Research on Tokamaks

The present generation of large machines (TFTR in the USA, JT-60U in Japan, JET in Europe), supplemented by a large number of average-sized machines, are concentrating their research on a few major points:

• Confinement
  By studying how matter and heat are transported through the magnetic field, scientists can analyse and describe the plasma particle and energy confinement. This means surveying a large number of parameters (magnetic field, current, density, temperature, etc.) and measuring the space-time profiles of numerous characteristics of the plasma.
• Plasma purity
  Impurities released by wall-plasma interactions increase radiation losses and dilute the fuel. By operating with a magnetic limiting device, the plasma can be kept away from the material components with which it would otherwise come into contact. Coating the wall with low atomic number materials (B, Be, C) helps to reduce plasma contamination due to release of heavier metal atoms from the confining structures (as SS).
• Steady-state or virtually steady-state operation
  A transformer can induce a current in the plasma for only a short time. However, if the current is generated by non-inductive methods (such as a neutral beam or radiofrequency waves) it can be maintained.
• Disruptions
  Tokamaks operate within a limited parameter range. Outside this range sudden losses of energy confinement can occur. These, known as disruptions, cause major thermal and mechanical stresses to the structure and walls. If the early warning signs of such disruptions can be identified, preventive measures will be possible in future devices.
• Heating by Alpha Particles
  No experiment has yet been carried out in which the plasma is heated mainly by alpha particles. In the D - T phase of the JET programme, in 1996, alpha particles should account for 20% of the total heating power.
  Where a large number of alpha particles are present, new plasma instabilities can arise and further fuel dilution occurs. An effective system for extracting the helium «ash» will be needed.

Current Tokamaks Performances

The JET machine, the largest tokamak in operation (plasma volume : 140m³) has achieved the best performance for the product nTt namely 9 x 10²⁰m^-3keVs. For ignition to take place, this product would have to be six times higher.

In November 1991, the first experiments with a D - T fuel mix were carried out in the JET (limiting the tritium concentration to 11 %). This yielded a fusion power of 1.7 MW, with a total fusion energy production of 2 MJ. It is therefore to be hoped that, in the future, with a tritium concentration of 50 %, «breakeven» will be reached. (This is the situation where as much fusion energy is produced as is required for heating the plasma).

Two years later, in November 1993, similar experiments were conducted on the TFTR tokamak at Princeton. Since then reproducible D-T discharges made it possible to carry out detailed studies with an a particle heated plasma. A peak of fusion power of 10.7 MW was also achieved.

Four European tokamaks in which the nominal plasma current exceeds 1 MA. Like their less powerful counterparts, these tokamaks are assigned specific research tasks.

	ASDEX-U: a view inside the torus (Euratom-IPP, Garching, Germany)

TORE SUPRA (Euratom-CEA, Cadarache, France)

	FTU (Euratom-ENEA, Frascati, Italy)

TCV (Euratom-Switzerland, Lausanne, CH)

	TEXTOR (Euratom-KfA, Jülich, Germany, with the collaboration of Euratom-ERM Belgium, Brussels-B)

COMPASS (Euratom-UKAEA, Culham, UK)

	RTP (Euratom-FOM, Nieuwegein, Netherlands)

ISTTOK (Euratom-IST, Lisbon, Portugal)