Kenneth A. Mikkelson

Use of a radial self-field diode geometry for intense pulsed ion beam generation at 6 MeV on Hermes III

We investigate the generation of intense pulsed focused ion beams at the 6 MeV level using an inductive voltage adder (IVA) pulsed-power generator, which employs a magnetically insulated transmission line (MITL). Such IVA machines typical run at an impedance of few tens of Ohms. Previous successful intense ion beam generation experiments have often featured an “axial” pinch-reflex ion diode (i.e., with an axial anode-cathode gap) and operated on a conventional Marx generator/water line driver with an impedance of a few Ohms and no need for an MITL. The goals of these experiments are to develop a pinch-reflex ion diode geometry that has an impedance to efficiently match to an IVA, produces a reasonably high ion current fraction, captures the vacuum electron current flowing forward in the MITL, and focuses the resulting ion beam to small spot size. A new “radial” pinch-reflex ion diode (i.e., with a radial anode-cathode gap) is found to best demonstrate these properties. Operation in both positive and negat...

Design and Testing of a Load Current Multiplier on Zebra Facility

The Load Current Multiplier concept (LCM) was validated for the first time on a high‐voltage nanosecond pulse‐power generator. The designed new device allowed to increase the load current from the nominal 0.8–0.9 MA up to 1.6 MA in static loads with constant inductance and up to 1.4 MA in a planar wire‐array plasma loads. These results were achieved without modifying the generator energetic or architecture. LCM allowed both the load magnetic energy increase and the increase of soft X‐ray radiation from z‐pinch plasmas.

Polarity inversion on Saturn

Saturn is a pulsed power accelerator consisting of 36 parallel modules producing 10 MA at 1.7 MV in a 40 ns power pulse[1]. Saturn is built to operate in negative polarity. Two methods of inverting the polarity in vacuum and driving up to 2.5 MA into a triplate MITL with a low impedance load have been built and demonstrated. Both rely on the use of a ballast inductance to invert the polarity[2]. The first method uses a dual post-hole convolute while the second method uses no convolutes.

A High-Speed, Eight-Frame Electro-Optic Camera With Multipulsed Ruby Laser Illuminator

We have developed a high-speed, electro-optic camera that features a large format; eight frames; and an accompanying eight-pulse, ruby laser illuminator. This system offers greater resolution and versatility than has been previously available. The eight 75-mm frames provide high dynamic spatial resolution (15 line pairs/mm) and time resolution (as short as 10-ns exposure time) to record fine physical details of rapid events. Four independent two-frame (tube) cameras complete with objective optics comprise the eight-frame array. Finally, pulsed ruby laser illumination with spectral filtering allows photography in intense ambient light.

Inverse diode for combination of multiple modules and fusion driver-target standoff

A newly invented, multi-megampere inverse diode converts the currents in many electron beams to current in a single Magnetically Insulated Transmission Line (MITL) for driving a common load. Electrons are injected through a transparent anode, cross a vacuum gap, and are absorbed in the cathode of the inverse diode. The cathode current returns to the anode through a load and generates electric and magnetic fields in the anode-cathode gap. Counter streaming electron flow is prevented by self-magnetic insulation in most of the inverse diode and by self-electrostatic insulation where the magnetic field is insufficient. Two-dimensional simulations with a 40 MA, 4 MeV, 40 ns electron beam at 3.5 kA/cm2 current density, 5 degree beam divergence, and up to 60 degree injection angle show 85% of the injected electron beam current is captured and fed into the MITL. Exploratory experiments with a 2.5 MA, 2.8 MeV, 40 ns electron beam at 2 kA/cm2at injection normal to the anode gave 70+/−10% collection efficiency in an unoptimized inverse diode. The inverse diode appears to have the potential of coupling multiple pulsed power modules into a common load at rates of change of current ∼1.6× 1015 A/s required for a fusion energy device called the Plasma Power Station with a Quasi Spherical Direct Drive fusion target.

Demonstration of current doubling and pulse sharpening on saturn

Electrically driven flux compression was demonstrated on the Saturn Accelerator pulsed power driver at Sandia National Laboratories. Proprietary 16-cm-long ‘skinny-armature’ re-entrant flux-compression cartridges were installed on Saturn to amplify and sharpen the current into a 3-nH static inductive load.

