Molecular Beam Injection Research Articles

The neuroendocrine-immune regulatory network characterized as simple structure but complex function in oyster Crassostrea gigas

The characteristics of the divertor particle and heat fluxes are investigated during ELM bursts in ELMy H-mode plasmas with the lower single null (LSN) configuration in Korea Superconducting Tokamak Advanced Research (KSTAR). The particle and heat fluxes are evaluated from the electric probe measurements at the divertor region. It is found that the peak amplitude of the divertor flux during an ELM burst obtained near the outer strike point (OSP) decreases up to about 20% as the ELM frequency increases by a factor of ∼6.5 due to the ELM mitigation and the plasma shaping, which is similar to the trend of the amplitude versus the frequency of the ELM observed in other tokamaks. The ELMs are mitigated by using several methods as magnetic perturbation (MP) field, supersonic molecular beam injection (SMBI) and electron cyclotron heating (ECH) at the edge region. In addition, the particle flux, evaluated at the far scrape-off layer (SOL) region, is less than 1% of the divertor particle flux. In this work, results from the experimental investigations of particle and heat fluxes during ELM bursts from the electric probe measurements at the divertor and far SOL regions are presented.

Study on divertor particle and heat fluxes from electric probe measurements during ELMy H-modes in KSTAR

The characteristics of the divertor particle and heat fluxes are investigated during ELM bursts in ELMy H-mode plasmas with the lower single null (LSN) configuration in Korea Superconducting Tokamak Advanced Research (KSTAR). The particle and heat fluxes are evaluated from the electric probe measurements at the divertor region. It is found that the peak amplitude of the divertor flux during an ELM burst obtained near the outer strike point (OSP) decreases up to about 20% as the ELM frequency increases by a factor of ∼6.5 due to the ELM mitigation and the plasma shaping, which is similar to the trend of the amplitude versus the frequency of the ELM observed in other tokamaks. The ELMs are mitigated by using several methods as magnetic perturbation (MP) field, supersonic molecular beam injection (SMBI) and electron cyclotron heating (ECH) at the edge region. In addition, the particle flux, evaluated at the far scrape-off layer (SOL) region, is less than 1% of the divertor particle flux. In this work, results from the experimental investigations of particle and heat fluxes during ELM bursts from the electric probe measurements at the divertor and far SOL regions are presented.

Comparison of ELM-Filament Mitigation Between Supersonic Molecular Beam Injection and Pellet Injection on HL-2A**supported by National Natural Science Foundation of China (Nos. 11075046, 10975049, 11375054, 11275060), the National Magnetic Confinement Fusion Science Program of China (Nos. 2013GB112008, 2013GB107000, 2013GB104002, 2014GB107000, 2014GB108000), and the China-Korean Joint Research Program (No. 2012DFG02230)

On HL-2A, two different injections (supersonic molecular beam injection (SMBI) and pellet injection (PI)) are used to mitigate edge localized mode (ELM)-filament convective transport. The changes of their characteristics are studied in this paper. A high spatiotemporal resolution probe shows there are many similar phenomena, and the filament density amplitude and radial velocity are both suppressed. Our statistical results indicate that: the velocity suppression comes from the decrease of filament density and temperature; the transient particle and heat fluxes drop strongly; and long-range correlation along a magnetic flux surface also decreases, when the electron-ion collisionality increases significantly, which may have a role on the filament parallel current during ELM mitigation.

Effect of supersonic molecular beam injection deposition on edge localized mode mitigation in the HL-2A H-mode plasmas

The ELM (edge localized mode) mitigation by SMBI (supersonic molecular beam injection) has been studied in the HL-2A H-mode plasmas. The ELM mitigation effect and its relationship with the deposition position of SMBI in the H-mode pedestal are reported for the first time experimentally. We found that when the deposition of SMBI is about 20% into the pedestal, the best ELM mitigation effect is achieved, which is identified by a significant increase of the ELM frequency and also a decrease of the ELM amplitude. The theoretical inference that no deep injection is needed is confirmed. The sand-pile model is used to simulate the ELM burst and explain the mitigation effect for different SMBI deposited positions. It is found that the gradient threshold is a key parameter in the process of the ELM mitigation, and there should be a local gradient threshold in the middle of the pedestal. When the deposition of SMBI is close to the edge of this region, the best ELM mitigation effect can be observed.

