A.L. Velikovich

Neutron production and implosion characteristics of a deuterium gas-puff Z pinch

Experiments on the Z accelerator with deuterium gas puff implosions have produced up to 3.9×1013(±20%) neutrons at 2.34 MeV (±0.10MeV). Experimentally, the mechanism for generating these neutrons has not been definitively identified through isotropy measurements, but activation diagnostics suggest multiple mechanisms may be responsible. One-, two-, and three-dimensional magnetohydrodynamic (MHD) calculations have indicated that thermonuclear outputs from Z could be expected to be in the (0.3–1.0)×1014 range. X-ray diagnostics of plasma conditions, fielded to look at dopant materials in the deuterium, have shown that the stagnated deuterium plasma achieved electron temperatures of 2.2keV and ion densities of 2×1020cm−3, in agreement with the MHD calculations.

Z-pinch plasma neutron sources

A deuterium gas-puff load imploded by a multi-MA current driver from a large initial diameter could be a powerful source of fusion neutrons, a plasma neutron source (PNS). Unlike the beam-target neutrons produced in Z-pinch plasmas in the 1950s and deuterium-fiber experiments in the 1980s, the neutrons generated in deuterium gas-puffs with current levels achieved in recent experiments on the Z facility at Sandia National Laboratories could contain a substantial fraction of thermonuclear origin. For recent deuterium gas-puff shots on Z, our analytic estimates and one- and two-dimensional simulations predict thermal neutron yields ∼3×1013, in fair agreement with the yields recently measured on Z [C. A. Coverdale et al., Phys. Plasmas (to be published)]. It is demonstrated that the hypothesis of a beam-target origin of the observed fusion neutrons implies a very high Z-pinch-driver-to-fast-ions energy transfer efficiency, 5 to 10%, which would make a multi-MA deuterium Z-pinch the most efficient light-ion ac...

Efficient argon K-shell radiation from a Z pinch at currents >15 MA

The first observations of gaseous load implosions with over 15 MA in >110 ns on the Z generator [R. B. Spielman et al., Phys. Plasmas 5, 2105 (1998)] are reported. Starting from a diameter of over 8 cm, an argon double-shell Z pinch imploded to under 0.5 cm K-shell emission diameter. With a load mass of 0.8 mg/cm, K-shell x-ray output reached 274±24 kJ in a 15 TW peak power, 12 ns pulse. This record-high yield is consistent with the current-squared scaling predicted for the “efficient” emission regime.

Measurements of magneto-Rayleigh–Taylor instability growth during the implosion of initially solid metal liners a)

A recent publication [D. B. Sinars et al., Phys. Rev. Lett. 105, 185001 (2010)] describes the first controlled experiments measuring the growth of the magneto-Rayleigh–Taylor instability in fast (∼100 ns) Z-pinch plasmas formed from initially solid aluminum tubes (liners). Sinusoidal perturbations on the surface of these liners with wavelengths of 25–400 μm were used to seed single-mode instabilities. The evolution of the outer liner surface was captured using multiframe 6.151 keV radiography. The initial paper shows that there is good agreement between the data and 2-D radiation magneto-hydrodynamic simulations down to 50 μm wavelengths. This paper extends the previous one by providing more detailed radiography images, detailed target characterization data, a more accurate comparison to analytic models for the amplitude growth, the first data from a beryllium liner, and comparisons between the data and 3D simulations.

Radiation properties and implosion dynamics of planar and cylindrical wire arrays, asymmetric and symmetric, uniform and combined X-pinches on the UNR 1-MA zebra generator

In the following experiments, we studied implosions of different wire arrays and X-pinches produced on the 1-MA Zebra generator at the University of Nevada, Reno. Diagnostics included both spatially-resolved and time-gated X-ray imaging and spectroscopy, and laser probing. In particular, we compared planar wire arrays, to which little energy could be coupled via the conventional magnetic-to-kinetic conversion mechanism, to cylindrical wire arrays of comparable dimensions and mass. The planar wire arrays were shown to radiate much higher peak power and more energy in subkiloelectronvolt and kiloelectronvolt spectral ranges than cylindrical wire arrays. We tested the theoretical conjecture that enhanced resistivity due to the small-scale inhomogeneity of wire-array plasmas has a major effect on dynamics, energy coupling and radiation performance of wire-array Z-pinches. The study of Al, Alumel, and W cylindrical wire arrays shows a wide variety of characteristic behaviors in plasma implosions discussed hereinafter. Additional experimental results for symmetric and asymmetric, uniform stainless steel, Cu, Mo, combined Al/Mo, Mo/Al, Al/W, W/Al, and Mo/W X-pinches are also presented. New data for the total radiation yield are obtained. The planar structures of X-pinch plasma and the corresponding electron beam was observed for most of X-pinches. The generation of hot spots along original wires positions-cooler than those from the cross-wire region-and arc structures with hot spots between wires were found for X-pinches composed from Al, Cu, and W wires.

