Simulations uncover obstacle to harnessing laser-driven fusion

Mar 26, 2013
These images from their simulations highlight the trajectories of randomly-selected electrons for a thin cone (left) and thick cone (right), each attached to a copper wire. Background colors show the strength of the electric fields pointing away from the cone and wire. For thin cones, the electric fields act to guide energetic electrons forward into the wire while for thick cones -- a more realistic case -- these fields are too distant to be effective. Credit: Ohio State

(Phys.org) —A once-promising approach for using next-generation, ultra-intense lasers to help deliver commercially viable fusion energy has been brought into serious question by new experimental results and first-of-a-kind simulations of laser-plasma interaction.

Researchers at The Ohio State University are evaluating a two-stage process in which a pellet of fusion fuel is first crushed by lasers on all sides, shrinking the pellet to dozens of times its original size, followed by an ultra-intense burst of light to ignite a chain reaction. This two-stage approach is called Fast Ignition, and there are a few variants on the theme. In a recent paper, the Ohio State research group considered the long-discussed possibility of using a hollow cone to maintain a channel for the ultra-intense "ignitor pulse" to focus laser energy on the compressed pellet core. Drawing on both experimental results from studies at the Titan Laser at Lawrence Livermore National Laboratory in California, and massively-parallel of the laser-target interaction performed at the (OSC) in Columbus, Ohio, the research team found compelling evidence that the cone-guided approach to Fast Ignition has a serious flaw.

"In the history of fusion research, two-steps-forward and one-step-back stories are a common theme," said Chris Orban, Ph.D., a researcher of the High Physics research group at Ohio State and the lead theorist on the project. "But sometimes progress is about seeing what's not going to work, just as much as it is looking forward to the next big idea."

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Since the ultra-intense pulse delivers energy to the fuel through relativistic electrons accelerated by the laser interaction, the Ohio State study focused on the coupling of the laser light to electrons and the propagation of those electrons through the cone target. Rather than investigating how the interaction would work on a high-demand, high-cost facility like the National Ignition Facility (NIF), which is also based at Lawrence Livermore National Laboratory and one of the largest scientific operations in the world, the researchers considered experiments just across from NIF at the Titan laser, which is much smaller and easily accessible.

Despite its size and despite having lower total energy, for a brief moment the Titan laser is many thousands of times more intense than NIF, which makes it a decent stand-in as a second-stage ignitor pulse. The OSU-led experimental team focused the Titan pulse on hollow cone targets attached at the tip to copper wires and observed the burst of X-ray photons coming from the copper as a measure of the to relativistic electron conversion efficiency.

The X-ray signal was much lower from the hollow cones with thicker cone walls. "This was strong evidence to the experimental team that the typical approach to cone-guided Fast Ignition wouldn't work, since thicker cones should be more realistic than thin cones," said Orban. "This is because electrons are free to move around in a dense plasma, much like they do in a normal metal, so the thicker cone target is like a thin cone embedded in a dense plasma."

These intuitions were tested in simulations performed at OSC. Whereas earlier efforts to simulate the laser-target interaction were forced to simplify or shrink the target size in order to make the calculations more feasible, Orban used the LSP code to perform the first-ever, full-scale 2D Particle-In-Cell simulations of the entire laser-target interaction using fully realistic laser fields.

These simulations also included a sophisticated model for the pre-heating of the target from stray laser light ahead of the ultra-intense pulse developed by collaborators at the Flash Center for Computational Science at the University of Chicago.

"We were delighted to help Chris use the FLASH code to provide realistic initial conditions for his Particle-In-Cell simulations," said Don Lamb, director of the Flash Center. "This is an outstanding example of how two groups can collaborate to achieve a scientific result that neither could have achieved alone."

To conduct the simulations, the Ohio State researchers accessed OSC's flagship Oakley Cluster supercomputer system. The HP-built system features 8,300+ Intel Xeon cores and 128 NVIDIA Tesla GPUs. Oakley can achieve 88 teraflops, tech-speak for performing 88 trillion calculations per second, or, with acceleration from the NVIDIA GPUs, a total peak performance of 154 teraflops.

"The simulations pointed to the electric fields building up on the edge of the cone as the key to everything," said Orban. "The thicker the cone is, the further away the cone edge is from the laser, and as a result fewer energetic electrons are deflected forward, which is the crucial issue in making cone-guided Fast Ignition a viable approach."

