Production Of Runaway Electrons Research Articles

Production of runaway electrons and x-rays during streamer inception phase

Streamers play a key role in the formation and propagation of lightning channels. In nature streamers rarely appear alone. Their ensemble behavior is very complex and challenging to describe. For instance, the intricate dynamics within the streamer zone of negative lightning leaders give rise to space stems, which help advance the stepped-leader. Another example is how the increasing morphological complexity of sprites can lead to higher sprite current and greater energy deposition in the mesosphere. Insights into the complex dynamics of a streamer corona can be obtained from laboratory experiments that allow us to control the conditions of streamer formation. Based on simultaneous nanosecond-temporal-resolution photography, and measurements of voltage, current, and x-ray emissions, we report the characteristics of negative laboratory streamers in 88 kPa of atmosphere. The streamers are produced at peak voltages of 62.2 ± 3.8 kV in a point-to-plane discharge gap of 6 cm. While all discharges were driven to the same peak voltage, the discharges occurred at different stages of the relatively slow voltage rise (177 ns), allowing us to study discharge properties as a function of onset voltage. The onset voltage ranged between 24 and 67 kV, but x-ray emissions were observed to only occur above 53 kV, with x-ray burst energies scaling quadratically with voltage. The average delay between the current pulse and x-ray emission was found to be 3.5 ± 0.5 ns, indicating that runaway electrons are produced during the streamer inception phase or no later than the transition stage, when the inception cloud is breaking into streamer filaments. During this short time span, runaway electrons can traverse the gap, hit the ground plate and produce bremsstrahlung x-ray photons. However, streamers themselves cannot traverse more than 3.5 mm across the gap, which supports the idea that runaway electron production is not associated to streamer connection to the ground electrode.

Simulation of runaway electron production with CQL3D coupled to NIMROD

A coupling between two distinctly different codes—one magnetohydrodynamic (MHD) and another kinetic—is achieved and applied for simulation of runaway electron (RE) production. The 3D initial value MHD code NIMROD simulates a DIII-D pure neon shattered pellet injection plasma quench including the propagation and ablation of the fragments, ionization and recombination of the impurities, and the radiated and transported energies. The field data from NIMROD is then used by the bounce-averaged Fokker–Planck Collisional QuasiLinear 3D (CQL3D) kinetic code to simulate the production of REs and their radial transport. The coupling procedure involves mapping of data between different grids and adjustment of the NIMROD toroidal electric field when REs appear. It is shown that without the radial transport, a large RE current is generated, up to 30% of the pre-pellet ohmic current. However, when the radial transport is included in CQL3D, the RE current is reduced to undetectable level, consistent with experiment. Various forms of the radial diffusion are surveyed to determine conditions when the fast electrons would not have time to be accelerated to relativistic energies before they are lost to chamber wall.

Elevation of runaway electron current by electron cyclotron resonance heating during disruptions on J-TEXT

Runaway electrons represent a serious problem for the reliable operation of future tokamaks. Radio frequency (RF) auxiliary heating can help suppress runaway electrons in the flat-top phase through a reduction in loop voltage. However, if a disruption occurs during RF auxiliary heating, it may lead to an increase in the runaway electron production due to the generation of a suprathermal population with energies in the order of ∼100 keV. The elevation of runaway current during disruptions by electron cyclotron resonance heating (ECRH) has been investigated on the J-TEXT tokamak. The results show that the conversion efficiency of runaway current increases with increased ECRH power. When the applied ECRH power is at the level of 400 kW, the conversion efficiency of Ohmic to runaway current increases from 35% to 75%, the thermal quench duration is decreased from 0.24 ms to 0.11 ms, and the Absolute Extreme Ultraviolet Spectrometer (AXUV) radiative power and electron density are about two times higher than the cases without ECRH heating.

Dispersive shell pellet injection modeling and validation for DIII-D disruption mitigation

Dispersive shell pellet injection for disruption mitigation on the DIII-D tokamak is modeled with the 3D MHD code NIMROD. The simulations attempt to reproduce as closely as possible a pellet velocity scan that was performed on the DIII-D tokamak [Hollmann, et al., Phys. Rev. Lett. 122, 65001 (2019)]. This scan involved injecting a hollow diamond shell filled with boron dust that is released in the core of the plasma. Trends that are reproduced in the simulation include improved mitigation, reduced plasma current spike amplitude, and increased likelihood of hot-tail runaway electron production with increased pellet speed. The simulations provide additional physics insight into these findings. Limitations of the present model and requirements for a predictive model are discussed.

