Stephan Neff

Analysis of Conical Wire Array Z‐Pinch Stability with a Center Wire

Adding a center wire on the axis of a conical wire array produces conditions suitable for studying shear flow stabilization of the Z‐pinch. The conical wire array produces and axial plasma flow while the center wire introduces a radial variation of the axial velocity. Experiments of this array configuration were preformed on the 1 MA Zebra Z‐pinch generator and showed stabilization of the kink instability when a center wire was present. Comparison with equivalent cylindrical wire arrays indicates that the shear flow stabilization plays a role in the stabilization of the kink instability.

Effect of an Axial Wire on Conical Wire Array Z‐Pinch Radiation

Adding a wire on the axis of wire arrays significantly affects the x‐ray emission of the conical arrays, and much less that of the cylindrical ones. The radiation of the conical wire arrays increases with the thickness of the central wire, surpassing that of the equivalent cylindrical arrays. Significant energy is emitted early on, around the time of the conical shock formation, before the pinch stagnation.

Faraday Cup Measurements of the Energy Spectrum of Laser-Accelerated Protons

Several experiments at the Nevada Terawatt Facility (NTF) study the interaction of laser-created plasmas with large external magnetic fields. Examples include a “solar wind” experiment that studies the development of a shock when an ablation plasma flow interacts with a strong magnetic field, and an isochoric heating experiment, in which the effect of a confining external magnetic field on target heating will be investigated. The plasmas can be created with one of our two multi-terawatt laser systems, Tomcat (up to 10TW) and Leopard (up to 100TW). Analyzing the experiments requires a thorough understanding of the initial conditions of the laser plasma. The laser absorption on target can be characterized by analyzing the energy spectrum of proton emitted from the target surface. The measurements, performed with Faraday cups, yield both the hot electron temperature (characteristic for electrons which directly absorb the laser energy) and the cold electron temperature (characteristic for the bulk of the target electrons, which are indirectly heated by the hot electrons). Energy spectrum measurements have been carried out with Tomcat pulses with up to 4J pulse energy and 4ps/1ps duration. For an optimized focal spot size, a hot electron temperature of more than 2.6keV was measured, corresponding to an absorbed laser intensity of 1016 W/cm2.

X-ray yield from pinch target implosions

The wire array z-pinch is an efficient x-ray source which has been proposed for use in indirect drive inertial confinement fusion schemes. Extensive research has focused on methods to enhance and manipulate the x-ray yield in a z-pinch. In recent experiments performed at the Nevada Terawatt Facility, it was observed that a center wire added as a target for conical array implosions resulted in an increase in x-ray yield when the diameter of the center wire was smaller than a threshold value which depended on the wire material. Investigation of this behavior was performed on Zebra, a 2 TW z-pinch generator which delivers a 1 MA current pulse to a load, with a 90 ns rise time. Aluminum cylindrical and conical wire arrays with similar implosion times were used to investigate the role of the center wire in the implosion. For each configuration the array wires diameter remained unchanged for all experiments, while Al, Ti, Cu, SS and W targets were used with diameters ranging from 10 µm – 1 mm. Comparing the soft x-ray yield (20 eV – 5 keV) without a center wire, the cylindrical arrays produced more x-rays than the conical wire arrays, which was expected since a portion of the kinetic energy of the conical implosion goes to producing a plasma jet. With the addition of a center wire, the conical wire array showed a positive correlation between the soft x-ray yield and the diameter of the target. This increase was significant enough to surpass the cylindrical wire array in soft x-ray yield. In conical wire array implosion the narrow end of the cone has an increased J×B force causing the narrow region to implode faster than the rest of the array, similar to an x-pinch. During the initial implosion, time-gated pinhole images recorded a bright x-ray burst at the narrow region of the cone. In addition there was also observed a bright, narrow, hard x-ray source along the length of the pinch. This talk will present the differences in soft (20 eV – 5 keV) and hard (1 keV – 5 keV) x-ray yield form different arrays and targets and will discuss the possible sources for the x-ray yield increase.

Effects of an axial target on the radiation of imploding wire arrays

Thin wires have been added on the axis of cylindrical and conical wire arrays to investigate the effect of sheared plasma flows on the z-pinch stability1. This addition significantly affected the x-ray emission of the conical wire arrays, while having little effect on that of the cylindrical ones. In addition, the material of the axial target had a strong influence on the radiation output. The experiments were performed on the 1 MA Zebra Z-pinch generator at the Nevada Terawatt Facility using conical and cylindrical wire arrays with and without a center wire. The arrays consisted of 8 aluminum wires 15 µm in diameter. Aluminum or copper wires of various thicknesses were used on axis.

Kelvin-Helmholtz instability in a sheared flow actuated by a magnetic field

The Kelvin-Helmholtz instability can lead to plasma transport across a magnetic field; one example is the solar wind transport across the earths magnetopause in the magnetotail. In an experiment done at the Nevada Terawatt Facility, we observed the Kelvin-Helmholtz instability in a laser produced plasma that interacted with an external magnetic field. This instability is evidenced by the presence of evenly spaced vortices on the plasma-field boundary. Due to the interaction with the external magnetic field, a velocity gradient perpendicular to the plasma velocity forms at this boundary. The presence of vortices in a region of sheared flow is characteristic of the development of the Kelvin-Helmholtz instability. The observed structure and its growth rate indicate that large ion Larmor radius effects contribute to its formation. Discussions of the mechanism producing the sheared flow and the resulting instability will be presented.

