Ion Diode Research Articles

Visible-light spectroscopy of pulsed-power plasmas (invited)

We describe the investigations of the plasma behavior in three pulsed-power systems: a magnetically insulated ion diode, and plasma opening switch, and a gas-puffed Z pinch. Recently developed spectroscopic diagnostic techniques allow for measurements with relatively high spectral, temporal, and spatial resolutions. The particle velocity and density distributions within a few tens of microns from the dielectric-anode surface are observed using laser spectroscopy. Fluctuating electric fields in the plasma are inferred from anisotropic Stark broadening. For the plasma opening switch experiment, a novel gaseous plasma source was developed which is mounted inside the high-voltage inner conductor. The properties of this source, together with spectroscopic observations of the electron density and particle velocities of the injected plasma, are described. Emission line intensities during the switch operation are discussed. In the Z-pinch experiment, spectral emission-line profiles of various charge-state ions are studied during the implosion phase. Radial velocity distributions are observed from the line Doppler shifts and widths.

Analyzing noisy spectral line shapes

In many situations it is necessary to analyze spectral line shapes where the data contain significant amounts of noise or statistical fluctuations. Our visible spectroscopy measurements exploring ion diode physics on the PBFA II accelerator typically result in noisy spectra because the harsh environment limits the number of photons collected. The spectral line profiles include contributions from Doppler (Ti∼1–3 keV), Stark (ne∼1017 cm−3), and instrument broadening, as well as from Stark shifting (E∼3–10 MV/cm) and Zeeman splitting (B∼2–10 T). We extract a range of parameters (e.g., ion temperature from Doppler broadening) that fit the data by determining a range of fits that are consistent with the uncertainty due to the noise in the data. The range of fits is generated by a Monte Carlo technique. This method effectively distinguishes between actual spectral features and artifacts due to noise. It provides not only estimates of physical parameters, but also their uncertainties. We evaluated the technique over a range of signal-to-noise ratios and found that it works well for our application.

Treatment of edge beams in a focusing self-magnetically Bθ-insulated ion diode

When focusing light ions with intense light ion-beam diodes, problems occur with ions emitted at the edge of the anode plasma. Numerical studies investigating this well-known experimental observation are presented. Going beyond the computational simulations introduced by Westermann (1991 Appl. Phys. Lett. 58, 696), two suggestions for the case of the self-magnetically Bθ-insulated ion diode are presented to focus the beamlets emitted from outer parts of the anode. The first one is based upon the idea of changing nonemitting parts of the anode surface and the second one of shifting the emitting anode surface into the anode body.

Photocathode transfer and storage techniques using alkali vapor feedback control

Photocathodes of quantum efficiency (QE) above 1% at the doubled YAG frequency of 532 nm are very sensitive to the local vacuum environment. These cathodes must have a band gap of less than 2.3 eV, and a work function that is also on the order of ∼ 2V or less. As such, these surfaces are very reactive because they provide many surface states for the residual gases that have positive electron affinities such as oxygen and water. In addition to this problem it is found that the optimal operating point for some of these cesium based cathodes is unstable. Three of the cesium series were tried, the CsAgBiO, the Cs3Sb and the K2CsSb. The most stable material found is the K2CsSb. The required vacuum conditions can be met by a variety of pumping schemes such as using sputter ion diode pumps and baking at 250°C or less for whatever time is required to reduce the pump currents to below 1 μA at room temperature. To obtain the required partial pressure of cesium, a simple, very sensitive, diagnostic gauge has been developed that can discriminate between free alkali atoms and other gases. This Pressure Alkali Monitor (PAM) can be used with cesium sources to provide a low partial pressure using standard feedback techniques. Photocathodes of arbitrary composition have been transferred to a separate vaccuum system and preserved for over 10 days with less than a 25% loss to the QE at 543.5 nm.

