FPN02-81

High Average Power Laser Progress

December 21, 2002

Research on inertial confinement fusion to date has concentrated on the physics of fusion pellet implosions on a single shot basis. The National Ignition Facility (NIF) is aimed at igniting such a pellet in a single shot (FPN02-78). For commercial power, however, these pellets will have to be ignited using "drivers" which pulse repetitively. There are several types of such drivers: heavy ion accelerators, pulsed power z-pinches and new types of lasers, under development. A recent meeting to review progress in "high average power lasers" was recently held at the U.S. Naval Research Laboratory (NRL). A summary of this meeting, prepared by John Sethian of NRL, follows:

Results from Recent High Average Power Laser Workshop-- a blizzard can't stop a flurry of good news.

On December 5 and 6, over 60 participants from 25 institutions slogged through a Washington DC snowstorm to attend the High Average Power Laser Workshop at the Naval Research Laboratory. Area schools were closed, the government was on liberal leave, even the mail was delayed... but nothing could stop this group from presenting and discussing their recent advances.

The High Average Power Laser Program (HAPL) is a national program dedicated to developing Inertial Fusion Energy (IFE) with lasers, direct drive targets and solid wall chambers. The program is funded by the US Department of Energy/NNSA/Defense Programs. One of the principles of the program is to develop the primary components together as a coherent system, as this is the most efficient approach to realizing a viable power fusion energy source. The components under development are: the lasers, target design, target fabrication, target injection, final optics, chambers and relevant materials. The HAPL team meets periodically (3 times/year) to discuss their progress in a community setting. Several significant advances were announced at the recent workshop:

KrF LASER ACHIEVED FIRST LIGHT: The Naval Research Laboratory announced that the "Electra" Krypton Fluoride (KrF) laser has achieved "first light" in a rep-rate mode. The system produces 400 Joules single pulse and 300 Joules repetitively. The Electra group has also demonstrated high electron beam deposition efficiency (>75%, vs. < 40% achieved in previous systems) and eliminated an electron beam instability that has plagued large area electron beams for decades.

DPSSL LASER ACHIEVED FIRST LIGHT: Lawrence Livermore National Laboratory announced that the "Mercury" Diode Pumped Solid State Laser (DPSSL) has also achieved "first light" in a rep-rate mode, producing 31.6 Joules in a single pulse and 20.6 Joules in 20 ns at 10 Hz. This is the highest energy ever achieved by a DPSSL in nanosecond pulses. Four diode arrays producing up to 320 kW of peak power were operated and significant advances were made in the growth of large, high quality laser crystals needed for present and future systems.

TARGET DESIGN--"PICKET FENCE" PULSE SHAPE STABILIZES TARGET AND REDUCES IMPRINT: In the areas of target design, Lawrence Livermore National Laboratory reported that a high intensity spike, or "picket", at the leading edge of the laser pulse can significantly enhance the target stability with an acceptable reduction in yield. The University of Rochester presented similar results. The Naval Research Laboratory reported the "picket" may have the added benefit of reducing imprint of the laser beam. (The NRL and Rochester work is funded through the ICF program, LLNL is funded through HAPL).

ULTRA-SMOOTH DT ICE LAYERS ON FOAM: Los Alamos National Laboratory has produced ultra-smooth deuterium-tritium (DT) ice layers by growing the DT ice on a foam base. This arrangement replicates the current fusion energy target designs. The liquid DT is wicked into a low-density plastic foam, frozen at 19.7 degrees above absolute zero, and then slowly cooled to equilibrate at 19.25 above absolute zero. With this approach Los Alamos observed less than 0.6 microns variation in the DT ice surface finish, which is about 2 times better than what has been achieved before. This is a significant advance, as the inner surface of the shell must be very smooth in order to maximize the fusion energy released from the target.

HELIUM RETENTION IN FIRST WALL MAY NOT BE A PROBLEM FOR IFE: One of the biggest challenges with the solid wall chamber approach is long term survival of the first wall. High energy helium ions from the exploding target will be driven into the wall. The helium cannot move easily inside the material, and thus after many shots coalesces into bubbles which eventually cause the wall surface to fracture. This has been documented with tungsten, a prime candidate for the wall material. Those experiments were performed with tungsten at 800 degrees C. At this meeting, experimenters from Oak Ridge National Lab (ORNL) reported that if the tungsten is cyclically heated to the higher temperatures (> 2000 degrees C) expected in an IFE chamber, the amount of helium retained can be reduced by a factor of two or more. Moreover, researchers from the University of California at Santa Barbara reported that their modeling shows the bubble formation could be reduced to the point that there may be no problem at all. This is because the helium ions produced by an IFE target have a wide spectrum of energies, and thus the helium will be driven to a range of depths into the first wall, rather than into just one location. It should be noted that all of this work is preliminary and more experimental verification is needed--for example the effects of neutron irradiation on helium mobility need to be evaluated. So even though we are not out of the woods yet, at least we can now walk between the trees.

For a complete summary of the workshop, as well as further information about the High Average Power Laser Program, see the website http://aries.ucsd.edu/HAPL/ or contact John Sethian at sethian@this.nrl.navy/mil.