R. Jaynes

Electra: repetitively pulsed 700 J, 100 NS electron beam pumped KRF laser

Summary form only given. Electra is a repetitively pulsed, electron beam pumped krypton fluoride (KrF) laser at the Naval Research Laboratory that is developing the technologies that can meet the inertial fusion energy (IFE) requirements for durability, efficiency, and cost. The technologies developed on Electra should be directly scalable to a full size fusion power plant beam line. The laser system consists of a main amplifier with an aperture of 30times30 cm2 , a 10times10 cm2 pre-amplifier, and a commercial discharge laser serving as the seed oscillator. The main amplifier, currently operated as an oscillator, has demonstrated single shot and rep-rate laser energies exceeding 700 J, with a pulse width of 100 ns at 248 nm. Continuous operation of the KrF laser has lasted for more than 2.5 hours without failure at 1 Hz, and tests at higher repetition rates are ongoing

Repetition-rate angularly multiplexed krypton fluoride laser system

The first results are reported from a repetitively pulsed, electron-beam-pumped angularly multiplexed krypton fluoride (KrF) laser system. This laser system, called Electra, was constructed at the U.S. Naval Research Laboratory. The technologies developed on Electra are scalable to a full-size fusion power plant beam line and should meet the inertial fusion energy (IFE) requirements for durability, efficiency, and cost. As in a full-size fusion power plant beam line, Electra is a multistage laser system that consists of a commercial discharge laser, a 175-keV electron-beam-pumped (40-ns flat-top) preamplifier, and a 500-keV (100-ns flat-top) main amplifier. Angular multiplexing is used in the optical layout to provide pulse length control and to maximize laser extraction from the amplifiers. The laser system initially demonstrated 452 J in a single shot and 1.585 kJ total energy in a one-second, 5-Hz burst. The preamplifier alone produces a 25-J KrF output with two angularly multiplexed beams. Extraction volumes were calculated for both a single-pass and a double-pass angularly multiplexed amplifier. A standard ray trace must be used to calculate the extraction volumes for the double-pass amplifier with focusing elements.

Scalloped Hibachi and Vacuum-Pressure Foil for Electra: Electron Beam Pumped KrF Laser

We are developing a new type of “scalloped” hibachi structure to be deployed on Electra, a 700 Joule/pulse electron beam pumped KrF laser system, to improve the durability and efficiency of the pressure foil. In an e-beam pumped laser, an electron beam is generated in a high vacuum diode, and then passed through a pressure foil to pump the gain medium in the gas laser cell. Previous hibachi structures used flat “picture frame” topologies in which the foil is laid flat on the frame. The natural bulging of the foils under pressure introduces significant stress concentrations at the corners of the rib openings. In our new design, the hibachi frame is scalloped, so the foil between the ribs approximates a section of a cylindrical pressure vessel. This arrangement eliminates these stress concentrations and, because the stress can in principle be made purely cylindrical, lowers the overall stress as well. This allows use of a thinner foil to transport the e-beam more efficiently. Two techniques were developed to seal this non-planar vacuum surface: utilizing a bonded gasket-foil fixture or employing a quad or double seal o-ring. The former is less expensive, but only proved viable for thicker foils. These methods have been shown to support foils of various materials including aluminum, stainless steel, and titanium with thicknesses ranging from 12 μm to 75 μm. Foils have been tested under high vacuum and with up to 30 psi differential applied to the foil.

Electra: durable repetitively pulsed 700 J, 100 ns electron beam pumped KrF laser system

Electra is a repetitively pulsed, electron beam pumped Krypton Fluoride (KrF) laser at the Naval Research Laboratory that is developing the technologies that can meet the Inertial Fusion Energy (IFE) requirements for durability, efficiency, and cost. Electra in oscillator mode has demonstrated single shot and rep-rate laser energies exceeding 700 J with 100 ns pulsewidth at 248 nm. Continuous operation of the KrF laser has lasted for more than 2.5 hours without failure at 1 Hz and 2.5 Hz. The measured intensity and energy per shot is reproducible in rep-rate runs of 1 Hz, 2.5 Hz and 5 Hz for greater than thousand shot durations. The KrF intrinsic efficiency is predicted to be 12% with measurements and modeling (Orestes Code). In addition we have compared Orestes with initial results of 23 J for the Electra Pre-Amplifier. The positive agreement between Orestes and our results lead allow us to predict that large KrF laser systems will meet the efficiency requirements for inertial fusion energy driver. The focal profile measurements show for single shot conditions recovery in less than 200 ms, the time needed for 5 Hz operation. Rep-rate focal profile measurements at 1 Hz show reproducibility in spatial extent and energy.

The Electra KrF laser system

This paper presents a brief overview of the Electra laser system and reports on the most recent results. The laser system consists of an electron beam pumped main amplifier with an aperture of 30×30 cm2, an e-beam pumped 10×10 cm2 pre-amplifier, and a KrF discharge laser serving as the seed oscillator. Full laser system shots have been completed with laser energies of 452 J. In addition, 25,000 continuous shots at 2.5 Hz from a single sided diode into laser gas have been achieved with the main amplifier operating as an oscillator.

Electra: Repetitively Pulsed 700 J, 100 ns Electron Beam Pumped KrF Laser

This paper presents a brief overview of the Electra laser system and report on the most recent results. The laser system consists of an electron beam pumped main amplifier with an aperture of 30times30 cm2, an e-beam pumped 10times10 cm2 pre-amplifier, and a commercial discharge laser serving as the seed oscillator. The main amplifier, currently operated as an oscillator, has demonstrated single shot and rep-rate laser energies exceeding 700 J, with a pulse width of 100 ns at 248 nm. Continuous operation of this laser in an oscillator mode has lasted for more than 2.5 hours without failure at 1 and 2.5 Hz. Tests at higher repetition rates and longer runs are ongoing. The pre-amplifier uses a fast gas Marx, pulse forming lines, a single stage magnetic switch, and transit time isolators and operates with a rep-rate of up to 5 Hz.

