Erik McKee

Note: Infrared laser diagnostics for deuterium gas puff Z pinches

Deuterium gas puff Z pinches have been used for generation of strong neutron fluxes on the MA class pulse power machines. Due to the low electron density of deuterium Z-pinch plasma, regular laser diagnostics in the visible range cannot be used for observation and study of the pinch. Laser probing at the wavelength of 1064 nm was used for visualization of deuterium plasma. Infrared schlieren and interferometry diagnostics showed the deuterium gas puff plasma dynamics, instabilities, and allowed for the reconstruction of the profile of the plasma density.

Characterization of a compact gas-puff nozzle and plasma gun assembly for staged Z-pinch experiments

Summary form only given. We discuss the design and characterization of a compact gas-puff nozzle and plasma gun assembly that will be used in Staged Z-pinch experiments1, where gas liners are imploded onto an on-axis target-plasma column. Previous work on the Staged Z-pinch demonstrated that gas liners can efficiently couple the energy and produce a uniform implosion on a target-plasma.The valve assembly produces an annular, high-atomic number, neutral gas-puff, co-axially with a target-plasma. The neutral gas-puff outlet consists of an annular solenoid valve and a converging-diverging nozzle, designed to achieve a Mach number around M=5 in the annular gas jet. The on-axis, cylindrical plasma gun is powered by a high-voltage capacitor. Breakdown occurs spontaneously when high-pressure gas is puffed between the electrodes, i.e., the deflagration mode3. The discharge current generates an annular magnetic field, producing the axial J×B force, which accelerates the plasma out of the gun. Measurements of the gas-puff and plasma-jet density distributions and ejection velocities by open-shutter camera, Mach-Zehnder interferometry, and Faraday cup diagnostics are made and compared with CFD simulations.

Design and optimization of a liner-on-target injector for staged Z-pinch experiments using computational fluid dynamics and MHD simulations

Summary form only given. Previous Staged Z-pinch experiments have demonstrated that gas liners (or puffs) can efficiently couple energy to a target plasma and implode uniformly, producing plasmas in High Energy Density (HED) regimes. In these experiments, a 50 kJ, 1.5 MA, 1 μs current driver was used to implode a magnetized, Kr liner onto a D+ target, producing 1010 neutrons per shot. Time-of-flight data suggested that primary and secondary neutrons were produced. MHD simulations show that, using optimized liner and plasma target conditions, neutron yield could be further increased in Staged Z-pinch implosions using the Zebra machine, a 1.5 MA and 100 ns rise time current driver. In this work we present the design and optimization of an injector for these experiments. The injector is composed of an annular high atomic number (e.g. Ar, Kr) gas-puff and an on-axis plasma gun that delivers the ionized deuterium target. The gas-puff nozzle optimization was performed using the computational fluid dynamics (CFD) code Fluent and the MHD code MACH2. The CFD simulations produce density profiles as a function of the nozzle shape and gas. These profiles are initialized in MACH2 to find the optimal liner density profile for a stable, uniform implosion that produces high neutron yield.

Neutron yield detectors characterized with MCNP

In preparation for measuring neutrons generated in experiments on the Zebra 1-MA Z-pinch generator at the Nevada Terawatt Facility, we use the Monte Carlo N-Particles (MCNP) radiation transport code to characterize neutron activation detectors. For reasonable assumptions regarding the emitted neutron pulse characteristics, the MCNP code is used to predict the expected neutron signal taking into account scattering events and thermalization effects from the neutron scatter. These effects can bolster neutron yield derivations via unique room geometries and, in general, higher absorption cross-section values for lower neutron energies. We report on neutron yield detector characterization, calibrated against NSTecs dense plasma focus for a known neutron yield, and the build-up factors calculated with MCNP necessary for a measurement of a correct neutron yield at NTF.