Suppression Of Runaway Electrons Research Articles

Suppression of runaway electrons by injection of argon gas in the low-density discharges in the STOR-M tokamak

Behaviors of runaway electrons have been investigated by injection of argon impurity gas during the low-density hydrogen discharges in the STOR-M tokamak. Argon gas is puffed using a fast electromagnetic gas puffing valve with various amounts of gas puffed at different injection times. It has been found that puffing of an appropriate amount of gas can suppress the already generated runaway electrons evidenced by the intensity reduction of the hard X-ray (HXR) radiation caused by the impact of runaway electrons on the STOR-M chamber wall. On the other hand, excessive argon gas puffing causes earlier termination of the discharge.

Magnetohydrodynamic features and particle transport during thermal quench in HL-2A with density source

Diverse disruption mitigation strategies based on massive material injection have been developed in recent decades, aiming to uniformly deplete the thermal energy stored within plasmas during the thermal quench (TQ) while simultaneously elevating electron density to facilitate runaway electron suppression. Irrespective of the detailed dynamics of the material delivery scheme, deposition location and subsequent density mixing are pivotal in achieving highly efficient mitigation, however, which are markedly influenced by magnetohydrodynamic (MHD) activities. In order to assess the influence of MHD-induced transport on disruption mitigation, a simulation of TQ triggered by pure deuterium (D) deposition is conducted using a three-dimensional (3D) nonlinear reduced MHD code, JOREK. Steady density sources (the deposition rate of 1024 D atoms per second is greater than in real experiments) are introduced at various locations to explore the dynamics. The findings distinctly reveal two types of TQ processes, contingent on locations of deposition (LoD) of the neutral D source. Evidently, the results underscore the effectiveness of proper density mixing and moderated MHD in disruption mitigation. Nonlinear mode coupling emerges as a significant factor in shaping the final outcomes of TQ. Specifically, the 5/2 mode contributes to edge collapse, whereas the 3/2 mode is instrumental in core collapse. Additionally, the investigation indicates that the rise in core density is contingent on LoD, exhibiting threshold behavior. This threshold is observed within the q = 2/1 surface of equilibria, and a rapid increase in core density is witnessed when the density source crosses this threshold. The outcomes point toward the important role of E × B convection due to the 1/1 mode evolution in the process.

Collisional-radiative simulation of impurity assimilation, radiative collapse and MHD dynamics after ITER shattered pellet injection

Recent studies suggest significant time delay between the Shattered Pellet Injection (SPI) fragment arrival and the temperature radiative collapse could exist in ITER, depending on the impurity assimilation and the plasma thermal reservoir. Hence in some cases the fragments could reach the core even before the edge radiative collapse occurs and triggers strong stochastic transport. This could be beneficial for heat load mitigation and hot-tail runaway electron suppression. To investigate the expected assimilation and radiation, thus the magneto-hydrodynamic (MHD) response after SPIs in 3D, we carry out simulations of collisional–radiative impurity mixed SPIs into ITER L-mode equilibrium. Localized cooling around the fragments is found to cause current perturbations which destabilize MHD modes. Meanwhile, slower injections are found to result in stronger and more complete radiative collapse, thus stronger MHD amplitude. Due to the q = 1 surface enclosing a significant volume, the resistive kink mode is shown to couple with outer modes to bring global stochasticity and convective core density mixing, although a transport barrier outside of the q = 1 surface prevents immediate temperature relaxation over the whole plasma. The impact of various physical assumptions and numerical treatments, such as the use of the flux-averaged ambient plasma parameters for ablation calculation, the exclusion of the magnetic constraining effect in ablation, the localization of the density source and the use of constant parallel thermal conduction instead of the Braginskii one and different injection velocities are also investigated. In general, stronger and more localized ablation results in stronger radiation, faster radiative collapse and a more violent MHD response, while the assimilation changes little due to a self-regulation effect.

