of the low-energy neutral behavior using the low-energy neutral particle analyzer on EAST

• The neutral energy spectrums with an energy range of 20–3000 eV was measured by the low energy neutral particle analyzer (LENPA). • In the LENPA detection range, more than 85 % of neutral particles are in the energy range of 20–1000 eV. • The integrated neutral flux in the energy range of 20–1000 eV increases with line-averaged density in ohmic discharges due to the increased charge exchange reaction rates in a higher density plasma. • Due to the deeper penetration depth by SMBI fueling, the neutral particles are generated closer to the core plasma and have higher energy. • The neutral flux increases with heating power in all energy range. The neutral energy spectrums with an energy range of 20–3000 eV was measured by the low energy neutral particle analyzer (LENPA) in the 2021 EAST summer campaign. The proportion of the neutral particles below 1000 eV measured by LENPA is more than 85 %. In the ohmic discharges, the integrated neutral flux increases with increasing the line-averaged density. However, the neural particles with higher energy (1000–3000 eV) are insensitive to the line-averaged density. In addition, the neutral flux in the energy range of 20–1000 eV of the discharges with supersonic molecular beam injection (SMBI) was lower than those without SMBI, which was different from neutrals in the range of 1000–3000 eV. The neutral flux increases with higher auxiliary heating power in all energy range. Compared to the ohmic discharges, the mean energy of neutral particles in auxiliary heated discharges is increased significantly.

Real-time disruption prediction in the plasma control system of HL-2A based on deep learning

A real-time disruption predictor based on deep learning method is implemented into the Plasma Control System (PCS) of HL-2A. It has a total accuracy of 89.0% during the online testing of Shot Nos. 38,650–39,347 in HL-2A. 32 false alarms are triggered during the 142 non-disruptive shots and 10 disruptions are missed in the 240 disruptive shots. In several shots, this disruption prediction algorithm is used to trigger the disruption mitigation system, namely, massive gas injection (MGI) and supersonic molecular beam injection (SMBI) in this experiment. For MGI, there is a 12-millisecond delay between the trigger signal and the mitigated disruption. In the 240 disruptive shots, 81.5% of them are predicted with more than 12 ms in advance and thus can be successfully mitigated. For SMBI, the delay is 4 millisecond and 91.3% of the disruptions can be successfully mitigated. In general, although dedicated work is needed to close the gap between online disruption prediction and the offline algorithms, the effectiveness of deep learning-based disruption predictors in real-time environments is validated in this research.

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 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 of the RMP intensity profiles of the RMP field is consistent with the phase profile of the toroidal rotation perturbation.

Development and implementation of Divertor Fast Particles Injection for EAST tokamak

The divertor is the main device in the tokamak that is subjected to heat loads, excessive high heat flux could cause damage to the divertor target. Divertor heat flux control is very important for magnetically confined fusion devices. As a method for divertor heat flux control, the Divertor Fast Particles Injection (DFPI) is successfully developed and implemented on the Experimental Advanced Superconducting Tokamak (EAST). The components of DFPI are a high-pressure gas source (0.6–1 MPa), a pulse solenoid valve, a nozzle and the pipeline etc. Compared with the Supersonic Molecular Beam Injection (SMBI) particle injection from midplane, the particles injected by DFPI are more difficult to enter the plasma core and have less impact on the performance of the core plasma; Compared with the gas puffing (GP) particle injection from the divertor, the particles injected by DFPI can enter the divertor area faster, the delay time of DFPI is about 25 ms. The successful implementation of DFPI provides an important tool for divertor heat flux control on EAST.

Progress of HL-2A experiments and HL-2M program

Since the last IAEA Fusion Energy Conference in 2018, significant progress of the experimental program of HL-2A has been achieved on developing advanced plasma physics, edge localized mode (ELM) control physics and technology. Optimization of plasma confinement has been performed. In particular, high-β N H-mode plasmas exhibiting an internal transport barrier have been obtained (normalized plasma pressure β N reached up to 3). Injection of impurity improved the plasma confinement. ELM control using resonance magnetic perturbation or impurity injection has been achieved in a wide parameter regime, including types I and III. In addition, impurity seeding with supersonic molecular beam injection or laser blow-off techniques has been successfully applied to actively control the plasma confinement and instabilities, as well as plasma disruption with the aid of disruption prediction. Disruption prediction algorithms based on deep learning are developed. A prediction accuracy of 96.8% can be reached by assembling a convolutional neural network. Furthermore, transport resulting from a wide variety of phenomena such as energetic particles and magnetic islands has been investigated. In parallel with the HL-2A experiments, the HL-2M mega-ampere class tokamak was commissioned in 2020 with its first plasma. Key features and capabilities of HL-2M are briefly presented.

Physics design of new lower tungsten divertor for long-pulse high-power operations in EAST

A new lower tungsten divertor has been developed and installed in the EAST superconducting tokamak to replace the previous graphite divertor with power handling capability increasing from <2 MW m−2 to ∼10 MW m−2, aiming at achieving long-pulse H-mode operations in a full metal wall environment with the steady-state divertor heat flux of ∼10 MW m−2. A new divertor concept, ‘corner slot’ (CS) divertor, has been employed. By using the ‘corner effect’, a strongly dissipative divertor with the local buildup of high neutral pressure near the corner can be achieved, so that stable detachment can be maintained across the entire outer target plate with a relatively lower impurity seeding rate, at a separatrix density compatible with advanced steady-state core scenarios. These are essential for achieving efficient current drive with low-hybrid waves, a low core impurity concentration and thus a low loop voltage for fully non-inductive long-pulse operations. Compared with the highly closed small-angle-slot divertor in DIII-D, the new divertor in EAST exhibits the following merits: (1) a much simpler geometry with integral cassette body structure, combining vertical and horizontal target plates, which are more suitable for actively water-cooled W/Cu plasma facing components, facilitating installation precision control for minimizing surface misalignment, achieving high engineering reliability and lowering the capital cost as well; (2) it has much greater flexibility in magnetic configurations, allowing for the position of the outer strike point on either vertical or horizontal target plates to accommodate a relatively wide triangularity range, δ l = 0.4–0.6, thus enabling to explore various advanced scenarios. A water-cooled copper in-vessel coil has been installed under the dome. Five supersonic molecular beam injection systems have been mounted in the divertor to achieve faster and more precise feedback control of the gas injection rate. Furthermore, this new divertor allows for double null divertor operation and slowly sweeping the outer strike point across the horizontal and vertical target plates to spread the heat flux for long-pulse operations. Preliminary experimental results demonstrate the ‘corner effect’ and are in good agreement with simulations using SOLPS-ITER code including drifts. The EAST new divertor provides a test-bed for the closed divertor concept to achieve steady-state detachment operation at high power. Next step, a more closed divertor, ‘sharp-cornered slot’ divertor, building upon the current CS divertor concept, has been proposed as a candidate for the EAST upper divertor upgrade.