Effect Of Runaway Electrons Research Articles

Subnanosecond breakdown of air-insulated coaxial line initiated by runaway electrons in the presence of a strong axial magnetic field

Flow of runaway electrons (RAEs) propagating in a radial, air-filled gap of coaxial line (CL) changes the dynamics of breakdown in the field of traveling voltage pulse. However, despite the effect of RAEs, breakdown does not occur if subnanosecond pulse is less in duration and amplitude than some values. In this work, we study the influence of an external axial magnetic field (B z) on the breakdown development. We demonstrate the transformation of the voltage pulse reflection from the ionized (breakdown) zone with changing B z. Due to gyration of fast electrons in an applied magnetic field, the gas region ionized by RAEs does not reach the anode. The ionized bridge between the cathode and anode is gradually replaced by a near-cathode plasma layer representing a discrete, reflecting/absorbing inhomogeneity in the CL.

Reflectometry of Picosecond Emission and Discharge Processes in a Gas-Filled High-Voltage Coaxial Line

Emission and discharge processes during the passage of high-voltage subnanosecond pulses with variable amplitude and duration along a gas-filled coaxial transmission line (CTL) are studied. Homogeneous CTL as well as CTL with special disk inserts (electric field enhancers initiating electron emission processes) on its central electrode were used. Data on the propagation and distortion of pulses were obtained and analyzed for different gases (nitrogen, hydrogen, SF6, and air) in a wide range of pressures. The voltage/field ranges and pulse durations are established where the role of runaway electrons (RAEs) appearing near the enhancer is essential in the breakdown development. It is shown that the effect of RAEs can be suppressed by shortening the voltage pulse even for hydrogen, which has a reduced static breakdown field and a high rate of ionization. It is also demonstrated that the RAE generation is abruptly replaced by the explosive electron emission from the enhancer edge with decreasing pressure from atmospheric to technical vacuum.

Effect of runaway electrons on tearing mode stability: with or without favorable curvature stabilization

An MHD-RE hybrid model is developed and implemented into the MARS-F code (Liu et al 2000 Phys. Plasmas 7 3681), where the runaway electrons (REs) are treated as a fluid species. The model is utilized to study the tearing mode (TM) stability in a tokamak plasma, with the numerical results compared to that of the analytic theory. The RE effect on the n = 1 TM is systematically investigated in the absence or presence of the favorable average curvature stabilization (the GGJ-effect). Without the GGJ-effect, for a force-free toroidal plasma equilibrium, REs are found to destabilize the TM and induce a finite real frequency to the mode (in a plasma with vanishing equilibrium flow). The RE contribution is found to significantly enhance the toroidal mode coupling. In the presence of the GGJ-effect, for a plasma with finite equilibrium pressure, the MARS-F hybrid model finds that REs destroy the symmetry in the TM spectrum and introduce extra branch of the TM solution due to the eigenvalue bifurcation. This happens because REs lead to coupling to a stable fluid TM mode and destabilize this mode. Despite complicated mode spectrum introduced by REs, we find that the latter play an overall destabilizing role for the TM also in the presence of the GGJ-effect. The MARS-F computed eigenvalues are well fitted with the analytic dispersion relation derived by Liu et al (2020 Phys. Plasmas 27 092507) for the force-free equilibrium. Less satisfactory is the fitting result with a TM dispersion relation that includes both the GGJ-effect and the RE contribution, suggesting further improvement of the proposed dispersion relation.

The phenomenon of runaway electrons in partially ionized non-ideal plasma

The effect of runaway electrons in partially ionized hydrogen plasma is investigated on the basis of pseudopotential models. The conditions of runaway electrons were determined. Dependences of an electron free path on the plasma density and coupling parameter were obtained. It is shown that if the quantum-mechanical and screening effects in non-ideal partially ionized plasma are taken into consideration, the collision frequency curve for electrons has maxima and free path curves for electrons have minima.

