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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 plasmas in a high-power self-magnetic pinch diode. The impact of an intense electron flux on the anode surface leads to rapid heating of contaminant material and diode impedance loss. [ABSTRACT FROM AUTHOR]

Abstract: Longitudinal compression factors in excess of 50 of a 300-keV, 20-mA K+ ion beam have been demonstrated in the Neutralized Drift Compression Experiment (NDCX) in agreement with LSP particle-in-cell simulations using the experimental tilt voltage waveform. Here, pre-formed plasma provides beam neutralization for a 1–2-m drift length. To achieve simultaneous transverse and longitudinal compression, we must understand and account for the impact of the applied velocity tilt on the transverse phase space of the beam. Of equal importance to achieving warm dense matter and heavy ion fusion conditions, is quantifying the effect of beam plasma interactions, including stability and neutralization, on the beam transport throughout the drift section up to the target. Critical new issues relate to transverse focusing of the axially compressing ion beam in a high-field (3–15T) solenoid that is filled with plasma. Integrated LSP simulations that include modeling of the diode, magnetic transport, induction bunching module, plasma neutralized transport, solenoidal focusing and beam target interaction, are assisting in the design of a near-term warm dense matter experiment. We discuss the simulation algorithms and present calculations of designs for such an experiment that will heat an aluminum target up to roughly 1-eV temperature. [Copyright &y& Elsevier]

This paper describes the configuration and operation of a seven-cavity linear transformer driver (LTD) system. This LTD system is configured to deliver ∼1 MV and 125 kA into a critically damped load. A detailed transmission line model coupled to particle-in-cell simulations is used to assess the system electrical performance. The evolution of the electron power flow in negative polarity is simulated, and the impact of this flow on the operation of the system with a large-area hollow electron beam diode is examined. The simulation results are compared with available electrical measurements and with dose rate measurements where a flash X-ray pulse is produced by an annular electron beam diode. These comparisons suggest that the LTD system meets the design specifications and is a robust pulsed power architecture. Additionally, the positive polarity operation for the LTD system driving a rod-pinch diode load is modeled to further assess the utility of the LTD system. [ABSTRACT FROM AUTHOR]

In this paper, we present the results from particle-in-cell simulations of the nominally 2-MV 40-kA CYGNUS radiographic machine using a standard rod pinch diode. The rod pinch electron-beam diode consists of a small-diameter high-atomic-number anode extending through a thin cathode aperture. A significant shot-to-shot variation of the photon spectrum emerging from the anode can introduce an error in interpreting the radiograph. We present the results from these integrated simulations of the electron and photon generation and quantify the sensitivities on the photon distribution. [ABSTRACT FROM PUBLISHER]

A series of simulations and experiments to resolve questions about the operation of arrays of closely spaced small aspect ratio rod pinches has been performed. Design and postshot analysis of the experimental results are supported by 3-D particle-in-cell simulations. Both simulations and experiments support these conclusions. Penetration of current to the interior of the array appears to be efficient, as the current on the center rods is essentially equal to the current on the outer rods. Current loss in the feed due to the formation of magnetic nulls was avoided in these experiments by design of the feed surface of the cathode and control of the gap to keep the electric fields on the cathode below the emission threshold. Some asymmetry in the electron flow to the rod was observed, but the flow appeared to symmetrize as it reached the end of the rod. Interaction between the rod pinches can be controlled to allow the stable and consistent operation of arrays of rod pinches. [ABSTRACT FROM PUBLISHER]

Abstract: This paper presents a survey of the present theoretical understanding based on advanced analytical and numerical studies of collective processes and beam–plasma interactions in intense heavy ion beams for applications to ion-beam-driven high energy density physics and heavy ion fusion. The topics include: discussion of the conditions for quiescent beam propagation over long distances; and the electrostatic Harris instability and the transverse electromagnetic Weibel instability in highly anisotropic, intense one-component ion beams. In the longitudinal drift compression and transverse compression regions, collective processes associated with the interaction of the intense ion beam with a charge-neutralizing background plasma are described, including the electrostatic electron–ion two-stream instability, the multispecies electromagnetic Weibel instability, and collective excitations in the presence of a solenoidal magnetic field. The effects of a velocity tilt on reducing two-stream instability growth rates are also discussed. Operating regimes are identified where the possible deleterious effects of collective processes on beam quality are minimized. [Copyright &y& Elsevier]

Abstract: Intense, space-charge-dominated ion beam pulses for warm dense matter and heavy ion fusion applications must undergo simultaneous transverse and longitudinal bunch compression in order to meet the requisite beam intensities desired at the target. The longitudinal compression of an ion bunch is achieved by imposing an initial axial velocity tilt on the drifting beam and subsequently neutralizing its space-charge and current in a drift region filled with high-density plasma. The Neutralized Drift Compression Experiment (NDCX) at Lawrence Berkeley National Laboratory has measured a sixty-fold longitudinal current compression of an intense ion beam with pulse duration of a few nanoseconds, in agreement with simulations and theory. A strong solenoid is modeled near the end of the drift region in order to transversely focus the beam to a sub-millimeter spot size coincident with the longitudinal focal plane. The charge and current neutralization provided by the background plasma is critical in determining the total achievable transverse and longitudinal compression of the beam pulse. Numerical simulations show that the current density of an NDCX ion beam can be compressed over a few meters by factors greater than 105 with peak beam density in excess of 1014 cm−3. The peak beam density sets a lower bound on the local plasma density required near the focal plane for optimal beam compression, since the simulations show stagnation of the compression when n beam>n plasma. Beam–plasma interactions can also have a deleterious effect on the compression physics and lead to the formation of nonlinear wave excitations in the plasma. Simulations that optimize designs for the simultaneous transverse and longitudinal focusing of an NDCX ion beam for future warm dense matter experiments are discussed. [Copyright &y& Elsevier]

Abstract: Properties of the multi-species electromagnetic Weibel instability are investigated for an intense ion beam propagating through background plasma. Assuming that the background plasma electrons provide complete charge and current neutralization, detailed linear stability properties are calculated within the framework of a macroscopic cold-fluid model for a wide range of system parameters. [Copyright &y& Elsevier]

The design of a 6.5-MV linear transformer driver (LTD) for flash-radiography experiments is presented. The design is based on a previously tested 1-MV LTD and is predicted to be capable of producing diode voltages of 6.5 MV for a 50-Ω radiographic-diode load. Several fault modes are identified, and circuit simulations are used to determine their effect on the output pulse and other components. For all the identified fault modes, the peak load voltage is reduced by less than 5%. [ABSTRACT FROM AUTHOR]