M. D. Johnston

Sandia National Laboratories, Albuquerque, New Mexico, United States

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Publications (51)2.25 Total impact

  • T. J. Renk · M. D. Johnston · B. V. Oliver · N. Bruner · D. Welch
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    ABSTRACT: form only given. The self-magnetic-pinch (SMP) diode is being developed on the Radiographic Integrated Test Stand (RITS-6) at Sandia National Laboratories. The time history of SMP load impedance is affected by the evolution of populations of high-energy electrons and ions from the cathode and anode, respectively, as well as by electrode plasma evolution. Framing camera images of optical emission spectra from electrode plasmas has been measured, and anode plasma dynamics has been modeled. Experimental data suggest that the interaction of high-energy and anode plasma ions can result in premature impedance collapse. These effects vary with the cathode radius and anode-cathode (A-K) gap. There is experimental evidence that changes to the anode material composition can affect the load impedance evolution. The standard anode structure consists of thin Al foil placed 0.8 mm in front of a Ta plate. We are experimenting with Al coatings placed directly upon the Ta surface, as well as with 'limited anodes', where the Al foil/Ta plate is replaced with a graphite plate of similar electron range, and with either tungsten or tantalum insert at small radius. Preliminary results with the Al-coated Ta anode indicate equivalent or better impedance history compared to the standard anode. Experiments with anode materials modifications are ongoing, and latest results will be presented.
    IEEE International Conference on Plasma Science 01/2011; DOI:10.1109/PLASMA.2011.5993078
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    ABSTRACT: In this paper we review several high power laser ablation techniques that have been utilized to fabricate high current (1-80 kA) electron beam cathodes for accelerators and microwave sources: 1) Projection Ablation Lithography (PAL) cathodes, 2) Ablation Line Focus (ALF) cathodes, and 3) Metal-Oxide-Junction (MOJ) cathodes. Laser-ablative micromachining techniques (PAL and ALF) have been utilized to generate micron-scale features on metal substrates that provide electric field (beta) enhancement for Fowler-Nordheim emission and plasma cathodes. Since these laser-ablated patterns are directly, laser-written on the substrate metal they exhibit much higher thermal conductivity for higher current capability and increased damage thresholds. Metal-Oxide-Junction (MOJ) cathodes exploit the triple-point electron emission that occurs at the interface between metal, insulator and vacuum.The ablation laser is a KrF excimer laser with a pulse energy of 600 mJ and pulselength of 20 ns. Cathode experiments were performed on the MELBA-C accelerator: V = -300 kV, pulselength = 0.5 microsecond. Data will be presented for PAL, ALF and MOJ cathodes.
    10/2010; 1278(1). DOI:10.1063/1.3507098
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    B V Oliver · K Hahn · M D Johnston · S Portillo
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    ABSTRACT: Recent experiments at Sandia National Laboratories have demonstrated an electron beam diode X-ray source capable of producing > 350 rad at one meter with 1.7 mm FWHM X-ray source distribution, with a 50 ns pulse-width and X-ray photon endpoint energy spectrum in the 6–7 MeV range. The diode operates at current densities of ≈ 1 MA/cm 2 . The intense electron beam rapidly (≈ 5 ns) heats the X-ray conversion anode/target, liberating material in the form of low density ion emission early in the pulse and high density plasma later. This environment gives rise to beam/plasma collective effects which dominate the diode and beam characteristics, affecting the radiation properties (dose and spot-size). A review of the diode operation, the measured source characteristics and the simulation methods and diagnostics used to guide its optimization is given.
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    ABSTRACT: Researchers at The University of Michigan have constructed and tested a 1‐MA Linear Transformer Driver (LTD), the first of its type to reach the USA. The Michigan Accelerator for Inductive Z‐pinch Experiments, (MAIZE), is based on the LTD developed at the Institute of High Current Electronics in collaboration with Sandia National Labs and UM. This LTD utilizes 80 capacitors and 40 spark gap switches, arranged in 40 “bricks,” to deliver a 1 MA, 100 kV pulse with 100 ns risetime into a matched resistive load. Preliminary resistive‐load test results are presented for the LTD facility.
