Ion Diode Research Articles

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/cm2 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-P0 correlation between variations in the beam energy and transverse momentum. The instability Played a dominant role in the limitation of the focused lithium power.

A filtration method for improving film dosimetry in photon radiation therapy.

Successful radiotherapy requires accurate dosimetry for treatment verification. Existing dosimeters such as ion chambers, TLD, and diodes have drawbacks such as relatively long measurement time and poor spatial resolution. These disadvantages become serious problems for dynamic-wedged beams. Thus the clinical use of dynamic wedges requires an improved dosimetry method. X-ray film may serve this purpose. However, x-ray film is not clinically accepted as a dosimeter for photon beams, because it overresponds to photons with energies below about 400 keV. This paper presents and develops a method which was initially proposed by Burch to improve the dose response of x-ray film in a phantom. The method is based on placing high-atomic number foils next to the film. The foils are used as filters to preferentially remove low-energy photons. The optimal film and filter configuration in a phantom was determined using a mathematical scheme derived in this study and a Monte Carlo technique (ITS code). The optimal configuration thus determined is as follows: the filter-to-film distance of 6 mm and the filter thickness of 0.15 mm for percent depth-dose measurement; the distance of 1 cm and the thickness of 0.25 mm for off-axis (dose) ratio measurement. The configuration was then tested with photon beams from a 4 MV linac. The test result indicates that the in-phantom dose distribution based on the optimal configuration agrees well with those measured by ion chambers.

New physical possibilities in compact neutron sources

Some non-traditional physical approaches to increase the yield parameters of compact impulse neutron sources based on (d,t) or (d,d) nuclear reactions are considered. For obtaining a high level of accelerating fields and intense ion current in a small accelerating volume, the effective magnetic suppression of the electronic component in an acceleration gap (ion diode with magnetic isolation), the collective acceleration of deuteron ions from laser plasma, and the acceleration of plasmoids are studied. Mainly these approaches have been realized by using the laser irradiation to create a plasma in an ion source. Experimental results and numerical calculations point out that utilizing these methods can result, using a campact laboratory apparatus (e.g. based on sealed neutron tubes), in a yield of 10 8–10 10 n/pulse, pulse duration 10 −8–10 −6 s, pulse rate 10–10 3 Hz and lifetime about 10 7 shots.

Deposition of carbon nitride films by vacuum ion diode with explosive emission

Carbon nitride films were synthesized using a novel technique based on the pulsed high voltage ion/electron diode with explosive emission (pulsed voltage 200–700 kV pulsed current 100–500 A cm −2 (ions) 150–2000 A cm −2 (electrons)). The method and its novel features are discussed as well as its application to the formation of the crystalline β-phase in C 3N 4 films. Mixed elemental nitrogen and carbon films are formed by sequential deposition then subjected to ion and/or electron beam mixing to synthesize the C 3N 4 structure. The experimental conditions used for this pulsed process are described and the efficiency of the method for nitrogen incorporation is demonstrated. The results presented indicate that β-C 3N 4 crystallites are formed in an amorphous matrix.

Analytical methods for the determination of urinary 2,4-dichlorophenoxyacetic acid and 2-methyl-4-chlorophenoxyacetic acid in occupationally exposed subjects and in the general population.

Two methods for the quantitative analysis of 2,4-dichlorophenoxyacetic acid (2,4-D) and 2-methyl-4-chlorophenoxyacetic acid (MCPA) in urine were compared. The first was an high-performance liquid chromatography method using a C8 column with ion suppression and diode array detection. The urine extracts were first purified by solid-phase extraction (SPE) on silica capillary columns. The detection limit of the method was 15 micrograms/L for both compounds. The percentage coefficient of variation of the whole analysis evaluated at a concentration of 125.0 micrograms/L was 6.2% for 2,4-D and 6.8% for MCPA. The mean recovery of analysis was 81% for 2,4-D and 85% for MCPA. The second was a gas chromatographic (GC) method in which the compounds were first derivatized with pentafluorobenzylbromide to pentafluorobenzyl esters, which were determined with a slightly polar capillary column and electron capture detection. Before GC analysis, the urine extracts were purified by SPE on silica capillary columns. This method had a detection limit of 1 microgram/L for both compounds and a percentage coefficient of variation of the whole analysis, evaluated at a concentration of 30.0 micrograms/L, of 8% for 2,4-D, and of 5.5% for MCPA. the mean recovery was 87% for 2,4-D and 94% for MCPA. The low detection limit made the second method suitable for assaying the two herbicides in the general population. Duplicate analysis of ten urine samples from occupationally exposed subjects by the two methods gave identical results for a wide range of concentrations.