Numerical simulation of the hot-tail runaway electron production mechanism using CQL3D and comparison with Smith–Verwichte analytical model

The hot-tail mechanism of runaway electron (RE) production (Harvey et al 2000 Phys. Plasmas 7 4590) is the primary source of RE in the case of rapidly cooling tokamak plasma. Quantifying this mechanism is very important as it can provide most of the post-thermal-quench (TQ) current, or a seed current for the secondary source of RE through the avalanche mechanism. An analytic model which omits pitch-angle scattering is often used in literature for estimating the hot-tail RE density (Smith and Verwichte 2008 Phys. Plasmas 15 072502). In the present study, we use the CQL3D bounce-averaged Fokker–Planck code (Harvey and McCoy 1992 Proc. IAEA Technical Committee Meeting on Advances in Simulation and Modeling of Thermonuclear Plasmas p 527) to test the limits of validity of the model. In particular, we examine the cases of Z= 1 and Z= 18 ions, for sets of different initial temperature, density, electric field and the characteristic time of temperature decay. We show that for Z = 1 plasma, the ratio of RE density computed by CQL3D to that estimated from the model is within 0.6–6.0 in studied cases. For the Z = 18 case, this factor is systematically a much smaller number, typically 0.02–0.6. We suggest a simple correction to the model that narrows down the range of this ratio to 0.3–3.8 in all of the cases, including Z = 1 and Z = 18 plasmas.

Influence of Elastic Scattering on Electron Swarm Distribution in Electrified Gases.

The propagation of energetic electrons through air is one key component in the generation of high‐energy atmospheric phenomena such as lightning‐generated X‐ray bursts, terrestrial gamma ray flashes (TGFs), and gamma ray glows. We show here that models for this propagation can be considerably affected by the parameterization of the differential cross section of elastic scattering of electrons on the molecular components of air. We assess existing parameterizations and propose a more accurate one that builds upon the most up‐to‐date measurements. Then we conclude that by overweighting the forward scattering probability, previous works may have overestimated the production of runaway electrons under high electric fields close to the thermal runaway threshold.

A study on combined effects of stochastic magnetic fluctuations and synchrotron radiation on the production of runaway electrons

The dynamics of relativistic runaway electrons are analyzed using the relativistic Fokker–Planck equation including deceleration due to the synchrotron radiation and radial diffusion loss caused by stochastic magnetic fluctuations (SMFs). SMFs are treated as friction force in (Martín-Solís et al 1999 Phys. Plasmas 6 3925). However, we think SMFs act as a ‘porter’ in configuration space, but do not directly affect the runaway electrons (REs) in momentum space. Both critical electric fields for sustainment of the existing REs and for avalanche onset are enhanced, and the modified avalanche growth rate is reduced by the combined effects of SMFs and synchrotron radiation as compared to the case with only synchrotron radiation (Aleynikov et al 2015 Phys. Rev. Lett. 114 155001).

The non-thermal origin of the tokamak low-density stability limit

DIII-D plasmas at very low density exhibit the onset of n = 1 error field (EF) penetration (the ‘low-density locked mode’) not at a critical density or EF, but instead at a critical level of runaway electron (RE) intensity. Raising the density during a discharge does not avoid EF penetration, so long as RE growth proceeds to the critical level. Penetration is preceded by non-thermalization of the electron cyclotron emission, anisotropization of the total pressure, synchrotron emission shape changes, as well as decreases in the loop voltage and bulk thermal electron temperature. The same phenomena occur despite various types of optimal EF correction, and in some cases modes are born rotating. Similar phenomena are also found at the low-density limit in JET. These results stand in contrast to the conventional interpretation of the low-density stability limit as being due to residual EFs and demonstrate a new pathway to EF penetration instability due to REs. Existing scaling laws for penetration project to increasing EF sensitivity as bulk temperatures decrease, though other possible mechanisms include classical tearing instability, thermo-resistive instability, and pressure-anisotropy driven instability. Regardless of the first-principles mechanism, known scaling laws for Ohmic energy confinement combined with theoretical RE production rates allow rough extrapolation of the RE criticality condition, and thus the low-density limit, to other tokamaks. The extrapolated low-density limit by this pathway decreases with increasing machine size and is considerably below expected operating conditions for ITER. While likely unimportant for ITER, this effect can explain the low-density limit of existing tokamaks operating with small residual EFs.