    01/2009; 1088(1):259-262. DOI:10.1063/1.3079742
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    ABSTRACT: Experiments have been conducted at Sandia National Laboratories' RITS-6 accelerator facility (operating at 7.5 MV and 180 kA) investigating plasma formation and propagation in relativistic electron beam diodes used for flash X-ray radiography. High resolution, visible and ultraviolet spectra were collected in the A-K gap of the self-magnetic pinch (SMP) diode. Time and space resolved spectra are compared with time-dependent, collisional-radiative (CR) calculations and Lsp, hybrid particle-in-cell code simulations. Results indicate the presence of a dense (1times1017cm-3), low temperature (few eV), on-axis plasma, composed primarily of protons from electrode surface contaminants, which rapidly expands (10- 30cm/mus) from the anode to the cathode. In addition, a cathode plasma sheath is observed which extends several millimeters into the A-K gap. It is believed that the interaction of these electrode plasmas causes a premature impedance collapse of the diode and subsequent reduction in radiation output. Diagnostics include high speed imaging and spectroscopy using nanosecond gated ICCD cameras, streak cameras, and photodiode arrays.
    IEEE International Conference on Plasma Science 01/2009; DOI:10.1109/PLASMA.2009.5227636
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    ABSTRACT: The RITS-6 induction voltage adder (IVA) electron accelerator has an output voltage of 7-12 MV and beam currents from about 10 kA to 170 kA depending on the diode employed and the two options impedance of the magnetically insulated transmission line (MITL). The determination of the diode voltage has traditionally be done by a combination of parapotential flow theory in the MITL and translating the voltage pulse to the diode or solving radiographers equations for the x-ray dose rate. However the time-integrated voltage can also be inferred from the electron beam energy spectrum unfolded from measurements of the absorbed dose as a function of depth in the anode material. Depth dose measurements using radiochromic film sandwiched in an aluminum anode were performed on RITS in the high voltage mode. Results are presented for the unfolded electron spectrum using a modified least-squares optimization method with Monte Carlo radiation transport code-generated mono-energetic depth-dose profiles. Variations in the beam spatial profile are observed. Time-resolved measurements of the beam current density profile were performed using gated CCD cameras which observed the beam-generated Cherenkov light pattern inside electron range-thin fused silica. These measurements were made at similar beam current but lower voltage than the depth-dose shots. Various cathode surface treatments were used to see if beam profile depended strongly on the electron source.
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    ABSTRACT: Experiments are being conducted to investigate fine wires as potential plasma sources for applications such as intense electron beam transport and propagation. For these studies, a microsecond long, low inductance, capacitive discharge (40kA, 50kV) is driven through a wire(s) to generate a plasma. The plasma expansion is determined by JxB forces which are controlled via changes in geometry and current. High resolution visible spectroscopy is used to spatially and temporally measure plasma parameters throughout the pulse. Lineshapes, ratios, and intensities are compared with time-dependent CR calculations to obtain plasma densities and temperatures. Results are compared with MHD calculations and scaling laws for mass ablation rates from wires.
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    ABSTRACT: The immersed-Bz diode is being developed as a high-brightness, flash x-ray radiography source at Sandia National Laboratories. This diode is a foil-less electron-beam diode with a long, thin, needlelike cathode which is inserted into the bore of a solenoid. The solenoidal magnetic field guides the electron beam emitted from the cathode to the anode while maintaining a small beam radius. The electron beam strikes a thin, high-atomic-number anode and produces forward-directed bremsstrahlung. In addition, electron beam heating of the anode produces surface plasmas allowing ion emission. Two different operating regimes for this diode have been identified: A nominal operating regime where the total diode current is characterized as classically bipolar with stable impedance [see D. C. Rovang et al., Phys. Plasmas 14, 113107 (2007) ] and an anomalous operating regime characterized by a rapid impedance collapse where the total diode current greatly exceeds the bipolar limit. The operating regimes are approximately separated by cathode diameters greater than 3 mm for the nominal regime and less than 3 mm for the anomalous impedance collapse regime. Results from a comprehensive series of experiments conducted at 4–5 MV characterizing the transition from this nominal operating regime to the anomalous operating regime as the cathode diameter is reduced are presented. Results from experiments investigating the effects of anode-cathode gap, anode material, and cryogenic modification of the anode surface are also presented. Although these investigations were unsuccessful in completely mitigating the anomalous behavior, insight gained from these experiments has elucidated several key physics issues that are discussed.