Use of ion chromatography for monitoring microbial spoilage in the fruit juice industry

Fruit juices and purees are defined as fermentable, but unfermented, products obtained by mechanical processing of fresh fruits. The presence of undesired metabolites derived from microbial growth can arise from the use of unsuitable fruit or from defects in the production line or subsequent contamination. This involves a loss in the overall quality that cannot be resolved by thermal treatment following the start of fermentation. With these considerations, together with microbiological control, the analysis of different metabolites, which can be considered as microbial growth markers, such as alcohols (i.e. ethanol, etc.), acids (i.e. acetic, fumaric, lactic, etc.). is fundamental in order to achieve a better evaluation of product quality. Enzymatic determination and other single-component analytical techniques are often used for the determination of these metabolites. When the microbial spoilage is not well known, this results in a long and cumbersome procedure. A versatile technique that is capable of determining many metabolites in one analysis could be helpful in improving routine quality control. For this purpose, an ion chromatographic technique, such as ion exclusion, for separation, and diode array spectrophotometry and conductivity, for detection, were evaluated. Both different industrial samples and inoculated samples were analyzed.

Results of vacuum cleaning techniques on the performance of LiF field-threshold ion sources on extraction applied-B ion diodes at 1-10 TW

Uncontrolled plasma formation on electrode surfaces limits performance in a wide variety of pulsed power devices such as electron and ion diodes, transmission lines, radio frequency (RF) cavities, and microwave devices. Surface and bulk contaminants on the electrodes in vacuum dominate the composition of these plasmas, formed through processes such as stimulated and thermal desorption followed by ionization. We are applying RF discharge cleaning, anode heating, cathode cooling, and substrate surface coatings to the control of the effects of these plasmas in the particular case of applied-B ion diodes on the SABRE (1 TW) and PBFA-X (30 TW) accelerators. Evidence shows that our LiF ion source provides a 200-700 A/cm/sup 2/ lithium beam for 10-20 ns which is then replaced by a contaminant beam of protons and carbon. Other ion sources show similar behavior. Our electrode surface and substrate cleaning techniques reduce beam contamination, anode and cathode plasma formation, delay impedance collapse, and increase lithium energy, power, and production efficiency. Theoretical and simulation models of electron-stimulated and thermal-contaminant desorption leading to anode plasma formation show agreement with many features from experiment. Decrease of the diode electron loss by changing the shape and magnitude of the insulating magnetic field profiles increases the lithium output and changes the diode response to cleaning. We also show that the LiF films are permeable, allowing substrate contaminants to affect diode behavior. Substrate coatings of Ta and Au underneath the LiF film allow some measure of control of substrate contaminants, and provide direct evidence for thermal desorption. We have increased lithium current density by a factor of four and lithium energy by a factor of five through a combination of in situ surface and substrate cleaning, substrate coatings, and field profile modifications.