Long‐duration X‐ray emissions observed in thunderstorms

AbstractIn 1995, a series of four balloon flights with an X‐ray spectrometer and an electric field meter were conducted to examine if strong electric fields could accelerate, and perhaps multiply, cosmic ray secondary electrons and produce bremsstrahlung X‐rays. X‐ray intensities between 10 and 1000 times that of normal background were observed in conjunction with strong electric fields. Both negative and positive polarity electric fields (as referenced to the vertical field) produced X‐rays, which lasted for time scales on the order of tens of seconds. It was also observed that the increased X‐ray intensity would return to near background levels after lightning reduced the local electric field. The observations indicate that X‐rays observed above background are most likely produced by a runaway electron process occurring in the strong static electric field present in thunderstorms. The production of runaway electrons can occur over long periods of time without causing an electrical breakdown. This may provide a leakage current that limits the large scale electric field to values near the runaway threshold, especially in regions where the thunderstorm charging rate is low.

Runaway electrons from a ‘beam-bulk’ model of streamer: application to TGFs

The generation of x- and gamma-rays in atmospheric discharges has been studied intensively since the discovery of terrestrial gamma-ray flashes (TGFs) by the Compton gamma-ray Observatory in 1991. Emissions are bremsstrahlung from high energy particles accelerated in large scale atmospheric electric fields associated with thunderstorms. Whereas observations now are many, both from lightning and the laboratory, the phases of the discharge where emissions are generated are still debated and several processes for electron acceleration have been put forward by theorists. This paper address the electron acceleration in streamer region of lightning. We present the first ‘beam-bulk’ model of self-consistent streamer dynamics and electron acceleration. The model combines a Monte Carlo Collision code that simulates the high-energy electrons (100 eV) and a fluid code that simulates the bulk of the low-energy electrons and ions. For a negative streamer discharge, we show how electrons are accelerated in the large electric field in the tip of the streamer and travel ahead of the streamer where they ionize the gas. In comparison to the results obtained with a classical fluid model for a negative streamer, the beam-bulk model predicts a decrease of the magnitude of the peak electric field and an increase of the streamer velocity. Furthermore, we show that a significant number of runaway electrons is lost by diffusion outside of the streamer tip. The results presented here do not yet include extra amplification nor acceleration far away from the streamer to explain the electron energies seen in TGFs. Still, in the light of those results, we emphasize that the production of runaway electrons from streamers needs to be simulated including the self-consistent feedback of runaways on the streamer. Simulations with a beam-bulk model may not only help to understand the fundamental atmospheric processes behind TGFs, but also pave the way for the interpretation of remote sensing of the most energetic discharges in the Earthʼs atmosphere and thus help to address their environmental impact.

Experimental study of the runaway current in the J-TEXT Tokamak

Major plasma disruptions in tokamaks often generate runaway currents, which contain electrons with energies of several tens of megaelectron-volts (MeV). These currents can cause substantial damage when control is lost and the current hits the limiters or the vessel wall. The interaction between the runaway electrons and the impurities inside the plasma results in soft X-ray emission, which can provide detailed information about the runaway generation process and the confinement of runaway electrons. A vertical soft X-ray array at the top of Joint Texas Experimental Tokamak (J-TEXT) was used to study the runaway beams resulting from major disruptions. Runaway electron production and confinement of runaway current were observed by using soft X-ray images.

Modeling terrestrial gamma ray flashes produced by relativistic feedback discharges

This paper reports a modeling study of terrestrial gamma ray flashes (TGFs) produced by relativistic feedback discharges. Terrestrial gamma ray flashes are intense energetic radiation originating from the Earth's atmosphere that has been observed by spacecraft. They are produced by bremsstrahlung interactions of energetic electrons, known as runaway electrons, with air atoms. An efficient physical mechanism for producing large fluxes of the runaway electrons to make the TGFs is the relativistic feedback discharge, where seed runaway electrons are generated by positrons and X‐rays, products of the discharge itself. Once the relativistic feedback discharge becomes self‐sustaining, an exponentially increasing number of relativistic electron avalanches propagate through the same high‐field region inside the thundercloud until the electric field is partially discharged by the ionization created by the discharge. The modeling results indicate that the durations of the TGF pulses produced by the relativistic feedback discharge vary from tens of microseconds to several milliseconds, encompassing all durations of the TGFs observed so far. In addition, when a sufficiently large potential difference is available in thunderclouds, a self‐propagating discharge known as the relativistic feedback streamer can be formed, which propagates like a conventional positive streamer. For the relativistic feedback streamer, the positive feedback mechanism of runaway electron production by the positrons and X‐rays plays a similar role as the photoionization for the conventional positive streamer. The simulation results of the relativistic feedback streamer show that a sequence of TGF pulses with varying durations can be produced by the streamer. The relativistic streamer may initially propagate with a pulsed manner and turn into a continuous propagation mode at a later stage. Milliseconds long TGF pulses can be produced by the feedback streamer during its continuous propagation. However, the continuous propagation of the streamer tends to be unstable, because it does not expand like the conventional positive streamer. Its head electric field continues to increase and can reach half of the conventional breakdown threshold field, which results in an ion density of 3– 10×1014m−3 in the channel immediately following the head. The spatial width of the high field region in the streamer head is about 100 m and the streamer speed is about 5×105 m/s. As a result, conventional positive streamers can be initiated from thundercloud hydrometeors or inhomogeneities of enhanced conductivities of millimeter sizes in the relativistic feedback streamer head and the positive streamers may further result in the formation of leaders. In addition, a relativistic feedback streamer can result in a charge moment change of several tens of coulomb‐kilometers in a few tens of milliseconds, indicating that the relativistic feedback discharge process could be an important component for thundercloud charge dynamics.

