Townsend coefficient and runaway of electrons at relativistic velocities

An electron-multiplication regime at large field strengths, in which case an electron can acquire a relativistic kinetic energy over the multiplication length, is considered. It is shown that, even in such superstrong fields, the Townsend electron-multiplication mechanism is valid if the distance between the electrodes is rather large. The Townsend coefficient and the drift velocity in helium are obtained in such fields. The electron-escape curve, which separates the region of efficient electron multiplication from the region where electrons escape from the gap without undergoing multiplication, is obtained.     

Townsend coefficient and efficiency of the formation of runaway electrons in neon

Basic ionization and drift properties are simulated for neon by the method of multiparticle dynamics. This calculation revealed that, in neon—in just the same way as in other gases that were studied previously—the Townsend ionization regime is realized even in strong fields if the distance between electrodes is rather large. The dependences of basic ionization and drift properties on the reduced electric-field strength are obtained. The results agree with available experimental data. The escape curve separating the region of efficient electron multiplication from the region in which electrons leave the discharge gap without undergoing multiplication is found for neon. The efficiency of the formation of a runaway-electron beam in helium and neon is simulated.     

Townsend coefficient,escape curve,and efficiency of runaway-electron beam formation in argon

The ionization and drift characteristics of electrons in argon are simulated by the method of multi-particle dynamics. It is shown that, in argon (as well as in other gases studied earlier), the Townsend regime of ionization sets in even in strong fields if the electrode distance is much larger than the reciprocal Townsend coefficient. The dependences of the basic ionization and drift characteristics on the reduced field intensity are obtained, and an escape curve is constructed separating the region of effective electron multiplication from the region where the electrons leave the discharge gap having no time to multiply. The formation efficiency of a runaway-electron beam is calculated. It is shown that the dependence of the electrode voltage generating a given fraction of runaway electrons on the product of the pressure by the electrode distance has a form that qualitatively agrees with the runaway curve. When the efficiency is not too high (≤20%), the runaway curves virtually coincide with isoefficiency curves.     

The townsend coefficient and runaway of electrons in electronegative gas

We have studied the character of variation of the number of electrons formed in an electronegative gas (SF₆) under the action of an external electric field. At any value of the electric field strength E, the number of generated electrons exponentially increases with the distance from the cathode, while the average velocity and energy of electrons attain constant values. At small values of the reduced field strength, E/p<94 V/(cm Torr) (p is the gas pressure), the regime of electron attachment prevails that is characterized by negative values of the exponent (negative Townsend coefficients). For E/p>94 V/(cm Torr), the electron multiplication proceeds in the usual Townsend regime with positive exponents. In the intermediate region of E/p=40–160 V/(cm Torr), the electron multiplication coefficient exhibits a linear dependence on E/p. Numerical calculations based on a simple model show that the Townsend multiplication regime takes place even in very strong fields where the drag caused by ionization can be ignored. A universal function describing the electron runaway in SF₆ is obtained.     

Commensurability oscillations by runaway and pinned electrons

We report on an investigation of commensurability oscillations in antidot square-lattices, and show that the commensurate peaks in resistivity ρ_xxderive from two kinds of electron: runaway and quasipinned electrons. The runaway electrons, which skip away along the antidot arrays, increase the value of conductivity σ_xx; and the quasipinned electrons, which orbit some antidots for a long time, decrease the value of σ_xx. Therefore, by competition between the two different types of electron, the conductivity σ_xxis determined. The conductivity σ_yxin the antidot lattice always has dips at the peaks in ρ_xx. As a result, by combining the of values of σ_xxand σ_yx, the resistivity is determined through the orthodox relation of ρ_xx =  σ_xx/ (σ_xx²  +   σ_yx²).     

On the mechanism of the runaway of electrons in a gas: The upper branch of the paschen curve

Basing on the simulation results, it is shown that the Townsend mechanism of electron multiplication in a gas at sufficiently large interelectrode distances is valid at least up to such large values of E/p at which relativistic electrons are generated. Correspondingly, the runaway electron producing in a gas is determined not by the local criteria accepted presently, but by the ratio of interelectrode distance and the characteristic electron multiplication length. It is shown that the critical discharge voltage U, at which the runaway electrons appear in a given gas, is a function of the product of the interelectrode distance by the gas pressure. This function (U-pd dependence) defines not only well-known Paschen curve but also an additional branch, which describes the absence of a self-sustained discharge at a high voltages sufficiently rapidly supplied across the electrodes. Critical discharge voltage dependence for helium and xenon are presented.     

Transverse runaway of hot electrons and the electron-temperature approximation

This paper discusses the transverse runaway effect in the electron-temperature approximation. The combinations of scattering mechanisms and the corresponding threshold electric fields for which transverse runaway develops are determined. It is shown that the transverse-runaway effect is not associated with any approximation. Zh. éksp. Teor. Fiz. 113, 688–692 (February 1998)     

Positron creation and annihilation in tokamak plasmas with runaway electrons

It is shown that electron-positron pair production is expected to occur in post-disruption plasmas in large tokamaks, including JET and JT-60U, where up to about 10(14) positrons may be created in collisions between multi-MeV runaway electrons and thermal particles. If the loop voltage is large enough, they are accelerated and form a beam of long-lived runaway positrons in the direction opposite to that of the electrons; if the loop voltage is smaller, the positrons have a lifetime of a few hundred ms, in which they are slowed down to energies comparable to that of the cool ( less, similar 10 eV) background plasma before being annihilated.     