Quantitative cleaning characterization of a lithium-fluoride ion diode

An ion source cleaning testbed was created to test plasma-cleaning techniques, and to provide quantitative data on plasma-cleaning protocols prior to implementation on the SABRE accelerator. The testbed was designed to resolve issues regarding the quantity of contaminants absorbed by the anode source (LiF), and the best cleaning methodology. A test chamber was devised containing a duplicate of the SABRE diode. Radio-frequency (RF) power was fed to the anode, which was isolated from ground and thus served as the plasma discharge electrode. RF plasma discharges in 1-3 mtorr of Ar with 10% O/sub 2/ were found to provide the best cleaning of the LiF surface. X-ray photoelectron spectroscopy (XPS) showed that the LiF could accrue dozens of monolayers of carbon just by sitting in a 2/spl times/10/sup -5/ vacuum for 24 h. Tests of various discharge cleaning protocols indicated that 15 min of an Ar/O/sub 2/ discharge was sufficient to reduce this initial 13-45 monolayers of carbon impurities to 2-4 monolayers. Rapid recontamination of the LiF was also observed. Up to ten monolayers of carbon returned in 2 min after termination of the plasma discharge and subsequent pumping back to the 10/sup -5/ torr range. Heating of the LiF also was found to provide anode cleaning. Application of heating combined with plasma cleaning provided the highest cleaning rates.

Pulsed high current ion beam processing equipment

A pulsed high-voltage ion source is considered for use in ion beam processing for the surface modification of materials, and deposition of conducting films on different substrates. The source consists of a Arkad'ev-Marx high voltage generator, a vacuum ion diode based on explosive ion emission, and a vacuum chamber as substrate holder. The ion diode allows conducting films to be deposited from metal or alloy sources, with ion beam mixing, onto substrates held at a pre-selected temperature. The main variables can be set in the ranges: voltage, 100–700 kV; pulse length, 0.3 us; beam current, 1–200 A, depending on the ion chosen. The applications of this technology are discussed in the context of semiconductor, superconductor and metallizing applications as well as the direction of future development and cost of these devices for commercial application.

Absorption spectroscopy characterization measurements of a laser-produced Na atomic beam

A pulsed Na atomic beam source developed for spectroscopic diagnosis of a high-power ion diode is described. The goal is to produce a ∼1012-cm−3-density Na atomic beam that can be injected into the diode acceleration gap to measure electric and magnetic fields from the Stark and Zeeman effects through laser-induced fluorescence or absorption spectroscopy. A ∼10 ns full width at half-maximum (FWHM), 1.06 μm, 0.6 J/cm2 laser incident through a glass slide heats a Na-bearing thin film, creating a plasma that generates a sodium vapor plume. A ∼1 μs FWHM dye laser beam tuned to 5890 Å is used for absorption measurement of the Na I resonant doublet by viewing parallel to the film surface. The dye laser light is coupled through a fiber to a spectrograph with a time-integrated charge-coupled-device camera. A two-dimensional mapping of the Na vapor density is obtained through absorption measurements at different spatial locations. Time-of-flight and Doppler broadening of the absorption with ∼0.1 Å spectral resolution indicate that the Na neutral vapor temperature is about 0.5–2 eV. Laser-induced fluorescence from ∼1×1012 cm−3 Na I 3s-3p lines observed with a streaked spectrograph provides a signal level sufficient for ∼±0.06 Å wavelength shift measurements in a mock-up of an ion diode experiment.

Intense ion beams for inertial confinement fusion

Intense beams of light and heavy ions are being studied as inertial confinement fusion (ICF) drivers for high yield and energy. Heavy and light ions have common interests in beam transport, targets, and alternative accelerators. Self-pinched transport is being jointly studied. This article reviews the development of intense ion beams for ICF. Light-ion drivers are highlighted because they are compact, modular, efficient and low cost. Issues facing light ions are: (1) decreasing beam divergence; (2) increasing beam brightness; and (3) demonstrating self-pinched transport. Applied-B ion diodes are favored because of efficiency, beam brightness, perceived scalability, achievable focal intensity, and multistage capability. A light-ion concept addressing these issues uses: (1) an injector divergence of /spl les/24 mrad at 9 MeV; (2) two-stage acceleration to reduce divergence to /spl les/12 mrad at 35 MeV; and (3) self-pinched transport accepting divergences up to 12 mrad. Substantial progress in ion-driven target physics and repetitive ion diode technology is also presented. Z-pinch drivers are being pursued as the shortest pulsed power path to target physics experiments and high-yield fusion. However, light ions remain the pulsed power ICF driver of choice for high-yield fusion energy applications that require driver standoff and repetitive operation.