Effect of the relative phase between pre-existing 2/1 and 3/1 magnetic islands on the suppression of runaway electrons on J-TEXT

In the experiments of actively triggering plasma disruption by massive gas injection, the externally applied resonant magnetic perturbation has been used to mitigate the hazard of runaway electron (RE). Motivated by the experiment of multimode coupling to suppress REs on J-TEXT, some typical simulation cases with non-ideal MHD with rotation-open discussion (NIMROD) code are carried out to explore the influential mechanism of different relative phases between m/n = 2/1 and m/n = 3/1 magnetic islands on the confinement of REs. Results show that the RE confinement is drastically affected by the relative phase between 2/1 and 3/1 magnetic islands. When the O point phase of 2/1 and 3/1 magnetic islands is toroidal 330°, REs can be effectively lost. The fitting curve of the remaining ratio of REs vs. the relative toroidal phase is predicted to approximate a sine-like function dependence. Further studies indicate that the phase difference between coexisting 2/1 and 3/1 islands can affect the radial transport of impurities. The loss of runaway electrons is closely related to the deposition effect of impurity. The impurity is easier to spread into the core region with smaller poloidal phase difference between the radial velocity of impurity and the impurity quantity of Ar.

Numerical Study on the Effect of Plasma Density on Runaway Electron Suppression in the ADITYA-U Tokamak

Runaway Electrons (REs) generated during plasma disruptions in fusion grade tokamaks have the potential to severely damage the plasma-facing components. Designing optimal plasma discharge scenarios for RE suppression in future experiments requires interpretative modeling of current experiments. Multiple experiments have been carried out on ADITYA-U tokamak to design optimal plasma discharge scenarios for RE avoidance and suppression. In this article, we have numerically studied two representative pairs of ADITYA-U plasma discharges. In the first pair of discharges, plasma density was increased by gas puffs during the flap-top phase which is shown to suppress RE generation. In the second pair of the representative discharges, the effect of a lower ratio of peak electric field during breakdown to the pre-fill pressure is shown on RE generation. We simulate these plasma discharges using the PREDICT code to study the dynamics of Res. The results are consistent with the experimental Hard X-Ray diagnostic observations for both the pair of representative discharges. Additionally, the lowest RE-current is obtained in the discharges with a low ratio of the peak electric field during breakdown to the pre-fill pressure and high plasma density during the flap-top phase due to gas puffs. The suppression of REs is demonstrated by showing the effect of increasing plasma density on the separatrix in momentum space between thermal electrons and REs.

Deterministic scattering of relativistic electron beams by off-resonant circularly polarized electromagnetic waves

A mechanism is presented whereby relativistic electron beams localized in phase space are deterministically scattered by coherent circularly polarized electromagnetic waves without stochastic processes. It is shown via an exact single-particle analysis that the condition for maximal scattering is an off-resonant condition, contrary to previous kinetic analyses that predict maximal diffusion or interaction at exact resonance or its harmonics. The mechanism, verified by single-particle simulations, enables a fast, nonlinear redistribution of the beam particles. A possible application of this mechanism to runaway electron suppression is presented.

The effect of 2/1 pre-existing magnetic islands width on the suppression of runaway electrons in disruption simulations of J-TEXT

The suppression of runaway electrons (REs) is influenced by the size of pre-existing 2/1 magnetic islands. Simulations of argon (Ar) massive gas injection into J-TEXT plasmas with pre-existing 2/1 magnetic islands of different width are performed with the 3D magnetohydrodynamic code called NIMROD (Sovinec et al 2004 J. Comput. Phys. 195 355). Results show that the ratio of remaining REs is not monotonically dependent on the width of pre-existing 2/1 magnetic islands. Large enough pre-existing 2/1 magnetic islands can lead to strong magnetic perturbation and strong magnetic surface stochasticity to expel the REs. The critical width for the suppression of REs is to be 0.31 as the minor radius. Under the condition of suitable small pre-existing 2/1 magnetic islands width, with island width ranging from 0.07 to 0.11 times the minor radius, the mitigation of REs is achieved. The duration of magnetic perturbations δB/B(n = 1) (exceeding (4 to 6) × 10−3) is a key for RE loss, and high amplitude of high n modes plays an important role in RE loss.

