Hydromagnetic stability and collisional properties of current-driven plasma jets
Keywords: Mechanical Engineering.

Pulsed plasma accelerators have been a research focus for many decades. They have the unique ability to generate high velocity, hydromagnetic jets with tunable compression and collimation. These features allow for both high acceleration efficiency and directed heat fluxes that are ideal for applications like space propulsion or ablative material testing. Such devices have led to fundamental contributions in the production of laboratory astrophysical jets, plasma jet driven magnetized target fusion, z-pinch schemes, and spheromak compact toroids. This work focuses on investigating two distinct issues with hydromagnetic flows that have conventionally limited both the scalability and efficiency of applications, namely, flow stability and collisionality. The basis of these studies is first informed by a detailed characterization of the plasma jet produced in the Stanford gun experiment. A combination of line broadening, high bandwidth imaging, electrical probes, and modeling is used to measure the equilibrium properties of the plasma jet. It is shown that the Stanford gun device can produce plasma jets with characteristic velocities of V ~ 100 km/s, number densities up to n ~ 10^23 m^-3, and magnetic Reynold's numbers of Re_m ~ 100. A detailed scaling of these parameter against both machine operating conditions and corresponding astrophysical phenomena is also given. The manner in which magnetized plasma jets evolve remains key to better understanding the stability of hydromagnetic systems and providing new insights into how they can be dynamically controlled. The underlying theory, apparatus, and optical features of a novel schlieren diagnostic capable of cinematically visualizing dense plasma jets is described. This diagnostic features the unique ability to simultaneously resolve both the characteristic Alfvenic timescales and spatial flow features with continuous acquisition over the lifetime of a jet. This diagnostic is employed to visualize the formation and evolution of stable hydromagnetic jets produced from a plasma gun device. Dynamic coherent flow features are identified and tracked over time throughout the evolutionary progression of plasma jets. To explain the apparent stabilizing behavior, a linear stability analysis of the magnetohydrodynamic equations is performed to identify the underlying mechanism contributing to the observed behavior. The results indicate that a stable dense plasma jet can be maintained for timescales over which a steady pinch current can be sustained, even at levels which would otherwise lead to rapid unstable mode growth and resultant pinch disassembly. Collisionality is first described in the plasma accelerator by considering the impact of the initial neutral gas distribution within the gun volume. A model based on the Rankine-Hugoniot formulation in combustion is presented and used to predict both characteristic operating regimes observed experimentally. Precise neutral gas triggering is used to change the gas distribution within the accelerator and modify the initial conditions governing the breakdown process. Both bulk energy transfer and time-of-flight measurements show that with increasing gas diffusion time, the directed energy in the flow decreases and the mode transitions from a deflagration or jet to a detonation or ``snowplow'' mode. Neutral gas simulations indicate that neutral gas governs the transition between these operating modes. This informs strategies to maintain high acceleration efficiency in pulsed plasma accelerators and eliminate shocking conditions caused by higher gas loadings. Finally, the role of flow collisionaliy is also considered in the context of using plasma guns to simulate high heat loadings caused by plasma disturbances in fusion reactors. Plasma gun devices have been used extensively to replicate ELM conditions in the laboratory, however feature higher density, lower temperatures, and thus higher flow collisionality than those expected in fusion conditions. Experimental visualizations are shown that indicate strong shocks form in gun devices during material stagnation over spatial and temporal scales that precede ablation dynamics. These measurements are used to validate magnetohydrodynamic simulations that capture the production of plasma jets and the shielding effect collisionality plays in particle transport to material surfaces. Simulations show that self-shielding effects in plasma guns reduce the free streaming heat flux by up to 90\% and further reduce the incoming particle kinetic energy impinging on material surfaces. These simulations are performed over a range of operating conditions for gun devices and a discussion is provided regarding how existing experimental measurements can be interpreted when extrapolating to fusion conditions.