From: Andy Soos, ENN
Published March 22, 2012 08:12 AM

Laser Induced Fusion

Inertial confinement fusion (ICF) is a process where nuclear fusion reactions are initiated by heating and compressing a fuel target, typically in the form of a pellet that most often contains a mixture of deuterium and tritium. In doing so fusion is initiated. This is basically a small controlled fusion reactor. To compress and heat the fuel, energy is delivered to the outer layer of the target using high-energy beams of laser light, electrons or ions, although for a variety of reasons, almost all ICF devices to date have used lasers. The National Ignition Facility (NIF), the world's most energetic laser, surpassed a critical milestone in its efforts to meet one of modern science's greatest challenges: achieving fusion ignition and energy gain in a laboratory setting. NIF's 192 lasers fired in perfect unison, delivering a record 1.875 million joules (MJ) of ultraviolet laser light to the facility's target chamber center.

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If a source of compression can be found, other than a nuclear bomb, then the size of the reaction could be scaled down. This idea has been of intense interest to both the bomb-making and fusion energy communities. It was not until the 1970s that a potential solution appeared in the form of very large, very high power, high energy lasers, which were then being built for weapons and other research. The mix in such a system is known as a target, containing much less fuel than in a bomb design (often only micro or milligrams), and leading to a much smaller explosive force.

Generally ICF systems use a single laser, the driver, whose beam is split up into a number of beams which are subsequently individually amplified by a trillion times or more. These are sent into the reaction chamber (called a target chamber) by a number of mirrors, positioned in order to illuminate the target evenly over its whole surface. The heat applied by the driver causes the outer layer of the target to explode.

This historic laser shot involved a shaped pulse of energy 23 billionths of a second long that generated 411 trillion watts (TW) of peak power (1,000 times more than the United States uses at any instant in time).

The record-breaking shot was made March 15.

"This event marks a key milestone in the National Ignition Campaign's drive toward fusion ignition," said NIF Director Edward Moses. "While there have been many demonstrations of similar equivalent energy performance on individual beams or quads during the completion of the NIF project, this is the first time the full complement of 192 beams has operated at this sound barrier."

The ultraviolet energy produced by NIF was 2.03 MJ before passing through diagnostic instruments and other optics on the way to the target chamber. As a result, NIF, located at Lawrence Livermore National Laboratory, is now the world's first 2 MJ ultraviolet laser, generating nearly 100 times more energy than any other laser in operation.

The 1.875 MJ shot exceeds NIF's original design specification and sets the stage for full-power experiments over the coming months. Not only did the shot achieve the highest recorded energy threshold, it also was one of the most precise ever fired at NIF: The energy produced was within 1.3 percent of its goal. Such precision is vital because the energy distribution among the beams determines how symmetrical an implosion is obtained in capsules containing fusion fuel. Implosion symmetry is a critical factor in achieving the pressures and temperatures required for ignition. 

"Our facility's ability to demonstrate this level of precision performance as part of routine operations is a testament to the efforts of multiple teams supporting laser operations, target chamber operations, transport and handling and optics refurbishment," Moses said.

Van Wonterghem points in particular to the enormous progress NIF scientists and engineers have made in economically maintaining the facility's optics system while operating at unprecedented energy levels.

For further information:  https://www.llnl.gov/news/newsreleases/2012/Mar/NR-12-03-02.html

Photo:  Wikipedia

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