Edge localized mode characteristics during edge localized mode mitigation by supersonic molecular beam injection in Korea Superconducting Tokamak Advanced Research

It has been reported that supersonic molecular beam injection (SMBI) is an effective means of edge localized mode (ELM) mitigation. This paper newly reports the changes in the ELM, plasma profiles, and fluctuation characteristics during ELM mitigation by SMBI in Korea Superconducting Tokamak Advanced Research. During the mitigated ELM phase, the ELM frequency increased by a factor of 2–3 and the ELM size, which was estimated from the Dα amplitude, the fractional changes in the plasma-stored energy and the line-averaged electron density, and divertor heat flux during an ELM burst, decreased by a factor of 0.34–0.43. Reductions in the electron and ion temperatures rather than in the electron density were observed during the mitigated ELM phase. In the natural ELM phase, frequency chirping of the plasma fluctuations was observed before the ELM bursts; however, the ELM bursts occurred without changes in the plasma fluctuation frequency in the mitigated ELM phase.

Overview of recent HL-2A experiments

Since the last Fusion Energy Conference, the HL-2A experiment has made significant progress in the following areas: (i) physics on low-intermediate-high (L-I-H) transition and pedestal dynamics; (ii) magnetohydrodynamic (MHD) and energetic-particle (EP) physics; (iii) interaction between neoclassical tearing modes (NTMs) and non-local transport; (iv) and edge localized mode (ELM) mitigation by supersonic molecular beam injection (SMBI). Two types of limit-cycle-oscillations (LCOs) were observed in the intermediate phase (I-phase), which indicated a second type of predator-prey process between turbulence and pressure gradient in addition to the conventional predator–prey involving zonal flows and turbulence. It was found that a kink-type MHD mode often crashed prior to the I-H transition, which played a crucial role in triggering H-mode by increasing the edge pressure gradient and E × B flow shear and consequently suppressing turbulent transport. It was also found that impurity concentration played an important role in the multi-transitions between ELM-free H-mode and I-phase. Besides, a quasi-coherent mode around 50–100 kHz was found to be associated with pedestal density gradient saturation in ELM-free H-mode. For the first time, two types of magnetic activities with n = 0 were observed in the presence of strong tearing modes. Fourier bicoherence analysis suggested that these modes were generated by the nonlinear coupling between Alfvén eigenmodes (AEs) and low-frequency MHD modes. Up- and down-sweeping reverse shear Alfvén eigenmodes (RSAEs) were identified experimentally. The transitions between fishbone and long-lived mode (LLM) were also investigated. The results showed that fishbone could transit from/to LLM and even trigger tearing modes (TMs). For non-local transport, key characteristics of enhanced avalanches in the theory of self-organized criticality (SOC) were identified, including high Hurst exponents and large-scale radial propagation. Experimental results revealed a close correlation between NTMs and non-local transport. In addition, studies on ELM mitigation by SMBI have also progressed substantially.

Evidence of enhanced self-organized criticality (SOC) dynamics during the radially non-local transient transport in the HL-2A tokamak

Self-organized criticality (SOC) dynamics have been investigated in non-local transport regimes induced by the supersonic molecular beam injection in the HL-2A tokamak. Experimental evidence shows that the SOC or avalanche behaviors, such as the Hurst parameter, self-similarity and large-scale radial correlations in turbulence, are remarkably enhanced during the prompt non-local phase, together with an increase of inward propagation of turbulent events. These results highlight the importance of SOC paradigm during the transient non-local thermal transport in magnetically confined plasmas.

