LLE Achieves Cryogenic Target Milestone

October 2006

Cutaway view of a cryogenic target showing the DT fuel contained within a uniform shell of deuterium ice and a deuterated plastic outer layer.

In July 2006, scientists at the University of Rochester’s Laboratory for Laser Energetics (LLE) achieved a major breakthrough in laser-fusion research that could have a significant impact on future energy production.

LLE researchers used the laboratory’s fusion laser, OMEGA, to successfully fire on targets containing frozen fusion fuel. The target designs are energy-scaled to those that will eventually be used on the National Ignition Facility to achieve thermonuclear fusion ignition and gain. The National Ignition Facility is a multi-billion dollar laser-fusion experimental facility nearing completion at the Lawrence Livermore National Laboratory in California. The LLE breakthrough is the result of many years of research and development.

The cooling module for the target fill/transfer station (FTS).

In the early 1970s, scientists at the LLE and other research facilities around the world began to develop techniques for forming frozen-fuel targets that could be used for laser fusion experiments. Fusion is the fundamental energy-producing process of the stars. Scientists all over the world have been working for several decades to tame this process so that it can be used to produce economically affordable and safe energy.

Laser fusion uses high power laser beams to heat and compress targets containing near-equal mixtures of the two heavy isotopes of hydrogen–deuterium and tritium (DT). At high temperature and density, these isotopes fuse to form the element helium and energetic neutrons which release a large amount of energy. At sufficiently high temperature (100 million degrees) and density (nearly a thousand times the density of solid DT) the fusion fuel ignites and generates a thermonuclear burn wave that consumes a large fraction of the fuel and releases more energy than was invested in the laser to start this process.

Moving Cryogenic Target Cart (MCTC) being docked at the lower pylon structure of the OMEGA target chamber.

To achieve fusion ignition with the lowest possible laser energy, targets comprised of DT fuel frozen to temperatures below - 430 F and uniformly layered inside spherical targets must be used. In the late 1980s, LLE experiments with relatively small, glass-shell targets containing thin layers of DT demonstrated compressed fusion fuel densities in excess of 100 times the density of frozen DT. The cooling module for the target Fill/Transfer Station (FTS).These experiments verified the feasibility of the "direct-drive" approach to laser fusion. Direct drive involves the direct illumination of spherical targets by many symmetrically disposed laser beams and is the alternative to the "indirect drive" approach. The indirect-drive approach uses x rays produced by internally illuminating a metal cylinder with many laser beams to compress and heat a spherical capsule placed within the cylinder.

A target suspended inside the lower half of a Gold coated layering sphere.

The early 1990s saw LLE begin to develop an advanced cryogenic target handling system that could be used for ignition–scaling direct-drive experiments on OMEGA. The first cryogenic implosion experiments using this system were conducted at LLE in 2000.

The subsequent program focused on minimizing target vibration, investigating and refining the target manipulation at cryogenic temperatures, testing the control software for the retraction of the thermal shroud around the target, optimizing the timing sequence for firing the laser, and producing and characterizing a smooth ice layer in the target. LLE researchers recently succeeded in addressing these issues and created nearly perfect targets of the type that eventually can be used on the NIF to demonstrate ignition and gain.

A target in a Gold coated Beryllium C-Mount as compared to a penny.

March 2006 marked the first time that a beta-layered, DT target was used in a laser-driven implosion. Beta-layering is a technique that uses the radioactive decay of the tritium fuel to locally heat the DT and smooth out variations in the DT ice thickness. The first two layered DT cryogenic targets that were shot contained only 13 percent tritium. Over the next few months, a series of tests were conducted to assure the safe and reliable operation of the tritium gas fill system and the percentage of tritium was increased in several steps to more than 50 percent.

Pins, alignment fixtures, target racks, and collets for mounting and preparing targets.

In July 2006, LLE scientists achieved a major milestone in cryogenic target fabrication. In four trials, the OMEGA laser successfully imploded several fully beta-layered cryogenic-DT targets that contained more than 50 percent tritium.

The quality of the inner ice surface of the most recent implosions met the requirements for direct-drive ignition on the NIF. The 3-D ice characterization based on multiple shadowgraphs indicated an inner-ice surface roughness as low as 0.72-µm rms at DT’s triple point, i.e., the temperature and pressure at which gas, liquid, and solid coexist in thermodynamic equilibrium. This is well within the smoothness required for direct-drive ignition experiments on the NIF.

Monochrome and false color thermal image analysis of a target during characterization.

Prior to imploding these targets, the ice was cooled to approximately a quarter of a degree below the triple point. The pre-shot characterizations indicated that the layer quality at the time of the shot was ~1.0-µm rms. These are the first targets imploded on any laser facility that can be scaled to ignition on the NIF. The NIF targets will be almost three times larger.

These and future experiments significantly enhance the prospects of achieving fusion ignition and gain on the NIF with direct-drive targets within the next decade and provide a strong impetus to explore the engineering and economic feasibility of laser-fusion energy production.