Flux Of Runaway Electrons Research Articles

On Thermal Runaway Electrons and the Polarization of X-ray Emission in Solar Flares

A model for the thermal runaway of electrons in solar flares based on an approximate analytical solution of the kinetic equation with a linearized Landau collision integral is considered. Coulomb collisions are shown to make the flux of runaway electrons with a very high temperature nearly isotropic. The derived electron distribution function is used to estimate the polarization of the hard X-ray bremsstrahlung of solar flares. Since the anisotropy of the flux of electrons is low, the polarization of their bremsstrahlung does not exceed 3–4% at an energy of about 15 keV, which creates great difficulties for future extra-atmospheric polarization observations. Furthermore, the Compton scattering of hard X-ray emission by the photosphere and magnetic field inhomogeneity can severely distort the expected results and can make their interpretation difficult. The prospects for space experiments to measure the polarization of the hard X-ray emission from solar flares are discussed.

Tore Supra/WEST Toroidal Field Coil Quench Following a Plasma Disruption With Runaway Electrons

Tore Supra/WEST is a Tokamak operating at CEA/IRFM since 1988. Its toroidal field system is composed of 18 NbTi toroidal field coils (TFC) cooled by superfluid helium bath at 1.8 K. The system is used to operate at full current during every day of a plasma campaign, and is protected by a quench detection system (QDS) which can trigger a fast safety discharge (FSD). In December 2017, an FSD was triggered. After analysis, we could show that it was caused by the quench of TFC9, and that the quench had been triggered by a high flux of runaway electrons at the end of a plasma shot. The detection and protection systems were effective to protect the TF magnet, which is presently back in operation at nominal current without any fault. This paper presents the analyses conducted to determine the origin of the quench, the details of the QDS operation, and the impact of the quench on our cryomagnetic system.

The effect of direct electron‐positron pair production on relativistic feedback rates

AbstractRunaway electron avalanches developing in thunderclouds in high electric field become self‐sustaining due to relativistic feedback via the production of backward propagating positrons and backscattered X‐rays. To date, only positrons created from pair production by gamma rays interacting with the air have been considered. In contrast, direct electron‐positron pair production, also known as “trident process,” occurs from the interaction of energetic runaway electrons with atomic nuclei, and so it does not require the generation of a gamma ray mediator. The positrons produced in this process contribute to relativistic feedback and become especially important when the feedback factor value approaches unity. Then the steady state flux of runaway electrons increases significantly. In certain cases, when the strong electrostatic field forms in a narrow area, the direct positrons become one of processes dominating relativistic feedback. Calculations of the direct positron production contribution to relativistic feedback are presented for different electric field configurations.

The effect of an electrostatic field on runaway electrons in the Uragan-3M stellarator

The possibility of suppressing the flux of runaway electrons at the stage of their generation in an Uragan-3M stellarator has been experimentally studied and theoretically analyzed.

The relativistic feedback discharge model of terrestrial gamma ray flashes

As thunderclouds charge, the large‐scale fields may approach the relativistic feedback threshold, above which the production of relativistic runaway electron avalanches becomes self‐sustaining through the generation of backward propagating runaway positrons and backscattered X‐rays. Positive intracloud (IC) lightning may force the large‐scale electric fields inside thunderclouds above the relativistic feedback threshold, causing the number of runaway electrons, and the resulting X‐ray and gamma ray emission, to grow exponentially, producing very large fluxes of energetic radiation. As the flux of runaway electrons increases, ionization eventually causes the electric field to discharge, bringing the field below the relativistic feedback threshold again and reducing the flux of runaway electrons. These processes are investigated with a new model that includes the production, propagation, diffusion, and avalanche multiplication of runaway electrons; the production and propagation of X‐rays and gamma rays; and the production, propagation, and annihilation of runaway positrons. In this model, referred to as the relativistic feedback discharge model, the large‐scale electric fields are calculated self‐consistently from the charge motion of the drifting low‐energy electrons and ions, produced from the ionization of air by the runaway electrons, including two‐ and three‐body attachment and recombination. Simulation results show that when relativistic feedback is considered, bright gamma ray flashes are a natural consequence of upward +IC lightning propagating in large‐scale thundercloud fields. Furthermore, these flashes have the same time structures, including both single and multiple pulses, intensities, angular distributions, current moments, and energy spectra as terrestrial gamma ray flashes, and produce large current moments that should be observable in radio waves.

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.

