M.C. Myers

Reduction of edge emission in electron beam diodes

This paper presents measurements of the enhanced current density along the edges of a large area electron beam as well as successful techniques that eliminated this edge effect/beam halo. The beam current is measured with a Faraday cup array at the anode, and the spatial, time-integrated current density is obtained with radiachromic film. Particle-in-cell simulations support the experimental results. Experiments and simulations show that recessing the cathode reduces the electric field at the edge and eliminates the edge effect. However, the cathode recess structure itself emits under long-term repetitive operation. In contrast, using a floating, metallic, electric field shaper that is electrically insulated from the cathode eliminates the beam halo and mitigates electron emission from its surface during repetitive operation.

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.

A durable gigawatt class solid state pulsed power system

A unique all solid-state pulsed power system has been tested at the Naval Research Laboratory that produced 200 kV, 4.5 kA, and 300 ns pulses, continuously for more than 11,500,000 shots into a resistive load at a repetition rate of 10 pps. The Marx has an efficiency of 80% based on calorimetric measurements. This pulser is used to evaluate components and advance solid state designs for a next generation solid-state pulsed power system to drive an electron beam pumped KrF laser system for inertial fusion energy. The solid state pulser, designed and constructed by PLEX LLC, consists of a 12 stage Marx, coupled with a 3rd harmonic stage to sharpen the Marx output waveforms, a main magnetic switch, a compact pulse forming line used as a transit time isolator, and a resistive load. Each Marx stage uses an APP Model S33A compact high voltage switch that consists of 12 series connected thyristors. A life test on individual thyristors showed operation of >; 300 M shots at 20 Hz without failure.

Emission of an intense large area electron beam from a slab of porous dielectric

Inserting a thick slab of porous dielectric (e.g., ceramic honeycomb) in front of the emitting surface of a large-area planar diode improves the electron beam emission uniformity, decreases the beam current rise and fall times, and maintains a more constant diode impedance. Particle-in-cell simulations of the first few nanoseconds of diode operation show that initially numerous secondary electrons and ions load the ceramic honeycomb. The electrons and ions were confined within the ceramic pores, redistributing the electric field by reducing it within the ceramic pores and increasing it on the cathode surface (by a factor of 2–3). After the initial stage, plasma fills the ceramic pores and the space between the cathode and the ceramic. A space-charge-limited electron beam was then emitted from the ceramic honeycomb. No surface plasma was detected outside the pores inside the diode vacuum. The introduction of dielectric into the diode solves two additional problems associated with large-area planar diodes: ...

Electron beam pumped krypton fluoride lasers for fusion energy

High-energy electron beam pumped krypton fluoride (KrF) gas lasers are an attractive choice for inertial fusion energy (IFE). Their short wavelength and demonstrated high beam uniformity optimizes the laser-target physics, and their pulsed power technology scales to a large system. This paper presents the principals of this type of laser and the progress toward developing technologies that can meet the IFE requirements for repetition rate (5 Hz), efficiency (>6%), and durability (>3/spl times/10/sup 8/ shots). The Electra laser at the Naval Research Laboratory (NRL) has produced >500 J of laser light in short 5-Hz bursts. Research on Electra and the NRL Nike laser (3000 J, single shot) has shown that the overall efficiency should be greater than 7%. This is based on recent advances in electron beam stabilization and transport, electron beam deposition, KrF laser physics, and pulsed power. The latter includes the development of a new solid-state laser triggered switch that will be the basis for a pulsed power system that can meet the IFE requirements for efficiency, durability, and cost. The major remaining challenge is to develop long-lived hibachi foils (e-beam transmission windows). Based on recent experiments, this may be achievable by periodically deflecting the laser gas.

