Patrick M. Burns

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.

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

Space-based, multi-wavelength solid-state lasers for NASA's Cloud Aerosol Transport System for International Space Station (CATS-ISS)

Fibertek has designed and is building two space-based lasers for NASA’s CATS-ISS mission. This space-based lidar system requires lasers capable of provide 4-5 kHz output at 1064 nm, 532 nm and 355 nm with each wavelength having ~2-2.5 mJ pulse energy. The lasers will be based on the ISS for a mission lasting up to 3 years.

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.

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.

ICESat-2 laser technology development

A number of ICESat-2 system requirements drove the technology evolution and the system architecture for the laser transmitter Fibertek has developed for the mission.. These requirements include the laser wall plug efficiency, laser reliability, high PRF (10kHz), short-pulse (<1.5ns), relatively narrow spectral line-width, and wave length tunability. In response to these requirements Fibertek developed a frequency-doubled, master oscillator/power amplifier (MOPA) laser that incorporates direct pumped diode pumped Nd:YVO4 as the gain media, Another guiding force in the system design has been extensive hardware life testing that Fibertek has completed. This ongoing hardware testing and development evolved the system from the original baseline brass board design to the more robust flight laser system. The final design meets or exceeds all NASA requirements and is scalable to support future mission requirements.

ICESat-2 laser technology readiness level evolution

We report on the completion of the space qualification testing program for NASA Goddard Space Flight Center’s (GSFC) Ice, Cloud, and Land Elevation Satellite 2 (ICESat-2) program. This paper describes the final performance results of the fully integrated (laser and electronics) flight laser system with an emphasis on the system design evolution from a breadboard demonstration to a fully space-qualified laser system. The 532 nm ICESat-2 laser transmitter generates diffraction limited pulse energies of 1 mJ, pulsewidths of < 1.5 ns, and 10 kHz pulse repetition frequency and has minimum lifetime of 1 trillion pulses on-orbit. A combination of engineering design units and correlated structural thermal optical analysis was used to systematically improve reliability and performance over the operating environment. The laser system qualification and acceptance test programs included electromagnetic interference (EMI), vibration, and thermal vacuum (TVAC) testing. This paper presents key laser performance results and lessons learned on the multi-year laser development to facilitate future space-qualified laser developments, improve reliability, and increase performance.

High efficiency laser designs for airborne and space-based lidar remote sensing systems

The increasing use of lidar remote sensing systems in the limited power environments of unmanned aerial vehicles and satellites is motivating laser engineers and designers to put a high premium on the overall efficiency of the laser transmitters needed for these systems. Two particular examples upon which we have been focused are the lasers for the ICESat-2 mission and for the Laser Vegetation Imaging Sensor-Global Hawk (LVIS-GH) system. We have recently developed an environmentally hardened engineering unit for the ICESat-2 laser that has achieved over 9 W of 532 nm output at 10 kHz with a wall plug efficiency to 532 nm of over 5%. The laser has a pulse width of <1.5 ns and an M2 of <1.5. For the LVIS-GH lidar, we recently delivered a 4.2 W, 2.5 kHz, 1064 nm laser transmitter that achieved a wall plug efficiency of 8.4%. The laser has a pulse width of 5 ns and an M2 of 1.1 We provide an overview of the design and environmental testing of these laser transmitters.

The Electra KrF Laser System

Electra is an electron beam pumped KrF laser system at the Naval Research Laboratory used to advance the technology towards a KrF laser driver for inertial fusion energy. Electra consists of two e-beam pumped amplifiers and one commercial discharge laser. The main amplifier includes two identical pulsed power systems, each with a capacitor bank, a 12:1 step-up transformer, parallel pulse forming lines, and laser triggered output switches. Each system generates a 500 kV, 110 kA, 140 ns electron beam that pumps the laser gas from two sides. Depending on the cathode and hibachi (laser gas/vacuum interface structure) configuration, the laser output energy ranges from of 200 to 700 J per shot. Operating as a laser oscillator, the main amplifier has run at repetition rates of up to 5 Hz, and has operated at 2.5 Hz for 25,000 continuous shots. The pre-amplifier pulsed power system utilizes a fast 10-stage Marx, pulse forming lines, and a magnetic switch. The system powers two diodes, which each operate at 175 kV, 70 kA, and 50 ns. Initial pre-amplifier laser experiments with a first generation cathode and hibachi, and with the 1 J discharge laser serving as the seed oscillator, produced an output energy of 20 J. Our planned improvements to both the cathode and hibachi should lead to an increase in laser energy of greater than 30 J per shot. This paper will present results of the overall laser system and discuss the performance of the pulsed power systems, the laser triggered output switches, cathode development, vacuum diode effects, laser system durability, and thermal management of the pulsed power components, hibachi, and the laser gas during long rep rates.

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.