M. D. Johnston

Sandia National Laboratories, Albuquerque, New Mexico, United States

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Publications (68)3.9 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: This talk will be an overview of spectroscopic results obtained on the RITS-6 accelerator at Sandia National Laboratories on the Self-Magnetic Pinch (SMP) electron beam diode. The SMP diode produces a focused (<3mm diameter), e-beam at 7MeV and 150kA, which is used as an intense, flash x-ray source. During the ˜45ns electron beam pulse, plasmas are generated on the electrode surfaces which propagate into the A-K vacuum gap, affecting the diode impedance, x-ray spectrum, and pulse-width. These plasmas are measured using a series of optical diagnostics including: streak cameras, ICCD cameras, and avalanche photodetectors. Visible spectroscopy is used to gather time and space information on these plasmas. Density and temperature calculations are made using detailed, time-dependent, collisional-radiative (CR) and radiation transport modelings. The results are then used in conjunction with hybrid PIC/fluid simulations to model the overall plasma behavior. Details regarding the data collection, system calibration, analyses, and interpretation of results will be presented. [4pt] ^1Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.
    10/2012;
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    ABSTRACT: This paper describes a convenient and accurate method to calibrate fast (<1 ns resolution) streaked, fiber optic light collection, spectroscopy systems. Such systems are inherently difficult to calibrate due to the lack of sufficiently intense, calibrated light sources. Such a system is used to collect spectral data on plasmas generated in electron beam diodes fielded on the RITS-6 accelerator (8-12MV, 140-200kA) at Sandia National Laboratories. On RITS, plasma light is collected through a small diameter (200 μm) optical fiber and recorded on a fast streak camera at the output of a 1 meter Czerny-Turner monochromator. For this paper, a 300 W xenon short arc lamp (Oriel Model 6258) was used as the calibration source. Since the radiance of the xenon arc varies from cathode to anode, just the area around the tip of the cathode ("hotspot") was imaged onto the fiber, to produce the highest intensity output. To compensate for chromatic aberrations, the signal was optimized at each wavelength measured. Output power was measured using 10 nm bandpass interference filters and a calibrated photodetector. These measurements give power at discrete wavelengths across the spectrum, and when linearly interpolated, provide a calibration curve for the lamp. The shape of the spectrum is determined by the collective response of the optics, monochromator, and streak tube across the spectral region of interest. The ratio of the spectral curve to the measured bandpass filter curve at each wavelength produces a correction factor (Q) curve. This curve is then applied to the experimental data and the resultant spectra are given in absolute intensity units (photons/sec/cm(2)/steradian/nm). Error analysis shows this method to be accurate to within +∕- 20%, which represents a high level of accuracy for this type of measurement.
    The Review of scientific instruments 08/2012; 83(8):083108. · 1.52 Impact Factor
  • Tim J. Webb, Mark D. Johnston, Bryan V. Oliver
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    ABSTRACT: The 7 MV, 160 kA induction voltage adder RITS-6 is used as a test bed for research and development of sub-100 ns flash x-ray radiography of which the self-magnetic pinch (SMP) diode is an example. The x-ray source properties such as dose, source spatial distribution, and energy spectrum couple with the imaging detector sensitivity and blur to form the radiologic system performance which is also highly dependent on the imaging geometry. The system performance of some SMP diode configurations will be presented.
    11/2011;
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    ABSTRACT: The self magnetic pinch (SMP) electron beam diode is being investigated as an intense flash x-ray radiographic source. The diode produces a focused e-beam (<3mm diameter) at 7 MeV and 150kA with a 45ns FWHM pulsewidth. Since the vacuum gap is small (˜ 1cm), plasmas formed on the electrode surfaces affect the diode impedance, x-ray spectrum, pulsewidth, and e-beam dynamics. Temporal and spatially resolved optical spectra are collected and analyzed using self-consistent, time-dependent, collisional radiative (CR) models which provide information about plasma species, densities, and temperatures. This data is used to verify plasma conditions and help benchmark hybrid PIC codes which simulate these plasma environments. Recent experimental results obtained for the SMP diode fielded on the RITS-6 accelerator are presented.
    11/2011;
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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;
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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.
    AIP Conference Proceedings. 10/2010; 1278(1).