Electra: a KrF electron-beam-pumped high-average-power laser system for inertial confinement fusion applications

Electra is an electron beam pumped laser being developed at the Naval Research Laboratory as an inertial confinement fusion (ICF) driver. Two opposing 500 kV, 100 kA electron beams pump the main amplifier, which achieves energies of 730 J over a 100 ns pulse at 248 nm when run in an oscillator configuration. KrF lasers have been shown to have intrinsic efficiencies of greater than 12% and, based on that, wall plug efficiencies of >7% are projected for an IFE system based on our established improvements in laser physics and pulsed power technologies. The Electra main amp has run at rep-rates of 1 Hz, 2.5 Hz, and 5 Hz in runs exceeding 10,000 shots. This paper will present an overview of the Electra accomplishments and highlight recent research, including integrating Electras amplifiers into a durable full laser system, interferometric measurements of the near field spatial distortions in the amplifiers and their effect on the far field profile, and spatially and temporally resolved temperature measurements of the electron beam transmission foil.

Electra: durable repetitively pulsed angularly multiplexed KrF laser system

Electra is a repetitively pulsed, electron beam pumped Krypton Fluoride (KrF) laser at the Naval Research Laboratory that is developing the technologies that can meet the Inertial Fusion Energy (IFE) requirements for durability, efficiency, and cost. The technologies developed on Electra should be directly scalable to a full size fusion power plant beam line. As in a full size fusion power plant beam line, Electra is a multistage laser amplifier system which, consists of a commercial discharge laser (LPX 305i, Lambda Physik), 175 keV electron beam pumped (40 ns flat-top) preamplifier, and 530 keV (100 ns flat-top) main amplifier. Angular multiplexing is used in the optical layout to provide pulse length control and to maximize laser extraction from the amplifiers. Single shot yield of 452 J has been extracted from the initial shots of the Electra laser system using a relatively low energy preamplifier laser beam. In rep-rate burst of 5 Hz for durations of one second a total energy of 1.585 kJ (average 317 J/pulse) has been attained. Total energy of 2.5 kJ has been attained over a two second period. For comparison, the main amplifier of Electra in oscillator mode has demonstrated at 2.5 Hz rep-rate average laser yield of 270 J over a 2 hour period.

Development of a Large Area, Durable Electron Emitter for High Average Power KRF Lasers

The high energy, repetitively pulsed krypton-fluoride (KrF) lasers being developed for use in inertial fusion energy research require large area electron emitters that are both durable and efficient. Specifically, emitters of 1000s of cm2 must reliably produce 100s of kA at voltages up to 1 MV in flat-topped, sub-microsecond pulses, at pulse repetition frequencies (PRF) of 1-10 Hz for 105 -108 continuous shots, Emission takes place in a vacuum diode that is immersed in an external magnetic field. To achieve the needed durability and efficiency, solutions for gas evolution, gap closure, beam halo formation, current density uniformity, beam current rise time, temperature management, and beam patterning must be found. On-going experiments using the Electra main amplifier at the Naval Research Laboratory have suggested that using a segmented secondary emission cathode may satisfy the above requirements. The secondary emission cathodes initially developed for repetitively pulsed laser operations on Electra consisted of a velvet dielectric primarv emitter coupled with a monolithic ceramic honeycomb secondary emitter. These 3000 cm2 cathodes allowed continuous runs of thousands of shots at 1 Hz producing 110 kA peak current at a diode voltage of 500 kV. However, reliability and efficiency were limited at greater PRF due to problems with gas evolution, temperature management, and ultimately gap closure. This paper reports on the experimental development of an improved secondary-emission cathode that largely eliminates these problems and is designed to operate reliably at PRF of 2.5 -5 Hz for extended, continuous runs. Time-resolved electrical and optical measurements of electron emission will be presented and discussed. Further work to convert the monolithic design to a segmented cathode that will allow high efficiencies and high laser outputs will also be presented.

KrF Laser Amplification in the Multi-Staged Electra Facility

Summary form only given. Electra is a rep-rated, e-beam pumped KrF laser system at the Naval Research Laboratory investigating the physics and technology required for inertial fusion energy. To date the main cell (30times30 cm2 aperture) has operated as an oscillator and achieved ~1.5 kW at 5 Hz for 2000 shots. It has also performed continuously at 2.5 Hz for over 22,000 shots. The next step is to convert Electra into an amplifier system. The planned design has three components. A commercial KrF discharge oscillator (1.3times3 cm2 aperture, ~1 J output) will provide the initiating laser pulse. This pulse is then sequentially amplified through a preamp (10times10 cm2 aperture) in a single pass, followed by a double pass through the main amp. The final system output laser energy depends upon the e-beam pumping power in each amp, which in turn depends on the particular cathode and hibachi configurations. To examine this dependency the KrF kinetics/laser simulation code Orestes has been used to follow the growth of the low energy oscillator pulse as it sequentially propagates through the two amplifiers. Initial calculations predict final output energies of ~200 J for monolithic (uniform emitting) cathodes and ~700 J for stripped cathodes designed for maximum gas deposition efficiency. In addition to the energy, the final output laser pulse shape is a complex product of the amplification and timing between the multiplexed laser pulses and the e-beam pulsed power. Simulations for the preamp have demonstrated partial agreement with the profiles and provide a test of the molecular/plasma kinetic processes used in Orestes.