Use of the BFCPIC and BFCRAY codes to describe space charge limited emission in electron guns for gyrotrons

Numerical modelling of an electron gun in the space charge limited regime requires determining the current density distribution as well as the electric fields and electron trajectories. This is a rather complicated self-consistent problem, since the space charge influences the electric field, which in turn influences the electron trajectories. Previous simulations of magnetron electron guns using the BFCPIC and BFCRAY codes used a simple emission model (constant current density) that is approximately valid for thermionic emission. The code has been modified to include space charge limited emission. Several different ways of doing this are considered. One of the models considered uses Gauss’s law to force the electric field on the emitter to vanish; it was used in the original version of BFCPIC for the simulation of ion diodes. A second is based on the use of Child’s law (locally), which may be more appropriate for extension to fully electromagnetic particle-in-cell (PIC) codes. Calculations were performed with both models, and the results compared with each other and with experiments performed at FZK.

High-accuracy time- and space-resolved Stark shift measurements (invited)

Stark-shift measurements using emission spectroscopy are a powerful tool for advancing understanding in many plasma physics experiments. We use simultaneous two-dimensional space- and time-resolved spectra to study the electric field evolution in the 20 TW Particle Beam Fusion Accelerator II ion diode acceleration gap. Fiber optic arrays transport light from the gap to remote streaked spectrographs operated in a multiplexed mode that enables recording time-resolved spectra from eight spatial locations on a single instrument. Design optimization and characterization measurements of the multiplexed spectrograph properties include the astigmatism, resolution, dispersion, and sensitivity. A semiautomated line-fitting procedure determines the Stark shift and the related uncertainties. Fields up to 10 MV/cm are measured with an accuracy ±2%–4%. Detailed tests of the procedure confirm that the uncertainty in the wavelength-shift error bars is less than ±20%. Development of an active spectroscopy probe technique that uses laser-induced fluorescence from an injected atomic beam to obtain three-dimensional space- and time-resolved measurements of the electric and magnetic fields is in progress.

Light ion beam driven inertial confinement fusion: Requirements and achievements

In this paper we compare the requirements for a light ion beam driven inertial confinement fusion (ICF) reactor with the present achievements in pulsed power technology, ion diode performance, beam transport, and target physics. The largest gap exists in beam quality and repetition rate capability of high-power ion diodes. Beam quality can very likely be improved to a level sufficient for driving a single-shot ignition facility, if the potential of two-stage acceleration is used. Present schemes for repetition rate ion diodes allow either too low power densities or create too large beam divergence. On the other hand, repetitively operating pulsed-power generators meeting the requirements for an ICF reactor driver can be built with present technology. Also, a rather mature target concept has been developed for indirect drive with light ion beams.

Basic and applied atomic spectroscopy in high-field ion diode acceleration gaps

Achieving inertial confinement fusion using a light-ion-beam driver requires continued improvement in understanding ion diode physics. The power delivered to a light-ion beam target is strongly influenced by the evolution of the charge-particle distributions across the ion beam acceleration gap. Our strategy is to determine this evolution from time- and space-resolved measurements of the electric field using Stark-shifted line emission. In addition to diode physics, the unique high-field (∼10 MV/cm, ∼6T) conditions in present experiments offer the possibility to advance basic atomic physics, for example by measuring field ionization rates for tightly bound low-principal-quantum-number levels. In fact, extension of atomic physics into the high-field regime is required for accurate interpretation of diode physics measurements. This paper describes progress in ion diode physics and basic atomic physics, obtained with visible-light atomic spectroscopy measurements in the ∼20 TW Particle Beam Fusion Accelerator II ion diode.

Very high rate coating deposition with intense ion and/or electron bombardment

The principles of a new method of high rate coating deposition are presented which are based on a pulsed ion diode with explosive emission. The equipment is flexible and can work in three regimes: pulsed ion beams, pulsed electron beams, and the deposition of films and coatings. Coatings can be deposited with ion beam mixing (coating and/or substrate), and these can be post treated with intense pulsed electron beams which allows the properties to be extensively modified. Experimental results are presented on a variety of coatings deposited on the characteristics of the diode and on different substrate materials at different temperatures; specific examples include high temperature superconductors, TiN, TiB 2 and diamond-like carbon films.

