Computational and experimental investigation of plasma deflagration jets and detonation shocks in coaxial plasma accelerators

Abstract

We present a magnetohydrodynamic (MHD) numerical simulation to study the physical mechanisms underlying plasma acceleration in a coaxial plasma gun. Coaxial plasma accelerators are known to exhibit two distinct modes of operation depending on the delay between gas loading and capacitor discharging. Shorter delays lead to a high velocity plasma deflagration jet and longer delays produce detonation shocks. During a single operational cycle that typically consists of two discharge events, the plasma acceleration exhibits a behavior characterized by a mode transition from deflagration to detonation. The first of the discharge events, a deflagration that occurs when the discharge expands into an initially evacuated domain, requires a modification of the standard MHD algorithm to account for rarefied regions of the simulation domain. The conventional approach of using a low background density gas to mimic the vacuum background results in the formation of an artificial shock, inconsistent with the physics of free expansion. To this end, we present a plasma-vacuum interface tracking framework with the objective of predicting a physically consistent free expansion, devoid of the spurious shock obtained with the low background density approach. The interface tracking formulation is integrated within the MHD framework to simulate the plasma deflagration and the second discharge event, a plasma detonation, formed due to its initiation in a background prefilled with gas remnant from the deflagration. The mode transition behavior obtained in the simulations is qualitatively compared to that observed in the experiments using high framing rate Schlieren videography. The deflagration mode is further investigated to understand the jet formation process and the axial velocities obtained are compared against experimentally obtained deflagration plasma front velocities. The simulations are also used to provide insight into the conditions responsible for the generation and sustenance of the magnetic pinch. The pinch width and number density distribution are compared to experimentally obtained data to calibrate the inlet boundary conditions used to set up the plasma acceleration problem.

Distinctive Features of Graphene Synthesized in a Plasma Jet Created by a DC Plasma Torch

“…Differences in the cases of helium and argon are investigated using equilibrium plasma composition as a function of temperature in the range 2000-4000 K. As a matter of fact just in this range the precursor turns into molecular hydrogen and condensed phase of carbon, identified in the database IVTANTERMO [36] as graphite. Modeling of the plasma flow in the reactor is made using quasi-one-dimensional approach under the local thermodynamic equilibrium [28,[37][38][39].…”

Effects of flow collisionality on ELM replication in plasma guns

“…At the Stanford facility, a suite of diagnostics have been employed to uncover the underlying plasma properties of the plume that interacts with material surfaces. Namely, spectroscopic line broadening has been employed to determine both plasma size and density [20], time-of-flight to measure velocity [17,20], and [20,21]. b Pulsed plasma gun coupled with long magnetic drift tube [7].…”

“…distributed probes/detailed numerical simulations to quantify both magnetic field and plasma temperature [21,22]. A survey of these results are included in table 1 along with measurements from similar facilities.…”

“…Numerical methodology For the sake of brevity, presented here are certain salient features of the numerical model. A detailed description of the MHD computational tool can be found in [21,34], where it is used to study mode transition in plasma guns. The equations are spatially discretized using a cell-centered finite volume scheme on generalized unstructured grids [34].…”

Dynamic formation of stable current-driven plasma jets

“…Figure 1c) depicts the plasma compression caused by the radial Lorentz force, which forms a hot, dense pinch at the core of the jet. The accelerated plasma surrounding the pinch maintains a quasi-steady axial shear flow featuring characteristic velocities of 10 5 m/s, while the axial flow velocity through the pinch itself is comparatively lower due to its position in the wake of the central electrode 22 . The resulting dense plasma jet is inherently unstable to both the m = 0 ‘sausage’ and m = 1 ‘kink’ MHD modes; however, a stable jet is observed for timescales over which a steady current is sustained.…”