02/04/2011

Chen on his way to fast ignition

Bob Hirschfeld, LLNL, (925) 422-2379, hirschfeld2@llnl.gov



Cliff Chen

When laser researchers discuss fusion, they usually are talking about the National Ignition Facility (NIF) at LLNL.

NIF, which is on the verge of achieving its goal of fusion ignition, is based on a technology known as "indirect drive," in which its 192 laser beams are aimed so they surround their target rather than hit it straight on.

But waiting in the wings is another technology called "fast ignition," a process developed at LLNL by Max Tabak and Jim Hammer.

It will still be years before a fast ignition facility is actually built, and design specifications for the process are still being researched.

The Lab's Titan laser at the Jupiter Laser Facility is a key tool in the quest to investigate several key parameters for fast ignition. Among the researchers there is Cliff Chen, a postdoc from MIT, who provided a glimpse into this work in a presentation titled Bremsstrahlung Measurements of the Properties of Laser-Generated Hot Electrons for Fast Ignition. (Click here to see the presentation.)

Chen received the Luis Alvarez award for "best experimental research by a postdoc" from the American Physical Society's California-Nevada section, at its 2010 annual meeting at Cal Tech in Pasadena. To compare the indirect drive and fast ignition methods, Chen uses the analogy of automobile engines.

NIF's indirect drive process resembles a diesel engine, he says, in which the fuel compresses and ignites on its own as a result of the compression.

In contrast, fast ignition is more like a conventional gasoline engine, in which the fuel is first compressed by a piston, and then ignited by a spark plug.

In the two-steps of fast ignition, laser beams first compress the fuel, and then an ultra-fast beam (1,000 times quicker than the first one) heats the fuel at the height of its compression.

Current research suggests fast ignition may offer certain advantages, such as higher energy gain compared with indirect drive, more robust ignition, and lower energy compression lasers, and thus, smaller sized facilities.

Chen is studying the characteristics of electrons generated by the ultra-fast laser. This laser enters the target through a cone -- made of gold or high-density carbon -- embedded in the fuel capsule. At the tip of the cone the laser generates a beam of electrons that propagate to the fuel core. It's crucial that the divergence of these electrons be spread in as narrow an angle as possible, to ensure that the laser's electrons can reach the fuel core as efficiently as possible. At 31 years old, Chen first came to the Lab as a summer student in 2002, then returned as a grad student in 2006 while working on his doctorate in plasma physics, and has been here ever since.

He says receiving the Luis Alvarez award was a complete surprise. He started getting congratulatory calls even before officially being notified of the honor.

"Maybe it got delayed en route to my computer," he says.




Founded in 1952, Lawrence Livermore National Laboratory is a national security laboratory, with a mission to ensure national security and apply science and technology to the important issues of our time. Lawrence Livermore National Laboratory is managed by Lawrence Livermore National Security, LLC for the U.S. Department of Energy's National Nuclear Security Administration.