International encounter around nuclear fusion

Interior view of EPFL tokamak

Interior view of EPFL tokamak

This week, physicists from 15 American and European universities are sharing EPFL’s Tokamak to develop the future of hydrogen fusion.

The school is participating in the international quest for the controlled fusion of hydrogen, a virtually inexhaustible source of energy. On the 15th and 16th September, Swiss, European and American scientists submit more than 80 projects that propose sharing the use of EPFL’s Tokamak. This imposing confinement chamber enables the heating of hydrogen to more than 170 million degrees, at which temperature the gas can potentially fusion. The international teams are preparing in particular to use the ITER experimental fusion reactor.

About thirty Tokamaks are in use in Europe, Asia and North America. The principle is broadly the same for all of them. Confined in a round chamber, in the form of a donut, a cloud of hydrogen is maintained in suspension by an intense magnetic field. The procedure allows the heating of the gas to extreme temperatures. Like at the heart of the sun, isotopes can fusion into helium, a reaction that produces an enormous quantity of energy.

Modeling a hydrogen cloud
Plasma physics is a well-organized world of its own. Each Tokamak has its own particularities in terms of power, flexibility of use or heating system. Because of this, physicists regularly exchange usage slots, to test their models in different conditions.

EPFL’s Tokamak has some interesting features. In particular, it’s by far the most flexible one in the world, in terms of how it creates the shape of hydrogen plasma – hence its full name, Variable Configuration Tokamak (TCV). This is a crucial factor in terms of performance.

H mode: to prepare ITER
Among the projects submitted to EPFL by the international teams, many of them concern “H mode”. This is sort of recipe for obtaining the ideal plasma that will be mainly used in ITER. Temperature, density, heating system and the shaping of the plasma are all elements that need to be minutely measured to achieve this result.

“Mode H is a little like insulating a house,” explains Yves Martin, assistant director at the Plasma Physics Laboratory at EPFL. “If you add a layer of insulation, you need less power to maintain a high temperature. But you also need kinds of valves through which the resulting helium can be evacuated, so that it doesn’t stifle the reaction.”

The challenge is a highly complex one. The TCV uses a microwave heating system, which is calibrated to transfer its energy to the electrons. By targeting small zones to be heated, the physicists influence the circulation of the electric current inside the plasma, which subsequently impacts the stability and requires adjustments to the magnetic field. “All the parameters are linked, and must be recalculated in real time, which requires a large computing infrastructure,” adds Yves Martin. “That’s the price we have to pay for improving performance.”

Other more experimental modes, like the IC mode (improved confinement mode) will be tested on the TCV. With approximately 1500 discharges of two seconds per year, EPFL’s Tokamak is designed to provide European and American teams with the possibility to test their models and prepare the era of ITER which, in 2019, will need to maintain plasmas for more than 400 seconds at a stretch.

Author: Lionel Pousaz

Source: EPFL