The Big Dipper: a new laser pulse design for high gain fusion experiments

What is it about?

The Big Dipper laser pulse (also known as shock-augmented ignition) is a theoretical concept that builds upon previous work in developing high "fusion energy-gain" in laser driven experiments. Fusion energy-gain is a measure of energy efficiency, from laser input to fusion output, and is one of the single most important metrics for an experiment to become a future energy source. Related to the Big Dipper, "Shock ignition" is a more established design for achieving high gain. Shock ignition uses a sudden increase in laser power to drive a late stage shock creating the hot, dense conditions in the core, required for high gain fusion. We say late stage as shock ignition has an early stage of target compression which is typical for laser driven fusion experiments. In the early stage, a series of laser pulses accelerate the hollow target (similar to a ping pong ball but the size of a BB pellet) inwards towards the core. The benefit of shock ignition is that we can relax some of the early stage pulse criteria, leading to a slower, more stable implosion. Possible critiques of shock ignition are that: the high laser intensities required lead to laser driven instabilities in the plasma, the high power requirement is hard to test at current implosion facilities and the approach is not easily compatible with indirect drive (one of the two major branches of laser fusion). Building beyond shock ignition, the Big Dipper is designed to maintain all the benefits while minimizing the challenges. Rather than a late stage increase in laser power, the Big Dipper first reduces laser power, "relaxing" the exterior of the target. This means that the laser power can be increased back up to the same level as before to shock the target without exceeding the intensities/powers of the early stage laser pulses. Overall, the Big Dipper is simulated to have similar benefits to shock ignition and removes the drawbacks.

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Photo by Manki Kim on Unsplash

Photo by Manki Kim on Unsplash

Why is it important?

Nuclear fusion at its core offers the ultimate energy solution. It provides a decentralized resource through Deuterium found in seawater, which helps prevent conflict zones and improve international energy security. Fusion plants do not have the large physical footprint of solar and wind and the reactants are inherently stable, making super-critical meltdown impossible. Similar to current nuclear fission reactors, they have the potential to be the zero carbon baseload or laser fusion in particular has the potential to vary output on a sub-minute timescale providing a high-value, demand dependent source. It is also one of the few energy sources renewable or otherwise capable of scaling with the predicted population growth.

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