Experimental Characterization of the Stagnation Layer between Two Obliquely Merging Supersonic Plasma Jets

Abstract

We present spatially resolved measurements characterizing the stagnation layer between two obliquely merging supersonic plasma jets. Intra-jet collisionality is very high (λii ≪ 1 mm), but the inter-jet ion-ion mean free paths are on the same order as the stagnation layer thickness (a few cm). Fast-framing camera images show a double-peaked emission profile transverse to the stagnation layer, with the central emission dip consistent with a density dip observed in the interferometer data. We demonstrate that our observations are consistent with collisional oblique shocks.Colliding plasmas have been studied in a variety of contexts, e.g., counterstreaming laser-produced plasmas supporting hohlraum design for indirect-drive inertial confinement fusion [1][2][3], forming and studying astrophysically relevant shocks [4][5][6][7][8], and for applications such as pulsed laser deposition [9] and laser-induced breakdown spectroscopy [10]. Physics issues arising in these studies include plasma interpenetration [11][12][13][14][15][16], shock formation [17], and the formation and dynamics of a stagnation layer [18][19][20]. In this work, we present experimental results on two obliquely merging supersonic plasma jets, which are in a different and more collisional parameter regime than many of the colliding plasma examples mentioned above. Ours and other recent jet-merging experiments [21,22] were conducted to explore the feasibility of forming imploding spherical plasma liners via an array of merging plasma jets [23][24][25][26][27], which could have applications in forming cm-, µs-, and Mbar-scale plasmas for fundamental high-energy-density-physics studies [28] and as a standoff driver for magnetoinertial fusion [23,[29][30][31][32]. Prior experiments studying the stagnation layer between colliding laser-produced or wire-array z-pinch [33] plasmas were on smaller spatial scales (mm or smaller) that could not be fully resolved by measurements. New results in the present work are the experimental identification and characterization of a few-cm thick stagnation layer between colliding plasmas, and the demonstration that our observations are consistent with hydrodynamic oblique shock theory [34].Experiments reported here are conducted on the Plasma Liner Experiment (PLX) [35], in which two supersonic argon plasma jets are formed and launched by plasma railguns [36]. Plasma jet parameters at the exit of the railgun nozzle (peak n e ≈ 2 × 10 16 cm −3 , peak T e ≈ 1.4 eV, V jet ≈ 30 km/s, Mach number M ≡ V jet /C s,jet ≈ 14, diameter = 5 cm, and length ≈ 20 cm) and their evolution during subsequent jet propagation have been characterized in detail [35]. The jet magnetic field inside the railgun is ∼3 T, but the classical magnetic diffusion time is a few µs [35], and thus • apart, two merging plasma jets, R-Z coordinates used in the paper, approximate interferometer/spectrometer linesof-sight (Z ≈ 84 cm), and CCD camera field-of-view.we ignore the effects of a magnetic field by the time of jet merging (> 20 µs). Experimental data ...

Particle-in-cell simulations of collisionless shock formation via head-on merging of two laboratory supersonic plasma jets

“…Although not considered in this paper, we point out that the LANL experiment should also be able to observe unmagnetized collisional shocks with higher density merging jets and has observed such phenomena. 43 …”

Laser-driven, magnetized quasi-perpendicular collisionless shocks on the Large Plasma Device

“…9,10 Other experiments studied the diamagnetic cavity generated by a sub-Alfv enic (M A ¼ v=v A < 1, where v A is the Alfv en speed) laser-driven plasma in an external magnetic field, [11][12][13][14][15] while more recent work has focused on studying collisionless shocks by combining a laser-plasma with a Z-pinch 16 or through field-reversed configuration (FRC) plasma guns. 17 At the University of California, Los Angeles (UCLA), we utilize a unique experimental platform based on an earlier design study 3 that combines a high-power, laser-driven super-Alfv enic magnetic piston with a large preformed, magnetized ambient plasma. Though dimensionless parameters of relevance to magnetized space shocks, such as the Alfv en Mach number M A or shock formation length scale, are only marginally satisfied experimentally, this has the advantage not readily accessible to spacecraft of providing a regime where the microphysics of shock formation can be studied in detail.…”

Magneto-Inertial Fusion

“…LANL also leads a team that is exploring a standoff concept of using a spherically convergent array of gun-driven plasma jets to Fig. 1 Many of the MIF concepts presently being explored in the USA achieve assembly and implosion of a plasma liner (PLX) without the need to destroy material liners or transmission lines on each shot [17][18][19][20]. A private company, general fusion (GF) in Canada, with many Americans working for it, is developing a merging compact toroid plasma source and envisions repetitively fired acoustic drivers that would drive a liquid liner compression of a magnetized target [21][22][23].…”

“…A 2.7-meter diameter spherical vacuum chamber is the centerpiece of the plasma liner experiment (PLX) at LANL, which has also conducted basic plasma shock experiments [19,20,36,37] using two plasma railguns that were developed by HyperV Technologies Corporation. The PLX team will conduct 36-60 coaxial-gun experiments that aim to address the key MIF-relevant scientific issues of spherically imploding plasma liners as a standoff driver.…”