    Physics of Plasmas 09/2008; 15(9):093105-093105-11. DOI:10.1063/1.2980418 · 2.25 Impact Factor
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    ABSTRACT: We present designs, resistive-load test results and experimental plans of the first 1 MA z-pinch in the USA to be driven by a Linear Transformer Driver (LTD). The Michigan Accelerator for Inductive Z-pinch Experiments, (MAIZE), is based on the LTD developed at the Institute for High Current Electronics in collaboration with Sandia National Labs. This LTD utilizes 80 capacitors and 40 spark gap switches to deliver a 1 MA, 100 kV pulse with
    IEEE 35th International Conference on Plasma Science, 2008, Karlsruhe, Germany; 06/2008
  • B. V. Oliver · K. Hahn · M. D. Johnston · J. Leckbee · S. Portillo · D. R. Welch
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    ABSTRACT: Recent experiments at Sandia National Laboratories have demonstrated electron beam diode X-ray sources capable of producing > 350 rad@m with 1.7mm FWHM x-ray source distributions, and endpoint energy spectrum in the 6–7 MeV range. A review of our present theoretical understanding of the diode(s) operation and our experimental and simulation methods to investigate them will be presented. Emphasis is given to e-beam sources used on state-of-the-art Induction Voltage Adder (IVA) pulsed-power accelerators. In addition, a brief discussion of a proposed radiographic system based on Linear Transformer Driver (LTD) technology is discussed.
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    ABSTRACT: We report construction and initial testing of a 1-MA Linear Transformer Driver (LTD), The Michigan Accelerator for Inductive Z-pinch Experiments, (MAIZE). This machine, the first of its type to reach the USA, is based on the joint HCEI, Sandia Laboratories, and UM development effort. The compact LTD uses 80 capacitors and 40 spark gap switches, in 40 ``bricks'', to deliver 1 MA, 100 kV pulses with 70 ns risetime into a matched resistive load. Test results will be presented for a single brick and the full LTD. Design and construction will be presented of a low-inductance MITL. Experimental research programs under design and construction at UM include: a) Studies of Magneto-Raleigh-Taylor Instability of planar foils, and b) Vacuum convolute studies including cathode and anode plasma. Theory and simulation results will be presented for these planned experiments. Initial experimental designs and moderate-current feasibility experiments will be discussed. *Research supported by U. S. DoE through Sandia National Laboratories award document numbers 240985, 768225, 790791 and 805234 to the UM. MRG supported by NNSA Fellowship and JCZ supported by NPSC Fellowship / Sandia National Labs.
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    ABSTRACT: Intense electron beam-driven diodes are being developed for flash x-ray radiography at 7.5 – 12 MV on the RITS-6 accelerator at Sandia National Laboratories. One of several diodes under investigation is the paraxial diode in which a 40-kA electron beam is formed in vacuum, propagated through a thin anode foil, and focused inside a gas-filled transport cell onto a high-atomic-number target to generate x-rays. Two key objectives are to produce a small spot size, 600 R@1m. Particle-in-cell (PIC) simulations have shown that the primary limitation in spot size is the finite decay of the plasma return current which causes the beam focal position to sweep axially during the pulse, hence leading to an increased time-integrated spot size [1]. Recently obtained time-resolved measurements of the spot size which support this hypothesis are reported. Interpretation of these results was aided by PIC simulations in which additional physical processes were included, such as stimulated ion emission from the foil and increasing area of cathode emission [2]. Other outstanding issues for the paraxial diode on RITS-6 are also presented which include electron power-flow coupling to the diode, cavity modes which may affect focusing, and gas-breakdown physics models. Measurements of dose, dose rate, time-integrated (and time-resolved) spot size, and current are reported from experimental results primarily at 7.5 MV. Time-resolved images of the cathode emission are also presented.
