D. G. Phipps

Initialization and Operation of Mercury, A 6-MV MIVA

Mercury became operational in a stepwise manner to test the machine components after modifications and reassembly at NRL. To avoid damaging the MIVA, extensive testing of the laser and PFL output switches was performed using dummy loads. Finally, the PFLs were connected to the MIVA and Mercury was fired into a simple cylindrical diode load with a Marx charge voltage up to 75 kV. Measured MIVA currents and voltages compare well with a circuit model of the MIVA fed by the measured PFL outputs and with PIC simulations of the MIVA and the diode load.

Time-resolved voltage measurements of Z-pinch radiation sources with a vacuum voltmeter

A vacuum-voltmeter (VVM) was fielded on the Saturn pulsed power generator during a series of argon gas-puff Z-pinch shots. Time-resolved voltage and separately measured load current are used to determine several dynamic properties as the load implodes, namely, the inductance, L(t), net energy coupled to the load, E(coupled)(t), and the load radius, r(t). The VVM is a two-stage voltage divider, designed to operate at voltages up to 2 MV. The VVM is presently being modified to operate at voltages up to 6 MV for eventual use on the Z generator.

Status of the Mercury pulsed-power generator, a 6-MV 360-kA, magnetically-insulated inductive voltage adder

Mercury is a nominal 6-MV, 360-kA, 2.2-TW magnetically-insulated inductive voltage adder that is being assembled at the Naval Research Laboratory. Mercury, originally known as KALIF-HELA, was located at the Forschungszentrum in Karlsruhe, Germany. Once assembled, Mercury will be used as a testbed for development of high-power electron- and ion-beam diodes. Applications include source development for high-resolution flash radiography, nuclear weapons effects simulation, and particle-beam transport research. This paper highlights the progress of the Mercury assembly and supporting activities, including modifications from the original design, circuit modeling to optimize the Mercury circuit, power-flow simulations to understand and optimize Mercury power flow and load coupling, and MITL theory and modeling to develop a transmission-line code capability for modeling transient effects in MITLs.

Ion diode performance on a positive polarity inductive voltage adder with layered magnetically insulated transmission line flow

A pinch-reflex ion diode is fielded on the pulsed-power machine Mercury (R. J. Allen, et al., 15th IEEE Intl. Pulsed Power Conf., Monterey, CA, 2005, p. 339), which has an inductive voltage adder (IVA) architecture and a magnetically insulated transmission line (MITL). Mercury is operated in positive polarity resulting in layered MITL flow as emitted electrons are born at a different potential in each of the adder cavities. The usual method for estimating the voltage by measuring the bound current in the cathode and anode of the MITL is not accurate with layered flow, and the interaction of the MITL flow with a pinched-beam ion diode load has not been studied previously. Other methods for determining the diode voltage are applied, ion diode performance is experimentally characterized and evaluated, and circuit and particle-in-cell (PIC) simulations are performed. Results indicate that the ion diode couples efficiently to the machine operating at a diode voltage of about 3.5 MV and a total current of about...

Conversion of Mercury (a 2-TW inductive voltage adder) to positive polarity

After 616 shots in a negative polarity configuration, Mercury, a 6-MV and 300-kA inductive voltage adder (IVA), has been converted to positive polarity in order to extract ion beams. Conversion to positive polarity was achieved by rotating all six of the adder cells by 180°. In principle, we could have chosen to instead insert the center conductor from the other end of the adder to change polarity, but rotating the cells minimized the time required to make the transition. Although most of the same pieces were used, the center conductor had to be reconfigured in order to align the transition pieces with the cell feed gaps. Because the electron flow was anticipated to be very different in positive polarity, a result of emission from surfaces of different potential, a simple blade diode was fielded for the initial shots to gain a better understanding of operation in positive polarity. The blade diode consisted of the same cathode used as a dummy load in the first negative polarity shots on Mercury, but with a different carbon anode that just covered the end of the center conductor. After a few short circuit and initializing shots, a series of shots were taken where only the blade diode AK gap was varied in order to characterize self-limited and load-limited operation and to compare measurements with theory and simulation.

A comparison of planar, laser-induced fluorescence, and high-sensitivity interferometry techniques for gas-puff nozzle density measurements

The distribution of argon gas injected by a 12-cm-diameter triple-shell nozzle was characterized using both planar, laser-induced fluorescence (PLIF) and high-sensitivity interferometry. PLIF is used to measure the density distribution at a given time by detecting fluorescence from an acetone tracer added to the gas. Interferometry involves making time-dependent, line-integrated gas density measurements at a series of chordal locations that are then Abel inverted to obtain the gas density distribution. Measurements were made on nominally identical nozzles later used for gas-puff Z-pinch experiments on the Saturn pulsed-power generator. Significant differences in the mass distributions obtained by the two techniques are presented and discussed, along with the strengths and weaknesses of each method.

