Sergey A. Pikuz

HIGH-LUMINOSITY MONOCHROMATIC X-RAY BACKLIGHTING USING AN INCOHERENT PLASMA SOURCE TO STUDY EXTREMELY DENSE PLASMAS (INVITED)

A new diagnostic method for dense plasmas, monochromatic x-ray backlighting, is described. In this method, shadow images of a bright, dense plasma can be obtained with high spatial resolution using monochromatic radiation from a separate plasma, permitting a major reduction in the required backlighting source power. The object plasma is imaged utilizing spherically bent mica crystals as x-ray optical elements. Experimental results, namely images of test objects obtained using x-ray radiation having different photon energies, are presented. Shadow images of exploding Al wire plasmas in the 1s2–1s2p line radiation of He-like Al XII are also shown. Spatial resolution as fine as 4 μm is demonstrated. The scheme described here is useful for backlighting extended high density plasmas, and could be a less costly alternative to using x-ray lasers for such purposes.A new diagnostic method for dense plasmas, monochromatic x-ray backlighting, is described. In this method, shadow images of a bright, dense plasma can be obtained with high spatial resolution using monochromatic radiation from a separate plasma, permitting a major reduction in the required backlighting source power. The object plasma is imaged utilizing spherically bent mica crystals as x-ray optical elements. Experimental results, namely images of test objects obtained using x-ray radiation having different photon energies, are presented. Shadow images of exploding Al wire plasmas in the 1s2–1s2p line radiation of He-like Al XII are also shown. Spatial resolution as fine as 4 μm is demonstrated. The scheme described here is useful for backlighting extended high density plasmas, and could be a less costly alternative to using x-ray lasers for such purposes.

Laboratory formation of a scaled protostellar jet by coaligned poloidal magnetic field

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. A scaled-down plasma experiment shows that axial magnetic fields in young stars can shape their bipolar jet outflows. Stellar outflows replicated in miniature Astronomers observe tight bright jets beaming from the poles of many celestial objects. But what focuses them so well? Albertazzi et al. recreated a scaled-down plasma jet in a laboratory setting to match the behavior of those in young stellar objects. The experiments show that the jets are collimated by a poloidal magnetic field aligned with the same axis. A conelike shock also emerges, as the expanding plasma is abruptly confined by the magnetic field. Science, this issue p. 325

Bragg X-Ray Optics for Imaging Spectroscopy of Plasma Microsources

Bragg x-ray optics based on crystals with transmission and reflection properties bent on cylindrical or spherical surfaces are discussed. Applications of such optics for obtaining one- and two-dimensional monochromatic images of different plasma sources in the wide spectral range 1-20 Å are described. Samples of spectra obtained with spectral resolution of up to λ/Δλ ~ 10,000 and spatial resolution of up to 18 μm are presented.

Multiwire

The rebuilt COBRA pulsed-power generator, which has a variable current-pulse waveform and amplitude (95-150-ns rise time and ~1-MA peak current), has extended the range of the current-pulse parameters that can be used to study the X-pinches. X-pinches with 4-12 wires of several different wire materials (from Al to W) with diameters from 25 to 75 mum have been studied. The influence of the rate of the rise of current and the X-pinch wire mass on the X-pinch plasma formation and pinch implosion dynamics has been studied using a set of diagnostics with spatial and/or temporal resolution. Multiwire X-pinches were placed in the diode center, and/or two four-wire X-pinches were placed in one of the four parallel return-current circuits of the diode. The experiments showed that the most important factor determining the first X-ray burst timing is the linear mass density of the X-pinch wires, and an optimal value for obtaining a single high-energy X-ray burst was found. Radiographic images of the different test objects, wires in wire-array Z-pinches, and the X-pinches, themselves, were obtained using a micrometer-scale spatial resolution

X

In high-spectral resolution experiments with the petawatt Vulcan laser, strong x-ray radiation of KK hollow atoms (atoms without n = 1 electrons) from thin Al foils was observed at pulse intensities of 3 × 10(20) W/cm(2). The observations of spectra from these exotic states of matter are supported by detailed kinetics calculations, and are consistent with a picture in which an intense polychromatic x-ray field, formed from Thomson scattering and bremsstrahlung in the electrostatic fields at the target surface, drives the KK hollow atom production. We estimate that this x-ray field has an intensity of >5 × 10(18) W/cm(2) and is in the 3 keV range.