The Science and Technologies for Fusion Energy With Lasers and Direct-Drive Targets

We are carrying out a multidisciplinary multi-institutional program to develop the scientific and technical basis for inertial fusion energy (IFE) based on laser drivers and direct-drive targets. The key components are developed as an integrated system, linking the science, technology, and final application of a 1000-MWe pure-fusion power plant. The science and technologies developed here are flexible enough to be applied to other size systems. The scientific justification for this work is a family of target designs (simulations) that show that direct drive has the potential to provide the high gains needed for a pure-fusion power plant. Two competing lasers are under development: the diode-pumped solid-state laser (DPPSL) and the electron-beam-pumped krypton fluoride (KrF) gas laser. This paper will present the current state of the art in the target designs and lasers, as well as the other IFE technologies required for energy, including final optics (grazing incidence and dielectrics), chambers, and target fabrication, injection, and tracking technologies. All of these are applicable to both laser systems and to other laser IFE-based concepts. However, in some of the higher performance target designs, the DPPSL will require more energy to reach the same yield as with the KrF laser.

Emission of an intense electron beam from a ceramic honeycomb

Inserting a slab of honeycomb ceramic in front of the emitting surface of a large-area cathode improves the electron beam emission uniformity, decreases the beam current rise and fall times, and maintains a more constant diode impedance. Moreover, changing the cathode material from velvet to carbon fiber achieved a more robust cathode that starts to emit at a higher electric field without a degradation in beam uniformity. In addition, an 80% reduction in the postshot diode pressure was also observed when gamma alumina was deposited on the ceramic. A possible explanation is that reabsorption and recycling of adsorbed gases takes place.

Electra: Repetitively pulsed, 500 J, 100 ns, KrF oscillator

Electra is a repetitively pulsed, double-sided, electron-beam pumped krypton fluoride laser. Electra has recently operated as an oscillator with an output pulse of 510 J, with 100 ns pulse duration for single shots. At a 1 Hz repetition rate for a ten-shot burst, the laser output averaged 500 J per shot. The dependence of the laser energy on the partial pressures of Kr, Ar, and F2 were examined. Over a 10 to 30 psi total pressure range, the laser output energy decreases with decreasing argon concentration. Specifically, the laser output drops slightly as the argon concentration reduces from 60% to 40%, and then drops more noticeably as it is reduced to 0%. For the 60% Ar case, the optimal fluorine concentration is 0.25%, with a significant falloff in the laser energy from 0.25% to 0.1% and a gradual falloff from 0.25% to 0.7% fluorine. The present burst results indicate that the KrF kinetics is not very sensitive to the gas temperature at a total pressure of 20 psi.

Forced Convective Cooling of Foils in a Repetitively Pulsed Electron-Beam Diode

Electron-beam (e-beam)-pumped high-power gas lasers require the use of a transmission window/foil to separate the vacuum diode from the laser cell. Under repetitive operation, the foil is subject to an e-beam heat load and would eventually fail without cooling. This paper investigates forced convective cooling of a foil in the main amplifier of the Electra KrF laser by flowing the laser gas around a closed loop. The experimental data were taken with one of the two diodes operating at 500 kV, 110 kA, a full-width at half-maximum of 140 ns, and with an external axial magnetic field of 0.14 T. Type-T thermocouples are used to measure the temperature of the foil under a variety of conditions including flow-velocity enhancement due to louver inserts, repetition rate, cathode configuration, gas composition, and height along the foil. A first-order model that considers cooling due to turbulent flow, as well as internal foil thermal conduction and radiation, reproduces the general trends observed in the data. The goal is to keep the temperature of a 25-mum-thick stainless steel foil below the tensile strength and long-term thermal fatigue limits when operating at 5 Hz. The data, in combination with the model, predict that this goal can be achieved by diverting the laser gas to flow at high velocity along the foil surface.

Development of a durable, large area cathode for repetitive, uniform electron beam generation

Electra is a large aperture krypton-fluoride laser under development for inertial fusion energy research. The laser will require dual 500 kV, 36 A/cm/sup 2/, uniform electron beams operating at 5 Hz. Experimental studies have been performed to develop a /spl sim/3000 cm/sup 2/ cathode capable of generating a 110 kA, 100 ns flat top beam pulse with minimal current density variation (<10%), fast rise time (<40 ns), negligible gap closure (<1 cm//spl mu/s), and long lifetime (ultimately 10/sup 8/ shots). Time resolved electrical and optical data from the study of various dielectric fiber, carbon, and metal/dielectric cathodes are discussed.