With both the experiment and the simulations telling the same story, the evidence is compelling that the cone-guided route to Fast Ignition is an unlikely one. While other studies have come to similar conclusions, the group was the first to identify the plasma surrounding the cone as a severe hindrance. Thankfully, there are still many other ideas for successfully igniting the fusion pellet with current or soon-to-be-constructed laser facilities. Any future efforts to spark fusion reactions with these lasers using a two-stage fast-ignition approach must be mindful to consider the neutralizing effect of the free electrons in the dense plasma.

"We could not have completed this project without the Oakley Cluster," Orban noted. "It was the perfect combination of speed and RAM and availability for us. And thanks to the profiling I was able to do, the compute time for our production runs went from two weeks in November 2011 to three or four days as of February 2012."

"Energy and the environment is one of the primary focus areas of the center, and this research fits perfectly into that domain," said Brian Guilfoos, the client and technology support manager for OSC. "Many of our systems were designed and software packages selected to best support the type of computing required by investigators working in fields related to our focus areas."

Explore further: Physicist demonstrates dictionary definition was dodgy

More information: The paper describing the study, "Coupling of high-intensity laser light to fast electrons in cone-guided fast ignition," was recently published in Physical Review E, a journal of the American Physical Society.

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User comments : 20

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ValeriaT
1.2 / 5 (19) Mar 26, 2013
Well, the better for cold fusion.. Some of us knew about it before five years already, but the money must be spent and job places generated anyway...
Q-Star
3.3 / 5 (14) Mar 26, 2013
Well, the better for cold fusion.. Some of us http://arxiv.org/...4229.pdf before five years already, but the money must be spent and job places generated anyway...


Uuuh, Zephyr,,,, the article is about yet another failure, not a success. I understand English isn't your first language, or 2nd or 3rd, but it would help help if ya read the entire article, not just the headlines.
Shootist
2.8 / 5 (9) Mar 26, 2013
Physicists I know have poo-pooed laser induced fusion for 25 years, and still do.

Polywell for the win.
TheGhostofOtto1923
2.5 / 5 (11) Mar 26, 2013
the article is about yet another failure, not a success
I love how much pessimists and religionists have in common. What exactly do you mean by 'failure'?

Scientists know the value of research:

"But sometimes progress is about seeing what's not going to work, just as much as it is looking forward to the next big idea."

-Plasma physics is about learning everything possible about plasma. Future tech will be doing all sorts of things with plasma. The more we know about it, the better.

And if we need to use a possibly spurious excuse like domestic power generation to secure funding for these megaprojects, then so what? People thought all that money spent on the 1000s of tons of fissile material was to prevent nuclear war. And now we have ample quantities of the most useful material a civilization at our stage of development, can possess.

Future gens will applaud the foresight of Leaders who made this necessity happen in the only manner possible.
Lurker2358
2.6 / 5 (10) Mar 26, 2013
Simulation =/= discovery.

We know from countless examples that computer physics models rarely make the correct prediction compared to the real world experimental results.
Q-Star
3.7 / 5 (9) Mar 26, 2013
"But sometimes progress is about seeing what's not going to work, just as much as it is looking forward to the next big idea."


Oops, sorry I was sort of unclear. My comment was on the poster of the post the post I was commenting on.

I agree with ya 100%. Often there is as much to be learned from a failure, as from a success. It's worthwhile research even if it isn't "paying" off as soon as it's done.
Mannstein
2.6 / 5 (7) Mar 26, 2013
@ ValeriaT

"but the money must be spent and job places generated anyway..."

Indeed they all have to put their kids through college. I hope Andrea Rossi is successful with LENR and puts this boondogle into the grave.

JeremyC
1 / 5 (3) Mar 26, 2013
ok....
FainAvis
2.7 / 5 (3) Mar 27, 2013
Article: "...shrinking the pellet to dozens of times its original size..." Puzzling that.

IanC1811
2.8 / 5 (6) Mar 27, 2013
Yes, I get tired of reporters using phrases like "... 10 times smaller ..." when they mean "... one tenth the size of..". I also hate people using "decimated" to mean "reduced to one tenth the original size". It doesn't mean that, it means "reduced by one tenth the original size", i.e. reduced to 90% of the original size. I know the language is evolving but this sort of thing leaves the language poorer, not richer.