Generation and suppression of runaway electrons in MST tokamak plasmas

This paper explores the behavior of runaway electrons in tokamak plasmas at low electron density, plasma current, and magnetic field using experimental data from the Madison Symmetric Torus (MST) and computational data from the NIMROD nonlinear resistive 3D MHD code. Density thresholds for the onset and suppression of runaway electrons are determined experimentally in steady tokamak plasmas, and in plasmas with a population of runaway electrons, resonant magnetic perturbations with different poloidal mode numbers are applied. Poloidal mode number m = 3 perturbations suppress the runaway electrons, while perturbations with m = 1 have little effect. This difference is consistent with the difference in computed magnetic topologies. The m = 1 RMP has little effect on the topology, while the m = 3 RMP produces a broad region of stochasticity, which can allow for rapid loss of the runaway electrons.

Overview of the recent experimental research on the J-TEXT tokamak

Recent J-TEXT research has highlighted the significance of the role that non-axisymmetric magnetic perturbations, so called three-dimensional (3D) magnetic perturbation (MP) fields, play in a fundamentally 2D concept, i.e. tokamaks. This paper presents the J-TEXT results achieved over the last two years, especially on the impacts of 3D MP fields on magnetohydrodynamic instabilities, plasma disruptions and plasma turbulence transport.On J-TEXT, the resonant MP (RMP) system, capable of providing either a static or a high frequency (up to 8 kHz) rotating RMP field, has been upgraded by adding a new set of 12 in-vessel saddle coils. The shattered pellet injection system was built in J-TEXT in the spring of 2018. The new capabilities advance J-TEXT to be at the forefront of international magnetic fusion facilities, allowing flexible study of 3D effects and disruption mitigation in a tokamak.The fast rotating RMP field has been successfully applied for avoidance of mode locking and the prevention of plasma disruption. A new control strategy, which applies pulsed RMP to the tearing mode only during the accelerating phase region, was proved by nonlinear numerical modelling to be efficient in accelerating mode rotation and even completely suppresses the mode. Remarkably, the rotating tearing mode was completely suppressed by the electrode biasing. The impacts of 3D magnetic topology on the turbulence has been investigated on J-TEXT. It is found that the fluctuations of electron density, electron temperature and plasma potential can be significantly modulated by the island structure, and a larger fluctuation level appears at the X-point of islands. The suppression of runaway electrons during disruptions is essential to the operation of ITER, and it has been reached by utilizing the 3D magnetic perturbations on J-TEXT. This may provide an alternative mechanism of runaway suppression for large-scale tokamaks and ITER.

Suppression of runaway electrons by mode locking during disruptions on J-TEXT

The generation of runaway electrons during disruptions poses a serious threat for the operation of ITER. The efficiency of the injection of large amounts of impurities by massive gas injection or shattered pellet injection to achieve runaway suppression might be compromised due to low gas mixture efficiency and the high Rosenbluth density for runaway suppression. The transport of runaway electrons is dominated by magnetic perturbations. The magnetic perturbations have the advantage of expelling the runaway seeds before they reach high energy. Robust runaway suppression has been reached on J-TEXT with mode locking by the application of m/n = 2/1 resonant magnetic perturbations before the thermal quench. The mode locking implemented large magnetic islands inside the plasma which acted as an explosive bomb during disruptions and led to stronger stochasticity in the whole plasma cross section. The NIMROD simulation indicates that this strong stochasticity expels the runaway seeds and results in runaway free disruptions on J-TEXT. This might provide an alternative runaway suppression technique during disruptions for large-scale tokamaks.

