John B. Trenholme

Measurements and modeling of gain coefficients for neodymium laser glasses

Small-signal gain coefficients are reported for neodymium in silicate, phosphate, fluorophosphate, and fluoroberyllate laser glasses. Measurements were made in a disk amplifier under identical conditions. Using spectroscopic data as the input, amplifier gain is calculated as a function of flashlamp energy, pumping pulse duration, disk thickness, and Nd-doping. The agreement between predicted and measured gains is generally with ±10 percent, consistent with experimental uncertainties in the model and the parameters used. The operating conditions which optimize amplifier performance and efficiency for a given laser glass may be found using spectroscopic data alone. This process can be extended to derive the most cost-effective staging of amplifier chains for fusion lasers. A discussion of the model and examples of calculations are presented.

A glass-disk-laser amplifier

The details of the analysis, design, and operation of a Nd-glass-disk-laser amplifier which has been constructed at the Naval Research Laboratory are presented. Gain and fluorescence measurements have been compared to theoretical predictions; these show that 0.6-J/cm3energy storage is achieved in the disk (assuming a cross section of 3.0 \times 10^{-20} cm2). The effects of unsuppressed parasitic oscillations are demonstrated and an effective method of preventing their occurrence is shown. The disk amplifier has demonstrated 320-J output in a 1-ns pulse with 110-J input.

NIF optical specifications: the importance of the RMS gradient

The performance of the National Ignition Facility (NIF), especially in terms of laser focusability, will be determined by several key factors. One of these key factors is the optical specification of the thousands of large aperture optics that will comprise the 192 beamlines. We have previously reported on the importance of the specification of the power spectral density (PSD) on NIF performance. Recently, we have been studying the importance of long spatial wavelength phase errors on focusability. We have concluded that the preferred metric for determining the impact of these long spatial wavelength phase errors is the rms phase gradient. In this paper, we outline the overall approach to NIF optical specifications, detail the impact of the rms phase gradient on NIF focusability, discuss its trade-off with the PSD in determining the spot size, and review measurements of optics similar to those to be manufactured for NIF.

Parasitic Suppression in Large Aperture Nd:Glass Disk Laser Amplifiers

Threshold conditions for bulk and surface parasitic oscillations, which may limit energy storage in large aperture Nd:glass disk lasers, have been developed as a function of material parameters. An expression describing the energy storage distribution within a disk was used to determine the mode that will be most limiting for a particular disk design. Additional modes that may be limiting in special cases were identified and their effects evaluated. These results are useful in developing disk laser designs that minimize parasitic effects.

Damage mechanisms avoided or managed for NIF large optics

Abstract After every other failure mode has been considered, in the end, the high-performance limit of all lasers is set by optical damage. The demands of inertial confinement fusion (ICF) pushed lasers designed as ICF drivers into this limit from their very earliest days. The first ICF lasers were small, and their pulses were short. Their goal was to provide as much power to the target as possible. Typically, they faced damage due to high intensity on their optics. As requests for higher laser energy, longer pulse lengths, and better symmetry appeared, new kinds of damage also emerged, some of them anticipated and others unexpected. This paper will discuss the various types of damage to large optics that had to be considered, avoided to the extent possible, or otherwise managed as the National Ignition Facility (NIF) laser was designed, fabricated, and brought into operation. It has been possible for NIF to meet its requirements because of the experience gained in previous ICF systems and because NIF designers have continued to be able to avoid or manage new damage situations as they have appeared.

ND:GLASS LASER DESIGN FOR LASER ICF FISSION ENERGY (LIFE)

Abstract We have developed preliminary conceptual laser system designs for the Laser ICF (Inertial Confinement Fusion) Fission Energy (LIFE) application. Our approach leverages experience in high-energy Nd: glass laser technology developed for the National Ignition Facility (NIF)1, along with high-energy-class diode-pumped solid-state laser (HEC-DPSSL) technology developed for the DOE’s High Average Power Laser (HAPL) Program and embodied LLNL’s Mercury laser system.2 We present laser system designs suitable for both indirect-drive, hot spot ignition and indirect-drive, fast ignition targets. Main amplifiers for both systems use laser-diode-pumped Nd:glass slabs oriented at Brewster’s angle, as in NIF, but the slabs are much thinner to allow for cooling by high-velocity helium gas as in the Mercury laser system. We also describe a plan to mass-produce pump-diode lasers to bring diode costs down to the order of

Size-selection initiation model extended to include shape and random factors

0.01 per Watt of peak output power, as needed to make the LIFE application economically attractive.

Solid-state disk amplifiers for fusion-laser systems

The Feit-Rubenchik size-selection damage model has been extended in a number of ways. More realistic thermal deposition profiles have been added. Non-spherical shapes (rods and plates) have been considered, with allowance for their orientation dependence. Random variations have been taken into account. An explicit form for the change of absorptivity with precursor size has been added. A simulation tool called GIDGET has been built to allow adjustment of the many possible parameters in order to fit experimental data of initiation density as a function of fluence and pulse duration. The result is a set of constraints on the possible properties of initiation precursors.

Pulse length and terminal-level lifetime dependence of energy extraction for neodymium-doped phosphate amplifier glass

We review the design, performance, and operation of large-aperture (10-46 cm) solid-state disk amplifiers for use in laser systems. We present design data, prototype tests, simulations, and projections for conventional cylindrical pump-geometry amplifiers and rectangular pump-geometry disk amplifiers. The design of amplifiers for the Nova laser system is discussed.

Modeling max-of-N fluence distribution using measured shot-to-shot beam contrast

On the basis of detailed numerical calculations, we have formulated an empirical expression for the saturation fluence in neodymium-doped phosphate amplifier glass that explicitly depends upon the ratio of the pulse width to the terminal-level lifetime. The empirical expression, when substituted within the Frantz-Nodvik solution for energy extraction, can be used to determine the impact of the lower level lifetime on the energy extracted from Nd:phosphate glass amplifiers. We used our empirical solution to model experimental gain-saturation data and determine the terminal-level lifetime. We compared this value with two independent experiments and found that all three experiments yielded similar results. The terminal-level lifetime of LG-750 at room temperature is found to be 253 ps /spl plusmn/50 ps.