Experimental studies of a microsecond plasma opening switch in the positive polarity regime with inductive load/extraction ion diode

Systematic studies of the microsecond plasma opening switch (MPOS) operation in the positive polarity of its inner electrode with an inductive load/B-applied ion diode of the extraction type at a level of 0.3 TW of dissipated power were performed at the DOUBLE generator (300 kA, 480 kV, 1 μs). The detailed measurements of ion flow parameters in the conductive phase of the MPOS showed the considerable enhancement of the ion current amplitude over the thermal flow limit (3–10 times) which is coupled with a significant decrease of electron conductivity in the MPOS across its self-magnetic field. The positive polarity MPOS operation proved to be more critical to the stored current amplitudes and geometry of the electrodes in comparison with the negative polarity case. This fact resulted in limitations of satisfactory performance of the MPOS involving short high-voltage pulse duration, low stored current amplitudes, and a narrow region of acceptable electrode diameters. The variation of the diode anode-cathode (AC) gap provided a sensitive control of the MPOS + magnetically insulated diode (MID) system, which displayed very strong coupling, resulting in clamping of the output voltage in a wide region of diode impedances. The early long-duration (<300 ns) high-voltage (50–200 kV) prepulse improves plasma production at the anode of the MID prior to the application of the main pulse. The optimal performance of the MPOS+MID system was realized at the level of ZMPOS/ZMID = 2.5. The energy of the extracted high-power ion beam made up 3.5 kJ, its power being 120 GW with 40% efficiency of energy transfer from MPOS to the MID.

Production and investigation of TW proton beams from an annular diode using strong radial magnetic insulation fields and a preformed anode plasma source

A magnetically insulated proton diode in extraction geometry which has achieved an ion beam power of 0.8 TW and an ion production efficiency of greater than 90% is described. Using a Pd-Ti metal hydride film on the anode surface and diverting part of the generator current through it with the help of plasma opening switches, a proton-rich plasma is preformed on the anode before any voltage occurs across the diode gap. Thus it is possible to apply very strong magnetic insulation, V/sub c//V/sub op/>3. Ion current and diode voltage simultaneously reach a plateau for 40 ns, resulting in a nearly constant diode impedance for 60 ns. Thus, chromatic aberrations of the diode focusing system are minimized. The microscopic divergence of proton beamlets due to diode effects is determined to be 0.7 degrees . Geometric aberrations of spherical and aspherical anode shape have been determined. The experimental results are compared to simulations with a 2.5 dimensional stationary PIC (particle-incell) code.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Generation and Focusing of Intense Ion Beams with an Inverse Pinch Ion Diode

Generation and focusing of ion beams using an inverse pinch ion diode with a flat anode has been studied. The ion beams generated with the inverse pinch ion diode were found to be focused at 120 mm from the anode by the electrostatic field in the diode. The energy and maximum current density of the ion beams were 180 keV and 420 A/cm2, respectively. The focusing angle of the ion beams was 4.3°. The beam brightness was estimated to be 1.3 GW/cm2·rad2. The focusing distance of the ion beams was found to be controllable by changing the diameters of the anode and cathode.

Transport of intense light ion beams

Theoretical and experimental research concerning two techniques for transporting intense light ion beams is described. The first technique uses the magnetic field associated with a wall-stabilized z-discharge to radially confine and guide the beam, and the second technique uses the magnetic field from a central current-carrying wire. The ion beam for the experiments is generated and weakly focused onto the aperture of the transport system using a pinch-reflex ion diode on the Naval Research Laboratory Gamble II generator. Typically 1.2-MeV, 100-kA proton beams are transported within radii as small as 1 cm. High-efficiency transport of beams has been demonstrated for both techniques over distances of a few metes. Beam charge and current neutralization were also confirmed. Nuclear diagnostics and shadowboxes were used to measure beam transport efficiency and phase space information for comparison with theoretical predictions. Particle transport efficiencies as high as 100% for the z-discharge technique and 80% for the wire-guided technique were observed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

