N. Bruner

Role of plasmas in the operation of a self-magnetically pinched diode

The self-magnetic pinch diode is being developed as an intense electron beam source for high-power x-ray radiography. The diode is comprised of a ∼1-cm diameter, hollow cathode with a rounded tip from which a high-current electron beam is emitted. The beam self focuses in its own magnetic field as it propagates across a ∼1-cm vacuum gap where it deposits its energy onto a planar high-atomic-number bremsstrahlung target. Heating of the anode by the beam quickly provides an ion emitting plasma and bipolar diode operation. The dynamics of expanding electrode plasmas can affect the impedance lifetime of the diode. Realistic modeling of such plasmas is being pursued to aid in the understanding of the operating characteristics of these diodes as well as establishing scaling relations for reliable extrapolation to higher voltages. Here, a hybrid particle-in-cell code is used to study the evolution of electrode plasmas in the self-magnetic pinch diode for a nominal 6-MV voltage and different anode-cathode gaps. The impact of the intense ion beam on the cathode surface can lead to enhancement of the cathode plasma production and faster diode impedance loss.

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

The electron beam-driven self-magnetic pinch diode is presently fielded on the RITS-6 accelerator at Sandia National Laboratories and is a leading candidate for future flash x-ray radiographic sources. The diode is capable of producing sub 3-mm radiation spot sizes and greater than 350 rads measured at 1 m from the x-ray source. While RITS-6 is capable of delivering up to 11.5 MV using a magnetically insulated transmission line (MITL), the diode typically operates between 6 – 7 MV. Because the radiation dose has a power-law dependence on diode voltage, this limits the dose production on RITS. Coupling this low-impedance (∼ 40–60 ohms) diode to a MITL with similar or higher impedance affects its radiographic potential. The sensitivity in diode operation is compounded by the interaction of evolving plasmas from the cathode and anode, which seem to limit stable diode operation to a narrow 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 in order to better understand the underlying diode physics. An overview of these experiments and simulations is presented.

The Role of Plasma Evolution in the Operation of a Self Magnetically Pinched Diode

Summary form only given. The self-magnetic-pinch (SMP) diode is being developed as an intense electron beam source for high-power x-ray radiography. The diode is comprised of a ~1-cm diameter, hollow cathode with a rounded tip from which a high-current electron beam is emitted. The team self-focuses in its own magnetic field as it propagates across a ~1-cm vacuum gap where it deposits its energy onto a planar high-atomic-number target from which bremsstrahlung is produced. The evolution and dynamics of expanding electrode plasmas has long been recognized as a limiting factor in the impedance lifetimes of high-power vacuum diodes. Accurate modeling of such plasmas is being pursued to aid in the understanding of the operating characteristics of these diodes as well as establishing scaling relations for reliable extrapolation to higher voltages. Here, a hybrid particle-in-cell code is used to study the evolution of electrode plasmas in the SMP diode for a nominal 4 MV voltage and different cathode configurations. A low density plasma front is calculated to quickly evolve towards the anode producing an effective 1 mm anode-cathode gap. The simulated plasma motion is consistent with that expected from jxB acceleration. The small sheath enables a high intensity electron spot at and rapid heating of the anode. The interplay of the cathode plasma with the subsequently expanding anode plasma will determine the overall diode impedance.

Modeling the RITS-6 transmission line

Sandia National Laboratories’ Radiographic Integrated Test Stand (RITS-6) is a six-cell inductive voltage adder accelerator designed to produce currents of 186 kA at 7.8 MV in 70 ns in its low-impedance configuration. The six inductive-adder cells are connected in series to a coaxial magnetically insulated transmission line. Each cell has a single point feed to an azimuthal transmission line which distributes the pulse around the cell bore. To understand the extent to which power is distributed symmetrically around the coaxial transmission line and its effect on electron power flow downstream, particle-in-cell simulations were used to model the entire RITS-6 transmission line in 3D from pulse forming circuit to the diode load. Simulation results show electron flow current to be asymmetric by 16% at the exit to the sixth cell, but 3% or less at diagnostic positions near the load. Magnetic insulation in the trans-mission line does not appear to be impacted by the asymmetry, though flow impedance is not uniform axially.

