D. B. Seidel

Simulation codes for light-ion diode modeling

The computational tools used in the investigation of light-ion diode physics at Sandia National Laboratories are described. Applied-B ion diodes are used to generate intense beams of ions and focus these beams onto targets as part of Sandias inertial confinement fusion program. Computer codes are used to simulate the energy storage and pulse forming sections of the accelerator and the power flow and coupling into the diode where the ion beam is generated. Other codes are used to calculate the applied magnetic field diffusion in the diode region, the electromagnetic fluctuations in the anode-cathode gap, the subsequent beam divergence, the beam propagation, and response of various beam diagnostics. These codes are described and some typical results are shown.

A general theory of magnetically insulated electron flow

A theory of magnetically insulated flow is developed where two‐ or three‐dimensional spatial variation and temporal variation are allowed as long as the spatial variations in the flow direction are over many electron gyrolengths and temporal variations are over many electron gyroperiods. These criteria are met in most flows of interest. The theory is based upon describing the electron dynamics in terms of parameters which are constants of motion in one‐dimensional, time‐independent flows.

Applied‐B ion diode experiments on the Particle Beam Fusion Accelerator‐I

A series of experiments was performed with an Applied‐B ion diode on the Particle Beam Fusion Accelerator‐I, with peak voltage, current, and power of approximately 1.8 MV, 6 MA, and 6 TW, respectively. The purpose of these experiments was to explore issues of scaling of Applied‐B diode operation from the sub‐TW level on single module accelerators to the multi‐TW level on a low impedance, self‐magnetically insulated, multimodule accelerator. This is an essential step in the development of the 100‐TW level light ion beam driver required for inertial confinement fusion. The accelerator and the diode are viewed as a whole because the power pulse delivered by the 36 imperfectly synchronized magnetically insulated transmission lines to the single diode affects module addition, diode operation, and ion beam focusability. We studied electrical coupling between the accelerator and the diode, power flow symmetry, the ionic composition of the beam, and the focusability of the proton component of the beam. Scaling of...

Electromagnetic particle‐in‐cell simulations of Applied‐B proton diodes

Fully electromagnetic particle‐in‐cell simulations of Applied‐B ion diodes have been performed using the magic code. These calculations indicate that Applied‐B diodes can be nearly 100% efficient. Furthermore, the simulations exhibit an impedance relaxation phenomenon due to the buildup of electron space charge near the anode which causes a time‐dependent enhancement of the ion emission above the Child–Langmuir value. This phenomenon may at least partially explain the rapidly decreasing impedance that has been observed in Applied‐B ion diode experiments. The results of our numerical simulations will be compared to experimental data on Applied‐B ion diodes and to analytic theories of their operation.

A load-balancing algorithm for a parallel electromagnetic particle-in-cell code

Abstract Particle-in-cell simulations often suffer from load-imbalance on parallel machines due to the competing requirements of the field-solve and particle-push computations. We propose a new algorithm that balances the two computations independently. The grid for the field-solve computation is statically partitioned. The particles within a processors sub-domain(s) are dynamically balanced by migrating spatially-compact groups of particles from heavily loaded processors to lightly loaded ones as needed. The algorithm has been implemented in the quicksilver electromagnetic particle-in-cell code. We provide details of the implementation and present performance results for quicksilver running models with up to a billion grid cells and particles on thousands of processors of a large distributed-memory parallel machine.

Three‐dimensional particle‐in‐cell simulations of applied‐B ion diodes

The three‐dimensional particle‐in‐cell code quicksilver [Seidel et al., Computational Physics, edited by A. Tenner (World Scientific, Singapore, 1991), p. 475] has been used to study applied‐B ion diodes. The impedance behavior of the diode in these simulations is in good agreement with both analytic theory and experiments at peak power. The simulations also demonstrate the existence of electromagnetic instabilities which induce divergence in the ion beam. Early in time, there is an instability at high frequency relative to the ion transit time τi, and the resulting beam divergence is low. However, later in time, the system makes a transition to an instability with a frequency close to 1/τi, and the ion beam divergence rises to an unacceptably high value. The transition is associated with the build‐up of electron space charge in the diode, and the resulting increase in the beam current density enhancement (J/JCL). Using different schemes to inhibit the electron evolution, the transition has both been post...

Magnetic insulation of extraction Applied‐B ion diodes

The difference in the operation of extraction geometry versus radial geometry Applied‐B ion diodes is explained through the use of a general magnetic insulation condition for cylindrically symmetric ion diodes which is expressed in terms of the magnetic stream function. We find that Applied‐B extraction geometries attempted so far have suffered from a magnetic field configuration defect which explains their rather poor performance. We present solutions to the extraction Applied‐B ion diode problem, as well as 2D, electromagnetic particle‐in‐cell simulations of an example solution.

Design of a command-triggered plasma opening switch for terawatt applications

The crucial element of an inductive energy storage system is the opening switch. In microsecond and nanosecond pulsed power systems the plasma opening switch has been in use for many years. The development of the triggered switch addresses three important areas: complete de-coupling of the closed phase and the opening phase will allow improved performance, especially at longer conduction times; the simplified physics allows for easier modeling because of a better-defined geometry; and triggering will reduce jitter of the output pulse. Improving performance will allow longer conduction time, and triggering will negate the naturally increased self-operating jitter at longer conduction time. The triggered switch system is based on moving the plasma switch armature with a magnetic field. Up until the time the armature is pushed away, it is held in place against the drive current magnetic pressure by a second magnetic field. Our system is designed to deliver 1-2 terawatts of usable load power at multi-megavolt potentials. We define usable load power as the product of load voltage and load cathode current. The length of the vacuum storage inductor defines the 35 ns pulse length. This paper shows the design of the switch and trigger system, which is conservatively designed to provide a wide range of trigger signals. The trigger power for this system is important for cost reasons. The first experiments will use a trigger level of ten percent of the output pulse; we describe design features intended to reduce the amount of trigger power needed. Particle-in-cell simulations of the active trigger are also shown.

Theory of instability-generated divergence of intense ion beams from applied-B ion diodes

Over the course of the past few years, rapid progress has been made in the development of a theoretical understanding of the physics of applied-B ion diodes. Success in predicting diode current and voltage operating characteristics has been followed by new insight into the effects of electromagnetic instabilities on ion beam divergence

Self‐magnetic‐field‐enhanced ion diode

An intense ion source has been developed utilizing an ion diode that partially suppresses electron flow using the self‐magnetic field of the diode current. This is an extension of a diode class known as pinch reflex ion diodes. In this case the diode was coupled to the particle beam fusion accelerator and was configured in two different cylindrical designs. The first pinch ion diode was a straight, 42‐cm‐diam cylinder and the second, Obi, was a focusing, 26‐cm‐diam, aspheric barrel. In the Obi diode a central gas cell provided current‐neutralized beam transport. In addition, the accelerator was run in a low‐voltage, 0.8 MV, and a high‐voltage, 2.0 MV, mode. The best results showed that the Obi diode produced 3 TW of protons at 34% efficiency in the high‐voltage mode. We present an analytic model of ion efficiency, compare various diode impedance models, and discuss beam divergence mechanisms. The limitation of this ion source as a fusion driver is presently the 2–3° divergence that we measure using a shad...