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

Recent advances in the analytic theory and numerical simulation of applied-B ion diodes in general and, in particular, the diodes being tested experimentally on the Particle Beam Fusion Accelerator II at Sandia National Laboratories, Albuquerque, NM, are described. Numerical simulation of these diodes using the three-dimensional particle-in-cell code QUICKSILVER and analytic stability analysis have provided new insight into the physics governing instability-induced beam divergence. Experimental evidence of these instabilities is described and compared with the theory. When simulation parameters are chosen to model present experiments, QUICKSILVER calculates beam divergence near the measured value. A new understanding of the nature of the electromagnetic instabilities and their effects on beam divergence has led to suggestions for improved diode design. A simulation of one of these design concepts shows divergence levels in the vicinity of 10 mrad, a level considered adequate for ignition experiments.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The effect of a virtual cathode on the electromagnetic stability of high-power ion diodes

Previous stability analyses have shown magnetically insulated ion diode equilibria to be unstable. However, important equilibrium features such as a virtual-cathode and a charge–neutral region were not included. A stability analysis including these features is presented, which indicates that the stability behavior is strongly affected by the equilibrium model. In particular, the diocotron instability displaces the ion resonance instability found in the previous analyses. The calculated growth rates for the diocotron mode are consistent with a fast growing mode that has been observed in recently reported three-dimensional numerical simulations. These simulations exhibit a transition from the diocotron to an ion resonance mode and a subsequent increase in ion divergence due to the much lower frequency of the ion mode. The stability analysis presented in this paper demonstrates how the evolution of the electron sheath reduces the diocotron growth rate resulting in unstable ion modes, thus explaining the transition to ion modes observed in the simulations.

Advances in the Engineering of High Field Applied-B Ion Diodes for Inertial Confinement Fusion

Pulsed high field magnet coils are an integral part of the applied-B ion diode used in the light ion Inertial Confinement Fusion program at Sandia National Laboratories. Several factors have contri...

Performance of two helium ion diodes

Two magnetically insulated gas-puff diodes were tested. In one design the plasma source was a fast inductive coil; in the other a coaxial gun created the plasma. The plasma current density from both sources, as well as the accelerated beam from each, was comparable to the Child–Langmuir limit of ∼10 A/cm2.

Evolution and control of ion-beam divergence in applied-B diodes.

The time evolution of beam divergence induced by electomagnetic instabilities in applied-B ion diodes is examined using the 3D particle-in-cell code quicksilver. The evolution is generally characterized by a high-frequency, low-divergence phase, associated with the diocotron mode, followed by a low-frequency, high-divergence phase, associated with the two-stream-like ion mode. Limiting the extent of the electron density profile evolution allows the diocotron phase to be sustained with a resulting divergence of \ensuremath{\approxeq}10 mrad.

High-purity ion beam production at high current densities with a liquid-helium-cooled series-field-coil extraction ion diode

Experiments are described in which a high-purity, high-power (0.15 TW, 1 MeV) proton beam is generated from an ion source consisting of H2 gas frozen onto a liquid-helium-cooled copper anode at 4.2 K in a series-field-coil extraction diode on the 0.7 TW HydraMITE-II accelerator. Peak anode proton current densities of 2 kA/cm2 were measured. This current density is a factor of 100 higher than those obtained in previous liquid-helium-cooled cryogenic diode experiments on small accelerators and is in the range required for high-power ion beam applications. Thomson parabola, Faraday cup, and carbon activation measurements indicate an ion beam proton fraction close to 100% for the cryogenic source, compared to 50–70% for the standard hydrocarbon anode tested. The cryogenic proton source is believed to consist of no more than a few monolayers of molecular hydrogen. The hydrogen-coated cryogenic anode shows a faster initial anode turn-on than other materials. However, source-limited emission from the thin hydrogen layer results in a somewhat longer current risetime, reduced ion diode efficiency, lower proton current enhancement over the Child–Langmuir limit, and a proton spectrum of lower average energy than for the hydrocarbon anode. Techniques to overcome these limitations are discussed. Cryogenic ion sources consisting of frozen N2, CH4, and Ne have also been studied. In each case, high intensity beams consisting predominantly of components of the refrigerated gas were produced.

Simulating electron flow in the applied-B diode

Electron flow in and into an applied-B ion diode is calculated, including the effects of turbulence produced by streaming instabilities. A technique is developed for including these effects in a numerical simulation. The results of two-dimensional simulations using this technique in the electromagnetic particle-in-cell code magic are presented.