Recent ASDEX Upgrade research in support of ITER and DEMO

Recent experiments on the ASDEX Upgrade tokamak aim at improving the physics base for ITER and DEMO to aid the machine design and prepare efficient operation. Type I edge localized mode (ELM) mitigation using resonant magnetic perturbations (RMPs) has been shown at low pedestal collisionality . In contrast to the previous high ν* regime, suppression only occurs in a narrow RMP spectral window, indicating a resonant process, and a concomitant confinement drop is observed due to a reduction of pedestal top density and electron temperature. Strong evidence is found for the ion heat flux to be the decisive element for the L–H power threshold. A physics based scaling of the density at which the minimum PLH occurs indicates that ITER could take advantage of it to initiate H-mode at lower density than that of the final Q = 10 operational point. Core density fluctuation measurements resolved in radius and wave number show that an increase of R/LTe introduced by off-axis electron cyclotron resonance heating (ECRH) mainly increases the large scale fluctuations. The radial variation of the fluctuation level is in agreement with simulations using the GENE code. Fast particles are shown to undergo classical slowing down in the absence of large scale magnetohydrodynamic (MHD) events and for low heating power, but show signs of anomalous radial redistribution at large heating power, consistent with a broadened off-axis neutral beam current drive current profile under these conditions. Neoclassical tearing mode (NTM) suppression experiments using electron cyclotron current drive (ECCD) with feedback controlled deposition have allowed to test several control strategies for ITER, including automated control of (3,2) and (2,1) NTMs during a single discharge. Disruption mitigation studies using massive gas injection (MGI) can show an increased fuelling efficiency with high field side injection, but a saturation of the fuelling efficiency is observed at high injected mass as needed for runaway electron suppression. Large locked modes can significantly decrease the fuelling efficiency and increase the asymmetry of radiated power during MGI mitigation. Concerning power exhaust, the partially detached ITER divertor scenario has been demonstrated at Psep/R = 10 MW m−1 in ASDEX Upgrade, with a peak time averaged target load around 5 MW m−2, well consistent with the component limits for ITER. Developing this towards DEMO, full detachment was achieved at Psep/R = 7 MW m−1 and stationary discharges with core radiation fraction of the order of DEMO requirements (70% instead of the 30% needed for ITER) were demonstrated. Finally, it remains difficult to establish the standard ITER Q = 10 scenario at low q95 = 3 in the all-tungsten (all-W) ASDEX Upgrade due to the observed poor confinement at low βN. This is mainly due to a degraded pedestal performance and hence investigations at shifting the operational point to higher βN by lowering the current have been started. At higher q95, pedestal performance can be recovered by seeding N2 as well as CD4, which is interpreted as improved pedestal stability due to the decrease of bootstrap current with increasing Zeff. Concerning advanced scenarios, the upgrade of ECRH power has allowed experiments with central ctr-ECCD to modify the q-profile in improved H-mode scenarios, showing an increase in confinement at still good MHD stability with flat elevated q-profiles at values between 1.5 and 2.

Development of an ITER prototype disruption mitigation valve

Disruptions in tokamaks seem to be unavoidable. Consequences of disruptions are (i) high heat loads on plasma-facing components, (ii) large forces on the vacuum vessel, and (iii) the generation of runaway electron beams. In ITER, the thermal energy of the plasma needs to be evenly distributed on the first wall in order to prevent melting, forces from vertical displacement events have to be minimized, and the generation of runaway electrons suppressed. Massive gas injection using fast valves is a concept for disruption mitigation which is presently being explored in many tokamaks. Fast disruption mitigation valves based on an electromagnetic eddy current drive have been developed in Jülich since the 1990s and models of various sizes have been built and are in operation in the TEXTOR, MAST, and JET tokamaks. A disruption mitigation valve for ITER is of necessity larger with an estimated injected gas volume of ∼20kPam3[7] for runaway electron suppression and all materials used have to be resistant to much higher levels of neutron and gamma radiation than in existing tokamaks. During the last 5 years, the concept for an ITER prototype disruption mitigation valve has been developed up to the stage that a fully functional valve could be built and tested. Special emphasis was given to the development and functional testing of some critical items: (i) the injection chamber seal, (ii) the piston seal, (iii) the eddy current drive, and (iv) a braking mechanism to avoid too fast closure of the valve, which could damage the injection chamber seal. The concept of the valve will be introduced and detailed solutions and testing results for the critical items presented.

