D. V. Rose

Efficient electron beam deposition in the gas cell of the Electra laser

Extensive research has been performed to elucidate the transport of electron beam energy from a vacuum diode, through a foil support structure (hibachi), and into the Electra laser cell. Measurements and simulations of the energy deposition in the cell are reported for various krypton/argon mixtures, gas pressures, and the thickness and material of the hibachi foil. Two hibachi and several cathode configurations are investigated and electron energy deposition efficiencies into the gas of up to 75% have been achieved with a 500 kV, 180 ns full width at half maximum diode pulse. The experimental data are compared with one-, two-, and three-dimensional Monte Carlo transport calculations and particle-in-cell simulations. The importance of electron backscattering, radiation effects, and power deposition uniformity in the laser gas are discussed.

Ballistic transport and solenoidal focusing of intense ion beams for inertial confinement fusion

Light‐ion inertial confinement fusion requires beam transport over distances of a few meters for isolation of the diode hardware from the target explosion and for power compression by time‐of‐flight bunching. This paper evaluates ballistic transport of light‐ion beams focused by a solenoidal lens. The ion beam is produced by an annular magnetically insulated diode and is extracted parallel to the axis by appropriate shaping of the anode surface. The beam propagates from the diode to the solenoidal lens in a field‐free drift region. The lens alters the ion trajectories such that the beam ballistically focuses onto a target while propagating in a second field‐free region between the lens and the target. Ion orbits are studied to determine the transport efficiency ηt (i.e., the fraction of the beam emitted from the diode which hits the target) under various conditions relevant to light‐ion inertial confinement fusion. Analytic results are given for a sharp boundary, finite thickness solenoidal lens configura...

Self-pinched transport of an intense proton beam

Ion beam self-pinched transport (SPT) experiments have been carried out using a 1.1-MeV, 100-kA proton beam. A Rutherford scattering diagnostic and a LiF nuclear activation diagnostic measured the number of protons within a 5 cm radius at 50 cm into the transport region that was filled with low-pressure helium. Time-integrated signals from both diagnostics indicate self-pinching of the ion beam in a helium pressure window between 35 and 80 mTorr. Signals from these two diagnostics are consistent with ballistic transport at pressures above and below this SPT pressure window. Interferometric measurements of electron densities during beam injection into vacuum are consistent with ballistic transport with co-moving electrons. Interferometric measurements for beam injection into helium show that the electron density increases quadratically with pressure through the SPT window and roughly linearly with pressure above the SPT window. The ionization fraction of the helium plateaus at about 1.5% for pressures abov...

Numerical simulations of self-pinched transport of intense ion beams in low-pressure gases

The self-pinched transport of intense ion beams in low-pressure background gases is studied using numerical simulations and theoreticalanalysis. The simulations are carried out in a parameter regime that is similar to proton beam experiments being fielded on the Gamble II pulsed power generator [J. D. Shipman, Jr., IEEE Trans. Nucl. Sci. NS-18, 243 (1971)] at the Naval Research Laboratory. Simulation parameter variations provide information on scaling with background gas species, gas pressure, beam current, beam energy, injection angles, and boundaries. The simulation results compare well with simple analytic scaling arguments for the gas pressure at which the effective net current should peak and with estimates for the required confinement current. The analysis indicates that the self-pinched transport of intense proton beams produced on Gamble II (1.5 MeV, 100 kA, 3 cm radius) is expected to occur at gas pressures between 30 and 80 mTorr of He or between 3 and 10 mTorr of Ar. The significance of these results to ion-driven inertial confinement fusion is discussed.

