R. S. Coats

Time-resolved proton focus of a high-power ion diode

An improved understanding of the factors that control the axial focus of applied‐B ion diodes was obtained from time‐resolved diagnostics of ion‐beam trajectories. This resulted in a new selection of anode shape that produced a proton focus of 1.3‐mm diameter from a 4.5‐cm‐radius diode, which is a factor of 2 improvement over previous results. We have achieved a peak proton power density of 1.5±0.2 TW/cm2 on the 1‐TW Proto I accelerator. The radial convergence of this proton beam, defined as the ratio of the anode diameter to focused beam FWHM, is 70. Time‐resolved information about virtual cathode evolution, the self‐ and applied‐magnetic‐field bending, and the horizontal focus of the beam was also obtained. In addition, the diffusion of the magnetic field into the anode plasma is estimated by measuring the horizontal focal position as a function of time. Finally, we discuss the effects of gas cell scattering on the beam focus.

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.

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.

EIGER ™ : An open-source frequency-domain electromagnetics code

EIGERtrade is a general-purpose, 3D frequency-domain electromagnetics code suite consisting of a pre-processor (Jungfrau), the physics code (EIGER), and post processor (Moench). In order to better enable collaborative development, EIGERtrade version 2.0 has been approved for release as open source software under a GNU Public License. EIGERtrade is primarily an integral-equation code for both frequency-domain electromagnetics and electrostatics. This version includes the following Greens functions: 2D and 3D free space, symmetry-planes, periodic and layered media. There is a thin-wire algorithm as well as junction basis functions for attachment of a wire to a conducting surface, and also thin-slot models for coupling into cavities. The code is written in Fortran 90 using object-oriented design and has the capability to run both in parallel and serial.

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...

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

Ion coupling efficiency for an extraction applied-B ion diode on the HELIA linear-induction adder in positive polarity

Initial experiments to investigate coupling of the four-stage HELIA linear-induction accelerator to a uniformly insulated applied-B ion diode in planar extraction geometry are reported. Results describing the efficient operation of an applied-B extraction ion diode coupled to the HELIA linear induction accelerator operated in positive polarity are reported. Operation of a close-coupled, undermatched, applied-B diode on HELIA was consistent with magnetically insulated transmission line (MITL) electron flow intermediate between locally emitted flow and generalized flow, rather than with full-gap flow. Peak ion coupling efficiencies of 60-70% and peak ion power levels of 0.3-0.4 TW have been achieved. >

Electromagnetic Analysis of Forces and Torques on the Baseline and Enhanced ITER Shield Modules due to Plasma Disruption

An electromagnetic analysis is performed on the ITER shield modules under different plasma-disruption scenarios using the OPERA-3d software. The models considered include the baseline design as provided by the International Organization and an enhanced design that includes the more realistic geometrical features of a shield module. The modeling procedure is explained, electromagnetic torques are presented, and results of the modeling are discussed.

Lithium beam generation and focusing with a radial diode on PBFAII

The performance of a 15-cm-radius applied-magnetic-field ion diode was investigated on the PBFA II accelerator at a power of 23 TW. The power coupling between the accelerator and diode was measured and compared with numerical simulations that show the effects of the electron flow in the MITL. The power coupled to the cathode of the diode was 18 MW. Measurements of the lithium beam generated from an electric-field-emission LiF anode showed a lithium beam power of 9 TW. The lithium beam was ballistically focused in a gas cell filled with 2 torr argon. The resultant focused power density was ∼1.8 TW/cm 2 equivalent on a cylindrical target at the centerline of the diode. The focused power was limited by the 20- to 30-mR divergence of the beam caused by the LiF source used and by virtual cathode instabilities in the anode-cathode gap. The ion mode instability in the virtual cathode was studied extensively by measurement of waves in the ion emission pattern from the anode and of the E-P θ correlation between variations in the beam energy and transverse momentum. The instability played a dominant role in the limitation of the focused lithium power.