Sandia LabNews

Sandia’s Z machine receives funding aimed at fusion energy

OMEGA 20-cm disk amplifiers. // Technicians work on the Omega laser at the University of Rochester's Laboratory for Laser Energetics April 9, 2014 // photo by J. Adam Fenster / University of Rochester

Twenty-centimeter disk amplifiers (shown here) used to energize 60 laser beams glow brightly at the 100-meter-long OMEGA laser at the University of Rochester’s Laboratory for Laser Energetics, while amplified light passes through the large tubes of Sandia’s Z-Beamlet laser (not shown), one of the most powerful in the world. Both lasers will experiment with more efficiently preheating fusion fuel to improve the output of Sandia’s MagLIF (Magnetized Liner Inertial Fusion) technique (Photo courtesy of University of Rochester)

A $3.8 million, two-year grant to Sandia and the University of Rochester’s Laboratory for Laser Energetics (LLE) is expected to hasten the day of fusion break-even and eventually high-yield for energy production.

The grant was announced by DOE’s Advanced Research Projects Agency for Energy (ARPA-E).

Previous fusion work at both institutions had been funded by DOE’s National Nuclear Security Administration (NNSA) solely to support the Stockpile Stewardship Program, whose goal is to maintain a safe and reliable nuclear deterrent without nuclear testing.

Break-even means as much energy emerges from a fusion reaction as is put into it; high-yield means that much more energy emerges.

The work to be conducted at both laboratories is expected to advance a promising Sandia energy concept called MagLIF, for Magnetized-Liner Inertial Fusion.

Originally proposed in a 2010 theoretical paper by Sandia researcher Steve Slutz (1684) and colleagues, the concept uses a laser to heat fusion fuel contained in a cylinder, called a liner, as that cylinder itself is compressed  by the  huge magnetic field of Sandia’s massive Z accelerator. A secondary axial magnetic field embedded in the fuel and cylinder impedes the laser energy from escaping the resultant plasma, which would lower the temperature of the fuel and reduce the fusion output.

The combined heat and pressure, created by the laser preheating and liner imploding over a hundred or so nanoseconds, have been shown to force fuel to fuse in recent experiments on Z. The next step is to force it to fuse more efficiently and, at the same time, allow researchers to learn more about important  physical mechanisms at work.

ARPA-E’s bet, and Sandia’s and Rochester’s with it, is that a more efficient coupling of the laser energy to the fusion fuel will increase the number of neutrons produced, the gold standard in judging the efficiency of the fusion reaction.

Smoothing laser beams

As it happens, scientists at the LLE over many years have developed techniques to  “smooth” laser beams, a prerequisite for delivering more energy to fusion fuel.

“By smoothing the beam,” says project lead and Sandia senior manager Daniel Sinars (1680), “we eliminate hot spots in the laser beam that waste laser energy and potentially alter the beam path of some of the light. This altered path can disintegrate portions of the liner or other surrounding material. Some of that material then may contaminate the fuel and increase radiation losses, causing the fuel temperature to collapse below that needed for fusion reactions to occur.”

When optimized, the process should allow fusion reactions to occur at 1 to 2 percent of the density and pressure required in traditional inertial confinement fusion (ICF), which has used either laser-created X-ray pulses or direct laser illumination to compress a pea-sized capsule containing fusion fuel.

Says professor and LLE director Robert L. McCrory, “The ARPA-E award will fund research that will benefit from the existing strong collaborative effort between Sandia National Laboratories and LLE.” The two institutions already have traded scientific knowledge and laser components in pursuit of the grand challenge of laboratory-scale fusion. “LLE, with its 60-beam OMEGA and 4-beam high-energy OMEGA-EP lasers, and Sandia, with the world’s largest pulsed-power machine at Z, provide unique capabilities to explore a range of fusion parameters previously unexplored,” he says.

Nuclear fusion joins small atoms like hydrogen, releasing huge amounts of energy in the process. Unlike nuclear fission, which splits large atoms such as uranium, the dream of fusion is that it eventually could provide humanity unlimited energy from sea water and from such abundant elements as lithium with significantly less radioactive hazards than fission energy.

Unlike fission, fusion requires that matter be brought to enormous temperatures such as those found in the center of stars, approximately 50 to 100 million degrees. The challenge of fusion is to create matter at such temperatures at high enough pressures and for long enough times to release significant amounts of energy.

“Creating a high-yield reaction in a MagLIF plasma at Z should demonstrate the promise of the broader field of research we call magneto-inertial fusion — a potentially inexpensive form of fusion,” says Dan. “The overall grant objective is to improve techniques to compress and heat intermediate-density, magnetized plasmas, as well as to provide insights into relevant energy losses and instabilities. We hope that the results of our research will successfully motivate more investment by the Department of Energy and private companies in this field.”

An advantage of laser heating is that ideas involving lasers can be tested on multiple facilities across the country, allowing a much larger number of tests per year than is possible on the unique Z facility.

“It should easily be possible to do more than 200 laser shots a year split among the Z-Beamlet, OMEGA, and OMEGA-EP facilities, in contrast to the two dozen or so integrated MagLIF experiments a year realistically possible on Z,” Dan says.

A new path in fusion research

The LLE’s OMEGA laser, funded and operated as a national user facility with more diagnostics than Z’s Beamlet laser, is expected to greatly speed the work. “OMEGA can fire 12 times per day and can also provide better diagnostic access,” says Jonathan Davies, a research scientist and leader of the effort at LLE. “The ARPA-E project will bring together the resources of Sandia and the LLE to work on the same project  with completely different techniques.”

 Integrated laser experiments, where 40 of OMEGA’s 60 beams are used to compress the liner as well as heat the magnetized fusion fuel it contains, are also part of the ARPA-E program. “These experiments will allow us to study MagLIF on a much smaller scale and at a faster rate than on Z,” says Davies. “If the small-scale MagLIF experiments are successful and accurately modeled, we will have demonstrated magneto-inertial fusion principles over a very broad range of energy, space, and time scales.”

The collaboration will study fusion in a relatively unexplored intermediate density regime between the lower-than-air density of magnetic confinement fusion, which uses magnetic field to contain fusing plasma, and the greater-than-solid density of ICF, which uses X-rays or direct laser illumination to crush pellets of fusion fuel over times less than a billionth of a second. “With this collaboration, we will apply our expertise to explore a new path in fusion research,” Davies says.

The work will consist of four parallel efforts: achieving fuel pre-heating; determining whether MagLIF can reach fusion conditions on Z and on the OMEGA laser; and validating simulations against experiments.