A Different Way to Ignite Fusion: Focused Energy Announces a New Approach to Inertial Confinement
Focused Energy developed its initial high-gain target design based on direct-drive laser inertial fusion.
The Fusion Report recently interviewed Dr. Debbie Callahan, senior scientist at Focused Energy, on their latest press release, which detailed their achievement of two science and technology milestones as part of the US Department of Energy (DoE) milestone-based fusion development program. Prior to Focused Energy, Debbie was on the staff at the Lawrence Livermore National Laboratory (LLNL) for 35 years. In this role, she was one of the principal scientists working on laser technology at the LLNL National Ignition Facility (NIF), where they achieved net-positive ignition in 2022 with a Q (ratio of energy released by ignition vs energy expended to achieve ignition) of roughly 1.5.
For reference purposes, it is believed that a “Q” of 50 to 100 (or greater) will be needed to achieve commercialization of fusion energy. Like many other private companies worldwide, Focused Energy is working on steps to improve “Q” to enable commercial fusion energy power plants. The press release describes two milestones achieved under the DoE fusion development program: improving the gain of the fusion targets and improving the ability of the lasers to achieve ignition.
Step 1: Increasing Target Gain
One way to increase “Q” is to increase the coupling of input energy to the fusion target. At NIF, the fusion target is a design known as a ‘hohlraum’ (German for ‘cavity’). In the hohlraum approach, a frozen deuterium-tritium (D-T) pellet is held inside a cryogenically-cooled aluminum cylinder roughly 6.4mm x 3.6mm (about 0.33 inches by 0.18 inches), with holes on either side. The laser beams go through the holes, impinging on the inside of the hohlraum and heating it. The interior of the hohlraum then emits x-rays, which heat up and compress the D-T pellet, causing fusion; this is why the approach is called “indirect drive.” The negatives of the indirect drive are twofold: i) heating the aluminum to x-ray emission requires significant energy, not all of which is emitted as x-rays; and ii) it means that the heating of the fuel and the compression of the fuel has to happen at the same time, requiring more laser energy.
Alternatively, there are several benefits if the target could be heated and then compressed without a hohlraum (also known as “direct drive”). The direct drive approach is precisely what Focused Energy’s first milestone is about – they use one set of lasers to heat up the pellet, and then a second set of more powerful lasers compress the target. According to Dr. Callahan, who is heading the efforts at Focused Energy, the direct drive approach has several benefits:
It cuts the compression speed required to achieve fusion in half, reducing the laser energy needed to 25% of that needed with a hohlraum.
This also allows four (4) times the amount of D-T fuel to be put into the pellet, increasing the amount of time that the fuel stays “together” to fuse.
As we said earlier, the direct drive approach results in nearly 28% of the fuel in the pellet being fused versus 7% in the hohlraum-based indirect drive approach. This direct drive design was proven by simulations; the next step is to verify the expected results through physical experimentation and then fine-tune the design.
Step 2: Using Protons to Improve Inertial Confinement
The laser-generated proton focusing was the second milestone achieved under the DoE Milestone Program. Dr. Callahan described the approach as taking a curved piece of foil and then bombarding the outside of the curved surface with a short-pulse high-power laser, producing a stream of high-energy protons. Since the foil surface is curved, the proton beam is focused, as if the foil is acting like a lens. The focused proton beam then impinges on the fusion target, achieving better “coupling” with the target.
This approach was confirmed through experiments at the Laboratory for Advanced Laser for Extreme Photonics (ALEPH) at Colorado State University, in conjunction with a number of laboratories worldwide. ALEPH is a petawatt-class, ultra-short pulse titanium sapphire (Ti:sapphire) laser system that produces 0.85 PW pulses of laser light with a 30 femtosecond duration and at a rep rate of 3.3 pulses per second. This “ultra-fast” laser also enabled the testing of automated alignment of targets with the laser, which is critical to achieving “shot rates” approaching 10 Hz, which is believed to be necessary for creating commercial power plants in the 500 MW to 1GW range.
The Result: Driving Q Towards Commercialization
As we said earlier, one of the primary goals of private fusion companies is to increase “Q” to the point where the power output of the fusion reaction is significantly greater than the energy put into the reaction; a “Q” of 50 to 100 or greater. Dr. Callahan stated that she believes the combination of direct drive targets and proton-based inertial confinement can improve “Q” by up to 80X, putting it in the range of 120. The next steps for Focused Energy is to take these technologies and build further experiments to validate the current results as they move towards commercial inertial confinement fusion energy. The company is building a new facility in the San Francisco Bay Area to bring together the top scientific and engineering minds to commercialize fusion.