But to get back to the original article ...
I have been reading about "advances" in nuclear fusion since I was a lad and it seems to me that it has been "two steps backwards and one step forwards" for the last 60 years. It is all very well to value knowing what doesn't work, but it would be nice to know something that did work. I have a sinking feeling that in the long run the conclusion will be that only gravitational confinement of the plasma will suffice.
rubberman
2 / 5 (4) Mar 27, 2013
"I have a sinking feeling that in the long run the conclusion will be that only gravitational confinement of the plasma will suffice." - Ian c

I says pardon? Were gonna make a star?
Q-Star
1 / 5 (4) Mar 27, 2013
the article is about yet another failure, not a success
Which is why I'm presenting it in context of article about another obstacle of laser fusion. Why do you have problem with it (together with many people who upvoted you)? I'm perfectly on topic here with it...


Yes ya are. And I am too. Maybe we are all confused. Maybe one of us are confused.

I hope Andrea Rossi is successful with LENR and puts this boondogle into the grave
I hope so too, but the boondogles are more vital, than you may think.


I hope so too, but now it's me that is confused.
daqman
1.3 / 5 (4) Mar 27, 2013
"shrinking the pellet to dozens of times its original size"

What sort of english is this? How can it shrink and be dozens of times the original size?

"shrinking the pellet to be dozens of times smaller than its original size"

There, that's better!
rkolter
1.3 / 5 (3) Mar 27, 2013
Article: "...shrinking the pellet to dozens of times its original size..." Puzzling that.


Loved this comment. I was impressed to find that apparently we now have the ability to shrink a mass to negative volume, which is what shrinking something by dozens of times it's original size would imply. I am anxiously waiting for the univese to core dump with a divide by zero error... :)
WillieWard
2.3 / 5 (4) Mar 27, 2013
Better would be multiple colliding-beams instead of lasers. Simple computer simulations performing few calculations per second would be enough to prove that multiple colliding-beams can result in much higher fusion rate than lasers.
dschlink
2 / 5 (4) Mar 27, 2013
@ ValeriaTI hope Andrea Rossi is successful with LENR and puts this boondogle into the grave.


He has been very successful; if you consider scamming millions of dollars from people without demonstrating any reproducible results success. Just like his trash-to-oil process in Italy: 12 million Euros to clean up the mess.

Right now, Mitsubishi and Toyota are the leaders in LENR.
ValeriaT
1 / 5 (4) Mar 27, 2013
Mitsubishi and Toyota are the leaders in LENR
You see, the Japanese did lost the WWW II instead and what? Does it change their credibility? So far we cannot buy /afford any cold fusion product, so I'd remain quiet in this matter. For me the technology itself is important, not the people behind it. I'm not blind proponent of A. Rossi, I simply would support everyone who will come first with something useful.
Parsec
3.7 / 5 (3) Mar 27, 2013
The huge win out of this research was clearly pointed out in the article, but I suppose most of you missed it by focusing on what was being shown would not work.

The point was that BEFORE the lab spent a ton of money and more work, the computers told them to try a new way, AND gained insight on how to make better and more detailed simulations that can help point exactly what that better way is. That is a big win for the good guys. This is the kind of lab results that pay for themselves many times over.
ValeriaT
1 / 5 (4) Mar 27, 2013
On the contrary: the physicists have built the NIF and Titan facilities first, they wasted billions of money into years of experiments and now they play smart before publics, when these money evaporated in futile research. Such a simulations should be done before twenty years, not by now.. You apparently missed the "detail", that this simulation models the experiments, which were done already - and which failed...;-)
Osiris1
1 / 5 (1) Mar 31, 2013
Someone wondered why computer models were not done this way twenty years ago. Twenty years ago I had a 66Mhz processor with one core, and a hard drive of 100 Megabytes. Industry did not have stuff a whole lot better. If this work had just started then with the tech available then, that work some wanted started then would only be a hundredth done now. So do not criticize before thinking of our old time capabilities. Remember the first lunar astronauts went to the moon litterally on an Apple][ . That sounds scary! I still OWN an Apple][ and it works....and I say that WAS scary knowing what I know how 'steve jobs' that incompetant bean counter mandated the Apple to be built--max profit from cheapest and most failable parts!! And we actually put that fruitcake into the ground with honors when he should have been used for fertilizer but the plants would have all died. My apple keyboard controller still fries and resets the system every hour on the hour...lol

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