Counter-propagating plasma jet collision and shock formation on a compact current driver

Abstract

In this paper we report on the ability of a compact current driver yielding 250 kA in 150 ns to produce counter-propagating plasma flows. The flows were produced by two vertically-opposed conical wire arrays separated by 1 cm, each comprised of 8 wires. With this array configuration, we were able to produce two supersonic plasma jets with velocities on the order of 100-200 km/s that propagate towards each other and collide. Aluminum wires were tested first; we observed a shock wave forming at the collision region that remained stationary for an extended period of time (~ 50 ns) using optical probing diagnostics and Extreme Ultraviolet imaging. After this period, a bow shock is formed that propagates at 20 km/s towards the cathode of the array, likely due to small differences in the density and/or speed of the jets. The inter-jet ion mean free path was estimated to be larger than the shock scale length for aluminum, indicating that the shock is not mediated by collisions, but possibly by a magnetic field, whose potential sources are also discussed. Radiative cooling and density contrast between the jets were found to be important in the shock wave dynamics. We studied the importance of these effects by colliding jets of two different materials, using aluminum in one and copper in the other. In this configuration, the bow shock was observed to collapse into a thin shell and then to fragment, forming clumpy features. Simultaneously, the tip of the bow shock is seen to narrow as the bow shock moves at a similar speed observed in the Al-Al case. We discuss the similarity criteria for scaling astrophysical objects to the laboratory, finding that the dimensionless numbers are promising.

Exploring astrophysics-relevant magnetohydrodynamics with pulsed-power laboratory facilities

“…Experiments with colliding plasma jets in which the jets were produced by two conical wire arrays are reported in Valenzuela et al (2015). These experiments also show development of clumpy structures, consistent with development of cooling instabilities in which the cooling time is shorter than the flow dynamical time.…”

Persistent mysteries of jet engines, formation, propagation, and particle acceleration: have they been addressed experimentally?

“…In this pulsed-power approach [92,227,203,228,229,230,231] jet formation happens via formation of a conical standing shock, similar to the configuration employed in laser-driven jet experiments. The main difference from the laser driven platforms is a more steady-state nature of the converging flow formation.…”

“…In a reference frame moving with the internal shock velocity, the interaction is equivalent to an interaction of two counter-propagating plasma flows, which is a more practical frame in which to perform laboratory experiments. Such interactions were produced and studied in [247,231] using pulsed-power platforms. In [247] the counter-streaming jets were formed by two ablating radial foils, driven by the same current pulse (Figure 7).…”

Theoretical and computational studies of the Weibel instability in several beam–plasma interaction configurations

“…Furthermore, the collision of relativistic beams with plasma mimicking the background interstellar medium near young stellar objects and Herbig-Haro objects has been investigated (Gregory et al 2008;Valenzuela et al 2015). Lepton beams into background electron-proton plasmas are a promising source of high-energy cosmic rays via Weibel-induced moving magnetic islands (Cui et al 2015) and can show preferential positron acceleration (Dieckmann et al 2020).…”

“…2008; Valenzuela et al. 2015). Lepton beams into background electron–proton plasmas are a promising source of high-energy cosmic rays via Weibel-induced moving magnetic islands (Cui et al.…”