Advances in H-mode physics for long-pulse operation on EAST

Since the 2012 International Atomic Energy Agency Fusion Energy Conference (IAEA-FEC), significant advances in both physics and technology has been made on the Experimental Advanced Superconducting Tomakak (EAST) toward a long-pulse stable high-confinement (H-mode) plasma regime. The experimental capabilities of EAST have been technically upgraded with the power enhancement (source power up to 26 MW) of the continuous-wave heating and current drive system, replacement of the upper graphite divertor with an ITER-like W monoblock divertor, and installation of a new internal cryopump in the upper divertor and a set of 16 in-vessel resonant magnetic perturbation (RMP) coils. This new upgrade enables EAST to be a unique operating device capable of investigating ITER-relevant long-pulse high-performance operations with dominant electron heating and low torque input within the next 5 years. Remarkable physics progress in controlling transient and steady-state divertor heat fluxes has been achieved on EAST, e.g. (i) edge-localized mode (ELM) mitigation/suppression with a number of attractive methods including lower hybrid wave (LHW), supersonic molecular beam injection (SMBI), RMPs, and real-time Li aerosol injection; and (ii) active control of steady-state power distribution by the synergy of LHW and SMBI. In the 2014 experimental campaign, a long-pulse high-performance H-mode plasma with H98 ∼ 1.2 has been obtained with a duration over 28 s (∼200 times the energy confinement time). In addition, several new experimental advances have been achieved in the last EAST campaign, including: (i) high-performance H-mode with βN ∼ 2 and stored plasma energy ∼220 kJ; (ii) H-mode plasma sustained by neutral beam injection (NBI) alone or modulated NBI with lower hybrid current drive (LHCD), for the first time in EAST; (iii) high current drive efficiency and nearly full noninductive plasmas maintained by the new 4.6 GHz LHCD system; (iv) demonstration of a quasi-snowflake divertor configuration; and (v) observation of a new edge-coherent mode and its effects on edge transport in H-mode plasmas.

A Supersonic Molecular Beam Injection System on the J-TEXT Tokamak

A supersonic molecular beam injection (SMBI) system has been developed as a new fueling method for the J-TEXT tokamak. According to experimental needs, the system can provide various supersonic gas jets with the pressure range 0.2–2.0 MPa and pulse duration 0.2–10 ms. In this paper, the structure of the system is described in detail, including injectors, controller and gas line. Wave forms of the controller output show a fast response of the system. The evolution of pressure curve in the vacuum chamber indicates the system works well. Test results also show that the particle number of SMBI is linear proportional to the product of gas pressure and pulse duration. For different pressure of 0.5, 1.0 and 1.66 MPa, the beam velocities are estimated about 0.9, 1.2 and 1.4 km/s, respectively. Besides, fuelling experiments with the SMBI system are performed. Significant density increase demonstrates the fuelling abilities of both LSMBI and HSMBI.

Comparisons between tokamak fueling of gas puffing and supersonic molecular beam injection in 2D simulations

Plasma fueling with high efficiency and deep injection is very important to enable fusion power performance requirements. It is a powerful and efficient way to study neutral transport dynamics and find methods of improving the fueling performance by doing large scale simulations. Two basic fueling methods, gas puffing (GP) and supersonic molecular beam injection (SMBI), are simulated and compared in realistic divertor geometry of the HL-2A tokamak with a newly developed module, named trans-neut, within the framework of BOUT++ boundary plasma turbulence code [Z. H. Wang et al., Nucl. Fusion 54, 043019 (2014)]. The physical model includes plasma density, heat and momentum transport equations along with neutral density, and momentum transport equations. Transport dynamics and profile evolutions of both plasma and neutrals are simulated and compared between GP and SMBI in both poloidal and radial directions, which are quite different from one and the other. It finds that the neutrals can penetrate about four centimeters inside the last closed (magnetic) flux surface during SMBI, while they are all deposited outside of the LCF during GP. It is the radial convection and larger inflowing flux which lead to the deeper penetration depth of SMBI and higher fueling efficiency compared to GP.

Enhanced confinement scenarios without large edge localized modes in tokamaks: control, performance, and extrapolability issues for ITER

Large edge localized modes (ELMs) typically accompany good H-mode confinement in fusion devices, but can present problems for plasma facing components because of high transient heat loads. Here the range of techniques for ELM control deployed in fusion devices is reviewed. Two strategies in the ITER baseline design are emphasized: rapid ELM triggering and peak heat flux control via pellet injection, and the use of magnetic perturbations to suppress or mitigate ELMs. While both of these techniques are moderately well developed, with reasonable physical bases for projecting to ITER, differing observations between multiple devices are also discussed to highlight the needed community R&D. In addition, recent progress in ELM-free regimes, namely quiescent H-mode, I-mode, and enhanced pedestal H-mode is reviewed, and open questions for extrapolability are discussed. Finally progress and outstanding issues in alternate ELM control techniques are reviewed: supersonic molecular beam injection, edge electron cyclotron heating, lower hybrid heating and/or current drive, controlled periodic jogs of the vertical centroid position, ELM pace-making via periodic magnetic perturbations, ELM elimination with lithium wall conditioning, and naturally occurring small ELM regimes.

