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The Independent

June 25, 2019

NIF Doubles the Energy From Its Target Shots

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Posted: Thursday, June 21, 2018 12:00 am

Experiments using new target designs have enabled researchers to double energy yields from thermonuclear targets after being hit by beams from Lawrence Livermore National Laboratory’s huge National Ignition Facility laser system, the Laboratory reported last week.

The research results appear to be an important step on the difficult road to NIF’s long sought goal of demonstrating ignition in the laboratory.

The experiments were “robust” in the sense that they were repeated several times with slight variations and still produced the same energy as measured by neutron output, according to the leader of the research team, LLNL’s Sebastien Le Pape.

Le Pape described himself as “optimistic” about continuing progress toward ignition but cautioned that the research is very difficult and success will have to be earned.

A report on the research was published in the journal Physical Review Letters.

Laura Berzak Hopkins, lead designer for the experiments, was enthusiastic about the results and emphasized the “incredible amount (learned) over the past few years.

“We have really been able to surmount a very large number of challenges,” she said.

On the other hand, like Le Pape, she cautioned against overconfidence. “I’m certain there will be more that we don’t know we don’t know. We are at an exciting time but that’s not to say that there aren’t more challenges…that we are going to need to work through and resolve.”

She said that there are inevitable uncertainties in the “incredibly powerful and capable” computer codes that predict and analyze experimental outcomes. For example, her calculations do not incorporate the effects of gravity or the structural details of tiny pins that hold the targets in place.

Nevertheless, the teams have learned to move past some of the difficult and even unknown physics that made computer simulations less reliable so that they are now in better agreement with results.

“We have tried to really simplify the system as much as we can to make it more predictable,” Le Pape said. “We removed some part of the unknown physics that is hard to model.”

‘Critical Step Forward’

Outside the program, experts also called the results impressive and promising. The journal Physics Today quoted MIT senior scientist Richard Petrasso, who was not involved in the research, as saying said that LLNL has taken “a critical step forward” in the quest to achieve ignition in the laboratory.

In the extraordinarily rapid implosion-explosion sequence of a laser driven target, measured in billionths of a second, ignition begins when the center of a rapidly compressing laser target becomes a “hotspot” of self-sustaining thermonuclear reactions that produce more energy than is lost from the system.

Achieving ignition is not necessary for NIF’s role in the nation’s nuclear weapons program, called Stockpile Stewardship, where it is used to explore weapons science on a laboratory scale.

In the absence of full-scale nuclear testing, it comes closer to recreating the extreme temperatures and pressures of a nuclear explosion than do other scientific instruments.

Achieving ignition would add considerable scientific value by generating energy bursts that bring it still closer to these extremes. It might also revive interest in pursuing the so-called inertial approach to controlling thermonuclear fusion for civilian energy.

Ignition has been a key goal for NIF, although a decade ago, when the system was still being built, Laboratory management was careful not to guarantee that it would be achieved. It stressed the facility’s value to the weapons program even without ignition.

Years later, however, some laser program managers acted as if ignition could be taken for granted and aggressively promoted NIF as a route to civil electrical power.

The failure to achieve ignition became an embarrassment not so much because the science was more difficult than expected, but to critics, it seemed to symbolize a tendency to over promise and under deliver in the ultracompetitive modern world of science.

Target Design

The successful experiments announced last week depended significantly on a target design involving an ultrathin, high density carbon capsule often called diamond, although it is not gem quality material.

The capsule contained heavy hydrogen (fusion) fuel and was seated inside a chamber, called a hohlraum, made of depleted uranium.

This design “allowed the researchers to greatly improve their control over the symmetry of the x-rays that drive the capsule, producing ‘rounder’ and more symmetric implosions,” the announcement read.

The x-rays are created when the laser system’s ultraviolet beams strike the inside of the hohlraum. This conversion involves loss of energy, but the Livermore approach to this research is based on years of evidence that improved symmetry and other characteristics that more than make up for the loss.

“By controlling the uniformity of the implosion, we’ve improved the compression of the hotspot” to temperatures and densities never achieved before in the laboratory, according to research leader Le Pape, first author of the technical article.

Speaking from France, his country of origin, where he was traveling, Le Pape said that “everybody (in the laser community) is thinking and talking about what we are doing on NIF.

“People are excited about it and want to contribute and join the effort.”

Besides LLNL, where most of the experimenters work, significant contributions were made by researchers from General Atomics, University of Rochester Laboratory of Laser Energetics, Los Alamos National laboratory, MIT and Diamond Materials GmbH of Freiburg, Germany.

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