B.V. Oliver

Magnetically insulated electron flow with ions with application to the rod-pinch diode

A one-dimensional, steady-state, relativistic electron-flow model is developed that describes magnetically insulated electron flow in the presence of ions produced by space-charge-limited emission from the anode. The model is applied to the rod-pinch diode which is a cylindrical pinched-beam diode consisting of a small radius anode rod extending through the hole of an annular cathode. The diode is designed to run at critical current so that electrons emitted from the cathode are magnetically insulated and flow axially along the anode rod until they pinch radially onto the rod tip. Ions are emitted along the length of rod and flow radially outward. Without these ions, magnetically insulated electron flow cannot be established and electrons cannot propagate to the rod tip. Both fluid and Vlasov treatments of the electrons are considered. An analytic expression for the critical current is derived and is compared with the critical current determined from experimental data and particle-in-cell simulations. Rea...

Particle‐in‐cell simulations of fast magnetic field penetration into plasmas due to the Hall electric field

Particle‐in‐cell (PIC) simulations are used to study the penetration of magnetic field into plasmas in the electron‐magnetohydrodynamic (EMHD) regime. These simulations represent the first definitive verification of EMHD with a PIC code. When ions are immobile, the PIC results reproduce many aspects of fluid treatments of the problem. However, the PIC results show a speed of penetration that is between 10% and 50% slower than predicted by one‐dimensional fluid treatments. In addition, the PIC simulations show the formation of vortices in the electron flow behind the EMHD shock front. The size of these vortices is on the order of the collisionless electron skin depth and is closely coupled to the effects of electron inertia. An energy analysis shows that one‐half the energy entering the plasma is stored as magnetic field energy while the other half is shared between internal plasma energy (thermal motion and electron vortices) and electron kinetic energy loss from the volume to the boundaries. The amount o...

Evolution of a Maxwellian plasma driven by ion-beam-induced ionization of a gas

The ionization of gas by intense (MeV, kA/cm2) ion beams is investigated for the purpose of obtaining scaling relations for the rate of rise of the electron density, temperature, and conductivity of the resulting plasma. Various gases including He, N, and Ar at pressures of order 1 torr have been studied. The model is local and assumes a drifting Maxwellian electron distribution. In the limit that the beam to gas density ratio is small, the initial stage of ionization occurs on the beam impact ionization time and lasts on the order of a few nanoseconds. Thereafter, ionization of neutrals by the thermal electrons dominates electron production. The electron density does not grow exponentially, but proceeds linearly on a fast time scale tth=U/(vbρ dE/dx) associated with the time taken for the beam to lose energy U via collisional stopping in the gas, where U is the ionization potential of the gas, vb is the beam velocity, ρ is the gas mass density, and dE/dx is the mass stopping power in units of eV cm2/g. T...

Electron production in low pressure gas ionized by an intense proton beam

Electron density measurements from previous ion-beam-induced gas ionization experiments [F. C. Young et al., Phys. Plasmas 1, 1700 (1994)] are re-analyzed and compared with a recent theoretical model [B. V. Oliver et al., Phys. Plasmas 3, 3267 (1996)]. Ionization is produced by a 1 MeV, 3.5 kA, 55 ns pulse-duration, proton beam, injected into He, Ne, or Ar gas in the 1 Torr pressure regime. Theoretical and numerical analysis indicates that, after an initial electron population is produced by ion beam impact, ionization is dominated by the background plasma electrons and is proportional to the beam stopping power. The predicted electron density agrees with the measured electron densities within the factor of 2 uncertainty in the measurement. However, in the case of Ar, the theoretically predicted electron densities are systematically greater than the measured values. The assumptions of a Maxwellian distribution for the background electrons and neglect of beam energy loss to discrete excitation and inner sh...

Equilibria for intense ion beam transport in low-pressure gas or vacuum

Two fluid equilibrium solutions for intense ion beam transport in low-pressure gas or vacuum are derived. The equilibria that are most relevant to beam transport have neutralizing electrons drifting in the same direction as the beam. These solutions require a small net positive charge within the beam channel to support an equilibrium radial electric field to allow the electrons to E×B drift axially. At the extremes of the domain of allowable solutions this electric field approaches zero and complete charge neutrality is achieved. In this case, two solutions are obtained. The first describes ballistic beam transport with complete neutralization of the beam current by the electrons, and the second describes pinched beam transport with no neutralizing electron current. Equilibria between these two extremes exhibit both a small net positive charge within the beam channel and partial current neutralization.

Vlasov model for the impedance of a rod-pinch diode

The rod-pinch diode [(R.A. Mahaffrey et al., 1978), (G. Cooperstein et al., 2001)] is a cylindrical, pinched-beam diode being developed as a radiography source [R.J. Commisso et al., 2002]. The diode consists of a small radius anode rod extending through the hole of an annular cathode. The diode has been operated at 1 to 5 MV with an impedance of 20 to 50 /spl Omega/, a FWHM pulse width of 20 to 50 ns, and an anode radius as small as 0.25 cm [(R.A. Mahaffrey et al., 1978), (G. Cooperstein et al., 2001), (R.J. Commisso et al., 2002)]. The diode is designed to run at critical current so that electrons emitted from the cathode flow axially along the anode rod and pinch radially onto the rod tip. Typically, ion emission from the anode is required for propagation of the pinch along the rod. Without ions, the pinch would occur on the anode rod just downstream of the cathode disk. In order to assure that a given diode will be properly designed to run at critical current requires a detailed knowledge of the diode impedance characteristics. Initially, a laminar flow model [B.V. Oliver et al., 2001] was developed to describe the rod-pinch diode. Although this model provides considerable insight into diode operation, PIC simulations show that the electron flow is not laminar [G. Cooperstein et al., 2001]. The model of [B.V. Oliver et al., 2001] was extended to include transverse electron pressure in order to consider the effects of nonlaminar flow [P.F. Ottinger et al., 2002]. However, a form for the transverse pressure tensor is required to close the equation set in this model and only special forms of the pressure tensor are analytically tractable. Here, a Vlasov model for the diode electron flow is developed using an electron distribution function with properties that are well characterized and directly related to a rod-pinch diode. In this model, the pressure tensor is self-consistently derived.