Load impedance dynamics in the RITS-6 self-magnetic -pinch diode

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 [1], and anode plasma dynamics has been modeled [2]. 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.

Ion kinetic effects in hybrid-PIC simulations of merging plasma jets in the plasma liner experiment

Merging plasma jets are envisioned for use in magneto-inertial fusion schemes as the source of an imploding plasma liner. In the upcoming plasma liner experiment (PLX) at Los Alamos National Laboratory a spherical array of 15–30 plasma jets generated by compact accelerators will be merged. We present simulation results of plasma jets in the PLX parameter regime (ni ∼ 1017 cm−3, Te, Ti ∼ 1 eV) using the Hybrid particle-in-cell (PIC) code LSP. We also describe synthetic diagnostics implemented into the code to facilitate comparison with PLX experimental results. In hybrid mode electron macroparticles are treated as Lagrangian fluid elements while ion macroparticles may be treated either as a fluid or a kinetic species. The kinetic approach for ions captures non-maxwellian behavior and finite mean-free-path effects such as inter-penetration in jet merging. An equation of state model is used to calculate the local effective charge-state and internal energy of the plasma. A radiation transport algorithm is coupled to the plasma to capture the effect of radiation cooling on the electron energy. We present results for acceleration, transport, and merging of argon plasma jets, and compare the results for simulations with fluid and kinetic ions.

Advanced particle-in-cell techniques for pulsed power device and hedp simulation

Understanding the operation of pulse power systems and components, magnetically insulated transmission lines (MITLs), plasma accelerators and high power diodes require accurate modeling of evolving electron flow and plasma motion. Challenges include gas breakdown, surface physics, high energy density plasma, interaction of energetic particles with surfaces, and complex geometries. In this paper, we will discuss the new capabilities and discuss the enabling algorithms.

Recent paraxial diode experiments on RITS-6

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, <5 mm, high forward-directed dose, >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.

Nonideal effects in the operation of a paraxial diode with gas cell focusing

Paraxial electron diodes with gas transport cells have been used to focus intense electron beams onto a high-Z target producing bremsstrahlung radiation. This configuration is being fielded on the recently commissioned RITS-6 accelerator producing 35-kA current and 10-MeV energy electron beams. Direct ionization of the beam and avalanche from the electron secondaries drive a break down of the gas that rapidly increases the gas cell conductivity. Non-ideal effects result in an axial sweep of the beam focus position that ultimately limits the radiographic spot. These effects include cathode plasma evolution, stimulated emission of anode ions, and magnetic diffusion in the gas cell. The gas breakdown is studied with hybrid particle-in-cell simulations using several different gas chemistry models including kinetic Monte Carlo interaction. In addition, we test the validity of these models against measured time- dependent beam spot and radiation production rates.

Simulations of paraxialdiode operation on the 10mv rits-6 accelerator

Summary form only given. Paraxial electron diodes with gas transport cells have been used to focus intense electron beams onto a high-Z target producing bremsstrahlung radiation. The quality of the resulting radiographic X-ray source increases linearly with the dose and inversely with the square of the spot size. This configuration is being fielded on the recently commissioned RITS-6 accelerator producing 35-kA current and 10-MeV energy electron beams. Direct ionization of the beam and avalanche from the electron secondaries drive a break down of the gas that rapidly increases the gas cell conductivity. Because of the delay and incompleteness of the gas breakdown, the gas cells typically operate with a small but finite and slowly increasing net current (sum of the plasma and beam currents). These non-ideal effects result in an axial sweep of the beam focus position that ultimately limits the radiographic spot. Other factors limiting the spot include intrinsic beam emittance, foil and gas scattering, and nonlinear magnetic field evolution in the gas cell. The gas breakdown is being studied with hybrid particle-in-cell simulations using several different gas chemistry models. In this paper, we numerically optimize the beam spot for radiographic utility. In addition, we assess the accuracy of these models via comparison with measured time-dependent beam spot and radiation production rates