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Laser Fusion
While some UCLA scientists are conceiving magnetic bottles in which
they hope someday to set tiny suns serenely burning, others, rather than
attempting to contain fusion’s fury, are hoping to set it free by building
microscopic hydrogen bombs and converting the blasts into electrical energy.
This approach, known as inertial confinement, is conceptually simple and,
according to Chand Joshi, a professor of electrical engineering
in UCLA’s School of Engineering and Applied Science, certain to work. “There’s
no question in my mind that this method will produce fusion,” he says.
“Indeed, it’s the one way fusion has been achieved on Earth. We’ve already
made hydrogen bombs.”
Because of the huge scale and expense of this research, most of the
nation’s efforts in inertial confinement fusion (ICF) are taking place
at the University of California-run Lawrence Livermore National Laboratory,
located in the San Francisco Bay Area. The $1-billion National Ignition
Facility (NIF), now under construction at Livermore, will be as large as
a football stadium and contain 192 extremely powerful lasers which, when
focused on a pea sized frozen pellet of deuterium and tritium fuel, should
kindle a fusion reaction.
Under Joshi’s leadership, UCLA runs the premier university program in
ICF. Over the past decade, the program has trained half a dozen students
who are now noted in the field. In addition, despite its relatively modest
resources, UCLA has made important technical contributions to the ICF effort.
“The key,” Joshi observes, “is to carefully choose problems that are important
to inertial confinement fusion, but that the big labs don’t have the time
to systematically solve.”
Joshi and his colleagues decided to look for ways of improving how the
energy of laser beams is absorbed by fuel pellets. When a laser is trained
on a fuel pellet, its beams instantly cause the outer layer of fuel to
vaporize into a plasma. The force of this rapid vaporization compresses
the pellet, causing it to implode. It is hoped that eventually lasers will
be designed to continue compressing and heating the pellet to hundreds
of times the density of ordinary matter. At such densities and temperatures,
the rapidly moving nuclei of deuterium and tritium should collide and fuse,
producing helium and releasing energy.
This process, unfortunately, is more complicated than it might seem.
The same cloud of plasma that starts the collapse of the fuel pellet shields
it from being struck by further laser light, much as the smoke from the
first volley in a fireworks display may obscure the light of subsequent
fireworks. “The laser light doesn’t reach the surface anymore, where it
can impart maximum momentum,” Joshi explains. “The plasma begins to act
like a mirror, reflecting the light.”
A related problem is that when laser light is reflected it creates waves
in the plasma, which in turn shoot energetic electrons to the core of the
fuel pellet. The electrons heat the core, causing it to expand -- the opposite
of the desired effect. “The trick,” Joshi says, “is to keep the target
cold, gently pushing on it until it reaches hundreds of times solid density.
Then . . . boom!”
Joshi and his collaborators have used complex computer programs to model
the interaction of laser light and plasmas, and they have run experiments
using a carbon dioxide laser. Their laser is not nearly powerful enough
to ignite fusion, but it has allowed them to make important advances that
will help clear the way for more powerful machines.
While the physics behind their advances is extremely complex, their
prescription is amazingly simple: They have found that adding a little
hydrogen to fuel pellets dramatically reduces the effects of reflection.
The results of their research are to be incorporated into the target design
at the National Ignition Facility.
In a working ICF reactor, five pellets will be detonated every second.
The heat from these explosions will drive turbines that generate about
10 gigawatts of electrical energy. According to Joshi, “Whether this leads
to a practical power plant will depend on our ability to make a cheap driver”
-- an alternative, that is, to the extremely costly and inefficient lasers
NIF will be using to ignite the first fusion pellets. Joshi and his colleagues
are betting that this problem, like others they have encountered, will
yield to the pressure of their investigation.
Power Plays...
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