Energy deposition onto HL-2A divertor plates in ELMy H-mode discharges using infrared thermography

Using infrared (IR) thermography, power loads onto the divertor plates have been investigated in ELMy H-mode plasmas on HL-2A. In the ELMy H-mode discharges, ELMs are the largest contributors to the divertor target energy load. Analysis of energy balance shows that up to 45% of the energy losses are deposited onto the divertor targets during ELMs and about 30% are found as plasma radiation. Moreover, divertor heat flux mitigation has been achieved during an ELMy H-mode phase by using Supersonic Molecular Beam Injection (SMBI), characterized by a sharp increase of ELM frequency and a reduction in peak heat flux. The increased plasma radiation energy losses, especially the doubled plasma radiation in the divertor region, should be responsible for the reduction of integrated energy deposition onto divertor targets.

Active control of divertor heat and particle fluxes in EAST towards advanced steady state operations

Significant progress has been made in EAST towards advanced steady state operations by active control of divertor heat and particle fluxes. Many innovative techniques have been developed to mitigate transient ELM and stationary heat fluxes on the divertor target plates. It has been found that lower hybrid current drive (LHCD) can lead to edge plasma ergodization, striation of the stationary heat flux and lower ELM transient heat and particle fluxes. With multi-pulse supersonic molecular beam injection (SMBI) to quantitatively regulate the divertor particle flux, the divertor power footprint pattern can be actively modified. H-modes have been extended over 30s in EAST with the divertor peak heat flux and the target temperature being controlled well below 2MW/m2 and 250°C, respectively, by integrating these new methods, coupled with advanced lithium wall conditioning and internal divertor pumping, along with an edge coherent mode to provide continuous particle and power exhaust.

The electron density profile measurement with a fast time-resolution in the KSTAR tokamak

A frequency-modulation reflectometer is installed in the KSTAR tokamak to measure the plasma density profile. The reflectometer operates in the frequency range of 32.3–107.3 GHz. A plasma density up to 7 × 1019 m−3 can be measured when the toroidal magnetic field is 2 T. The reflectometer measures the electron density profile with a time resolution of 25 μs for 0.16 s. This fast time-resolution is essential for the study of the transport barrier formation during the L-H transition. In this paper, the measured time evolutions of the sawtooth oscillation, the density increase due to supersonic molecular beam injection (SMBI), and the pedestal relaxation due to an edge-localized mode (ELM) are presented. The spatial profile of slow fluctuations (< 10 kHz) may be extracted from these fast profile measurements. In addition, the dependences of the pedestal structure, such as its width and height, on the plasma parameters are characterized.

ELM mitigation by means of supersonic molecular beam and pellet injection on the EAST superconducting tokamak

In this paper, we will present experimental results from EAST on the mitigation of edge localized modes (ELMs) using recently developed deuterium/lithium pellet injections as well as supersonic molecular beam injections (SMBI). Using a Laval nozzle, ELM mitigation with SMBI has been demonstrated in EAST in quasi-steady state. Using a D2 pellet injector, a giant ELM appears followed by a burst of high frequency ELMs at ∼300Hz with duration of a few tens of milliseconds. Furthermore, for the first time, a novel technology using a simple rotating impeller to inject sub-millimeter size lithium (Li) granules at speeds of a few tens of meters per second was successfully used to pace ELMs. These experiments indicate that, on EAST, several technologies can contribute to the database supporting ELMs control in future fusion devices, such as ITER.

Experimental study of the cold front propagation in the plasma shut-down experiment in the J-TEXT tokamak

Mitigation of major disruptions is essential in achieving fusion energy as a commercial energy source. Many tokamaks are using massive gas injection (MGI) as the disruption mitigation method since it is the most prospective potential disruption mitigation technique at present. However, mitigation efficiency by gas jet is limited by the shallow penetration of the gas jet which results in low gas mixing efficiency. In order to improve the mixture efficiency, the propagation of the cold front induced by supersonic molecular beam injection and the interaction between the cold front and the q = 2 surface have been studied in the J-TEXT tokamak.