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    ABSTRACT: Summary form only given. Development of intense electron beam-driven diodes for flash X-ray radiography is being carried out at 7.5-12 MV on the RITS-6 accelerator at Sandia National Laboratories. One of several diodes under investigation is the paraxial diode, which employs a gas-filled transport cell to focus an electron beam onto a high-atomic-number target to generate X-rays. Three key objectives are to produce a small spot size, 600 rads at 1 m, and efficiently couple this relatively high-impedance diode to the lower-impedance RITS-6 accelerator. Particle-in-cell (PIC) simulations have shown that the primary limitation in spot size is due to the finite decay of the plasma return current which causes the beam focal location to sweep axially during the timescale of the pulse, hence leading to an increasing spot size. Time-resolved measurements of the spot size which convey this trend are reported. Interpretation of these results was aided by PIC simulations in which additional physics was included. Other outstanding issues for the paraxial diode on RITS-6 are also presented which include electron powerflow coupling to the diode, cavity modes which may affect focusing, and gas-breakdown models. Measurements of dose, dose rate, time-integrated (and time-resolved) spot size, and current are reported for experimental results at 7.5 and 10.5 MV.
    IEEE International Conference on Plasma Science 01/2008; DOI:10.1109/PLASMA.2008.4590851
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    ABSTRACT: Summary form only given: A series of experiments were conducted at Sandia National Laboratories on the RITS-6 accelerator configured in the low impedance mode (7.5 MV, 180 kA) to investigate electrode plasma formation and propagation in relativistic electron beam diodes used for flash x-ray radiography. In particular the Self- Magnetic Pinch diode (SMP), which employed a hollow metal cathode positioned ~12 mm from a thin aluminum foil anode, in-front of a high atomic number bremsstrahlung x-ray converter, was studied. Anode and cathode plasmas composed of surface contaminants and metals with densities of up to 1017 cm-3 are formed and expand across the gap with velocities of 10's of cm/microsecond. It is believed that the dynamics and interactions of these plasmas are responsible for the observed impedance behavior of the diode. Visible and ultraviolet spectroscopy is used to spatially and temporally measure individual plasma species. Plasma densities and temperatures are determined using collisional-radiative models. Diagnostics include gated, intensified CCD camera imaging and gated/streaked spectroscopy using high resolution 1 meter Czerny-Turner monochromators. Recent results are presented.
    IEEE International Conference on Plasma Science 01/2008; DOI:10.1109/PLASMA.2008.4590862
  • A. D. Heathcote · A. D. Critchley · M. D. Johnston
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    ABSTRACT: Summary form only given. Optical emission diagnostics have been used to observe self- magnetic pinch X-ray radiography diodes during operation on the RITS-6 accelerator (11 MV, 150 kA) at Sandia National Laboratories (SNL). Studies of this type inform flash X-ray radiography diode research from which an improved understanding of diode behavior and X-ray spot size evolution can be realised. The formation and propagation of both anode and cathode plasma have been investigated during electron-beam propagation and resulting X-ray radiation pulse. To facilitate shot to shot comparisons from this campaign and previous SMP data, the diode has remained unchanged from its standard configuration. Information obtained from line and continuum emission can provide key plasma parameters such as temperatures, densities, charge states, and expansion velocities. This paper summarises time-resolved emission spectra collected in the UV and visible from discrete points from the diodes axis of symmetry and at radial positions within the 12 mm A-K gap.
    IEEE International Conference on Plasma Science 01/2008; DOI:10.1109/PLASMA.2008.4590863
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    ABSTRACT: Contact resistance of single and multi-wire array z-pinch has been measured for aluminum, stainless steel, and tungsten wires; diameters ranged from 7.5 to 30.5 micron. DC contact resistance in these experiments accounted for approximately 80% of load resistance, and resistance measurements varied from wire-to-wire by up to 15%. These DC measurements show that the resistance is highly dependent on both the wire material and the mass of the wire weights (0.8 g to 3.6 g). Marx pulses of 120 kV, 18 kA, 150 ns risetime were applied to the z-pinch. Wire plasma expansion velocity was measured using a streak camera, and expansion profile of the wires was determined using laser schlieren imaging. Electron temperature of individual wire plasmas is being determined by visible/UV spectra. Results will be presented of several methods being explored to reduce the contact resistance. *This work was supported by U. S. DoE through Sandia National Laboratories award number 240985 to the University of Michigan. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under Contract DE-AC04-94AL85000.