Charge-Exchange Atoms and Ion Source Divergence in a 20 TW Applied-B Ion Diode.

Space- and time-resolved spectroscopy is used to measure properties of several-keV Li atoms in a 20 TW ion diode. These measurements out in the diode anode-cathode gap are used in a charge-exchange model for the Li atom production in order to obtain the ${\mathrm{Li}}^{+}$ beam angular divergence within 50 \ensuremath{\mu}m of the LiF-coated anode surface. This ion divergence near the surface is surprisingly large, and accounts for about half the typical 25 mrad final accelerated-beam divergence. The measurements provide constraints for models attempting to explain highly diverging ion emission from thin alkali-halide films in $\ensuremath{\sim}10\mathrm{MV}/\mathrm{cm}$ applied fields.

Three-dimensional, particle-in-cell simulations of applied-B ion diodes on the particle beam fusion accelerator II

We have used the three-dimensional, particle-in-cell code QUICKSILVER [J. P. Quintenz, et al., Lasers and Particle Beams 12, 283 (1994)] to simulate radial applied-B ion diodes on the particle beam fusion accelerator II at Sandia National Laboratories. The simulations agree well with experiments early in the beam pulse, but differ substantially as the ion-beam current increases. This is attributed to the oversimplified ion emission model. We see the same instabilities seen in earlier simulations with idealized diode geometries; Early in time there is a diocotron instability, followed by a transition to an ‘‘ion mode’’ instability at much lower frequency. The instability-induced beam divergence for the ∼10 MeV beam during the diocotron phase is <10 mrad, significantly less than the total beam divergence in experiments early in the pulse, but increases to ≳25 mrad after the transition. The ion mode has a distinct harmonic structure along the applied field lines, making the instability transition sensitive to the diode geometry. The ion mode instability in our latest simulations is consistent with evidence of instabilities from recent experiments.

Ion divergence in magnetically insulated diodes

Magnetically insulated ion diodes are being developed to drive inertial confinement fusion. Ion beam microdivergence must be reduced to achieve the very high beam intensities required to achieve this goal. Three-dimensional particle-in-cell simulations [Phys. Rev. Lett. 67, 3094 (1991)] indicate that instability-induced fluctuations can produce significant ion divergence during acceleration. These simulations exhibit a fast growing mode early in time, which has been identified as the diocotron instability. The divergence generated by this mode is modest, due to the relatively high-frequency (≳1 GHz). Later, a low-frequency low-phase-velocity instability develops with a frequency that is approximately the reciprocal of the ion transit time. This instability couples effectively to the ions, and can generate unacceptably large ion divergences (≳30 mrad). Linear stability theory reveals that this mode has structure parallel to the applied magnetic field and is related to the modified two-stream instability. Measurements of ion density fluctuations and energy-momentum correlations have confirmed that instabilities develop in ion diodes and contribute to the ion divergence. In addition, spectroscopic measurements indicate that lithium ions have a significant transverse temperature very close to the emission surface. Passive thin-film lithium fluoride (LiF) anodes have larger transverse beam temperatures than laser-irradiated active sources. Calculations of the ion beam source divergence for the LiF film due to surface roughness and the possible loss of adhesion and fragmentation of this film are presented.

Gas breakdown effects in the generation and transport of light ion beams for fusion

The efficiency of delivering an ion beam to an inertial confinement fusion target depends on the ability to control the breakdown of both unintended (in the ‘‘vacuum’’ diode region) and intended (in the transport region) gas. The desorption and breakdown of anode-surface contaminants in an ion diode complicates the generation of a pure, high-brightness ion beam. Beyond the accelerator, the gas in the reactor vessel must provide excellent charge neutralization and specified current neutralization to permit the beam transport and focusing to a <1 cm radius, spherical target. Two schemes, in which controlling gas breakdown is essential, are ‘‘ballistic’’ and ‘‘self-pinched’’ ion transport. Results are discussed from hybrid particle-fluid simulations of anode contaminant desorption and ion beam transport.