Spatial and energy distributions of X‐ray emissions from leaders in natural and rocket triggered lightning

Energetic radiation is known to be produced by lightning. To investigate these emissions, ground‐based observations are being conducted at the International Center for Lightning Research and Testing (ICLRT) at Camp Blanding, FL where measurements of energetic radiation from both natural and rocket‐triggered lightning discharges are recorded. In the present study, data from two natural negative cloud‐to‐ground stepped leaders and one rocket‐triggered “chaotic” dart leader are analyzed in detail to investigate X‐ray energy spectra and spatial X‐ray distributions around the source. These measurements are compared with Monte Carlo simulations of runaway electron propagation with the goal of understanding the underlying mechanism of runaway electron production and their role in lightning initiation and propagation. We show that the energetic electrons that produce X‐rays exhibit a characteristic energy less than 3 MeV for two natural and one rocket‐triggered leaders investigated. In addition to studying these three leaders, energetic electron luminosity, total energy, and energetic electron per meter are compared to the following return stroke currents for 28 leaders from 12 different triggered flashes. Electron luminosity is found to increase exponentially with return stroke current up to about 10 kA and to be roughly constant for larger currents. The maximum electron luminosity, which is determined indirectly through X‐ray luminosities, is on the order of 1017 electrons/s, which is less than the value found from theoretical calculations.

High-Energy Atmospheric Physics: Terrestrial Gamma-Ray Flashes and Related Phenomena

It is now well established that both thunderclouds and lightning routinely emit x-rays and gamma-rays. These emissions appear over wide timescales, ranging from sub-microsecond bursts of x-rays associated with lightning leaders, to sub-millisecond bursts of gamma-rays seen in space called terrestrial gamma-ray flashes, to minute long glows from thunderclouds seen on the ground and in or near the cloud by aircraft and balloons. In particular, terrestrial gamma-ray flashes (TGFs), which are thought to be emitted by thunderclouds, are so bright that they sometimes saturate detectors on spacecraft hundreds of kilometers away. These TGFs also generate energetic secondary electrons and positrons that are detected by spacecraft in the inner magnetosphere. It is generally believed that these x-ray and gamma-ray emissions are generated, via bremsstrahlung, by energetic runaway electrons that are accelerated by electric fields in the atmosphere. In this paper, we review this newly emerging field of High-Energy Atmospheric Physics, including the production of runaway electrons, the production and propagation of energetic radiation, and the effects of both on atmospheric electrodynamics.

Disruption avoidance by means of electron cyclotron waves

Disruptions are very challenging to ITER operation as they may cause damage to plasma facing components due to direct plasma heating, forces on structural components due to halo and eddy currents and the production of runaway electrons. Electron cyclotron (EC) waves have been demonstrated as a tool for disruption avoidance by a large set of recent experiments performed in ASDEX Upgrade and FTU using various disruption types, plasma operating scenarios and power deposition locations. The technique is based on the stabilization of magnetohydrodynamic (MHD) modes (mainly m/n = 2/1) through the localized injection of EC power on the resonant surface. This paper presents new results obtained in ASDEX Upgrade regarding stable operation above the Greenwald density achieved after avoidance of density limit disruptions by means of ECRH and suitable density feedback control (L-mode ohmic plasmas, Ip = 0.6 MA, Bt = 2.5 T) and NTM-driven disruptions at high-β limit delayed/avoided by means of both co-current drive (co-ECCD) and pure heating (ECRH) with power ⩽1.7 MW (H-mode NBI-heated plasmas, PNBI ∼ 7.5 MW, Ip = 1 MA, Bt = 2.1 T, q95 ∼ 3.6). The localized perpendicular injection of ECRH/ECCD onto a resonant surface leads to the delay and/or complete avoidance of disruptions. The experiments indicate the existence of a power threshold for mode stabilization to occur. An analysis of the MHD mode evolution using the generalized Rutherford equation coupled to the frequency and phase evolution equations shows that control of the modes is due to EC heating close to the resonant surface. The ECRH contribution (Δ′H term) is larger than the co-ECCD one in the initial and more important phase when the discharge is ‘saved’. Future research and developments of the disruption avoidance technique are also discussed.

