And could a barrier be lurking?

Jennifer Granholm is the U.S. Secretary of Energy in President Joe Biden’s cabinet.

Last week, she said at a press conference:

“Simply put, this is one of the most impressive scientific feats of the 21st century.”

The “this” is the achievement by a team at Lawrence Livermore National Laboratory (LLNL) of fusion ignition. This means that the portion of the marginal energy output of the reaction that is directed toward heating the fuel exceeds the amount needed to make and keep it hot, so that the reaction is self-sustaining. The reaction can continue as long as fuel remains.

The temperature level required is about four times that of the interior of the sun. That is because nearly all the fusion output of the sun goes toward keeping the interior hot. The environment of the LLNL module has more scope for energy to escape. The sun has it easy. Lasers are used to confine and compress the fuel. How the laser input is modulated is our main discussion topic.

Breaking a Barrier or Just Crossing a Threshold?

Terrestrial fusion has endured many disappointments in the years since the 1970s, when Ken’s freshman physics class at Princeton toured their Tokamak fusion reactor. Until last year, the output had never reached 3% of the intensity needed for ignition. In 2016, the journal Physics Today published an article admitting that ignition could be beyond reach. It quoted predictions by Stephen Bodner, from when he led a fusion-seeking team at the US Naval Research Laboratory in 1995, that the need to control the reaction—as a hydrogen bomb need not do—set barriers on reaching ignition.

Last year, however, the National Ignition Facility (NIF) at LLNL jumped from under 3% to 70% of the ignition threshold. This left the question of whether further progress would be like the famous difficulty of breaking the sound barrier for aircraft. It was not. The essence of last week’s announcement is that the NIF team were able to progress smoothly from 70% to over 150%.

Is this a breakthrough? We quote the contrast made by Tom Hartsfield, a PhD physicist and writer for RealClearScience:

Here we go again. In 2021, the National Ignition Facility (NIF) announced a scientific breakthrough in its pursuit of fusion power technology. One year later, they’re making another announcement, heralded as “game-changing,” “transformative,” and “a moment of history.” … This week’s announcement is an increase in fusion energy output, relative to laser energy input, from 70% in 2021 to 154% in 2022. This incremental, possibly incidental, progress toward thermonuclear burn is not a breakthrough.

This gave Ken the image of serenely crossing the event horizon of a large black hole. The traveler feels nothing special according to the standard description in relativity. Part of the recent firewall paradox is that those observing the traveler from outside would see a catastrophe at the horizon. Well, no hint of catastrophe occurred at NIF. So any barriers must be elsewhere than the ignition point.

The Prior Shot

Now we continue Hartsfield’s opening to his article, where we broke it at the “…”

… But this is not a meaningful breakthrough for practical, commercial fusion power: NIF still drains at least 130 times more energy from the power grid than it produces.

The reason is that the ignition is a two-step process. To quote the LLNL’s page “Anatomy of a NIF Shot,” which is linked also by Hartsfield:

“When the shot is fired, the NIF capacitor bays deliver 400 megajoules of stored electrical energy to the main laser’s 7,680 flashlamps. In the laser bays, the amplifiers increase the one-joule input from the injection laser to nearly 4 million joules of infrared laser light. In the Target Bay, the final optics assemblies convert the infrared light to about 2 million joules of high-energy ultraviolet light and direct the laser beams to the target at the center of the Target Chamber.”

Here is our nontechnical way to explain this:

Suppose I am in the forest and need to have some wood gathered to make a fire. So I gather some wood and then I burn the wood. Say that it lasts one hour. The true measure of the fire should include the cost of gathering the wood. Not just the fact that the fire lasts one hour.

The initial shot is just like gathering the wood:

1. Energy is used to create a laser pulse.
2. The pulse heats the target.
3. The target makes output from fusion.

Using Hartsfield’s numbers, part (1) is about 384 to 400 megajoules (MJ) to create the pulse. The laser pulse (2) heats the target with 2.05MJ. This creates the fusion and (3) releases 3.15MJ. This is why they say it is ignition. But it is still way off since:

An Explodable But Reasonable Analogy

This led us—well, Ken—to wonder whether there might be an analogy to the “prior exchange” which explains the apparent paradox of quantum superdense coding. We like the diagram of this in the treatment by IBM Qiskit:

(We added Pauli gate legend to the diagram.)

Midway in the diagram, Alice chooses one of four options. Her only interaction with Bob after then is to send him one qubit. Bob is then able to tell which option she chose by measuring that qubit plus one that was previously in his possession. The paradox is Bob’s apparent gain of two bits of information from the transmission of only one qubit.

You could say that Alice and Bob have achieved “information ignition,” getting two bits out of one after Alice’s choice. The rub is that this is only possible because of the prior entanglement of their qubits mediated by Charlie. To transmit more information at the apparent rate, they have to gather more wood from Charlie. There is a hard-and-fast barrier saying this cannot be avoided: Alexander Holevo’s 1973 theorem bounding the extraction of classical information from quantum states.

Is there a substantive analogy to the situation with terrestrial fusion? Might there be a barrier to attaining the rate yielding thermonuclear burn needed to surmount the startup energy? Hartsfield pegs this rate at a factor of 1,000 higher, “100,000%” in his terms. What we wondered most particularly is whether, under the hood of the quantum field equations governing the behavior of the supercondensed fuel, there might be a transmuted form of the limitation on information output.

Ten years ago, we created the “Pip” persona as an icon for asking childlike questions. We figured the value of hitting on one novel question that is good would outshine the expenditure of energy on twenty squibs. In this case, however, we can already tell there’s no mathematically ingrained physical barrier just by thinking of a hydrogen bomb. The analogy goes boom.

Bodner, now retired, has evidently declined to transfer his previous thoughts of barriers to controlled fusion from ignition to the burn point. He already stated, after last year’s breakthrough, that:

“It demonstrates to the sceptic that there is nothing fundamentally wrong with the laser fusion concept. It is time for the U.S. to move ahead with a major laser fusion energy program.”

Open Problems

Do we need to include the gathering step too? Is that important to include in analyzing the whole physical system?

[author fix and a word change]