Intermittent convective transport suppressed by supersonic molecular beam injection on the HL-2A tokamak

Edge localized mode (ELM)-filament statistical characteristics and intermittent convective transport are studied during ELM mitigation (10–30 ms) induced by supersonic molecular beam injection (SMBI) on the HL-2A tokamak. Langmuir probe data show that the burst rate of large amplitude filaments (δne > 4σ) decreases by 30–50% and their radial velocity decreases by 40–60%, which is an indication of an amplitude reduction. A decrease of long-range correlation along the magnetic flux surface is observed after SMBI, indicating a role of electron–ion collisionality on the filament parallel current in the ELM mitigation time. The transient particle and heat fluxes decrease about 40–50% and 50–60%, respectively. These observations indicate that SMBI might be an effective method to suppress the intermittent convective transport.

The influence of Laval nozzle throat size on supersonic molecular beam injection

In this study, finite element analysis (FEA) has been used to investigate the effects of different Laval nozzle throat sizes on supersonic molecular beam. The simulations indicate the Mach numbers of the molecular stream peak at different positions along the center axis of the beam, which correspond to local minimums of the molecular densities. With the increase of the throat diameter, the first peak of the Mach number increases first and then decreases, while that of the molecular number density increases gradually. Moreover, both first peaks shift progressively away from the throat. At the last part, we discuss the possible applications of our FEA approach to solve some crucial problems met in modern transportations.

Effect of supersonic molecular-beam injection on edge fluctuation and particle transport in Heliotron J

Edge fluctuation in a supersonic molecular-beam injection (SMBI) fueled plasma has been measured using an electrostatic probe array. After SMBI, the plasma stored energy (Wp) temporarily decreased then started to increase. The local plasma fluctuation and fluctuation induced particle transport before and after SMBI have been analyzed. In a short duration (∼4 ms) just after SMBI, the density fluctuation of broad-band low frequency increased, and the probability density function (PDF) changed from a nearly Gaussian to a positively skewed non-Gaussian one. This suggests that intermittent structures were produced due to SMBI. Also the fluctuation induced particle transport was greatly enhanced during this short duration. About 4 ms after SMBI, the low frequency broad-band density fluctuation decreased, and the PDF returned to a nearly Gaussian shape. Also the fluctuation induced particle transport was reduced. Compared with conventional gas puff, Wp degradation window is very short due to the short injection period of SMBI. After this short degradation window, fluctuation induced particle transport was reduced and Wp started the climbing phase. Therefore, the short period of the influence to the edge fluctuation might be an advantage of this novel fueling technique. On the other hand, although their roles are not identified at present, coherent MHD modes are also suppressed as well by the application of SMBI. These MHD modes are thought to be de-exited due to a sudden change of the edge density and/or excitation conditions.

2D simulations of transport dynamics during tokamak fuelling by supersonic molecular beam injection

Time-dependent transport of both plasma and neutrals is simulated during supersonic molecular beam injection (SMBI) yielding the evolution of edge plasma and neutral profiles. The SMBI model is included as a module, called trans-neut, within the original BOUT++ boundary plasma turbulence code. Results of calculations are reported for the realistic divertor geometry of the HL-2A tokamak. The model can also be used to study the effect of gas puffing. A seven-field fluid model couples plasma density, heat, and momentum transport equations together with neutral density and momentum transport equations for both molecules and atoms. Collisional interactions between molecules, atoms, and plasma include dissociation, ionization, recombination and charge-exchange effects. Sheath boundary conditions and particle recycling are applied at both the wall and the divertor plates. A localized boundary condition of constant molecular flux (product of density times speed) is applied at the outermost flux surface to model the SMBI. Steady state profiles with and without particle recycling are achieved before SMBI. During SMBI, the simulation shows that neutrals can penetrate several centimetres inside the last closed (magnetic) flux surface (LCFS). Along the SMBI path, plasma density increases while plasma temperature decreases. The molecule penetration depth depends on both the SMBI flux and the initial plasma density and temperature along its path. As the local plasma density increases substantially, molecule and atom penetration depths decrease due to their higher dissociation and ionization rates if the electron temperature does not drop too low. Dynamic poloidal spreading of the enhanced plasma density region is observed due to rapid ion flow along the magnetic field (parallel) driven by a parallel pressure asymmetry during SMBI. Profile relaxation in the radial and poloidal directions is simulated after SMBI termination, showing that the plasma returns to pre-SMBI conditions on a time scale of 60 ms.