Matthew F. Wolford

Electron beam pumped KrF lasers for fusion energy

Abstract : Direct drive with krypton fluoride (KrF) lasers is an attractive approach to inertial fusion energy (IFE): KrF lasers have outstanding beam spatial uniformity, which reduces the seed for hydrodynamic instabilities; they have short wavelength (248 nm) that increases the rocket efficiency and raises the threshold for deleterious laser-plasma instabilities; they have the capability for zooming , i.e. decreasing the spot size to follow an imploding pellet and thereby increase efficiency; and they have a modular architecture, which reduces development costs. Numerical 1-D simulations have shown that a target driven by a KrF laser can have a gain above 125 [1,2], which is ample for a fusion system. Simulations of the pellet burn in 2-D and 3-D are underway. In addition to these laser-target advantages, the Sombrero Power Plant study showed a KrF based system could lead to an economically attractive power plant [3]. In view of these advances, several world-wide programs are underway to develop KrF lasers for fusion energy. These include programs in Japan [4, 5], China [6], Russia [7], and The United Kingdom [8]. There was also a large program in the United States [9]. The paper here concentrates on current research in the US with two lasers at the Naval Research Laboratory: The Electra laser [10] is a 400-700 J repetitively pulsed system that is being used to develop the technologies that meet the fusion requirements for rep-rate, durability, efficiency and cost. The Nike laser [11] is a 3-5 kJ single shot device that is used to study KrF issues with full-scale electron beam 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.

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 Continuous Multi-Thousand Shot Electron Beam Pumped KrF Rep-Rate Laser for Fusion Energy

Abstract The Electra laser system is currently being developed at the Naval Research Laboratory to serve as a test bed for laser driver technologies needed for an inertial fusion energy power plant. The main amplifier has produced 730 J of laser light operating in an oscillator mode. These results as well as advancement of the laser physics, electron beam deposition, and the pulse power technologies give us projections of >7% wall plug efficiency for an IFE system. The Electra main amplifier in oscillator configuration has run continuously at 1 Hz, 2.5 Hz, and 5 Hz for multi-thousand shot runs. This paper will discuss recent results of the Electra program at the Naval Research Laboratory including integrating the Electra main amplifier into a complete laser amplifier system. Issues addressed will include development paths for the cathode, window coating, and foil longevity to attain the durability required for a fusion power plant.

ELECTRA: AN ELECTRON BEAM PUMPED KrF REP-RATE LASER SYSTEM FOR INERTIAL FUSION ENERGY

Electra is a high average power KrF laser system at the Naval Research Laboratory funded under the HAPL program. The goal of Electra is to develop the laser driver technologies needed for an inertial fusion energy power plant. When run in an oscillator configuration the 500 kV, 100 kA e-beam pumped main amplifier produces 730 J with a 100ns pulse width at 248 nm. KrF lasers have been shown to have intrinsic efficiencies of 12%leading to a projected wall plug efficiency of >7% for an IFE system with demonstrated improvements in laser physics and pulse power technologies. As an oscillator the Electra main amplifier has run continuously at 1 Hz,2.5 Hz, and 5 Hz for multi-thousand shot runs. This paper will discuss recent results from Electra including operation as a complete laser amplifier system, first demonstration of a new method to efficiently cool the hibachi foil with indications of a reduced penalty in laser uniformity, and design modifications to increase durability.

Krypton Fluoride (KrF) Laser Driver for Inertial Fusion Energy

Abstract The United States Naval Research Laboratory (NRL) is developing the krypton fluoride (KrF) laser technology for a direct drive laser inertial fusion energy (IFE) power plant. The overall projected wall plug efficiency for KrF laser system is ~7%, including thermal management and optical losses. There are two KrF lasers at NRL. The first, Nike, provides up to 3 kJ of laser light per shot for experimental research in KrF laser-target interactions. The Electra Laser at NRL is a repetitively pulsed electron beam pumped 700 Joule KrF laser facility. The objective with Electra is to develop technologies to meet the IFE requirements for repetition rate, efficiency, and durability. Electra produces over 750 Joules in oscillator mode. Based on experiments, there is expected to be virtually no degradation in the laser focal profile, even at 5 Hz, high efficiency operation. Progress in durability has lead to achievement of KrF laser runs for 10 continuous hours at 2.5 Hz (90,000 shots) and 100 minutes at 5 Hz (over 30,000 shots). The main impediment to achieving long duration runs is the present pulsed power system that is based on spark gap switches. NRL has developed a new all solid state system that has operated for 11 million pulses continuously at 10 Hz and is based on components attaining 300 million pulses. These studies show an electron beam pumped KrF laser should be a viable approach for a laser fusion energy driver.

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

Electra is a repetitively pulsed, electron beam pumped Krypton Fluoride (KrF) laser at the Naval Research Laboratory that is developing technologies to meet the Inertial Fusion Energy (IFE) requirements for durability, efficiency, and cost. Electra has demonstrated single shot and rep-rate laser energies as an oscillator exceeding 700 J with 100 ns pulsewidth at 248 nm. Peak output of 731 J laser energy in single shot operation demonstrates a total pulse intrinsic efficiency of 8.3%, and an intrinsic efficiency during the flat-top region of 9.6%. Improvements in the window transmissivity from the present 93% to excimer grade fused silica with anti-reflection coatings on both sides could provide up to 17% enhancement of the output intensity, and likewise for the efficiency. The Electra KrF gain measured in 4 positions of the laser aperture in an amplifier configuration demonstrates near field uniformity. In addition, the amplifier and oscillator results are qualitatively consistent in the pressure dependence on KrF yield. A quantitative Rigrod analysis and comparison is provided at the peak oscillator yield conditions for both, oscillator and amplifier, configurations. Continuous operation of the KrF laser at 300 J has lasted for 2.5 hours without failure at 1 Hz. Successful operation at 5 Hz with 400 J per pulse output has occurred in bursts of 100 seconds. In all of these runs the laser pulse waveform is extremely consistent from the first shot to the last.

High yield rep-rate operation of electra; an electron beam pumped KrF laser

Electra has demonstrated high yield, 725 J, 1 Hz and 5 Hz operation for 100 second bursts. Analysis of fluorine dependence on yield shows replacing super-elastic collision processes with dissociative attachment processes improves agreement.