An overview of KSTAR results

Since the first H-mode discharges in 2010, the duration of the H-mode state has been extended and a significantly wider operational window of plasma parameters has been attained. Using a second neutral beam (NB) source and improved tuning of equilibrium configuration with real-time plasma control, a stored energy of Wtot ∼ 450 kJ has been achieved with a corresponding energy confinement time of τE ∼ 163 ms. Recent discharges, produced in the fall of 2012, have reached plasma βN up to 2.9 and surpassed the n = 1 ideal no-wall stability limit computed for H-mode pressure profiles, which is one of the key threshold parameters defining advanced tokamak operation. Typical H-mode discharges were operated with a plasma current of 600 kA at a toroidal magnetic field BT = 2 T. L–H transitions were obtained with 0.8–3.0 MW of NB injection power in both single- and double-null configurations, with H-mode durations up to ∼15 s at 600 kA of plasma current. The measured power threshold as a function of line-averaged density showed a roll-over with a minimum value of ∼0.8 MW at . Several edge-localized mode (ELM) control techniques during H-mode were examined with successful results including resonant magnetic perturbation, supersonic molecular beam injection (SMBI), vertical jogging and electron cyclotron current drive injection into the pedestal region. We observed various ELM responses, i.e. suppression or mitigation, depending on the relative phase of in-vessel control coil currents. In particular, with the 90° phase of the n = 1 RMP as the most resonant configuration, a complete suppression of type-I ELMs was demonstrated. In addition, fast vertical jogging of the plasma column was also observed to be effective in ELM pace-making. SMBI-mitigated ELMs, a state of mitigated ELMs, were sustained for a few tens of ELM periods. A simple cellular automata (‘sand-pile’) model predicted that shallow deposition near the pedestal foot induced small-sized high-frequency ELMs, leading to the mitigation of large ELMs. In addition to the ELM control experiments, various physics topics were explored focusing on ITER-relevant physics issues such as the alteration of toroidal rotation caused by both electron cyclotron resonance heating (ECRH) and externally applied 3D fields, and the observed rotation drop by ECRH in NB-heated plasmas was investigated in terms of either a reversal of the turbulence-driven residual stress due to the transition of ion temperature gradient to trapped electron mode turbulence or neoclassical toroidal viscosity (NTV) torque by the internal kink mode. The suppression of runaway electrons using massive gas injection of deuterium showed that runaway electrons were avoided only below 3 T in KSTAR. Operation in 2013 is expected to routinely exceed the n = 1 ideal MHD no-wall stability boundary in the long-pulse H-mode (⩾10 s) by applying real-time shaping control, enabling n = 1 resistive wall mode active control studies. In addition, intensive works for ELM mitigation, ELM dynamics, toroidal rotation changes by both ECRH and NTV variations, have begun in the present campaign, and will be investigated in more detail with profile measurements of different physical quantities by techniques such as electron cyclotron emission imaging, charge exchange spectroscopy, Thomson scattering and beam emission spectroscopy diagnostics.

Passive runaway electron suppression in tokamak disruptions

Runaway electrons created in disruptions pose a serious problem for tokamaks with large current. It would be desirable to have a runaway electron suppression method which is passive, i.e., a method that does not rely on an uncertain disruption prediction system. One option is to let the large electric field inherent in the disruption drive helical currents in the wall. This would create ergodic regions in the plasma and increase the runaway losses. Whether these regions appear at a suitable time and place to affect the formation of the runaway beam depends on disruption parameters, such as electron temperature and density. We find that it is difficult to ergodize the central plasma before a beam of runaway current has formed. However, the ergodic outer region will make the Ohmic current profile contract, which can lead to instabilities that yield large runaway electron losses.

Runaway Electron Suppression by ECRH and RMP in KSTAR

We have observed reduction of the runaway electron synchrotron radiation, hard X-ray (HXR) intensity, and HXR energy after applying 110 GHz 2nd harmonic electron cyclotron resonant heating (ECRH) during runaway electron (RE) discharges at low density with startup runaway electrons. However, we did not see a significant reduction of X-rays from 170 GHz 2nd harmonic ECRH at a higher field. A recently installed IR TV camera was used to observe the forward cone of synchrotron radiation from high energy REs in KSTAR. We have observed changes to the synchrotron images and reduction of the HXR by application of resonant magnetic perturbations (RMP) from in-vessel control coils (IVCC) installed inside KSTAR in the n = 1 configuration.