Peak Implosion Power as a Predictor of Plasma Radiation Source K-Shell Performance

Hawk is a current-stiff Z-pinch driver that uses a plasma opening switch to transfer inductively-stored current into the load, and employs a vacuum voltmeter to directly measure the voltage drop across the neon gas-puff Z-pinch. The load-voltage measurements permit calculation of the time-dependent load inductance, as well as the associated power Pimp and energy Eimp in the pinch during the implosion and stagnation phases. For experiments that determine the dependence of neon K-shell yield on the gas-puff density profile at constant load mass, Pimp is found to be a better measure of pinch quality and yield than peak current or implosion energy. For these data, and other data for which the load mass was varied with constant profile shape, Pimp and Eimp are combined with K-shell radiation measurements to provide insights into the physics of the pinch stagnation phase.

6 MV vacuum voltmeter development

A standard voltmeter designed for measuring up to ± 2 MV in vacuum was modified to operate at −4 to −6 MV, appropriate levels for measuring the convolute voltage during z-pinch experiments on Z. Field shaping structures are used to eliminate electron emission from the voltmeter grading rings. The voltmeter operated as expected during tests at −4 MV on the Mercury generator, indicating this voltmeter could work for the lower voltage range expected on Z. At −6 MV on Mercury, the voltmeter did not operate correctly, probably because of electron emission. Particle-in-cell modeling is consistent with the observed voltmeter response. An improved field shaper could extend the voltmeter operation to −6 MV.

Development of an intense, pulsed source of combined characteristic-gamma-rays and neutrons to induce fission

The detection of fissile material by passive and active techniques is an area of intense interest. Many active approaches detect the products of fission induced by photon and/or neutron irradiation.¹ One such approach has focused on using intense, pulsed, beam-target interactions to produce an irradiation source of 6.13-, 6.92-, and 7.13-MeV characteristic gamma rays from the ¹⁹F(p, αγ)¹⁶O reaction.^2,3 Initially, work at NRL utilized the Gamble-II, 3-Ω, water-line generator with a pinch-reflex ion diode that accelerates ions to 2 MeV at ~ 0.5-TW peak ion power.³ This pulsed source also simultaneously produces neutrons from: (1) a collective ion-acceleration mechanism associated with the diode and (2) the reactions resulting from the isotopic abundance of deuterons in the ion beam, e.g., ¹²C(d, n)¹³N and ¹⁹F(d, n)²⁰Ne. In this presentation, we review progress toward characterizing a similar mixed source using the Mercury inductive voltage adder operated in positive polarity producing ~ 4-MeV⁴ ion energy and ~ 0.3-TW peak ion power. An array of diagnostics is used to determine the number of gamma rays and neutrons produced by the source, including a shielded scintillator-photomultiplier for the gamma-ray time history and a rhodium-activation counter to measure the total number of neutrons produced. In addition, the gamma and neutron source strengths are inferred from delayed neutrons that result from fissions in depleted uranium and that are measured using ³He and ⁶Li neutron detectors.

Overview of Recent Large-Diameter, Gas-Puff Z-Pinch Research

Summary form only given. Large radius implosions of gas-puff Z-pinches are the subject of intense investigation. Such implosions are needed to achieve the high specific energy required to excite K-line radiation from high-atomic-number (e.g. Z>26) radiators, for proper matching with high-current (~10 MA and higher) generators, for continuum radiator concepts that require heating beyond the optimum He/H-like state for K-shell emission, and to utilize more efficiently long-current-rise-time (>100 ns) generators. The key physics issues involve R-T instability mitigation and energy coupling to the radiating plasma by the choice of the initial radial and axial mass profiles. In this talk, recent experimental and theoretical progress in this work are reviewed, including achieving 20-kJ of argon K-shell radiation from a 12-cm-diam. nozzle at 3.5 MA and 200-ns implosion time. The primary experimental testbeds are double-EAGLE (long-pulse mode, 3.5 MA, 210 ns) at TPSD and the Decade Quad (6 MA, 300 ns) at Arnold Engineering Development Center. Supporting experiments on Hawk (0.6 MA, 300 ns) at NRL are also reviewed. Besides the usual soft X-ray and electrical diagnostics, we have developed methods for measuring: the initial gas profile, the L-shell emitting region, the UV and continuum, the initial current flow paths, and the 2-D evolution of the pinch. Recent developments in 2-D radiation-hydrodynamic simulations are also reviewed