Developing flyer plate impact experiments for shockwave interaction studies

Summary form only given. The interaction of shock waves with inhomogeneous media is important in many astrophysical phenomena. Modeling these phenomena in the laboratory yields additional information to improve simulations as well as the interpretation of astrophysical observations. Scaled experiments using magnetically accelerated flyer plates impacting on low density foam targets have been proposed for the Z machine at the Sandia National Laboratories. Carrying out such experiments on smaller machines like the Zebra accelerator at the Nevada Terawatt Facility (NTF) would reduce costs significantly and thus enable a broader scan of experimental parameters. At the NTF, we have demonstrated flyer acceleration to velocities of up to 8 km/s; we have also carried out first impact tests with transparent targets and imaged the resulting shock waves with shadowgraphy. Simulations with a ID Lagrangian hydrodynamical simulation code show that we are able to drive strong shocks over several millimeters. We are currently developing additional diagnostics (VISAR and x-ray backlighting) for our experiments. Once these diagnostics are implemented, we plan to carry out shock interaction experiments with inhomogeneous low-density foam targets.

Suppressed instability growth in seeded kink Z-pinches

Among the limiting factors preventing the achievement of maximum energy density in Z-pinches is the turbulence generated from instabilities, such as the sausage and kink. The kink instability can be stimulated by introducing a thin helical obstacle on the axis of the pinch. Imploding a conical wire arrays with this obstacle creates a sheared axial flow that interacts with the developing kink instability. Experiments involving conical and cylindrical wire arrays were conducted at the Nevada Terawatt Facility in order to investigate the growth of the seeded instability. Observations show that the kink instability was indeed suppressed for conical wire arrays.

Creating solid density warm matter by laser heating in external magnetic field

Summary form only given. Solid state density matter can be heated to high temperatures by ultrafast energy deposition. Using 1018 W/cm2 laser pulses, volumes of the order of 105 I??m3 can in principle be heated to several hundred electronvolts for several picoseconds. This is achievable if the hot electrons generated by the intense laser can be confined laterally in the region of the laser focal spot. Collisional two-dimensional particle-in-cell simulations suggest novel ways of achieving this goal. The simulations have shown that high intensity laser-generated hot electrons are confined laterally by self-generated resistive magnetic fields [1]. While these resistive fields decay on a time scale comparable with the duration of the laser pulse, according to other simulations the confinement may be possible to be maintained for a longer time by applying external megagauss magnetic fields [2]. In addition, shock waves generated in layered solids by ultrafast laser deposition are predicted by simulations to enhance the local heating [1]. By ultrafast laser heating of solid targets, conditions can be achieved similar to those found in the interiors of stars and in the atmospheres of neutron stars. Based on simulation results, an experiment has been developed to study the isochoric heating based on the magnetic control of heat transport in laser irradiated targets. The experiments involve target irradiation with the 1018W/cm2, 0.35 ps laser Leopard and megagauss external magnetic fields created by the pulsed power generator Zebra (0.6 MA, 200 ns) [3]. To investigate the confinement efficiency and the heating of Si targets tamped with polyethylene, x-ray spectroscopy and diagnostics of proton beams were developed.

Magnetized Laser-Plasma Interactions to Create Solid Density Warm Matter

Summary form only given. Collisional particle-in-cell simulations predict that solid density matter irradiated with a short pulse high intensity laser can be heated to keV temperatures by applying an external magnetic field. The role of the magnetic field is to restrict the radial diffusion of the hot electrons accelerated by the laser field. The confinement can be effective if the gvro-period is less than the collision time. This reduces the radial diffusion of the hot electrons long enough so that they can couple to the cold electrons which in turn couple to the ions. To test these predictions, an experiment is being developed that takes advantage of the coupled Tomcat/Leopard -Zebra facility. According to simulations performed for achievable values of the parameters, with a laser intensity higher than 1017 W/cm2 and a magnetic field of the order of 1 MG material volumes of 105 mum can be heated for several ps to temperatures of several hundred eV. These parameters make this technique extremely appealing for fusion and opacity studies with numerous applications that include modeling the radiation transport in the interiors of stars. In preparation for the integrated experiment, magnetic fields higher than 1 MG were produced in vacuum with the pulsed power generator Zebra (0.6 MA, 200 ns) using horseshoe shaped coils. In the configuration used, no plasma was created on the surface of a CM laser target placed inside the coil. To date, the best parameters measured for the Tomcat compressed laser pulse are: energy 4 J, duration 0.8 ps, and focal spot FWHM 30 mum (measured with the unamplified beam), resulting in an irradiance on target around 1018 W/cm2. Higher irradiance will be soon available using the 100 TW laser Leopard, the pulse compression of which is currently under way. The jitter of Zebra was reduced to less than 15 ns rms assuring successful synchronization with the lasers. The goal of the experiment is to demonstrate enhanced heating of a solid target irradiated by an intense, short pulse laser in the presence of an external magnetic field. Several types of targets including homogeneous Si and CD targets, as well as layered targets CD-Si-CD will be used, and their heating compared. The electron temperature and ionization balance will be inferred from X-ray spectra. A von Hamos KAP crystal spectrograph was built and used to record single shot Al and Si spectra from laser irradiated targets. Neutron yield measurements with scintillator-photomultiplier detectors will be used to determine the deuteron temperature.