Novel rapid shutdown strategies for runaway electron suppression in DIII-D

New rapid shutdown strategies have been recently tested in the DIII-D tokamak to mitigate runaway electrons (REs). Disruptions in ITER are predicted to generate multi-MeV REs that could damage the machine. The RE population in large tokamaks is expected to be dominated by avalanche amplification which can be mitigated at high density levels by collisional drag. Particle injection schemes for collisional suppression of RE have been developed and tested in ITER-relevant scenarios: massive gas injection, shattered pellet injection (SPI) and shell pellet injection. The results show an improved penetration of particles injected with the SPI. Another strategy has been developed to harmlessly deconfine REs by applying a non-axisymmetric magnetic perturbation to worsen their confinement. This technique appeared to deconfine seed RE before the avalanche process could amplify the RE beam. The last method tested was to use the plasma position control system on the RE beam to prevent it from contacting the wall. This proved effective in preventing high current RE beam from touching the wall and providing more time to mitigate an existing RE beam but a successful ‘soft landing’ (without fast final losses) of the RE has not been observed yet.

Fuelling efficiency of massive gas injection in TEXTOR: mass scaling and importance of gas flow dynamics

Fuelling efficiency is an important parameter in designing a massive gas injection system for suppression of runaway electrons in ITER. In this work Z-dependence of fuelling efficiency is measured for TEXTOR. The dependence covers the following gases: He, Ne, Ar, Kr, Xe and a 10% Ar–D2 mixture. It is shown that the fuelling efficiency significantly decreases with the gas mass, from above 0.5 for He to below 0.03 for Xe.To explain the variation of the efficiency with the gas mass and pressure a simple model of gas flow from the valve to the plasma edge is developed. The flow model is validated using available laboratory flow measurements of a TEXTOR-like injection system. An unsteady gas flow and a premature plasma disruption are shown to explain the mass dependence of the efficiency.

Generation and suppression of runaway electrons in EAST lower hybrid current drive experiments

In a lower hybrid current drive experiment on the EAST tokamak in the spring of 2010, the process in which fast electrons transform into runaway electrons was observed, and the way in which lower hybrid waves (LHWs) affect the behavior of runaway electrons was studied. It was found that when the power of LHWs is below a certain threshold, fast electrons excited by LHWs can be accelerated to runaway electrons due to the primary generation mechanism of runaway electrons, but when the power is high enough, fast electrons cannot transform into runaway electrons anymore. No matter how high the power was, the production of runaway electrons due to the secondary generation mechanisms was suppressed.

Experimental study of disruption mitigation using massive injection of noble gases on Tore Supra

Disruptions are a major threat for future tokamaks, including ITER. Disruption-generated heat loads, electromagnetic forces and runaway electrons will not be tolerable for next-generation devices. Massive noble gas injection is foreseen as a standard mitigation system for these tokamaks. Disruption mitigation experiments have been carried out on Tore Supra to study various injection scenarios and to investigate gas jet penetration and mixing. Comparisons of different gases (He, Ne, Ar, He/Ar mixture) and amounts (from 5 to 500 Pa m3) were made, showing that light gases are more efficient regarding runaway electron suppression than heavier gases. Eddy currents in the limiter are moderately reduced by all the gases, and may be more dependent on the time constants of the structures than on the gas species. The density rise induced by the massive injection before the thermal quench is higher and faster with light gases. Gas jet penetration in the cooling phase is observed to be shallow and independent of the gas nature and amount. The gas cold front is stopped along the q = 2 surface where it triggers MHD instabilities, expelling thermal energy from the plasma core.

Experiments in DIII-D toward achieving rapid shutdown with runaway electron suppression

Experiments have been performed in the DIII-D tokamak [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] toward understanding runaway electron formation and amplification during rapid discharge shutdown, as well as toward achieving complete collisional suppression of these runaway electrons via massive delivery of impurities. Runaway acceleration and amplification appear to be well explained using the zero-dimensional (0D) current quench toroidal electric field. 0D or even one-dimensional modeling using a Dreicer seed term, however, appears to be too small to explain the initial runaway seed formation. Up to 15% of the line-average electron density required for complete runaway suppression has been achieved in the middle of the current quench using optimized massive gas injection with multiple small gas valves firing simultaneously. The novel rapid shutdown techniques of massive shattered pellet injection and shell pellet injection have been demonstrated for the first time. Experiments using external magnetic perturbations to deconfine runaways have shown promising preliminary results.