Synchrotron radiation intensity and energy of runaway electrons in EAST tokamak

A detailed analysis of the synchrotron radiation intensity and energy of runaway electrons is presented for the Experimental Advanced Superconducting Tokamak(EAST). In order to make the energy of the calculated runaway electrons more accurate, we take the Shafranov shift into account. The results of the analysis show that the synchrotron radiation intensity and energy of runaway electrons did not reach the maximum at the same time. The energy of the runaway electrons reached the maximum first, and then the synchrotron radiation intensity of the runaway electrons reached the maximum.We also analyze the runaway electrons density, and find that the density of runaway electrons continuously increased. For this reason, although the energy of the runaway electrons dropped but the synchrotron radiation intensity of the runaway electrons would continue rising for a while.     

Mechanism of generation of runaway electrons in a lightning leader

A mechanism is analyzed of the electric field enhancement in a lightning leader up to the level permitting runaway of low-energy electrons. The ionization wave propagation in the preionized domain in front of the leader makes it possible to overcome the limitation imposed on the field intensity by transversal expansion of the leader front. By means of numerical simulations, it is demonstrated that, at the final stage of formation of a new leader step, generation of an electric field is possible in the channels of the streamer zone ahead of the new step with intensity sufficient for electron runaway and, consequently, for producing the X-ray and γ-ray pulses observed in correlation with the lightning leader steps.     

Application of diffuse discharges of atmospheric pressure formed by runaway electrons for modification of copper and stainless steel surface

Physics of Atomic Nuclei - The results of studies devoted to the influence of a runaway electron pre-ionized diffuse discharge (REP DD) formed in air and nitrogen at atmospheric pressure on the...     

Energy limit of runaway electrons in the HT-7 tokamak

The energy limit of runaway electrons in the HT-7 tokamak is investigated by measuring the synchrotron radiation originated from the runaway electrons and the hard X-ray radiation (HXR) when they hit the first wall. An upper boundary on the runaway energy can appear due to the resonance between the electron gyromotion and the magnetic field ripple in the low field side. The experimental derived maximum energy in the core is about 26 MeV, and maximum energy in the edge region is blocked to no more than 5 MeV. This resonance interaction of runaways with the nth harmonic of the magnetic field ripple can account for the observed energy gap of the runaways.     

On the mechanism of the runaway of electrons in a gas: The upper branch of the self-sustained discharge ignition curve

Based on the results of simulation by the method of particles, it is shown that the Townsend mechanism of electron multiplication in a gas at a sufficiently large electrode spacing is valid at least up to such large values of E/p at which relativistic electrons are generated. On the other hand, the phenomenon of electron runaway in a gas is determined by the electrode spacing, which must be either comparable with or smaller than the characteristic electron multiplication length, rather than the local criteria accepted presently. It is shown that, for a particular gas, the critical voltage across the electrodes at which the runaway electrons comprise a significant fraction is a universal function of the product of the electrode spacing by the gas pressure. This function also determines the condition of self-sustained discharge ignition. It not only incorporates the known Paschen curve but also additionally contains the upper branch, which describes the absence of a self-sustained discharge at a high voltage sufficiently rapidly supplied across the electrodes.     

EAST 装置等离子体逃逸电子同步辐射行为的实验研究

通过红外可见内窥镜诊断系统对EAST 等离子体芯部逃逸电子的同步辐射功率谱进行了分析,得出低能段逃逸电子同步辐射主要在红外波段,随着逃逸电子能量的增加,同步辐射向短波方向移动进入可见光波段。在欧姆放电条件下,对逃逸电子同步辐射所产生的的红外可见光进行了成像分析,同时研究了EAST 等离子体在低杂波和中性束注入加热条件下的逃逸电子行为。实验结果显示,低杂波和NBI 的投入总体抑制电子的逃逸,但低杂波投入初期产生的快电子对逃逸电子的产生具有促进作用。     

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通过红外可见内窥镜诊断系统对EAST等离子体芯部逃逸电子的同步辐射功率谱进行了分析,得出低能段逃逸电子同步辐射主要在红外波段,随着逃逸电子能量的增加,同步辐射向短波方向移动进入可见光波段。在欧姆放电条件下,对逃逸电子同步辐射所产生的的红外可见光进行了成像分析,同时研究了EAST等离子体在低杂波和中性束注入加热条件下的逃逸电子行为。实验结果显示,低杂波和 NBI 的投入总体抑制电子的逃逸,但低杂波投入初期产生的快电子对逃逸电子的产生具有促进作用。     

Differential conductivity at the transverse runaway of hot electrons

Differential conductivity under the transverse runaway of hot electrons is investigated. Quasi-elastic scattering and electronic-temperature approximations are considered for equilibrium and heated phonon subsystem. In both approximations, the differential conductivity is shown to tend to infinity, keeping the sign. Phonon heating retards transverse runaway.