The term tokamak is in fact the acronym of the Russian terms “toroïdalnaïa kameras magnitnymi katushkami”, which can be translated as “toroidal chamber with magnetic coils”. It was in fact two Russian researchers who, in 1968, succeeded in attaining temperature levels and plasma confinement times – two of the parameters essential for fusion – never before obtained.“ITER” has largely benefited from feedback from existing facilities”, explains Joëlle Elbez-Uzan, responsible for the safety of the facility at ITER Organization. “Each of the preceding experiments has resolved, one by one, different challenges encountered and ITER is going to integrate all of the technologies developed. For example, the technology of the superconductivity of coils, which has in particular been tested in the French "Tore Supra" tokamak located in Cadarache, which has been operating since 1988, has made it possible to obtain a high magnetic field intensity at the centre of the plasma. The British JET tokamak, inaugurated in 1984, and the American TFTR tokamak, designed in the 1980s, were, for their part, the first to achieve fusion between deuterium and tritium.”
Other tokamaks have provided building blocks for this international project: KSTAR in South Korea, which produced its first plasma in 2002, EAST in China, which produced its first plasma in 2006, SST-1 in India or instead JT-60, in Japan, which in particular enhanced our knowledge of the physics of the plasma and the manner of maintaining its shape and its stability, away from the walls.
A plasma more energy productive than energy consuming
“Throughout the world, fusion facilities have continuously progressed. The Tore Supra tokamak, of CEA/Cadarache, holds the plasma time record at 6 minutes and 30 seconds”, continues Joëlle Elbez-Uzan.
“The Japanese JT-60 obtained the highest value obtained to date of the triple product of fusion – density, temperature, confinement time. In the United States, fusion facilities have obtained temperatures of several hundreds of millions of degrees Celsius.
All of these remarkable achievements have enabled the science of fusion to come close to the “breakeven” point, which corresponds to the moment when, in a fusion facility, a plasma releases as much energy as it has received to produce the energy. This “breakeven” point has never been reached to date.
The current record is held by the JET, which has managed to reproduce, in the form of energy, 70 % of the power supplied. ITER should be the first, producing 500 MW of energy for 50 MW of energy consumed.”
Once the ITER scientific experiment has demonstrated the feasibility of this fusion based energy system, it is planned to create, on a site that has not yet been determined, a prototype plant known as DEMO. “In ITER, the energy produced is not recovered”, continues Joëlle Elbez-Uzan. “In DEMO, we will not only recover the energy, but we will also test the capacity of this prototype to supply itself with tritium.”
The project is only at the feasibility and elaboration of plans stage. The first stone is not likely to be laid before 2060. And the first power plant based on this prototype could see the light of day by the end of the century.
Interview Benjamin Carreras: Combining empirical extrapolation and modeling
Benjamin Carreras is an American nuclear physicist and a specialist in plasma physics. He formerly worked in the multidisciplinary research Oak Ridge National Laboratory, which depends on the US Department of Energy. Currently a researcher at two Spanish institutions and physics professor at Fairbanks (Alaska, United States), he is also a consultant for IRSN on the ITER project.
What is the biggest technical challenge linked to plasma?
One of the major concerns regards what are known as disruptions, in other words the sudden appearance of plasma instabilities. In the initial tests, before radioactive elements are used, ITER will have to acquire experience on the probability of such disruptions and the way their consequences need to be managed.
How to predict these instabilities?
Aspects of the plasma, which we do not yet fully understand, need to be studied on ITER. Our knowledge is based on smaller devices that are not able to attain the temperatures at which ITER is going to work. We do not have at our disposal sufficient theoretical bases to allow us to predict or calculate the different aspects of the disruption. In order to evaluate the consequences, two means exist: extrapolating from empirical data from present tokamaks or modeling the process, as precisely as possible. However, both approaches have limits.
Can you give an example?
An important problem consists in determining the shortest time taken by the current to collapse during a disruption. The shorter this time, the greater the consequences of the disruption. It is thus vital to enhance the analysis of existing data and the level of modeling of these events and to arrive at three-dimensional models, for example, and at the same time to gather together as much information as possible during the first experimentation phase in order to test the empirical extrapolations already carried out.