We report spatially resolved measurements of the oblique merging of two supersonic laboratory plasma jets. The jets are formed and launched by pulsed-power-driven railguns using injected argon, and have electron density ∼ 10 14 cm −3 , electron temperature ≈ 1.4 eV, ionization fraction near unity, and velocity ≈ 40 km/s just prior to merging. The jet merging produces a few-cm-thick stagnation layer, as observed in both fastframing camera images and multi-chord interferometer data, consistent with collisional shock formation [E. C. Merritt et al., Phys. Rev. Lett. 111, 085003 (2013)].
*awetj@lanl.govOne-dimensional radiation-hydrodynamic simulations are performed to develop insight into the scaling of stagnation pressure with initial conditions of an imploding spherical plasma shell or "liner." Simulations reveal the evolution of high-Mach-number (M), annular, spherical plasma flows during convergence, stagnation, shock formation, and disassembly, and indicate that cmand µs-scale plasmas with peak pressures near 1 Mbar can be generated by liners with initial kinetic energy of several hundred kilo-joules. It is shown that radiation transport and thermal conduction must be included to avoid non-physical plasma temperatures at the origin which artificially limit liner convergence and thus the peak stagnation pressure. Scalings of the stagnated plasma lifetime (τ stag ) and average stagnation pressure (P stag , the pressure at the origin, is also found for a wide range of liner-plasma initial conditions.
We report experimental results on the parameters, structure, and evolution of high-Mach-number (M) argon plasma jets formed and launched by a pulsed-power-driven railgun. The nominal initial average jet parameters in the data set analyzed are density ≈ 2×10 16 cm −3 , electron temperature ≈ 1.4 eV, velocity ≈ 30 km/s, M ≈ 14, ionization fraction ≈ 0.96, diameter ≈ 5 cm, and length ≈ 20 cm. These values approach the range needed by the Plasma Liner Experiment (PLX), which is designed to use merging plasma jets to form imploding spherical plasma liners that can reach peak pressures of 0.1-1 Mbar at stagnation. As these jets propagate a distance of approximately 40 cm, the average density drops by one order of magnitude, which is at the very low end of the 8-160 times drop predicted by ideal hydrodynamic theory of a constant-M jet.
We describe a laboratory plasma physics experiment at Los Alamos National Laboratory that uses two merging supersonic plasma jets formed and launched by pulsed-power-driven railguns. The jets can be formed using any atomic species or mixture available in a compressed-gas bottle and have the following nominal initial parameters at the railgun nozzle exit: n e ≈ n i ∼ 10 16 cm −3 , T e ≈ T i ≈ 1.4 eV, V jet ≈ 30-100 km/s, mean chargeZ ≈ 1, sonic Mach number M s ≡ V jet /C s > 10, jet diameter = 5 cm, and jet length ≈20 cm. Experiments to date have focused on the study of merging-jet dynamics and the shocks that form as a result of the interaction, in both collisional and collisionless regimes with respect to the inter-jet classical ion mean free path, and with and without an applied magnetic field. However, many other studies are also possible, as discussed in this paper.
The stabilizing effect of a sheared axial flow is investigated in the ZaP flow Z-pinch experiment at the University of Washington. Long-lived, hydrogen Z-pinch plasmas are generated that are 1 m long with an approximately 10 mm radius and exhibit gross stability for many Alfvén transit times. Large magnetic fluctuations occur during pinch assembly, after which the amplitude and frequency of the fluctuations diminish. This stable behaviour continues for an extended quiescent period. At the end of the quiescent period, fluctuation levels increase in magnitude and frequency. Axial flow profiles are determined by measuring the Doppler shift of plasma impurity lines using a 20-chord spectrometer. Experimental measurements show a sheared flow that is coincident with low magnetic fluctuations during the quiescent period. The experimental flow shear exceeds the theoretical threshold during the quiescent period, and the flow shear is lower than the theoretical threshold at other times. The observed plasma behaviour and correlation between the sheared flow and stability persists as the amount of injected neutral gas and experimental geometry are varied. Computer simulations using experimentally observed plasma profiles show a consistent sheared flow stabilization effect. Plasma pinch parameters are measured independently to demonstrate an equilibrium consistent with radial force balance.
We present time-resolved observations of Rayleigh-Taylor-instability (RTI) evolution at the interface between an unmagnetized plasma jet colliding with a stagnated, magnetized plasma. The observed instability growth time (∼10 μs) is consistent with the estimated linear RTI growth rate calculated using experimentally inferred values of density (∼10(14) cm(-3)) and deceleration (∼10(9) m/s(2)). The observed mode wavelength (≳1 cm) nearly doubles within a linear growth time. Theoretical estimates of magnetic and viscous stabilization and idealized magnetohydrodynamic simulations including a physical viscosity model both suggest that the observed instability evolution is subject to magnetic and/or viscous effects.
A fusion space thruster based on the flow-stabilized Z-pinch may be possible in the near-term and provide many advantages over other fusion-based thruster concepts. The Zpinch equilibrium is classically unstable to gross disruption modes according to theoretical, numerical, and experimental evidence. However, a new stabilization mechanism has been discovered that can stabilize these modes with plasma flow. The stabilizing mechanism was developed for a Z-pinch plasma equilibrium which has an axial velocity profile that is linear in radius. When the velocity shear exceeds a threshold, the plasma modes are stabilized. The magnitude of the peak velocity is dependent on the mode wavelength but is sub-Alfvénic for the wavelengths of experimental interest, vmax > 0.1VAka where VA is the Alfvén speed, k is the axial wave vector, and a is the characteristic pinch radius. The flow Z-pinch experiment ZaP has been built at the University of Washington to experimentally verify the sheared flow stabilizing mechanism. The experiment has achieved plasma flow velocities of 10 5 m/s and stability for almost 2000 growth times. For more information the reader is encouraged to visit http://www.aa.washington.edu/AERP/ZaP. The extension of the flow Z-pinch to a space thruster is straight forward. The plasma in a flow Z-pinch would already be moving axially, fusing, and releasing a tremendous amount of nuclear energy. The end of the Z-pinch can be left open to allow the escape of the energetic plasma. Specific impulses in the range of 10 6 s and thrust levels of 10 5 N are possible.
We discuss the design, fabrication, and operation of a liner implosion system at peak currents of 16 MA. Liners of 1100 aluminum, with initial length, radius, and thickness of 4 cm, 5 cm, and 1 mm, respectively, implode under the action of an axial current, rising in 8 s. Fields on conductor surfaces exceed 0.6 MG. Design and fabrication issues that were successfully addressed include: Pulsed Power-especially current joints at high magnetic fields and the possibility of electrical breakdown at connection of liner cassette insulator to bank insulation; Liner Physics-including the angle needed to maintain current contact between liner and glide-plane/electrode without jetting or buckling; Diagnostics-X-radiography through cassette insulator and outer conductor without shrapnel damage to film.
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