l World Development in Fusion Power
Ø Future - Towards a Fusion Power Plant
In parallel to ITER construction and operation, an accompanying R&D programme will be carried out in both physics and technology in order to prepare for the subsequent step, DEMO, a fusion power reactor that will demonstrate electricity production.
This programme is likely to include an International Fusion Materials Irradiation Facility (IFMIF). This high-intensity neutron source is required to test and verify the performance of materials for future fusion reactors, in particular low activation materials. DEMO should demonstrate tritium fuel self-sufficiency and first electrical power production 30 to 35 years from the start of ITER construction, and will lead fusion into its industrial era.
The demonstration reactor, DEMO, is to be decided on around 2020. As compared to ITER, it is a power producing device which is fully equipped with reactor components, has an improved confinement system, is run in a steady state at a power of 2.5 GWth with an efficiency of Q=25, and has a mantle for generation of tritium.
A commercial power-producing reactor will be built after the performance of the DEMO has been shown to be satisfactory. However, a power-producing reactor cannot be fully designed until a reliable operation of the DEMO reactor has been proven. In any case, it will most likely be based on the DT reaction, and may have the form of a concept-improved tokamak, or possibly some other magnetic confinement system.
Despite the military implications, unclassified inertia fusion research has been conducted in Limeil (France), Garching (Germany), Rijnhuizen (Holland), and Darmstadt (Germany). The primary problems in making a practical inertia fusion device would be building a driver of the required energy and making its beams uniform enough to collapse a fuel target evenly.
The development of laser fusion to some extent is coupled with the development of high power lasers. In the 1970s, it was believed that laser driver as little as 1 kilojoules (kJ) would suffice to create the fusion conditions. As the various plasma instabilities and laser-plasma energy coupling loss modes were gradually understood, estimates of the laser energy needed to effectively compress the targets to ignition conditions has grown rapidly from the early estimates into the mega joule range (MJ). The latest research devices, such as the National Ignition Facility (NIF) in the US and the Laser Mégajoule (LMJ) in France, support investigations into this regime. These two devices adopt the indirect-drive scheme. Direct-drive scheme has been explored in the OMEGA up-grade in USA, the NIKE in USA, and the GEKKO XII in Japan.
In the traditional inertia fusion approach, the drivers (high energy beams) are used to both compress and heat the target. The driver compresses the fuel pellet to very high density and the shock wave created by this process further heats the compressed fuel (in analogous to a diesel engine in which the fuel is compressed until it ignites spontaneously). Efficient heating requires very symmetric compression process and driver energy in the range of mega joules is needed to create ignition conditions.
The recent works (e.g., GEKKO XII in Japan) demonstrated that significant savings in the required laser energy are possible using a technique known as "fast ignition", which decouples the heating and compression phases of the implosion. Instead of using the shock wave to heat the compressed fuel, this approach use a separate beam to directly heats the fuel to create the ignition conditions more efficiently. The laser system of GEKKO XII is being upgraded to further explore fast ignition to achieve ‘breakeven’ and ignition.
NIF, the world's largest laser, to be completed in 2009, will focus 192 giant laser beams delivering 1.8MJ of ultraviolet laser energy on a tiny D-T fuel pellet in the center of its target chamber – creating conditions similar to those that exist only in the cores of stars to initiate fusion reactions. The LMJ laser beam facility, being built in France, is designed to study high energy density plasmas and inertial confinement fusion. When completed, LMJ will have 240 laser beams that will deliver 1.8 MJ of energy to a target.
A European proposal for a High Power laser Energy Research facility (HiPER) aims to achieve high energy gains, providing the critical intermediate step between ignition and a demonstration reactor. It would consist of a long-pulse laser with an energy of 200 kJ to compress the fuel and a short-pulse laser with an energy of 70 kJ to heat it. If funded, the facility would be opened to the scientific community towards the end of next decade. At present, UK is the leading contender to host the HiPER laser facility.
Inertia fusion has reached near “breakeven” condition and the new devices are designed to demonstrate breakeven and ignition. There are major technical problems to be resolved before inertial fusion power plant can be realized. The lasers need to be much larger and more efficient than those existing today, the repetition rates need to be increased by orders of magnitude, the injection and tracking systems remains to demonstrate that the reactor requirements can be met, the manufacturing of the fuel pellets will require an entire industry using technologies not even demonstrated today, and the biggest challenge is to figure out how to integrate the system into a reactor.