-Pinches at 1-MA Current on the COBRA Pulsed-Power Generator

Distribution of matter in the discharge channel formed upon a nanosecond electrical explosion of a single wire in air and vacuum was studied experimentally. Simultaneous use of optical, UV, and X-ray diagnostics made it possible to distinguish qualitatively different regions of the discharge channel, such as the current-carrying layers and the region occupied by a weakly conducting cold plasma. Several series of experiments with 25-µm-diameter 12-mm-long wires made of different materials were performed. The charging voltage and the current amplitude were varied in the ranges of U0 = 10–20 kV and Imax ∼ 5–10 kA, respectively. Explosion regimes with a current pause and with and without current interruption, as well as with wire preheating in air and vacuum, were studied. Shadow and schlieren images of the discharge channel were obtained using optical probing at the second harmonic of a YAG: Nd+3 laser (λ = 0.532 µm, τ ∼ 10 ns). In the experiments carried out in vacuum, X-ray images of the discharge channel were also obtained using an X-pinch as a point source of probing radiation and UV images were recorded using a four-frame MCP camera.

Exotic dense-matter states pumped by a relativistic laser plasma in the radiation-dominated regime

The structure of the discharge channel during nanosecond wire explosions has been studied using laser probing. Wires of 25μm diameter and 12mm length were exploded in air and vacuum by 10kA current pulse having a 50A∕ns rate of rise. Upon electrical explosion of thin wires in the air, the development of shock waves was observed. The propagation of shock waves was analyzed, and it was possible to draw conclusions on the location of the flow of most of the current in the volume of the discharge channel. This permitted distinguishing between two scenarios (shunting and internal) of the interelectrode gap breakdown development. The scenario depends to a large extent on the properties of the exploding wire material. The same two scenarios are valid upon electrical explosion of wire in vacuum. Moreover, if secondary breakdown develops in the internal scenario, the value of the energy deposition in the wire material during explosion in vacuum may be comparable with that found during explosion in air.

Distribution of matter in the current-carrying plasma and dense core of the discharge channel formed upon electrical wire explosion

The effect of surrounding media of different densities and electric strengths on the heating dynamics of a micron wire during its nanosecond electric explosion is investigated. Tungsten wires with diameters of d = 25–50 μm were exploded in air and water at a current rise time of (dI/dt) ∼ 1010 A/s. The diagnostic complex is described.

Analysis of the discharge channel structure upon nanosecond electrical explosion of wires

Investigations of X-pinch plasma structure and generation of an accelerated electron beam in the X-pinch using the methods of x-ray emission spectroscopy are discussed. The diagnostics allowed to make an investigation of the x-ray radiation with high spectral and spatial resolution over the spectral range 0.5 to more than 10 A from a variety of wire materials.

Nanosecond electric explosion of a tungsten wire in different media

For the first time, the regime of a micrometer-size hot spot formation is impemented for an X-pinch in a plasma, which is fed from a current generator based on low-inductance capacitors and rapid current switches. The configurations of X-pinches, which can be used effectively as point sources of soft X-rays with this type of current generator, are determined. A prototype of a small-size radiation source for high-resolution point projection X-ray radiography has been constructed. The main parameters of X-pinch as a radiation source are analyzed and compared with X-pinch parameters in high-voltage setups with shaping lines. An analysis of the data on the operation of X-pinches in generators with different parameters has led to simple relations that can be used to select optimal initial X-pinch parameters.