Direct observation of mass oscillations due to ablative Richtmyer–Meshkov instability and feedout in planar plastic targets

Perturbations that seed Rayleigh–Taylor (RT) instability in laser-driven targets form during the early-time period. This time includes a shock wave transit from the front to the rear surface of the target, and a rarefaction wave transit in the opposite direction. During this time interval, areal mass perturbations caused by all sources of nonuniformity (laser imprint, surface ripple) are expected to oscillate. The first direct experimental observations of the areal mass oscillations due to ablative Richtmyer–Meshkov (RM) instability and feedout followed by the RT growth of areal mass modulation are discussed. The experiments were made with 40–99 μm thick planar plastic targets rippled either on the front or on the rear with a sine wave ripple with either 30 or 45 μm wavelength and with 0.5, 1, or 1.5 μm amplitude. Targets were irradiated with 4 ns long Nike KrF laser pulses at ∼50 TW/cm2. The oscillations were observed with our novel diagnostic technique, a monochromatic x-ray imager coupled to a streak c...

Planar Wire Array as Powerful Radiation Source

The radiative performance of Al, Ni, and W planar wire arrays, to which little energy could be coupled via the conventional magnetic-to-kinetic conversion mechanism, is investigated. However, the planar wire arrays were shown to radiate much more energy in a short intense peak than possible due to dissipation of the kinetic energy. The planar array gives the unique possibility of seeing the evolution of the small-scale inhomogeneity of wire-array plasmas during wire ablation and implosion phases and highlights the importance of the Hall plasma phenomena and their impact on the dynamics, energy coupling, and radiation performance of wire-array Z-pinches

Dynamics of a Xe cluster plasma produced by an intense ultrashort pulse KrF laser

The dynamics of Xe clusters with initial radius between 10 and 100 A irradiated by an IR subpicosecond laser pulse is investigated. The evolution of the cluster is modeled with a relativistic time-dependent three-dimensional particle simulation model. The focus of this investigation is to understand the energy absorption of clusters and how the absorbed energy is distributed among the various degrees of freedom. The consequence of the initial cluster radius on the absorbed energy, average charge per atom, mean electron and ion energies, ionization, removal of electrons from the cluster, and cluster expansion was studied. The absorbed energy per cluster scales as N5∕3, and the mean electron and ion energies scale as N1∕3 and N2∕3, respectively (N is the number of atoms per cluster). A significant fraction of the absorbed energy (∼90%) is converted into kinetic energy with comparable contribution to electrons and ions. The energy balance suggests that smaller clusters are more efficient as radiators, while ...

Perfectly conducting incompressible fluid model of a wire array implosion

An incompressible perfectly conducting magnetohydrodynamic model is applied to describe a multiwire array implosion on the (r,θ) plane using the theory of analytic functions. The plasma columns emerging from the electrical explosion of individual wires move and change the shape of their cross section in the magnetic field produced by the currents flowing on the surfaces of the columns and closing through a cylindrical return current can. Geometry of both the “global” and “private” magnetic fields and self-consistent distributions of the electric currents on the conducting surfaces are determined for any wire array configuration including nested wire arrays, wires close to the return current can, etc. The coupled equations of motion and magnetostatics for an essentially two-dimensional problem are reduced to one-dimensional parametric governing equations, written for the boundary of the fluid contours. The implosion dynamics is shown to be driven by a competition between the implosion pressure, making the ...

Valve and nozzle design for injecting a shell-on-shell gas puff load into a z pinch

We have developed a dual-plenum gas valve coupled to a double shell nozzle for the generation of “shell-on-shell” gas loads in z-pinch plasma radiation source experiments. The gas density profiles of the nozzles have been characterized with laser interferometry. This valve/nozzle combination has been successfully fielded on the Double-EAGLE and Saturn pulsed-power generators. The design and characterization of the shell-on-shell valve/nozzle are presented in this article.