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    ABSTRACT: Summary form only given. The rod-pinch diode is a co-axial relativistic electron-beam-driven diode, where the cathode is a hollow disk and the anode a rod protruding through the cathode. The diode operates at currents in excess of the critical current such that electrons emitted from the cathode are magnetically insulated from crossing the A-K gap and are forced to attach at the tip of the anode rod, where magnetic insulation is lost. Fielded on the RITS-6 accelerator at Sandia in negative polarity geometries, it has been operated at high electrical powers in excess 0.75 TW with voltages greater than 6 MV and currents of 120 kA. Pulse lengths are typically ~ 45 ns. For high atomic number anode/rod materials like Gold or Tungsten, specific energy deposition of order 1 MJ/g is achieved at the tip.Optical imaging of the thermodynamic expansion of the rod tip, for times after the electric pulse, suggest expansion speeds of the bulk material of order a few cm/Πs. Referring such velocities back to initial conditions for the material suggest near solid density at a couple eV temperatures. Particle-in-cell (PIC) simulations of electron beam energy deposition into the rod tip also suggest high average temperature of the bulk material. In addition, because the rod diameter is not a full electron range at these energies, the electrons reflex through the anode, producing, relatively uniform (radial) heating. A discussion of the possibility of using the rod-pinch as a warm dense matter environment and the planned measurements to verify its utility will be presented.
    IEEE International Conference on Plasma Science 01/2010;
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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.
    10/2009; 115(87).
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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.
    AIP Conference Proceedings. 01/2009; 1088(1):259-262.
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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;
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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.
    01/2009;
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    ABSTRACT: New numerical techniques for simulating the formation and evolution of cathode and anode plasmas have been successfully implemented in a hybrid code. The dynamics of expanding electrode plasmas has long been recognized as a limiting factor in the impedance lifetimes of high-power vacuum diodes and magnetically insulated transmission lines. Realistic modeling of such plasmas is being pursued to aid in understanding the operating characteristics of these devices as well as establishing scaling relations for reliable extrapolation to higher voltages. Here, in addition to kinetic and fluid modeling, a hybrid particle-in-cell technique is described that models high density, thermal plasmas as an inertial fluid which transitions to kinetic electron or ion macroparticles above a prescribed energy. The hybrid technique is computationally efficient and does not require resolution of the Debye length. These techniques are first tested on a simple planar diode then applied to the evolution of both cathode and anode plasmas in a high-power self-magnetic pinch diode. The impact of an intense electron flux on the anode surface leads to rapid heating of contaminant material and diode impedance loss.
    Physics of Plasmas. 01/2009; 16(12):123102.
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    ABSTRACT: Experiments at SNL are underway to investigate plasma formation in rod pinch diodes fielded in negative polarity on the RITS-6 accelerator (7.5MV and 180kA at 40 Ohms). The rod pinch diode consists of a small diameter metal rod which extends through a larger diameter metal cathode plate. Electrons formed at the cathode accelerate across the gap, become self-insulated, and are focused at the rod tip, generating x-rays used for flash radiography. Most of the previous rod pinch work has been performed in positive polarity; however, there is an interest in operating this diode in negative polarity at higher voltages, allowing more flexibility and incorporation into a wider variety of pulsed power devices. In an effort to better understand the basic physics, and the role ions play in the impedance behavior, a series of shots were taken looking at plasma formation in the diode. Diagnostics include optical imaging and spectroscopy using nanosecond gated ICCD cameras, streak cameras, and photodiode arrays. *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.
    01/2009;
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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.
    11/2008;
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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. · 2.38 Impact Factor
  • 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;
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    ABSTRACT: The six cell inductive voltage adder (IVA) RITS-6 is a test bed for studying and improving the radiographic performance of various flash X-ray sources. RITS will be used to study the effect of the pulse shape predicted for the Hydrus IVA on the Self Magnetic Pinch (SMP) and paraxial radiographic diodes. The modification of RITS's configuration to match the Hydrus IVA output pulse shape into simple diodes is described. Test results with the radiographic diodes will be presented in the future. The 10 cell Hydrus IVA was designed by L-3 Communications - Pulse Sciences for the Atomic Weapons Establishment (AWE) in England. The baseline Hydrus IVA is predicted to drive the nominally 40-Omega SMP to nearly 8 MV through its 80-Omega output MITL. The output pulse shape of the Hydrus IVA has two important features that have not been previously tested with radiographic diodes. One feature is a 20-ns duration foot preceding the fast rising main pulse. The other feature is a slowly rising ramp through the duration of the voltage pulse top which is predicted to counter the SMP diode's falling impedance, thereby flattening the diode voltage. The output pulse shape of the RITS-6 accelerator fitted with its 80-Omega output MITL was modified to mimic that of the Hydrus IVA by adjusting the sequential timing of its six 8-Omega PFLs and by altering the relative closing times of each PFL's self breaking main, sharpening, and prepulse switches. Circuit simulations predict that the modified RITS-6 will approximately reproduce the pulse shape of the Hydrus accelerator. Measured pulse shapes will be compared to predictions for both RITS and Hydrus with a large area electron beam diode.
    IEEE International Conference on Plasma Science 01/2008;
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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.
    01/2008;
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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 International Conference on Plasma Science 01/2008;
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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;