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.

Energy and fluxes of thermal runaway electrons produced by exponential growth of streamers during the stepping of lightning leaders and in transient luminous events

[1] In the present paper, we demonstrate that the exponential expansion of streamers propagating in fields higher than the critical fields for stable propagation of streamers of a given polarity leads to the exponential growth of electric potential differences in streamer heads. These electric potential differences are directly related to the energy that thermal runaway electrons can gain once created. Using full energy range relativistic Monte Carlo simulations, we show that the exponential growth of potential differences in streamers gives rise to the production of runaway electrons with energies as high as ∼100 keV, with most of electrons residing in energy range around several tens of keVs. We apply these concepts in the case of lightning stepped leaders during the stage of negative corona flash. The computation of electric field produced by stepped leaders demonstrates for the first time that those energetic electrons are capable of further acceleration up to the MeV energies. Moreover, the flux of runaway electrons produced by streamers suggests that stepped leaders produce a considerable number of energetic electrons, which is in agreement with the number of energetic photons observed from satellites in terrestrial gamma ray flashes (TGFs). The results suggest that previously proposed process of relativistic runaway electron avalanche is difficult to sustain in the low‐electric fields observed in thunderclouds and is generally not needed for explanation of TGFs. The present work also gives insights on relations between physical properties of energetic electrons produced in streamers and the internal electrical properties of streamer discharges, which can further help development and interpretation of X‐ray diagnostics of these discharges.

X-rays from laboratory sparks in air: The role of the cathode in the production of runaway electrons

[1] Very little is known about how X-rays are produced from long laboratory sparks. Up to now, nobody has studied separately the influence of positive and negative electrodes. Thus, the goal of this letter is to demonstrate the influence of the positive and the negative electrode in the production of runaway electrons in laboratory sparks. Results show that, emissions are affected by the distribution of the equipotential lines around the cathode. This mechanism could also be related to negative stepped leaders during their path to the ground and perhaps could also be present in TGF's development.

Observation of Runaway Electron Behaviour During Disruptions in LHCD Discharges in the HT‐7 Tokamak

AbstractProduction of runaway electrons during disruptions has been observed in the HT‐7 Tokamak. The runaway current plateaus, which can carry part of the pre‐disruptive current, are observed in lower‐hybrid current drive (LHCD) limiter discharges. It is found that the runaway current can mitigate the disruptions effectively. We can use gas puffing to increase the line‐averaged density to restrain the runaway electrons and rebuild the plasmas after the disruptions. Detailed observations are presented on the runaway electrons generated following disruptions in the HT‐7 tokamak discharges. The results indicate that the magnetic oscillations play a significant role in the loss of runaway electrons in disruptions. There are two important preconditions to rebuild plasmas by runaway electrons after the disruptions. One of them are weak magnetic oscillations; another one are LHWs (lower‐hybrid waves) (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Enhanced production of runaway electrons during electron cyclotron resonance heating and in the presence of supersonic molecular beam injection in the HL-2A tokamak

In the present paper, it is reported that a large production of runaway electrons has been observed during the flattop phase of electron cyclotron resonance heating (ECRH) discharges and in the presence of supersonic molecular beam injection (SMBI) in the HuanLiuqi-2A (commonly referred to as HL-2A) [Q. W. Yang, Nucl. Fusion 47, S635 (2007)] tokamak. For the set of discharges carried out in the present experiment, the ranges of ECRH power and plasma electron density are 0.8–1.0 MW and (3.0–4.0)×1019 m−3, respectively. A large number of superthermal electrons are produced through the avalanche effect [A. Lazaros, Phys. Plasmas 8, 1263 (2001)] during ECRH. The loop voltage increase due to SMBI gives rise to a decline in the critical runaway energy, which leads to that many superthermal electrons could be converted into runaway region. Therefore, this phenomenon may come from the synergetic effects of ECRH and SMBI. That is, the superthermal electrons created by ECRH are accelerated into runaway regime via t...