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Inertial confinement fusion
Controlling fusion reactions is one of the great scientific challenges of this century. The fusion reaction studied the most is the reaction between deuterium (D) and tritium (T), two hydrogen isotopes that, when they fuse together, form a helium nucleus and emit a very high energy neutron.
Principle of fusion. © CEA
Study of the fusion of two light hydrogen atoms: deuterium (D) and tritium (T). © CEA
This reaction can take place only under conditions of extremely high temperature and pressure.
To achieve these conditions, two types of experiments are carried out in the laboratory:
Direct-drive inertial confinement fusion (ICF) with hotspot ignition was chosen for the Megajoule Laser (as for its American counterpart, the National Ignition Facility, or NIF).
The energy of the LMJ, in the order of one megajoule, was calculated to achieve ignition with a gain of 10 between the energy produced by the thermonuclear reactions and the laser energy supplied to the target.
Principles of ICF
ICF is a process whereby fusion reactions are induced by a mixture of deuterium and tritium (DT) contained in a microcapsule (also known as a microballoon). To achieve this, the mixture is compressed for a very short period to obtain a density of around several hundreds of grams per cubic centimeter, and heated to a temperature of 10 million degrees Celsius.
The rocket effect is used, i.e., a rocket achieves speed by the violent propulsion of gases from its reaction engine. In the same way, the aim is to vaporize the external part of the microcapsule very rapidly. The reaction will cause the rest of the microcapsule to move towards the center, compressing and heating the DT mixture inside. The center reaches very high temperatures (hotspot) and fusion reactions occur. The hotspot behaves like a match and ignites the rest of the "fuel".
Direct or indirect drive?
Two methods are used to bring about very rapid implosion of the microcapsule:
ICF experiments require the use of cryogenic targets of a specific geometry.
A little history
Direct-drive inertial confinement fusion was studied mainly at the University of Rochester, New York, on the Omega laser (60 laser beams that focus 30 kJ on a target) and at Osaka University, Japan, on the Gekko laser (12 laser beams that focus 15 kJ on a target).
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