Z-discharge transport of intense ion beams for inertial confinement fusion

Ion inertial confinement fusion requires beam transport over distances of a few meters for isolation of the diode hardware from the target explosion and for power compression by time‐of‐flight bunching. This paper evaluates light ion beam transport in a wall‐stabilized z‐discharge channel, where the discharge azimuthal magnetic field radially confines the ion beam. The ion beam is focused onto the entrance aperture of the transport channel by shaping the diode to achieve beam convergence in a field‐free drift region separating the diode from the transport section. Ion orbits are studied to determine the injection efficiency (i.e., the fraction of the beam emitted from the diode which is transported) under various conditions. Ions that are focused onto the channel entrance at too large of an angle for confinement hit the wall and are lost. For a multimodular scheme (10–30 beams), individual transport channels are packed around the target with the exit apertures at some standoff distance from it. The fraction of the beam that is lost in this field‐free standoff region is also evaluated under various conditions. The standoff efficiency is then combined with the injection efficiency to give the dependence of the total transport efficiency ηt on diode, focusing, transport and standoff parameters. It is found that ηt can be in the range of 75%–100% for parameter values that appear to be achievable.Ion inertial confinement fusion requires beam transport over distances of a few meters for isolation of the diode hardware from the target explosion and for power compression by time‐of‐flight bunching. This paper evaluates light ion beam transport in a wall‐stabilized z‐discharge channel, where the discharge azimuthal magnetic field radially confines the ion beam. The ion beam is focused onto the entrance aperture of the transport channel by shaping the diode to achieve beam convergence in a field‐free drift region separating the diode from the transport section. Ion orbits are studied to determine the injection efficiency (i.e., the fraction of the beam emitted from the diode which is transported) under various conditions. Ions that are focused onto the channel entrance at too large of an angle for confinement hit the wall and are lost. For a multimodular scheme (10–30 beams), individual transport channels are packed around the target with the exit apertures at some standoff distance from it. The fracti...

Space-charge limited currents in coaxial diodes with electron backscatter

The effect of backscattered electrons on space-charge limited currents of cylindrical (coaxial) diodes with anode center conductors is studied. The scattered electrons are parametrized by a fractional current density α and fractional energy β of the incident electrons. For bipolar flow, current enhancement factors of 2.5 are calculated for α, β≃0.5. Comparison of the model equations to one-dimensional particle-in-cell simulations with fully integrated Monte Carlo scattering algorithms demonstrates very good agreement for a range of energies and anode materials. In the absence of backscattering, the bipolar diode impedance decreases for increasing ratio of cathode to anode radius rc/ra for ratios greater than about 20.

Numerical modeling of large-area electron-beam diodes for KrF lasers

Particle-in-cell simulations are used to model large-area, electron-beam diodes that pump krypton–fluoride (KrF) laser cells. The simulations include models for following the energy loss and scattering of beam electrons in foils, gas, and support structures. Estimates of energy deposition to the various components of the diode system are obtained and compared with available experimental data. The simulations are in very good agreement with the Faraday cup measurements of electron-beam transport in the KrF gas. Additionally, global energy deposited in the KrF gas predicted from the simulations is in good agreement with experimental measurements for several different diode configurations. The results from this work benchmark and establish the computational procedure as an important tool from which future KrF laser systems can be developed.

Effect of time-of-flight bunching on efficiency of light-ion-beam inertial-confinement-fusion transport schemes

The Laboratory Microfusion Facility (LMF) has been proposed for the study of high‐gain, high‐yield inertial‐confinement‐fusion targets. The light‐ion LMF approach uses a multimodular system with applied‐B extraction diodes as ion sources. A number of ion‐beam transport and focusing schemes are being considered to deliver the beams from the diodes to the target. These include ballistic transport with solenoidal lens focusing, z‐discharge channel transport, and wire‐guided transport. The energy transport efficiency ηt has been defined and calculated as a function of various system parameters so that point designs can be developed for each scheme. The analysis takes into account target requirements and realistic constraints on diode operation, beam transport, and packing. The effect on ηt of voltage ramping for time‐of‐flight beam bunching during transport is considered here. Although only 5 mrad microdivergence calculations are presented here, results for bunching factors of ≤3 show that transport efficienc...

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.