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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
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:

  • Magnetic confinement fusion, which uses a tokamak-type device (like the current Tore Supra, now known as West and the future ITER) to produce low-density plasma over a rather long period. Plasma may be defined as a material phase consisting of charged particles, ions and electrons.
  • Inertial confinement fusion (ICF) uses high-power lasers (such as the Megajoule Laser or its American counterpart, the NIF) to generate very dense plasma of a very short duration.

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.

Intervention  dans la chambre en mars 2012. Crédit CEA
A worker inside the chamber, March 2012. © CEA

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.

Principles of ICF. © CEA

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:

  • Simulation of microcapsule implosion by indirect drive. © CEA
    Simulation of microcapsule implosion by indirect drive. The entire process lasts 20 billionths of a second.© CEA
    Direct drive
    The laser delivers its energy directly to the microcapsule, causing a direct interaction between the laser beam and the microcapsule filled with the DT mixture.
  • Indirect drive
    The laser beams are directed against the inner walls of the gold hohlraum holding the microcapsule to produce intense X-rays. The radiation is trapped inside this cavity, where the temperature can reach several million degrees Celsius. It is the X-rays that interact with 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).
The indirect-drive method was studied more in particular at the DAM, on the Phébus laser (2 beams for 6 kJ) and at the Lawrence Livermore National Laboratory, California, on the Nova laser (10 beams for 30 kJ). The DAM's researchers are also studying direct drive and fast ignition in collaboration with the scientific community.

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MàJ: 21/11/2014
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