Although bipolar jets are seen emerging from a wide variety of astrophysical systems, the issue of their formation and morphology beyond their launching is still under study. Our scaled laboratory experiments, representative of young stellar object outflows, reveal that stable and narrow collimation of the entire flow can result from the presence of a poloidal magnetic field whose strength is consistent with observations. The laboratory plasma becomes focused with an interior cavity. This gives rise to a standing conical shock from which the jet emerges. Following simulations of the process at the full astrophysical scale, we conclude that it can also explain recently discovered x-ray emission features observed in low-density regions at the base of protostellar jets, such as the well-studied jet HH 154.

No abstract

Experimental results are presented from studies of the dynamics of X-pinch plasmas, formed using two fine wires that cross and touch at a single point (in the form of an X) as the load of a high current pulser. High-resolution (sub-ns in time and 2–3 μm in space) x-ray radiographs of X pinches driven by current pulses that rise to 200–250 kA peak current in 40 ns show that ⩽300 μm long Z pinches form in the region of the original wire cross-point a few ns prior to the first sub-ns intense x-ray bursts that are characteristic of an X pinch. The Z pinches implode to ⩽10 μm diam and then appear to develop gaps in the radiographic images in one or two places, coincident in time with the x-ray burst(s). The emission spectra of the intense x-ray bursts from different wire materials indicate electron temperatures of 500–1300 eV and densities in excess of 1022/cm3.

Wire-array Z-pinch implosion experiments begin with wire heating, explosion, and plasma formation phases that are driven by an initial 50–100 ns, 0–1 kA/wire portion of the current pulse. This paper presents expansion rates for the dense, exploding wire cores for several wire materials under these conditions, with and without insulating coatings, and shows that these rates are related to the energy deposition prior to plasma formation around the wire. The most rapid and uniform expansion occurs for wires in which the initial energy deposition is a substantial fraction of the energy required to completely vaporize the wire. Conversely, wire materials with less energy deposition relative to the vaporization energy show complex internal structure and the slowest, most nonuniform expansion. This paper also presents calibrated radial density profiles for some Ag wire explosions, and structural details present in some wire explosions, such as foam-like appearance, stratified layers and gaps.

A review of recent experiments on the MAGPIE generator (1 MA, 250 ns) aimed at studying the implosion dynamics of wire array Z-pinches is presented. The first phase of implosion is dominated by the gradual ablation of stationary wire cores and gradual redistribution of the array mass by the precursor plasma flow. It is found that the rate of wire ablation depends on the magnitude of the global (collective) magnetic field of the array, and increases with the field. The existence of the modulation of the ablation rate along the wires leads to the presence of a 'trailing' mass left behind by the imploding current sheath. The trailing mass provides an alternative path for the current, reducing the force available for compression of the pinch at stagnation. The observed dependence of the ablation rate on inter-wire separation suggests an explanation for the existence of the optimal wire number maximizing the x-ray power. Axially resolved spectroscopy shows the presence of the x-ray 'bright' spots (<150 µm) emitting intense continuum radiation.

No abstract

Magnetized laser-produced plasmas are central to many novel laboratory astrophysics and inertial confinement fusion studies, as well as in industrial applications. Here we provide the first complete description of the three-dimensional dynamics of a laser-driven plasma plume expanding in a 20 T transverse magnetic field. The plasma is collimated by the magnetic field into a slender, rapidly elongating slab, whose plasma-vacuum interface is unstable to the growth of the "classical", fluid-like magnetized Rayleigh-Taylor instability.The combination of high-power lasers with externally applied high-strength magnetic fields of up to kT [1,2] has been seminal in the development of many recent applications in laboratory astrophysics [3][4][5][6][7], in novel concepts in laser-[8-10] and magnetically-driven [11,12] inertial confinement fusion physics, and in industrial applications [13,14]. Beside understanding the dynamics of the plasma expansion across a magnetic field, of particular importance is to grasp the nature of rapidly growing instabilities which may develop and profoundly modify the morphology and characteristics of these plasmas. Indeed, the presence of striations and flutes have often been associated with the development of instabilities and in particular with the lower hybrid drift instability (LHDI) or one of its variants [15][16][17]. In addition, anomalous resitivity driven by the LHDI [18,19] can also affect the plasma microscopically, with potentially important consequences on magnetic field diffusion and the growth of other instabilities. Among those, the magnetic Rayleigh-Taylor instability (MRTI) [20,21] is known to play a key role on the dynamics of laboratory [22], as well as astrophysical plasmas [23,24]. So far however, it has not been isolated in laser-produced high energy density plasmas.A major parameter affecting the stability and dynamics of these plasmas is the relative direction of the applied magnetic field with respect to the plasma expansion axis. While for an aligned magnetic field the plasma is collimated into an axisymmetric, stable jet-like flow [4,5], for a transverse magnetic field both stable[25] and unstable flows [26] were observed and a clear understanding of the plasma evolution is still missing.Here, we provide the first complete description of the three-dimensional dynamics of a laser-driven plasma plume in a transverse 20 T magnetic field. We show that the plasma is collimated into a slender, rapidly expanding slab, and demonstrate that under these conditions, the growth of flute-like, interchange modes at the plasma-vacuum interface that extend in the form of spikes into the vacuum is due to the classical, fluid-like, magnetic Rayleigh-Taylor instability (MRTI). Interestingly, we find that to recover quantitatively in the simulations the penetration of these spikes into the vacuum, a subgrid-scale model of anomalous resistivity needs to be included. This anomalous resistivity could be induced by the micro-turbulence generated by the LHDI, which for our plasma condition...

The properties of high energy density plasma are under increasing scrutiny in recent years due to their importance to our understanding of stellar interiors, the cores of giant planets 1 , and the properties of hot plasma in inertial confinement fusion devices 2 . When matter is heated by X-rays, electrons in the inner shells are ionized before the valence electrons. Ionization from the inside out creates atoms or ions with empty internal electron shells, which are known as hollow atoms (or ions) 3,4,5 . Recent advances in free-electron laser (FEL) technology 6,7,8,9 have made possible the creation of condensed matter consisting predominantly of hollow atoms. In this Letter, we demonstrate that such exotic states of matter, which are very far from equilibrium, can also be formed by more conventional optical laser technology when the laser intensity approaches the radiation dominant regime 10 . Such photon-dominated systems are relevant to studies of photoionized plasmas found in active galactic nuclei and X-ray binaries 11 . Our results promote laser-produced plasma as a unique ultra-bright x-ray source for future studies of matter in extreme conditions as well as for radiography of biological systems and for material science studies 12,13,14,15 .