Runaway breakdown in strong electric field as a source of terrestrial gamma flashes and gamma bursts in lightning leader steps

The new model of lightning step leader is proposed. It includes three main processes developing simultaneously in a strong electric field: conventional breakdown, effect of runaway electrons and runaway breakdown (RB). The theory of RB in strong electric field is developed. Comparison with the existing observational data shows that the model can serve as a background for the explanation of gamma bursts in step leader and TGF.

On the effect of runaway electrons in dense plasma

AbstractThe effect of runaway electrons has been studied in this work. There were derived the conditions runaway electrons, the influence of electric field on the electron velocity distribution is considered for nonideal classical plasma models. The dependence of friction force on electrons on their velocities,electron‐ion collision frequency as a function of the coupling parameter and the strength of critical electric field on particle density and temperature are determined. The results are compared with the asymptotic theory. It has been shown that for the definite density and temperature ranges the difference between critical electric field values is essential for various plasma models. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effects of runaway electrons on plasma facing components

The electron gamma shower code EGS4 was applied to the simulation of energy deposition due to runaway electrons in the plasma facing components of tokamaks. By taking into account the magnetic field, we calculated the energy deposition in the layers of carbon and molybdenum irradiated by electrons with energies from 10 to 300 MeV and an incident angle from 0.5° to 25°. For a simplified model, the influence of the magnetic field was reduced with increase of the incident energies and angles of the runaway electrons. The energy deposition in the materials was observed to be independent on the incident angles and energies over 100 MeV. Moreover, we found no effects of the inclination of the magnetic field on the energy deposition in the substrate. For the proposed ITER divertor models, we found the energy deposition inside the water and coolant tube was high due to the magnetic fields. Because of the deep penetration of the runaway electrons due to the magnetic field, the deposited energy in the copper structure was increased. The present results suggest that the divertor materials and its structure need to be modified against the impacts of the runaway electrons.

Fueling of Fusion Reactors By Means of Pellet Injection

The fueling requirements of fusion reactors are analyzed on the basis of a recent assessment performed for NET (Next European Torus). Results of penetration depth calculations applied to single pellets under thermonuclear plasma conditions are described. Data corresponding to the commonly used neutral gas shielding ablation model and a magnetic shielding approximation are compared. The pellet size and pellet velocity requirements for central fueling are established. Scenario calculations are performed with the help of a 1D tokamak transport code. The pellet size, pellet velocity, and injection frequency requirements for continuous fueling or, for example, ignition with the help of pellets are obtained. The effect of runaway electrons, NB ions and alpha particles is estimated by calculating the ablation rates caused by these particles in fusion plasmas and comparing them with the ablation rate induced by thermal electrons.

Effect of runaway electrons on the ionisation equilibria and radiation of Tokamak impurities

The results of calculations of the ionisation equilibria and radiative cooling of Tokamak impurities are given as a function of electron temperature. In the relevant rate coefficients, the Maxwellian distribution of electron energies has been assumed to be distorted by a runaway tail caused by the finite electric field in a Tokamak. In order to investigate the effect of runaways on the start-up phase of a Tokamak, non-steady-state calculations have also been performed.

Elektrostatische Stabilität eines nicht-maxwellschen homogenen Plasmas

For studying the electrostatic stability or instability of a homogeneous plasma with a non-MAXWELLian velocity distribution of the electrons and ions, both undisturbed distribution functions are expanded into a series of HERMitian polynomials multiplied by a GAussian function. By this procedure the dispersion-relation derived from the linearized VLASOV equation is expressed by the coefficients of that expansion and by the temperature and the mean velocity of the electrons and ions. This dispersion-relation is discussed numerically for certain sets of these parameters which in particular include the effect of run-away electrons. They cause an instability just at the moment when they lead to a second maximum in the undisturbed distribution function. This second maximum does not have to be very large to get rather high growth rates (∼ 0.05 ωρ in the examples given) for the unstable modes, the real part of the frequency being of the order ωρ and the wave length of the order of the DEBYE length. The critical mean velocity for the onset of two-stream instability of the electrons against the ions tends to increase (rather than diminish) by the run-away electrons, contrary to what one might expect on first sight.