Mark D. Johnston

Overview of Self-Magnetically Pinched-Diode Investigations on RITS-6

The electron-beam-driven self-magnetically pinched diode is a candidate for future flash X-ray radiographic sources. As presently fielded on Sandia Laboratories six-cavity Radiographic Integrated Test Stand (RITS-6), the diode is capable of producing sub 3-mm radiation spot sizes and greater than 350 rads of hard X-rays at 1 m. The diode operates between 6 and 7 MV with a slowly decreasing impedance that falls from approximately 65 to 40 Ω during the main pulse. Sensitivity in diode operation is affected by the interaction of evolving plasmas from the cathode and anode, which seem to limit stable diode operation to a narrow parameter regime. To better quantify the diode physics, high-resolution time-resolved diagnostics have been utilized which include plasma spectroscopy, fast-gated imaging, X-ray p-i-n diodes, X-ray spot size, and diode and accelerator current measurements. Data from these diagnostics are also used to benchmark particle-in-cell simulations. An overview of results from experiments and simulations is presented.

Hybrid simulation of electrode plasmas in high-power diodes

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 ...

MAIZE: a 1 MA LTD‐Driven Z‐Pinch at The University of Michigan

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.Planned experimental research programs at UM include: a) Studies of Magneto‐Raleigh‐Taylor instability of planar foils, and b) Vacuum convolute studies including cathode and anode plasma.

Wire-Tension Effects on Plasma Dynamics in a Two-Wire

Heavier wire weights reduce contact resistance, which increases the energy deposition in wire plasma. Images from a two-wire Z-pinch showing the effects of wire tension on expansion performance are presented.

Z

Over the past 30 years, the paraxial diode has utilized a gas-filled transport cell to focus an electron beam onto a high-Z target in order to generate intense x-rays for flash radiography. One of the key objectives is to create a small radiographic spot, which in turn is related to the focusing dynamics of the beam. For gas-filled transport cells, the primary limitation to achieving a small time-integrated spot size is believed to be due to beam sweeping through the focal plane on the timescale of the pulse. This results in a larger than desired time-integrated radiographic spot. Recent simulations using LSP, a particle-in-cell code, have found that if a pre-ionized plasma transport cell is utilized with density on the order of 1016 cm-3, then the beam focal plane is essentially frozen in time, thereby limiting spot growth due to beam sweep. Recent experiments at Sandia National Laboratories have been conducted to investigate the spot behavior as a function of time for both gas and plasma-filled transport cells. For the plasma-filled experiments, a z-discharge of static hydrogen is used to create a highly ionized plasma in the transport cell. For the gas-filled experiments, the transport cell is filled with either hydrogen or air with pressures of 0.5-5 Torr. Time-resolved as well as time integrated measurement of the focal spot are presented and compared for both the gas and plasma-filled transport cells.

-Pinch

The self-magnetic-pinch diode is being developed as an intense electron beam source for pulsed-power-driven x-ray radiography. The basic operation of this diode has long been understood in the context of pinched diodes, including the dynamic effect that the diode impedance decreases during the pulse due to electrode plasma formation and expansion. Experiments being conducted at Sandia National Laboratories RITS-6 accelerator are helping to characterize these plasmas using time-resolved and time-integrated camera systems in the x-ray and visible. These diagnostics are analyzed in conjunction with particle-in-cell simulations of anode plasma formation and evolution. The results confirm the long-standing theory of critical-current operation with the addition of a time-dependent anode-cathode gap length. The results may suggest that anomalous impedance collapse is driven by increased plasma radial drift, leading to larger-than-average ion vr × Bθ acceleration into the gap.

Electron Beam Transport in Gas and Plasma-filled Cells on RITS

The self-magnetic pinch diode is currently fielded on the RITS-6 accelerator at Sandia National Laboratories operating between 7-12 MV and is the leading candidate for future radiographic source development at the Atomic Weapons Establishment. The diode is capable of producing sub 3-mm radiation spot sizes and greater than 350 Rads measured at 1m. Complex physical processes affect the diode operation which in turn may affect its radiographic potential. High-resolution, time-resolved diagnostics have been utilized to help quantify the diode physics which include plasma spectroscopy, gated imaging, X-ray p-i-n diodes, spot size, and diode current measurements. The data from these diagnostics are also used to benchmark particle-in-cell simulations in order to better understand the underlying physics of operation. An overview of these experiments and simulations including future plans is presented.

The impact of plasma dynamics on the self-magnetic-pinch diode impedance

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 10s 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.

Overview of self-magnetic pinch diode investigations on RITS-6

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...

Investigation of plasma formation and propagation in relativistic electron beam diodes

Spectroscopic investigations in the visible and near UV are underway to study plasmas present in X-ray radiography diodes during the time of the electron beam propagation. These studies are being performed on the RITS-3 accelerator (5.25 MV and 120 kA) at Sandia National Laboratories using several diode configurations. The proper characterization of the plasmas occurring during the time of the X-ray pulse can lead to a greater understanding of diode behavior and X-ray spot size evolution. By studying these plasmas along with the use of selective dopants, insights into such phenomena as impedance collapse, thermal and non-thermal species behavior, charge and current neutralization, anode and cathode plasma formation and propagation, and beam/foil interactions, can be obtained. Information from line and continuum emission and absorption can give key plasma parameters such as temperatures, densities, charge states, and expansion velocities. This information is important for proper modeling and future predictive capabilities for the design and improvement of flash X-ray radiography diodes. Diagnostics include a gated, intensified multichannel plate camera combined with a 1 meter Czerny-Turner monochromator with a multi-fiber spectral input, allowing for both temporal and spatial resolution. Recent results are presented.