Around the Lab
Funding Aimed at Fusion Energy Awarded to the Laboratory for
Laser Energetics–Sandia National Laboratories Collaboration
June, 2015
The Department of Energy’s Advanced Research Projects Agency-Energy (ARPA-E) has announced a two-year, $3.8 million award for Sandia National Laboratories and the University of Rochester’s Laboratory for Laser Energetics to study the potential of combining two different technologies to further advance their research efforts to produce controlled fusion reactions.
“The ARPA-E award will fund research that will benefit from the existing strong collaborative effort between Sandia National Laboratories and LLE,” said Professor and LLE Director Robert L. McCrory. “LLE, with its 60-beam OMEGA and four-beam high-energy OMEGA EP lasers, and Sandia, with the world’s largest pulsed-power machine, Z, provide unique capabilities to explore a range of fusion parameters previously unexplored.”
The award seeks to build upon the recent successes of Sandia’s magnetized liner inertial fusion (MagLIF) concept. MagLIF is an innovative MIF concept that exploits advances in pulsed-power technology. A MagLIF target is magnetized with an axial field of up to 30 T (~60,000× greater than the Earth’s magnetic field) before the shot. The target is imploded using a 20-MA, 100-ns current from the Z pulsed-power accelerator. The liner rapidly becomes plasma. Just as the liner’s inner surface begins to move, the multikilojoule, 1-TW, 527-nm Z-Beamlet laser is used to heat the deuterium gas.
Collaborators from Sandia National Laboratories and LLE perform experiments on the OMEGA EP Laser System that test the effects of applied magnetic fields on laser preheat in the MagLIF scheme
The OMEGA EP Laser System bathed in its own light as laser beams ignite during a target shot
The axial magnetic field suppresses electron heat transport from the hot fuel to the cold liner wall, enabling electron–ion equilibration and allowing a quasi-adiabatic compression to fusion temperatures (>50 million Kelvin) and fuel pressures of approximately gigabars (~5 billion times atmospheric pressure). The embedded axial magnetic field is compressed to ~13 kT. Target designs on the Z facility are predicted to reach fusion yields that approach and exceed the driver energy invested in the fuel if equimolar deuterium–tritium (DT) fuel is used. In these designs, the fuel is compressed radially by ~25 to 30 times before the fuel’s plasma pressure stops the implosion (the point of “stagnation”). At stagnation, the charged-particle products [alpha particles for DT reactions or 1-MeV tritons for deuterium–deuterium (DD) reactions] are magnetized, meaning that their gyro radii are less than the final fuel radius resulting in their confinement in the plasma.
“Creating a high-output 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,” said project lead and Sandia manager Dan Sinars. “We hope that the results of our research will motivate more efforts in this area.”
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,” said Jonathan Davies, a research scientist and leader of the effort at LLE. “The ARPA-E project will bring together the resources of Sandia and LLE to work on the same project—the coupling of laser energy and fusion fuel—with completely different techniques.”
“These experiments allow us to study MagLIF at a much smaller size and at a faster rate than on Z,” said 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.”
This information will help accelerate the development of the MagLIF concept as part of ARPA-E’s ALPHA (accelerating low-cost plasma heating and assembly) program portfolio. The ALPHA program funds the development of the tools to build foundations for new pathways toward fusion power. ALPHA is focused on approaches that exploit magnetic fields to reduce energy losses in the intermediate-ion-density regime between lower-density magnetic confinement fusion (MCF) and higher-density inertial confinement fusion (ICF). This intermediate-density regime is not as well explored as the more-mature MCF and ICF approaches, and it may offer new opportunities for fusion reactors with energy and power requirements that are compatible with low-cost technologies.
MIF has long been discussed as a promising fusion approach using plasma densities between those of MCF (~1014 ions and electrons per cubic centimeter) and traditional ICF (>1025 ions and electrons per cubic centimeter), but there remains a paucity of experimental data quantitatively compared to state-of-the-art simulations validating MIF, particularly in fusing plasmas. In late 2013, experiments began on the Z pulsed-power facility studying MagLIF targets. These experiments showed that a 70 km/s cylindrical liner implosion compressing laser-heated and axially magnetized deuterium gas could produce a final ion temperature of up to about 44 million Kelvin and up to 2 × 1012 thermonuclear DD neutrons. In addition, >1010 secondary DT neutrons were observed, which is only possible with significant fuel magnetization. By reducing electron thermal conduction losses and magnetizing the ions, MagLIF relaxes the constraints on traditional inertial fusion designs (final fuel pressure, areal density, convergence, driver power, and intensity).
Julie Fooks working on a MagLIF target
“It should easily be possible to do more than 200 laser experiments 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,” Sinars said.
The work will take place on several parallel tracks: performing scaled-down MagLIF experiments at the LLE Omega Laser Facility; improving performance of full-scale MagLIF experiments on Z through optimized laser preheating and improved axial magnetic-field hardware; and validating simulations against experiments.
Close-up of a MagLIF target
Said Sinars, “The overall grant objective is to ultimately improve techniques to compress and heat intermediate-density, magnetized plasmas, as well as provide insights into relevant energy losses and instabilities.”
ARPA, Sandia, and LLE believe that a more-efficient coupling of the laser energy to the fusion fuel would increase the number of neutrons produced. Over many years, scientists at LLE have developed techniques to “smooth” laser beams, which provide very uniform deposition of the applied laser energy. “By smoothing the beam,” said Sinars, “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.”
Other laser experiments will include changing the beam’s intensity, its distance to the liner’s entry port, and the size of the liner hole through which the beam must pass. If the beam entrance hole is too small, not enough energy reaches the target; if the entrance hole is too large, too much energy escapes. When optimized, the process should allow fusion reactions to occur at 1% to 2% of the density and pressure required in traditional ICF implosions.