Rugby Hohlraums Join the Ignition Scrum

Since NIF experiments began in 2010, the facility has pursued an indirect-drive approach to ignition using cylindrically shaped gold cans known as hohlraums.

In this configuration, all of NIF’s 192 laser beams enter the gold or depleted-uranium hohlraum through a pair of laser entrance holes and deposit their energy on the interior surface. The hohlraum heats up and efficiently creates soft x rays that bathe a suspended plastic (CH), high-density carbon (HDC), or beryllium (Be) capsule containing hydrogen fuel. The capsule compresses the fuel to conditions that approximate those at the center of the sun, with the goal of realizing energy gain from nuclear fusion.

One of the challenges of achieving energy break-even and net gain is ensuring that the hohlraum efficiently converts laser energy into the desired spectrum of x rays for driving a symmetrical capsule implosion and achieving high fuel compression. Because the laser pulses are relatively “long” at 10-20 nanoseconds (ns), the heated hohlraum walls have enough time to move toward the cylindrical axis and cause the laser beams to be absorbed well away from their initial location.

The motion of these “laser spots” is detrimental to achieving a round capsule implosion, so a helium gas fill often is used to hold back the gold walls just long enough while the NIF laser is on. If the laser pulse is short enough—about 10 ns or less—a “near-vacuum hohlraum” can be used to drive the capsule implosion. On the other hand, if the laser pulse is significantly longer than 10 ns, a high-density gas fill (at 1.6 mg/cc) is needed to hold back the walls.

Both of these hohlraum platforms have been used with success, but several challenges remain—including capsule symmetry, laser energy reflection, and missing x-ray drive energy (see “Climbing the Mountain of Fusion Ignition”). Because of these challenges, other hohlraum designs have been sought to achieve a balance of efficient and symmetric hohlraum drive. One candidate is a hohlraum shaped like a rugby ball, which resembles a cylinder with the corners of the can rounded off to minimize surface area. The major sink of energy in a hohlraum is through the wall surface area, and the rugby shape helps to appreciably reduce this energy loss.

As an alternative to the traditional cylindrical hohlraum, the rugby shape can be used to field a larger hohlraum that helps to delay arrival of the expanding gold wall and provides more radiation smoothing of the drive x rays at the capsule location. For these reasons a hohlraum with a rugby shape has gained interest as a potential additional platform for ignition studies at NIF.

In a Physics of Plasmas Letter published online on April 21, LLNL researchers describe an attempt to blend the design advantages of a rugby-shaped hohlraum with an intermediate gas-fill density for potentially optimized ignition performance. The design was chosen as a natural compromise between the two standard cylindrical hohlraum platforms (near-vacuum and high gas-fill) and leverages preliminary NIF experimental results in larger cylinders showing reduced laser backscatter losses and more benign implosion symmetry.

“Our letter proposes a novel ignition studies platform on the NIF using both rugby hohlraums and medium gas fills to combine the best of both worlds of the mainline cylindrical platforms with a novel shape,” said lead author Peter Amendt. “Progress to date has been outstanding.”

Numerical studies of the proposed ignition design show high energy gain, good resistance to target fabrication imperfections, and robust implosion symmetry. The immediate application is a testable platform for realizing more benign—and perhaps more efficient—hohlraum performance, moving closer to eventual energy break-even and gain greater than unity.

“The path to ignition is recognized as extremely challenging for any platform that is planned for testing on the NIF,” Amendt said. “However, the availability of an onsite world-class facility to test new ideas makes frontier research here at LLNL the norm, not the exception.”

Amendt was joined on the paper by LLNL colleagues Darwin Ho and Ogden Jones.