Electron flux spatial distribution in an ultrashort avalanche electron beam generated at atmospheric air pressure

The results of experimental study on generation of ultrashort avalanche electron beams (UAEB) in gas-filled diodes are considered. The spatial distribution of the flux of runaway electrons and X-rays generated in the gas diode fed by nanosecond high-voltage pulses was studied. It was shown that the UAEB in the gas-filled diode (at an air pressure of 1 atm) with sharply nonuniform electric field is generated from the interelectrode region into a solid angle exceeding 2π sr. Narrowing of the cathode-anode gap results in a decrease in the current amplitude of the beam generated to side walls of the gas diode and an increase in the beam current pulse duration in both axial and radial directions. Current pulses of the beam initiated from the side surface of the tubular cathode were detected.

Relativistic breakdown in planetary atmospheres

In 2003, a new electrical breakdown mechanism involving the production of runaway avalanches by positive feedback from runaway positrons and energetic photons was introduced. This mechanism, which shall be referred to as “relativistic feedback,” allows runaway discharges in gases to become self-sustaining, dramatically increasing the flux of runaway electrons, the accompanying high-energy radiation, and resulting ionization. Using detailed Monte Carlo calculations, properties of relativistic feedback are investigated. It is found that once relativistic feedback fully commences, electrical breakdown will occur and the ambient electric field, extending over cubic kilometers, will be discharged in as little as 2×10−5s. Furthermore, it is found that the flux of energetic electrons and x rays generated by this mechanism can exceed the flux generated by the standard relativistic runaway electron model by a factor of 1013, making relativistic feedback a good candidate for explaining terrestrial gamma-ray flashes and other high-energy phenomena observed in the Earth’s atmosphere.

The initiation of lightning by runaway air breakdown

A mechanism for lightning initiation by the sustained runaway breakdown of air is presented. Unlike earlier models that rely upon large cosmic‐ray air showers, this mechanism uses the runaway electrons produced by the steady background of atmospheric cosmic‐rays to amplify non‐uniformities in the electric field. The ionization of air from the runaway electrons creates a region of discharge that propagates in the opposite direction of the electrons, enhancing the electric field in front of it to the point where a conventional breakdown can occur. As the discharged region grows, positron feedback can become important, dramatically increasing the flux of runaway electrons and increasing the propagation speed of the discharged region up to 106 m/s. The ambient electric field strength needed for this mechanism is within the range of values observed in thunderclouds and is substantially lower than that required by most other mechanisms.

Implications of x‐ray emission from lightning

Dwyer et al. [2003, 2004a] recently reported measurements of bursts of x‐rays, with energies up to ∼250 keV, originating from dart leaders and possibly the return strokes of rocket‐triggered lightning. In this paper, these x‐ray observations are compared with the relativistic runaway electron avalanche (RREA) model. It is found that for dart leaders the standard RREA model is inconsistent with the observed spectrum and flux of the x‐ray emission. This result implies that the cold runaway electron model may be applicable to dart leaders. In this model, runaway electrons are directly accelerated out of the bulk electron population, produced by the ionization of the neutral atoms. However, this mechanism requires an electric field an order‐of‐magnitude larger than the conventional breakdown field. Because the flux of runaway electrons is very sensitive to the electric field strength, x‐ray observations of lightning provide a novel tool for determining the fields associated with the leaders.

Measurements of fluctuations in the flux of runaway electrons to the PLT tokamak limiter

Fluctuations in the flux of runaway electrons to the limiter are measured during many PLT (Princeton Large Torus) discharges. Oscillations at 60, 120, and 720 Hz are driven by variations in the vertical magnetic field which moves the plasma major radius. Fluctuations are seen with frequencies in the range of 2–20 kHz because of magnetohydrodynamic (MHD) magnetic islands which extend to the plasma surface. A continuous spectrum of fluctuations is observed up to 200 kHz which correlates with drift-wave turbulence. The magnitude of the driven fluctuations can be used to measure transport properties of the runaway electrons. The amplitude of electron motion due to the MHD and drift-wave oscillations, and hence a measure of the radial size of the instability, can be determined as a function of frequency. The slope of the frequency power spectrum of the drift-wave-induced fluctuations steepens with increasing runaway-electron drift orbit displacement during the current drop at the end of the discharge, and as the power in the MHD oscillations increases. A magnetic probe is used to confirm the presence of oscillating magnetic fields capable of perturbing the electron orbits.

Fluctuations of the runaway electron flux to the PLT tokamak limiter

The flux of runaway electrons into the PLT limiter is modulated at frequencies up to 200 kHz with a power spectrum similar to that of density fluctuations observed by microwave scattering.