Plasma Physics (physics.plasm-ph)

Abstract: We investigate the main physical processes that limit the repetition rate of capillary discharges used in laser accelerators of electrons theoretically and with computer simulations. We consider processes in the capillary. We assume that a cooling system independently maintains temperature balance of the capillary, as well as a gas supply system and a vacuum system maintain conditions outside the capillary. The most important factor, determining the highest repetition rates in this case, is the capillary length, which governs a refilling time of the capillary by the gas. For a short capillary, used for acceleration of sub-GeV electron beams, the repetition rate approximately equal to 10 kHz, which is inversely proportional to the square of the capillary length. The effects of the capillary diameter, gas type and the gas density are weaker.

Abstract: Understanding the origin and structure of mean magnetic fields in astrophysical conditions is a major challenge. Shear flows often coexist in such astrophysical conditions and the role of flow shear on dynamo mechanism is only beginning to be investigated. Here, we present a direct numerical simulation (DNS) study of the effect of flow shear on dynamo instability for a variety of base flows with controllable mirror symmetry (i.e, fluid helicity). Our observations suggest that for helical base flow, the effect of shear is to suppress the small scale dynamo (SSD) action, i.e, shear helps the large scale magnetic field to manifest itself by suppressing SSD action. For non-helical base flows, flow shear has the opposite effect of amplifying the small-scale dynamo action. The magnetic energy growth rate ($\gamma$) for non-helical base flows are found to follow an algebraic nature of the form, $\gamma = - aS + bS^\frac{2}{3}$ , where a, b > 0 are real constants and S is the shear flow strength and $\gamma$ is found to be independent of scale of flow shear. Studies with different shear profiles and shear scale lengths for non-helical base flows have been performed to test the universality of our finding.