Sandia–Univ. of Rochester Win Funding to Demonstrate Fuel Magnetization and Laser Heating Tools for Low-Cost Fusion Energy

Sandia and the Laboratory for Laser Energetics (LLE) at the University of Rochester will investigate the compression and heating of high energy-density, magnetized plasmas at fusion-relevant magneto-inertial [confinement] fusion (MIF) conditions, building on the recent Magnetized Liner Inertial Fusion (MagLIF) concept successes. The SNL-LLE team will conduct focused experiments based on the MagLIF approach at both SNL and LLE facilities, targeting key physics challenges in the intermediate plasma density regime. The team will also exploit and enhance a suite of simulation and numerical design tools validated by these experiments. Through this project, the team will provide critical information for improved compression and heating performance as well as insights on loss mechanisms and instabilities for hot, dense, magnetized plasmas. This information will help accelerate MagLIF concept development, and will also inform the continued development of intermediate-density approaches across the DOE’s Advanced Research Projects Agency–Energy (ARPA-E) ALPHA (Accelerating Low-cost Plasma Heating and Assembly) program portfolio.

Illustration of the stages of the MagLIF concept on Z. The target is a metal cylinder ~1 cm tall containing fusion fuel. The target is axially magnetized using magnetic field coils (not shown). The Z machine applies a current along the axial direction, which creates an azimuthal magnetic field outside the liner and the resulting magnetic pressure compresses the liner. Early in the implosion, a multi-kJ laser pulse from the Z-Beamlet laser is coupled to the fuel through a thin plastic window on one end. This energy is homogenized over the several tens of nanoseconds it takes to compress the fuel.
Illustration of the stages of the MagLIF concept on Z. The target is a metal cylinder ~1 cm tall containing fusion fuel. The target is axially magnetized using magnetic field coils (not shown). The Z machine applies a current along the axial direction, which creates an azimuthal magnetic field outside the liner and the resulting magnetic pressure compresses the liner. Early in the implosion, a multi-kJ laser pulse from the Z-Beamlet laser is coupled to the fuel through a thin plastic window on one end. This energy is homogenized over the several tens of nanoseconds it takes to compress the fuel.

ARPA-E’s ALPHA program funds developing 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 more attractive fusion reactors with energy and power requirements that are compatible with low-cost technologies.

As shown in the figure (right), 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 tesla (~60,000 times greater than the Earth’s magnetic field) before the shot. The target is imploded using a 20 million ampere, 100 nanosecond current from the Z pulsed-power accelerator. The liner rapidly becomes plasma. Just as the liner’s inner surface begins to move, the multiple-kilojoule, 1 terawatt, 0.53 mm Z-Beamlet laser is used to heat the deuterium gas.

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 kilotesla. 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 ~25–30 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.

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 kilometers per second 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).

OMEGA (left) stands 10 meters tall and is approximately 100 meters in length. This system delivers pulses of laser energy to targets in order to measure the resulting nuclear and fluid dynamic events. In this target chamber (center), OMEGA's 60 laser beams focus up to 40,000 joules of energy onto a target (right) that measures less than 1 millimeter in diameter in approximately one billionth of a second.
OMEGA (left) stands 10 meters tall and is approximately 100 meters in length. This system delivers pulses of laser energy to targets in order to measure the resulting nuclear and fluid dynamic events. In this target chamber (center), OMEGA’s 60 laser beams focus up to 40,000 joules of energy onto a target (right) that measures less than 1 millimeter in diameter in approximately one billionth of a second.

It is predicted that yields two orders of magnitude larger are possible on Z using deuterium fuel within the next three years. If equimolar DT were to be used for the fusion fuel instead of pure deuterium, simulations suggest the fusion yields could exceed the total amount of energy delivered to the fuel (fuel gain > 1). Such an achievement—modeled and understood—would clearly demonstrate MIF’s credibility as an ICF/MCF alternative. A key challenge is the Z facility’s relatively low shot rate. Our research will greatly increase MagLIF rate of progress by conducting many more experiments per year on Sandia’s Z-Beamlet laser and the University of Rochester’s OMEGA (shown in the figure above) and OMEGA-EP lasers. Our experiments will target key physics challenges such as laser preheating and fuel magnetization. We will also conduct fully integrated, scaled-down experiments of the MagLIF concept on LLE’s OMEGA facility. Testing MagLIF on OMEGA and leveraging NNSA-funded research on Z will demonstrate key MIF tools on multiple facilities at significantly different energy, time, and spatial scales. Another key project goal is to use state-of-the-art simulations and numerical design tools, tested by these experiments, to validate and improve fusion-target design parameters for magnetized plasmas.

ARPA-E funding is $3.8M over two years. At this point, negotiations are underway to reconcile the elements of Sandia and LLE ’s originally proposed research program to the scope of the ARPA-E funding award.

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July 7, 2015