M. Krishnan

Large crystal grain niobium thin films deposited by energetic condensation in vacuum arc

This article presents evidence for unprecedented, large crystal grain size in ∼1μm thick Nb films that were grown on sapphire and copper substrates using a vacuum arc process called coaxial energetic deposition CED™. Most other deposition techniques with low adatom energy produce amorphous or small crystal-grain films. Typically, high substrate temperatures and annealing steps are required to form the large, highly connected grains. The CED™ technique deposits from plasma consisting of a nonequilibrium, high energy (50–150eV) ion population produced from the ionized source material. At the substrate these fast ions break up columnar structures, intermix with the first few atomic layers of the substrate to improve adhesion, and form dense films at lower substrate temperatures than are typical for low adatom energy techniques, such as physical vapor deposition (PVD). Nanoscale features of the thin films were examined using electron backscatter diffraction (EBSD). The films’ cryogenic state electrical proper...

A Fast Pulse, High Intensity Neutron Source Based Upon The Dense Plasma Focus

Alameda Applied Sciences Corporation (AASC) has built a bench‐top source of fast neutrons (∼10–30 ns, 2.45 MeV), that is portable and can be scaled to operate at ∼100 Hz. The source is a Dense Plasma Focus driven by three different capacitor banks: a 40 J/30 kA/100 Hz driver; a 500 J/130 kA/2 Hz driver and a 3 kJ/350 kA/0.5 Hz driver. At currents of ∼130 kA, this source produces ∼1×107 (DD) n/pulse. The neutron pulse widths are ∼10–30 ns and may be controlled by adjusting the DPF electrode geometry and operating parameters. This paper describes the scaling of the fast neutron output with current from such a Dense Plasma Focus source. For each current and driver, different DPF head designs are required to match to the current rise‐time, as the operating pressure and anode radius/shape are varied. Doping of the pure D2 gas fill with Ar or Kr was shown earlier to increase the neutron output. Results are discussed in the light of scaling laws suggested by prior literature.

A renewed argon gas puff capability on Sandia's Z machine

Summary form only given. We have reestablished gas puff z-pinch capability on Sandias 20 MA Z machine, including a Sandia-operated driver system and an imaging interferometer to characterize nozzle mass flow [1]. Initial experiments have focused on developing a 3 keV Ar K-shell x-ray source. We have pursued a design-driven approach to planning these experiments, utilizing numerical simulation to predict Ar K-shell yield for various nozzle mass profile configurations. In particular, we study coupling to the generator and how the distribution of mass between the two shells impacts magnetic Rayleigh-Taylor instability evolution. Two-dimensional radiation-magneto-hydrodynamic (MHD) simulations at NRL for a number of density profiles produced by the nozzle have predicted yields in excess of 300 kJ, and indicated that a 1:1.6 outer-to innershell mass ratio would produce the most stable implosion with high enough temperature to optimize Ar K-shell output [2]. This result was also consistent with 3D MHD modeling using the Gorgon code [3] at Sandia. Both models used tabulated non-LTE atomic models for Ar K-shell photon emission. We will present Z experimental data from the first gas puff shots on the accelerator since 2006, and compare these to the numerical models. Spectral output is measured from 1-20 keV. Electrical current measurements at different positions along the power flow section provide information on current coupling to the load. Time-gated pinhole imaging and radially-resolved spectroscopy indicate ~60 cm/μs implosion velocities and >1 keV electron temperatures.