Operation of an extraction, applied-B diode using an externally-driven, metallic foil anode plasma source

Summary form only given. We are developing an extraction, applied-B ion diode, on the Gamble II generator at NRL, for ion-beam-transport research in support of the SNL light-ion ICF program. An ion beam with a voltage above 1 MV and a proton current of 150-200 kA is required for transport experiments. At present we are using hardware which allows a maximum anode area of /spl sim/60 cm/sup 2/. These parameters result in enhancement factors 2-3 times greater than those in similar experiments at Cornell, SNL, and KfK. In addition, the early, high-impedance phase of the diode must be minimized to prevent insulator flashover. Transport experiments with beam focusing also preclude ion beam angular momentum. A version of the EMFAP source, developed at Cornell and improved at KfK is used to provide prompt turn-on of the ion current. This source comprises a thin metallic foil on an insulating substrate. Driving current through this foil results in rapid heating, gas desorption and breakdown and thus a uniform plasma conformal to the anode surface. An external pulser is used to drive current through the foil. The applied magnetic field is calculated using the ATHETA code. Recently, these calculations have been refined by direct measurement of rA/sub 8/. To date we have obtained ion beams with rapid turn-on, high currents and current densities (350 kA at 6 kA/cm/sup 2/), high ion efficiencies (80%).

MHD and EMH effects in POS gap opening

Summary form only given. Conduction and opening processes of plasma opening switches (POS) are examined with the aid of numerical simulations. In a POS, a plasma initially bridges a small axial region of the transmission line of an inductive energy storage generator. A section of vacuum transmission line then connects the POS region to a load. The plasma initially carries the generator current, allowing no current to flow to the load. Modifications to the plasma or to the current distribution in the plasma eventually open the POS, allowing power to flow to the load. These modifications can occur by a variety of different mechanisms: by plasma deformation and displacement, by magnetic field transport, or by a combination of such effects. Deformation and displacement are carried out by J/spl times/B forces and by electrostatic forces (ion erosion) associated with electron vortices. Magnetic field penetration can occur by electron-magneto-hydrodynamic (EMH) effects or resistive diffusion. Both particle-in-cell (PIC) and MHD codes are used to demonstrate many of these effects. In these simulations, a wide range of plasma densities (n/sub e/=5/spl times/10/sup 12/-5/spl times/10/sup 15/ cm/sup -3/) are examined.

Operational window for light-ion-beam transport for LMF

Summary form only given, as follows. The proposed Laboratory Microfusion Facility (LMF) will require /spl ges/10 MJ of 30 MeV lithium ions to be transported and focused onto high-gain, high-yield inertial confinement fusion (ICF) targets. The light-ion LMF approach uses a multimodular system with individual ion extraction diodes as beam sources. Several transport schemes are being considered to deliver the individual ion beams to the centrally located ICF target. Given a set of parameters associated with each of the transport schemes, constraints on transportable ion beam power are examined to define an operational window in parameter space. Beam-driven instabilities, plasma hydrodynamics, beam energy losses during transport, and beam transport efficiency are considered for each transport scheme. System parameters include time-of-flight bunching of the beams, diode radius, beam microdivergence, number of modules, background gas species and pressure, etc. Results will be presented for the baseline transport scheme, ballistic transport with solenoidal lens focusing. Preliminary results for other transport schemes will also be shown.

Modeling of the conductivity of a plasma created by beam-induced ionization of a gas

Summary form only given, as follows. The conductivity of a plasma created by ion-beam induced ionization of a gas has been analyzed. For the purpose of inertial confinement fusion driven by high current ion beams, it is expected that charge and current neutralization of the beam can be achieved by propagating the beam through a low pressure background gas. In recent experiments conducted at the Naval Research Laboratory, the net currents produced by the transport of intense (1 MeV, 1 kA/cm/sup 2/) proton beams through various gases in the 0.25 to 4.0 Torr range were measured. B-dot probes located at three positions along the propagation axis and placed outside of the beam envelope measured net currents of 2% to 8% of the injected beam current and indicated plasma return current decay times of hundreds of nanoseconds. Interferometric measurements indicate ionization fractions within the beam region of up to 5% of the background gas density. The plasma response is modeled according to the measured temporal evolution of the return currents. Timescales are such that significant plasma ion motion does not occur and therefore the plasma is considered to be a resistive electron fluid with an immobile ion component. The 2-1/2 dimensional (axisymmetric) hybrid fluid/particle code SOLENZ is used to aid the analysis. Preliminary results suggest that the return current decay can be well described by a classical Spitzer resistivity but that the conducting wall boundaries play an important role in the net current characteristics of the beam/plasma system. A model for the early time development of the plasma conductivity is being considered.