Fast magnetic-field penetration into plasmas due to the Hall field

The enhancement of magnetic-field penetration into short-duration plasmas by the dissipationless Hall field is examined. Magnetic-field penetration along a background magnetic field is focused on, where the inductive Hall electric field enables the magnetic field to penetrate as a whistler wave. It is shown that the magnetic-field evolution, when governed simultaneously by both whistler wave propagation and collisional diffusion, is described by a diffusion equation with a complex diffusion coefficient. The imaginary part of this coefficient is proportional to the Hall resistivity associated with the background magnetic field. In the collisionless limit the governing equation is equivalent to the Schrödinger equation for a free particle, and the magnetic field propagates the way a free-particle wave packet expands by dispersion rather than by diffusion. This study was motivated by the enhanced magnetic-field penetration recently observed in the anode plasma of a magnetically insulated ion diode.

A Light Ion Beam Driver for the Laboratory Microfusion Facility+

The Laboratory Microfusion Facility (LMF) is being planned to develop high-gain, high-yield (200 MJ-1000 MJ) ICF targets for applications to nuclear weapons effects simulation, thermonuclear weapons physics, and energy production. It is expected that a 1000-MJ yield will require ∼ 10–20 MJ input energy to the target. The light-ion beam driver concept for the LMF consists of 36 accelerator modules that drive independent Li+ ion diodes. Each ion beam is extracted from an annular ion diode and propagated to a solenoidal lens located near the wall of the target chamber. This magnetic lens focuses the beam on to the pellet located at the center of the target chamber. The temporal shape of the power pulse delivered to the target is controlled by the synchronized firing of the accelerator modules. This paper presents a status of the light-ion beam LMF driver concept.

The 3-D, Electromagnetic, Particle-In-Cell Code, QUICKSILVER

Charged-particle simulations in three dimensions are now performed routinely in the Pulsed Power Sciences Directorate at Sandia with the QUICKSILVER suite of codes. QUICKSILVER is a multitasked, finite-difference, three-dimensional, fully relativistic, electromagnetic, particle-in-cell code developed at Sandia. It is targeted for use on current and near-term supercomputers, such as the Cray Y-MP/864 or the Cray X-MP/416, which are characterized by large, shared central memories and multiple processors. QUICKSILVER has already been used to simulate ion diodes, magnetically insulated transmission lines, microwave devices, and electron beam propagation. Before describing the suite in detail, it is worthwhile to highlight some salient architectural features of the QUICKSILVER simulation code. All grid-related quantities (electromagnetic fields and current density, for example) are stored in central memory and particle information may be stored out of memory if there is not sufficient central memory. In order to minimize the use of central memory, large conductor volumes in which fields are always zero must not be stored. This necessitates constructing the simulation region from blocks'' logically connected along arbitrary planes. The overhead required to accomplish this block connection in the field-solving routines uses a very small percentage of the total processing time. After the grid-related arrays have beenmore » allocated in memory, additional central memory is used to cache particle information. When this cache is full, it is necessary to shuffle particle information in and out of the memory cache and onto disk or other out-of-memory storage. 8 refs.« less

Optimization by Numerical Simulation of the Focusing Properties of Self-Magnetically Insulated Ion Diodes

We present two-dimensional electromagnetic computer simulation results performed in order to investigate the focusing properties of self-magnetically insulated ion diodes. It has been shown computationally that in the case of the self-magnetically insulated bθ-diode the power density can be improved by a factor 10 by changing the anode surface.

Optimization by numerical simulation of the focusing properties of self-magnetically insulated ion diodes

We present two-dimensional electromagnetic computer simulation results performed in order to investigate the focusing properties of self-magnetically insulated ion diodes. A particle-in-cell code self-consistently computes the electromagnetic fields inside the anode-cathode gap and an optimization code changes the shape of the anode in order to improve the focusing behavior. It has been shown computationally that in the case of the self-magnetically Bθ-insulated diode the power density can be improved by a factor of 7 by changing the anode surface.