Publications

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The electrons flowing in a coaxial magnetically insulated transmission line (MITL), if allowed to flow uncontrolled into a radiographic electron diode load, can have an adverse impact on the performance of the system. Total radiation dose, impedance lifetime, and spot quality (size, shape, position, and stability) can all be affected. Current approaches to deal with this problem require a large volume in the vicinity of the electron diode load. For applications where this volume is not available, an alternate method of controlling the feed electrons is needed. In this paper, we will investigate various ideas for dealing with this issue and present results showing the properties of the various schemes investigated. © 2011 IEEE.

The 7 cavity, 1 MV linear transformer driver for radiography at Sandia National Laboratories has recently been upgraded to 21 cavities with an output voltage of 2.5 MV. In this paper, results from 2-D, r-z particle-in-cell simulations of the full 21 cavity system are presented. Each cavity feed is driven with its own external RLC circuit that is independently triggered, and has a realistic 45° slanted vacuum/insulator. Electrons are emitted from the central cathode with a conventional space-charge-limited emission model. Detailed diagnostics monitor electron loss to the anode, cavity conductors, and the insulators. The most significant and encouraging result is that the simulations have absolutely no electron loss to the insulators, even with large random variations in the trigger timing. © 2011 IEEE.

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A new method for including electrode plasma effects in particle-in-cell simulation of high power devices is presented. It is not possible to resolve the plasma Debye length, {lambda}{sub D} {approx} 1 {mu}m, but using an explicit, second-order, energy-conserving particle pusher avoids numerical heating at large {delta}x/{lambda}{sub D} >> 1. Non-physical plasma oscillations are mitigated with Coulomb collisions and a damped particle pusher. A series of 1-D simulations show how plasma expansion varies with cell size. This reveals another important scale length, {lambda}{sub E} = T/(eE), where E is the normal electric field in the first vacuum cell in front of the plasma, and T is the plasma temperature. For {delta}x/{lambda}{sub E} < {approx}1, smooth, physical plasma expansion is observed. However, if {delta}x/{lambda}{sub E} >> 1, the plasma 'expands' in abrupt steps, driven by a numerical instability. For parameters of interest, {lambda}{sub E} << 100 {mu}m. It is not feasible to use cell sizes small enough to avoid this instability in large 3-D simulations.

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This report summarizes the work completed during FY2007 and FY2008 for the LDRD project ''Hybrid Plasma Modeling''. The goal of this project was to develop hybrid methods to model plasmas across the non-continuum-to-continuum collisionality spectrum. The primary methodology to span these regimes was to couple a kinetic method (e.g., Particle-In-Cell) in the non-continuum regions to a continuum PDE-based method (e.g., finite differences) in continuum regions. The interface between the two would be adjusted dynamically ased on statistical sampling of the kinetic results. Although originally a three-year project, it became clear during the second year (FY2008) that there were not sufficient resources to complete the project and it was terminated mid-year.

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An analytic model for electron flow in a system driving a fixed inductive load is described and evaluated with particle in cell simulations. The simple model allows determining the impedance profile for a magnetically insulated transmission line given the minimum gap desired, and the lumped inductance inside the transition to the minimum gap. The model allows specifying the relative electron flow along the power flow direction, including cases where the fractional electron flow decreases in the power flow direction. The electrons are able to return to the cathode because they gain energy from the temporally rising magnetic field. The simulations were done with small cell size to reduce numerical heating. An experiment to compare electron flow to the simulations was done. The measured electron flow is {approx}33% of the value from the simulations. The discrepancy is assumed to be due to a reversed electric field at the cathode because of the inductive load and falling electron drift velocity in the power flow direction. The simulations constrain the cathode electric field to zero, which gives the highest possible electron flow.

The ZR accelerator is a refurbishment of Sandia National Laboratories Z accelerator [1]. The ZR accelerator components were designed using electrostatic and circuit modeling tools. Transient electromagnetic modeling has played a complementary role in the analysis of ZR components [2]. In this paper we describe a 3D transient electromagnetic analysis of the ZR water convolute and stack using edge-based finite element techniques. © 2005 IEEE.

The Z driver at Sandia National Laboratories delivers one to two megajoules of electromagnetic energy inside its ∼10 cm radius final feed in 100 ns. The high current (∼20 MA) at small diameter produces magnetic pressures well above yield strengths for metals. The metal conductors stay in place due to inertia long enough to deliver current to the load. Within milliseconds however, fragments of metal escape the load region at high velocity. Much of the hardware and diagnostics inside the vacuum chamber is protected from this debris by blast shields with small view ports, and fast-closing valves. The water-vacuum insulator requires different protection because the transmission line debris shield should not significantly raise the inductance or perturb the self-magnetically insulated electron flow. This report shows calculations and results from a design intended to protect the insulator assembly. © 2005 IEEE.

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