J. L. Giuliani

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.

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.

An efficient tabulated collisional radiative equilibrium radiation transport model suitable for multidimensional hydrodynamics calculations

A computationally efficient method for transporting radiation in multidimensional plasmas has been developed and evaluated. The basis of this method is a uniform plasma approximation that allows one to utilize existing escape probability techniques that are successfully used in one-dimensional (1D) calculations to approximately solve the multidimensional radiation transport problem. This method is superior to diffusion methods because (1) the probability of escape technique insures that the plasma goes to the correct optically thin and thick limits, (2) the effects of line absorption due to photoexcitations are modeled, and (3) this method uses source functions that are based on a self-consistent nonlocal thermodynamic equilibrium calculation, not an ad hoc assumption that the source functions are Planckian. This method is highly efficient because equation of state information from 1D calculations is tabulated as a function of plasma internal energy, ion density, and the line probability of escape from a ...

Model of enhanced energy deposition in a Z-pinch plasma

In numerous experiments, magnetic energy coupled to strongly radiating Z-pinch plasmas exceeds the thermalized kinetic energy, sometimes by a factor of 2–3. An analytical model describing this additional energy deposition based on the concept of macroscopic magnetohydrodynamic (MHD) turbulent pinch heating proposed by Rudakov and Sudan [Phys. Reports 283, 253 (1997)] is presented. The pinch plasma is modeled as a foam-like medium saturated with toroidal “magnetic bubbles” produced by the development of surface m=0 Rayleigh-Taylor and MHD instabilities. As the bubbles converge to the pinch axis, their magnetic energy is converted to thermal energy of the plasma through pdV work. Explicit formulas for the average dissipation rate of this process and the corresponding contribution to the resistance of the load, which compare favorably to the experimental data and simulation results, are presented. The possibility of using this enhanced (relative to Ohmic heating) dissipation mechanism to power novel plasma r...

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.

Electron energy deposition in an electron-beam pumped KrF amplifier: Impact of beam power and energy

The electron deposition in an Ar–Kr–F2 mixture, based on a solution of the electron Boltzmann equation, is presented. The model is relevant to an electron-beam generated KrF* laser amplifier at atmospheric pressure. Sets of cross sections for Ar, Kr, and F2 have been compiled. Calculations have been performed to determine the electron energy distribution function, energy per electron–ion pair and the ionization and excitation rates. It is found that the inclusion of inner shell ionization and the subsequent Auger emission are essential for matching known results on both the energy per electron–ion pair Wei and the stopping power in pure Ar or Kr target gases. For the chosen Ar–Kr–F2 mixture, Wei is calculated to be 24.6 eV. The excitation-to-ionization ratio is calculated to be 0.38 for Ar and 0.54 for Kr at low input power density Pbeam (1 kW/cm3). Both ratios increase with Pbeam, particularly for Kr which attains 0.8 at 1 MW/cm3. The dependency on Pbeam and the excitation efficiency for Kr is significan...

Influence of L‐shell dynamics on K‐shell yields for imploding krypton Z‐pinch plasmas

The radiative performance of a Z‐pinch krypton puff gas heated by a proposed multiterawatt generator is investigated with the aid of a one‐dimensional (1‐D) non‐Local Thermodynamic Equilibrium (LTE) radiation hydrodynamic model self‐consistently coupled to a circuit model. For stable loads configured either as cylindrical annular shells or uniform fills, predictions are made for the K‐ and L‐shell soft x‐ray emission as a function of the L‐shell level structure. The results of numerical simulations show that both the thin annular shell and uniform fill are prolific K‐ and L‐shell radiators. It is also predicted that the L‐shell level structure profoundly affects the optimum K‐shell soft x‐ray yield, as well as the choices for load mass corresponding to optimum emission.

Electron energy deposition in an electron-beam pumped KrF amplifier: Impact of the gas composition

Calculations for electron deposition in electron beam generated KrF laser at atmospheric pressure have been performed. The impact of the Ar/Kr/F2 gas mixture on the electron energy distribution function, electron density, and mean energy, energy per electron–ion pair, attachment, dissociation, excitation, and ionization rates have been investigated. The F2 abundance controls the low energy (≲9 eV) component of the distribution function, while both the fluorine and krypton mole fraction affect the distribution in the midenergy domain (9 to ∼25 eV). Consequently, the F2 attachment rate coefficient varies with the F2 mole fraction (xF2) such that the electron density scales as 1/xF20.7. The rate coefficient for direct dissociation of F2 is smaller than for attachment but the former contributes more to the total power dissipation (∼8% at xF2=0.01). The excitation-to-ionization ratio for Kr is not constant, as generally assumed, but increases by a factor of two with a decrease in either the Kr or F2 abundance....

A PROBABILISTIC MODEL FOR CONTINUUM TRANSPORT IN DENSE, OPTICALLY THICK PLASMAS

Abstract Probability-of-escape concepts have previously been successfully employed for cell-to-cell transport of spectral lines in dense, optically thick media. In this paper, the same conceptual framework is extended to the transport of continuum radiation emitted in radiative recombination and to the transport of line radiation in the presence of an optically thick continuum. Specific comparisons with calculations using multifrequency probability-of-escape transport demonstrate the accuracy of the model.