Power flow studies with HERMES-III, 300-ns pulse operation

Summary form only given. Recent experiments with HERMES III confirmed that the accelerator can operate very successfully in a 300-ns, 8-MV, 400-kA mode. This was accomplished by shorting the PFLs and pulse sharpening switches and transferring the output power from the gas switches directly to the voltage adder cavities. The observed voltage and current are consistent with the parapotential and pressure balance theories. Extensive numerical simulations with TWOQUICK were done to study the power flow in the voltage adder and the 4-m extension coaxial transmission line. Apparently even for these low operating voltages (8 MV), the adder becomes MITL in a relatively short time. The precursor pulse is small (/spl sim/35-kA peak) and /spl sim/40 ns duration. The MITL operation can be numerically simulated by assuming an electron emission threshold of /spl sim/100 kV/cm. Results of the experiments and numerical simulations as well as plans for future experimentation will be presented.

Enhanced magnetic energy released in solid-state and plasma loads on a nanosecond pulse power generator

The requirements on lossless power transport through vacuum interface and MITLs limit from above the physical volume and hence inductance of the vacuum part of pulse power generators. This in turn limits the generator-to-load energy coupling and hence the magnetic energy available in vacuum loads used in high energy density physics research. We obtained on Zebra generator (1.9 Ohm, 1 MA, 100 ns) an enhanced load magnetic energy corresponding to the load current increase from the nominal 0.95 MA to 1.65 (plusmn0.05) MA. This improvement was achieved without changing the generator architecture, but through better generator-to-load energy coupling using the new Load Current Multipliers (LCM) technique. The average experimental load-to-generator current amplitude ratio in LCM with both a 7 nH constant-inductance load and with z-pinch loads was 1.7plusmn0.2. We report on new generator electrotechnical parameters with LCM and on characterization of the plasma dynamics and radiative properties of planar wire-array z-pinches at the achieved enhanced load magnetic energy level.

Results from recent argon-gas-puff experiments at ∼ 6 MA

Experiments employing a 12-cm-diameter, triple-shell, argon gas puff were previously carried out in 2006 on the Saturn generator at Sandia National Laboratories. Employing the same gas puff design, a second experimental campaign was conducted in 2007. In this presentation, analyses of the data from the 2007 experiment and a comparison to the 2006 results are presented. The goal of this work is to gain understanding to both maximize the K-shell yield from medium- to high-atomic-number z pinches (photon energies of 1-8 keV) and to develop z-pinch-based concepts for sources of higher photon energy (8-40 keV). Large initial-diameter implosions are needed to achieve these goals. The expected deleterious effects on final pinch formation associated with the increase in the Rayleigh-Taylor instability growth at larger diameter can be mitigated by using an initial mass profile that is strongly peaked on axis. The properties of the K-shell emitting region such as its axial and radial extent, electron temperature, ion density, and average charge state are determined from time integrated spectra and time-resolved continuum emission. Power flow will also be discussed.

Improvement of the Energy Coupling to Low-Impedance Loads using Flux Extruders (Current Multiplier) on Existing Pulse-Power Generators

Summary form only given. The current multiplier (CM) concept was proposed to increase the driver-to-load energy transfer efficiency. The usual pulse-power load may have the inductance substantially lower than that of the generator, The suggested CM requires additional volumes with high self-inductance (magnetic flux extruders). Toroidal flux extruders can be incorporated into the vacuum part of existing pulse-power generator prior to the load and they extrude the magnetic flux toward the load magnifying the load current. Recent experiments with one extruder confirmed the concept at MA load currents with microsecond rise time both in static load inductance and in a dvnamic load (z-pinch). The accumulated experience in practical multiplier designs for high impedance generators suggests experiments to evaluate the performance of this new device on low-impedance nanosecond MA generators used for radiation production with z-pinches and for material studies in isentropic compression experiments (ICE). In this work, we present the optimization procedure for two existing generators, Saturn of Sandia and Zebra of Nevada TF operating with one or two extruders. The procedure allows comparison of the energy transfer efficiencies for different geometrical arrangements of extruders in a CM and design of the corresponding hardware in concrete experiments. Analytical and numerical study performed for these facilities suggest that better generator-to-load energy coupling is possible for both z-pinch and ICE loads. Available experimental results for some realistic CM hardware configurations will be also presented.