Steven Falabella

Modeling plasma flow in straight and curved solenoids

The ‘‘flux‐tube’’ model originated by Morozov is a very simple and numerically efficient method for simulating ion motion in plasma filters. In order to test its utility as a design tool, we compare the predictions of the model to recent experimental measurements of plasma flow in both straight and curved solenoids.

Laser conditioning characterization and damage threshold prediction of hafnia/silica multilayer mirrors by photothermal microscopy

Laser conditioning has been shown to improve the laser damage threshold of some optical coatings by greater than 2x. Debate continues within the damage community regarding laser-conditioning mechanisms, but it is clear that nodular ejection is one of the byproducts of the laser conditioning process. To better understand why laser conditioning is so effective, photothermal microscopy was used to measure absorption of coating defects before and after laser exposure. Although a modest absorption reduction was expected due to the lower electric field peaks within a pit and the absence of potentially absorbing nodular seeds, surprisingly, absorption reductions up to 150x were observed. Photothermal microscopy has also been successfully used to correlate laser-induced damage threshold and absorption of defects in hafnia/silica multilayer optical coatings. Defects with high absorption, as indicated by high photothermal signal, have low damage thresholds. Previously a linear correlation of damage threshold and defect photothermal signal was established with films designed and damage tested at 1(omega) (1053 nm) and Brewsters angle (56.4 degree(s)), but characterized by photothermal microscopy at 514.5 nm and near-normal angle of incidence (10 degree(s)). In this study coatings designed, characterized by photothermal microscopy, and damage tested at the same wavelength, incident angle, and polarization did not have a correlation between defect photothermal signal and absorption.

Multilayer High-Gradient Insulators

Multilayer high-gradient insulators are vacuum insulating structures composed of thin, alternating layers of dielectric and metal. They are currently being developed for application to high-current accelerators and related pulsed power systems. This paper describes some of the high-gradient insulator research currently being conducted at Lawrence Livermore National Laboratory

Plasma transport in a new cathodic arc ion source, theory and experiment

Abstract We have developed a plasma transport code based upon the “flux tube” model and applied it to the ion transport in a recently developed metal-ion source based on the cathodic vacuum arc. This model is compared with ion current measurements made at various locations within the source. The agreement is sufficient to justify use of this model for future arc source design.

Beam-target interaction experiments for bremsstrahlung converter applications

For multi-pulse radiography facilities, we are investigating the possible adverse effects of (1) backstreaming ion emission from the bremsstrahlung converter target and (2) the interaction of the resultant plasma with the electron beam during subsequent pulses. These effects would primarily manifest themselves in a static focusing system as a rapidly varying X-ray spot. To study these effects, we are conducting beam-target interaction experiments on the ETA-II accelerator (a 6.0 MeV, 2.5 kA, 70 ns FWHM pulsed, electron accelerator) by measuring spot dynamics and characterizing the resultant plasma for various configurations.

Commissioning the DARHT-II scaled accelerator downstream transport

The DARHT-II accelerator will produce a 2-kA, 17-MeV beam in a 1600-ns pulse when completed mid-2007. After exiting the accelerator, the pulse is sliced into four short pulses by a kicker and quadrupole septum and then transported for several meters to a tantalum target for conversion to X-rays for radiography. We describe tests of the kicker, septum, transport, and multi-pulse converter target using a short accelerator assembled from the first available refurbished cells. This scaled accelerator was operated at ~8 MeV and ~1 kA, providing a beam with approximately the same v/gamma as the final 18-MeV, 2-kA beam, and therefore the same beam dynamics in the downstream transport. The results of beam measurements made during the commissioning of this scaled accelerator downstream transport are described.

Absorption and damage thresholds of low-defect-density hafnia deposited with activated oxygen

Motivation for this work included observations at Lawrence Livermore National Laboratory and elsewhere of a correlation between increasing laser damage thresholds (DT) and both decreasing nodular-defect density and absorption of coatings. We reduced the nodular-defect densities by a factor of over 4x in hafnia (HfO2) coatings deposited by reactive e-beam evaporation from a Hf target source. In order to increase the metal oxidation kinetics at the coated surface, Hf was e-beam deposited reactively with O2 activated by a (mu) -wave discharge. The effect of using activated O2 during the evaporation of a HfO2 target source was also evaluated. A series of HfO2 layers were made with various conditions; we alternated between two (mu) -wave configurations, Hf and HfO2 targets and two reactive O2 pressures. Laser DTs (1064 nm - 10 ns pulses), absorption (at 511 nm), and nodular- defect densities from these coatings are reported. The DT correlated inversely with the coating absorption.

Intense pulsed neutron emission from a compact pyroelectric driven accelerator

Intense pulsed D–D neutron emission with rates of >1010 n/s during the pulse, pulse widths of approximately hundreds of nanoseconds and neutron yields of greater than 10 000 per pulse, are demonstrated in a compact pyroelectric accelerator. The accelerator consists of a small pyroelectric LiTaO3 crystal that provides the accelerating voltage and an independent compact spark plasma ion source. The crystal voltage versus temperature is characterized and compares well with theory. Results show neutron output per pulse that scales with voltage as V∼1.7. These neutron yields match a simple model of the system at low voltages but are lower than predicted at higher voltages due to charge losses not accounted for in the model. Interpretation of the data against modeling provides understanding of the accelerator and in general pyroelectric LiTaO3 crystals operated as charge limited negative high voltage targets. The findings overall serve as the proof of principle and basis for pyroelectric neutron generators that...

Engineering Prototype for a Compact Medical Dielectric Wall Accelerator

A compact accelerator system architecture based on the dielectric wall accelerator (DWA) for medical proton beam therapy has been developed by the Compact Particle Acceleration Corporation (CPAC). The major subsystems are a Radio Frequency Quadrupole (RFQ) injector linac, a pulsed kicker to select the desired proton bunches, and a DWA linear accelerator incorporating a high gradient insulator (HGI) with stacked Blumleins to produce the required acceleration energy. The Blumleins are switched with solid state laser‐driven optical switches integrated into the Blumlein assemblies. Other subsystems include a high power pulsed laser, fiber optic distribution system, electrical charging system, and beam diagnostics. An engineering prototype has been constructed and characterized, and these results will be used within the next three years to develop an extremely compact 150 MeV system capable of modulating energy, beam current, and spot size on a shot‐to‐shot basis. This paper presents the details the engineerin...

Characterization of a Surface-Flashover Ion Source with 10-250 ns Pulse Widths

As a step towards developing an ultra compact D‐D neutron source for various defense and homeland security applications, a compact ion source is needed. Towards that end, we are testing a pulsed, surface flashover source, with deuterated titanium films deposited on alumina substrates as the electrodes. An electrochemically‐etched mask was used to define the electrode areas on the substrate during the sputtered deposition of the titanium films. Deuterium loading of the films was performed in an all metal‐sealed vacuum chamber containing a heated stage. Deuterium ion current from the source was determined by measuring the neutrons produced when the ions impacted a deuterium‐loaded target held at −90 kV. As the duration of the arc current is varied, it was observed that the integrated deuteron current per pulse initially increases rapidly, then reaches a maximum near a pulse length of 100 ns.