348 7. Fusion
The laser energy goes mainly into the compression of the d-t mixture. The
energy necessary to heat the plasma comes mainly from the fusion energy
release. This results in a reduction of the laser energy which is necessary to
make the target burn.
In 1992, in the United States, the Lawrence Livermore National Labora-
tory (LLNL) declassified the principle of inertial confinement fusion. At Ann
Arbor, the KMS laboratory (named after its creator, K.M. Siegel) was the
first, around 1973, to achieve the implosion of glass “micro-balloons” con-
taining gaseous deuterium-tritium. The experiment was then performed by
other laboratories. The 100 kJ Nova laser of LLNL reached a production of
10
13
neutrons per laser pulse.
Such experiments led to rapid development of computer-simulated explo-
sions. The experimental inputs to these calculations involved the observation
of X-rays and neutrons emitted by the target, the spectroscopy of tracers
incorporated in the d-t mixture, such as argon and neon, and pictures of
the α particles produced in the fusion reactions. Such measurements were
compared with the results of computer simulations, in order to validate the
assumptions entering the codes, in particular the fact that neutron emission
is of thermonuclear origin.
Technical problems. An important problem is that the irradiation of the
target must be as uniform as possible, in order for the implosion to be as
spherical as possible. Several methods are used.
• One can use a direct attack by several laser beams symmetrically dis-
tributed about the target.
• The indirect attack consists in sending the laser beams inside a cavity made
with a heavy element. This results in an emission of thermal X-rays. The X
rays irradiate the sphere in the center of the cavity much more uniformly.
Using gold, one can reach conversion rates of laser radiation into X-rays of
80%.
• Another approach consists in replacing the laser beams electron with ion
beams of ∼ 1 MeV. High power beams are easy to produce but sufficiently
accurate focusing of the beam is difficult to achieve. The construction of
a 10 GeV heavy ion accelerator (for instance uranium) that could deliver
1 MJ in 10 ns is under study.
Projects. Two important projects are underway.
The NIF (National Ignition Facility) project, in the United States, consists
of 192 laser beams. It will deliver an energy of 1.8 MJ in each pulse of 1 ns.
Its installation is scheduled for 2003; its cost should be 1200 million dollars.
Ignition is expected in 2008-2012.
The Megajoule laser project, LMJ, in France, will consist of 240 laser
beams. It should deliver 1.8 MJ in 18 ns. Its installation is expected in 2010,
for a cost of 1000 million euros. Ignition is expected in 2015.