  • A. D. Heathcote · A. D. J. Critchley · M. D. Johnston
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    ABSTRACT: Optical diagnostics are being developed and deployed at AWE to explore the limiting physics of electron beam diodes used for flash x-ray radiography. Photodiode and imaging systems have been used to observe plasma emission from the diode anode cathode (A-K) gap during x-ray production (˜100 ns) and for several microseconds beyond. There is currently a paucity of experimental data [1] direct from A-K gap plasmas; these data are needed to enhance understanding of the limiting physics of the diode geometries. Spatially and spectrally resolved intensity data from the A-K gap of self magnetic pinch (SMP) diodes has been collected and is presented. This has been used to study the role of foils used within the diode assembly and their role in inhibiting diode impedance collapse [2] for improved integrated dose output. [1] - Martin P, Short D, Jones A -- Theory Underpinning Ongoing Fundamental Pinch Physics Research at AWE, November 2005 to August 2006. Ref: AWE/HD02/B/0607/812. [2] - I. Crotch et al., ``Self Magnetic Pinch Diode Experiments at AWE'',14th IEEE Int. Pulsed Power Conf., 2003, pp. 507-509.
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    ABSTRACT: Sandia National Laboratories is investigating and developing high-dose, high-brightness flash radiographic sources. One of the diodes we are developing is the immersed-B z diode. This diode is a foil-less electron-beam diode with a long, thin, needle-like cathode inserted into the bore of a solenoid. The solenoidal magnetic field guides the electron beam emitted from the cathode to the anode while maintaining a small beam radius. The electron beam strikes a thin, high-atomic-number anode and produces bremsstrahlung. Recently, we reported on an extensive series of experiments<sup>1</sup> where an immersed-B z diode was fielded on the RITS-3<sup>2,3</sup> pulsed power accelerator, a 3-cell inductive voltage generator that produced peak voltages between 4 and 5 MV, ∼140 kA of total current, and power pulse widths of ∼50 ns. The analysis of those results is on-going. A summary of those results and analyses are presented.
    Pulsed Power Conference, 2007 16th IEEE International; 07/2007
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    ABSTRACT: Summary form only given. Investigations of the effect of initial wire contact-resistance on wire ablation dynamics are being conducted. Experiments utilize a 120 kV, 20 kA pulser with 140 ns rise-time, which drives a two-wire z-pinch load. Diagnostics for the experiment include laser shadowgraphv, streak camera images, and time-gated ICCD visible-UV wavelength spectroscopy. Both aluminum and stainless steel wires of 19-30 mum diameter have been studied. UM wire holders utilize slots and weights similar to those employed on Sandia's Z-Machine. Both precision DC-AC low current measurements and 20 kA pulsed experiments have been performed and compared. Typical wire contact resistances for this wire-holder design account for more than 85 percent of the total resistance of the load. Variations in contact resistance between two wires were observed for wires strung in a nearly identical fashion. DC experiments have shown that initial differences in the contact resistance between two wires results in one wire continually drawing more current than the other. Streak camera images with 20 kA pulses have shown that the wire with lower contact resistance draws more current prior to wire explosion, and that the wire drawing greater current exhibits a higher plasma expansion velocity. In a large number wire array, a difference in plasma expansion velocities between wires might be expected to lead to a non-uniform plasma shell, which would negatively affect plasma implosion dynamics. Ongoing experiments are aimed at characterizing the relationship between plasma expansion velocity and contact resistance between wires using both streak camera images and laser shadowgraphy. Spectroscopic measurements will examine variations in plasma temperature between the two plasmas. Methods will be explored to reduce variations in wire contact resistance, as well as to reduce the overall contact resistance of the wires.
    IEEE International Conference on Plasma Science 01/2007; DOI:10.1109/PPPS.2007.4345893
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    ABSTRACT: Experiments are underway to investigate wire ablation plasmas as potentially tunable sources for applications such as intense electron beam transport and focusing. For these studies, one or more fine wires (100 micron diameter) are driven by a microsecond long, capacitive discharge (80kA, 100kV) to generate a plasma. High resolution visible/uv spectroscopy is used to spatially and temporally characterize the plasma throughout the pulse. Measured lineshapes and intensities are compared with time-dependent, collisional-radiative calculations to obtain plasma densities and temperatures. Changes in drive current, wire geometry, and materials are studied to determine the extent to which ablation plasma parameters can be controlled. Results are compared to MHD calculations and scaling laws for plasma mass ablation rates from wires.