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Register a new userElectromagnetic fields created by the lightning leader channel in the final jump phase
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The dependences between the parameters of the Lightning discharge in the leader phase and the return stroke phase are obtained. It is shown that the peak values of the electric field created by lightning are determined by the parameters of the streamer zone of the leader at the final jump phase. It is shown that a large peak value and the steepness of the lightning return stroke current are generated in the final jump phase.
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Behavior of the potential and electric field intensity at the front of a lightning leader channel taking into account the influence of space charges injected by the streamers is investigated theoretically. Analytical solutions of Poisson equation are found for different laws of charge density distribution in the streamer zone. Influence of the streamer zone parameters on the formation of final jump phase of a leader is investigated.
It is well known that in the presence of strong external electromagnetic fields many processes forbidden in standard QED become possible. One example is the one-photon annihilation process considered recently by the present authors in the framework of a kinetic approach to the quasiparticle e-e+ gamma plasma created from vacuum in the focal spot of two counter-propagating laser beams. In these works the domain of large values of the adiabaticity parameter gamma >> 1 (corresponding to multiphoton processes) was considered. In the present work we estimate the intensity of the radiation stemming from photon annihilation in the framework of the effective mass model where gamma < 1, corresponding to large electric fields E < E_c=m^2/e and high laser field frequencies (the domain characteristic for X-ray lasers of the next generation). Under such limiting conditions the resulting effect is sufficiently large to be accessible to experimental observation.
We have extended our study of the competition between the drive and stabilization of plasma microinstabilities by sheared flow to include electromagnetic effects at low plasma $beta$ (the ratio of plasma to magnetic pressure). The extended system of characteristic equations is formulated, for a dissipative fluid model developed from the gyrokinetic equation, using a twisting mode representation in sheared slab geometry and focusing on the ion temperature gradient mode. Perpendicular flow shear convects perturbations along the field at the speed we denote as $Mc_s$ (where $c_s$ is the sound speed). $M > 1/ sqrt{beta}$ is required to make the system characteristics unidirectional and inhibit eigenmode formation, leaving only transitory perturbations in the system. This typically represents a much larger flow shear than in the electrostatic case, which only needs $M>1$. Numerical investigation of the region $M < 1/sqrt{beta}$ shows the driving terms can conflict, as in the electrostatic case, giving low growth rates over a range of parameters. Also, at modest drive strengths and low $beta$ values typical of experiments, including electromagnetic effects does not significantly alter the growth rates. For stronger flow shear and higher $beta$, geometry characteristic of the spherical tokamak mitigates the effect of an instability of the shear Alfv{e}n wave, driven by the parallel flow shear.
We use the Low Frequency ARray (LOFAR) to probe the dynamics of the stepping process of negatively-charged plasma channels (negative leaders) in a lightning discharge. We observe that at each step of a leader, multiple pulses of VHF (30~--~80 MHz) radiation are emitted in short-duration bursts ($<10 mu$s). This is evidence for streamer formation during corona flashes that occur with each leader step, which has not been observed before in natural lightning and it could help explain X-ray emission from lightning leaders, as X-rays from laboratory leaders tend to be associated with corona flashes. Surprisingly we find that the stepping length is very similar to what was observed near the ground, however with a stepping time that is considerably larger, which as yet is not understood. These results will help to improve lightning propagation models, and eventually lightning protection models.
QED cascades in a strong electromagnetic field of optical range and arbitrary configuration are considered. A general expression for short-time dependence of the key electron quantum dynamical parameter is derived, allowing to generalize the effective threshold condition of QED cascade onset. The generalized theory is applied to selfsustained cascades in a single focused laser pulse. According to numerical simulations, if a GeV electron bunch is used as a seed, an ordinary cascade can be converted into the selfsustained one. As an application, it would be also possible to produce this way bright collimated photon beams with up to GeV photon energies.
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