Fusion by Inertial Confinement

Principle

Laser or particle beams are focused onto the surface of a capsule a few millimetres in diameter, containing a small quantity of fuel. The evaporation and ionization of the outer layer of the material lead to the formation of a plasma crown. This expands and, as in a rocket, generates an inward-moving compression front which heats up the inner layers of material. The core of the fuel is thus compressed to as much as one thousand times its liquid density, and ignition occurs when the core temperature reaches about one hundred million degrees. Thermonuclear combustion spreads rapidly through the compressed fuel, producing energy equivalent to several times the amount deposited on the capsule by the beams. The period of time during which these thermonuclear reactions occur is limited by the inertia of the fuel itself ; hence the term ´fusion by inertial confinementª.

Methods of Illuminating the Capsule

There are two ways of depositing the beam energy of the beams on the capsule surface : direct illumination, in which a number of laser or charged-particle beams, arranged with maximum symmetry, are aimed at the capsule ; and indirect illumination, in which the capsule is placed inside a metal container. The beam energy is deposited on the inner surface of this container, producing black-body radiation which is absorbed by the capsule.

In this second method the radiation is isotropic, ensuring a spherically symmetrical implosion -much more difficult to achieve with direct illumination. An alternative form of indirect illumination involves filling the cavity with material having a low atomic number. When heated to a temperature of more than one million degrees, this material becomes transparent to X-rays, thus ´smoothingª the radiation energy flow.

Beam Systems Used for Illuminating the Capsule

• Laser Beams
  The first beam system to be used in inertial fusion research and that upon which most work has concentrated so far, is the laser. Powerful flashes of laser light, of various durations and forms and of a selected wavelength, can be focused on the tiny space occupied by the capsule or cavity. However, the low energy efficiency of laser beams (only a few percent) makes it unlikely that they would be used in an inertial confinement fusion reactor unless there is a great improvement in optical pumping efficiency. The World's most powerful laser fusion facility is the NOVA (Lawrence Livermore Laboratory, USA) which produces 40 kJ of energy at a wavelength of 351 nm for a period of 3 to 4 ns.
  The most impressive results so far declassified (made public) are compressions to densities of up to 600 times that of the D -T liquid.

	The ABC laser device (Euratom-ENEA, Frascati, Italy). In the implosion chamber, alignment with the capsule is achieved using a beam of green laser light.

A chain of laser amplifiers (CEA, Limeil, France).

• Light Ion Beams
  In an electrostatic accelerator, a high-energy electric excitation is gradually shortened to intervals of 10^-8 to 10^-9 s. The resulting excitation of a few tens of MV and a few MA is applied to a diode whose anode emits ions (H+, Li+, etc.) ; the estimated efficiency of such systems is between 20 and 25%.
  Pulses with peak potential of 30 MV and an energy of 1 MJ should be possible using the PBF accelerator (Albuquerque, USA) ; power densities of 1 TW/cm² with an energy of 40 kJ have already been obtained using the KALIF accelerator (FZK, Karlsruhe, Germany).

	The PBFA2 facility (Sandia laboratories, Albuquerque, USA) for studying inertial fusion using light ion beams.

• Heavy Ion Beams
  The high rate at which energy can be deposited in targets and the considerable experience already acquired in accelerator technology, have led scientists to design fusion experiments using heavy ions.
  In addition, complex accelerators have been shown to be highly energy-efficient (several tens of percentage points) over long operating periods.
  Since intense pulses (several kA) can be generated only briefly (´ 10 ns), the beam must be temporally compressed by five orders of magnitude, between the source and the target. Advanced theoretical and experimental studies are being performed to investigate the effects of the very considerable space charge of these beams. High intensities with a maximum energy of 20 MeV/nucleon can be studied at the GSI facility (Darmstadt, Germany) using a wide variety of ions, of which the heaviest is Uranium.

	RF Linac-type module for accelerating heavy ions in the GSI facility (GSI, Darmstadt, Germany).

Principal Topics of Research on Inertial Fusion

• Instabilities Caused by Asymmetrical Illumination
  During the acceleration phase of the implosion, the contact between the high-density ´plungerª material and the low-density expanding fluid can bring Rayleigh-Taylor instabilities, as a result of which, the bubbling fuel can pass through the plunger and tear it. In order to avoid that phenomena, sophisticated optical systems are being developed for direct homogeneous illumination.

	Computer simulation of a Rayleigh-Taylor instability, showing how a target is deformed during irradiation by direct illumination, due to the inhomogeneous nature of this technique (ENEA-Frascati, Italy).

• Targets
  The targets are micro-balloons filled with a low-density D - T mixture. The target wall consists of multiple layers of materials with a range of atomic numbers (Z) so as to ensure high-efficiency combustion, by optimising energy coupling conversion.
• Laser and advanced accelerator design
  Although lasers using solids - in particular neodymium glass - have dominated research to date, lasers using gases (such as krypton fluoride or iodine) are promising, in terms both of their wavelength range and of their effectiveness. A tremendous amount of development work on such lasers is under way in many laboratories.

	The combustion chamber at the Nova laser fusion facility (Lawrence Livermore Labo- ratory, USA). Inside the combustion chamber at the Nova laser fusion facility (Lawrence Livermore Laboratory, USA) The Euratom Joint Research Centres and Associated Centre

Inside the combustion chamber at the Nova laser fusion facility (Lawrence Livermore Laboratory, USA).

A number of light and heavy ion accelerator systems are being studied, with a view to obtaining high particle densities in phase space; one method being explored is to stack packages of beams in storage rings. In the final phase, a number of these packages (a few tens to a few hundred) will be extracted from these rings and aimed simultaneously at the target.