Wednesday, June 12, 2013

Laser Fusion for Sustainable Energy

View from inside the target chamber at NIF showing the pencil-shaped target positioner. Image from LLNL
Yesterday began two days of laser-driven fusion talks punctuated by a visit to the nearby National Ignition Facilitiy (NIF) at Lawrence Livermore National Lab (LLNL) as part of CLEO Applications and Technology Special Symposium: The Path to Sustainable Energy: Laser Driven Inertial Fusion Energy.

The session began with The Physics of Laser Driven Inertial Confinement Fusion (ICF) and continued with the Technology of ICF Drive Lasers and Laser Facilities, and Optical and Nuclear Diagnostics. After the tour of NIF today, the symposium will pick up again on Thursday culminating in Future Perspective of ICF as Sustainable Energy Source.
A tutorial on ICF on the NIF website gives a cute recipe for creating the temperatures and pressures needed for fusion on earth that are only found elsewhere in our universe in stars,

Recipe for a Star:
- Take a hollow, spherical plastic capsule about two millimeters in diameter (about the size of a small pea)

- Fill it with 150 micrograms (less than one-millionth of a pound) of a mixture of deuterium and tritium

-Take a laser that for about 20 billionths of a second can generate 500 trillion Watts

-Wait ten billionths of a second
-Result: one miniature star
Figure of the hohlraum and a cross-sectional view (right) showing the fuel capsule. Figure from LLNL

Of course the devil is always in the details. Ignition, in which more energy is generated from the reaction than went into creating it, has yet to be achieved.  In 2009, NIF reached its laser energy goal and thought ignition would be achieved by fall of 2012.

John Lindl, of LLNL began today's session speaking about many of these devilish details, particularly on NIF. For example, besides having the necessary peak power, the 20 ns, 500 TW laser needs to have the proper pulse shape, which is a strangely-shaped series of four pulses of tailored delay and power in order to deliver four shocks to the target at the proper intervals. 

The target capsule, which may seem to be a trivial piece of the puzzle, has undergone an intense 20- year effort. Different shells of ablator materials, size, shape, density, concentricity, and surface smoothness are all key factors in a symmetric collapse (the attempt to get the correct "spherical rocket"). Lindl, spent a good portion of his talk showing diagnostics images of the collapse, and efforts to optimize the system to better ensure symmetric spherical collapse and confinement. 
Other factors include whether to use direct drive (hitting the capsule directly with the many laser beams) or indirect drive (hitting a cavity called a hohlraum with the beams to generate a symmetric barrage of x-rays to initiate collapse). NIF uses a hohlraum and 192 beams. Omega in Rochester, NY uses direct drive, which accelerates more fuel to burn, potentially for better energy production (when that day comes). Beam configurations, target placement and position, and much more come into play.
Of course simulation has been a key factor in design, result interpretation, and future direction. The immense effort for ICF at NIF, as well as other the facilities in the U.S. and around the world are extremely impressive and the problems are complicated, beautiful, and rich.
Laser inertial fusion energy (LIFE) is a worthy goal which could deliver a sustainable carbon-free energy source. There is no enrichment, no radioactive waste, and no worries of a meltdown; unlike fission chain reactions, when you turn "off" fusion, it is "off". NIF is an experimental facility made to understand the physics and technology necessary for LIFE and not scalable to a power plant. Scaling ignition towards operable power plants is another direction of physics, engineering, and optics research.
Schematic of how laser ignition fusion may interface with a power plant to deliver a sustainble source of electricity. Image from LLNL
Currently, targets are fixed and the laser delivers a few shots a day so that experiments can be changed out, realigned, and optics and components can cool down. In a power plant facility, the hope is to use a higher-repetition rate laser to deliver 20 shots a second. Targets would be injected at speeds of greater than 100 m/s to continually burn fuel, which would heat up a low-activiation coolant of lithium-bearing liquid metals or molten salts surrounding the target in order to convert water to steam with which to turn a turbine.

Lindl said that NIF is just 2 to 3 times away from achieving ignition, meaning the output energy from the fusion reaction is one-third to one-half of the input photon energy from the laser. Though nature has provided some delays from what was previously thought, ignition is realistically around the corner. Laser ignition fusion power plants may be close as 2030.

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