Fusion energy is created by merging two atoms in a very hot gas, called plasma, with temperatures around 150 million Kelvin. You could surely call this a fire. This extreme scenario takes place in a reactor with a doughnut-like geometry, called a tokamak. Plasma-surface interactions pose one of the biggest obstacles for fusion experiments. The future reactor wall materials have to withstand incredible heat and particle fluxes. Scientists are currently investigating plasma scenarios in which the wall loads are more benign. One way to cool the plasma down is by means of impurity seeding.

THE POWER EXHAUST PROBLEM

The particles and heat flux coming from the plasma core are channelled downstream along magnetic field lines to a region called the “divertor”. Divertor targets must withstand power fluxes in the order of several MW/m2 in steady state conditions, and up to 1 GW/ m2 during intrinsic instabilities of the plasma. This is comparable to the friction that occurs during re-entry of a spacecraft into the Earth’s atmosphere. This holds true just for the steady-state regime, while plasma instabilities can lead to a hundred times the expected power loads of the plasma during ~0.5 millisecond.

Renato Perillo (right) in the control room. Picture: private

Renato Perillo (right) in the control room. Picture: private

INTERDISCIPLINARY RESEARCH AS A KEY TO SUCCESS

Since the early ‘80s, scientists have been trying to reduce the heat flux on the target by increasing the gas pressure in the divertor region. The plasma is cooled down throughout its path to several thousands of Kelvin due to radiation, momentum transfer and volume recombination processes. In this way, the heat loads become sustainable for the material. This phenomenon is called “plasma detachment”. In order to fundamentally understand detachment and PSI in a tokamak, various disciplines, such as material sciences, control and mechanical engineering, plasma physics and chemistry have to work together properly.

FROM MAGNUM-PSI TO ITER

The linear plasma machine Magnum-PSI, located at DIFFER (Eindhoven, NL), is capable of mimicking the plasma-surface interactions foreseen for ITER. The excellent diagnostics accessibility provides accurate insights into the mechanisms occurring in both the exposed material and in the plasma located in the vicinity of the target.

WANTED IMPURITIES IN THE DIVERTOR

Experiments in tokamaks over the course of the last two decades have shown that the injection of gas (so-called impurities) into the divertor region leads to an enhanced detachment. At DIFFER we are currently pursuing such experiments. In particular, we are investigating the influence of nitrogen seeding on a hydrogen plasma, focusing on the plasma chemical processes occurring in such scenario. So far, experiments and numerical simulations deliver promising results while achieving a more comprehensive understanding of plasma detachment in a fusion reactor. In the end, this will be a key factor in making this technology feasible within the second half of this century.

PLASMA INSTABILITIES

info iconA fusion plasma is an extremely hot electrified gas which naturally wants to expand. It is suspended in a strong magnetic field designed to keep it from touching the chamber walls. As the temperature and pressure builds, the plasma forms areas of increasing turbulence, called instabilities, that must be controlled. There are many different types and sources of instabilities that may cause plasma disruption. The worst instabilities are able to eject streams of hot plasma out of the magnetic confinement, severely eroding the wall materials.

Clear

authorbox_Renato-PerilloI am a driven PhD student working within the Plasma Edge Physics and Diagnostics group at the Dutch Institute for Fundamental Energy Research. I combine numerical simulations with experiments, addressing the issue of power exhaust in a fusion reactor. I do believe fusion energy is the most promising energy source for the future. Working day by day in such a stimulating environment is the best thing that could have happened to me.

Renato Perillo (27) from Italy is currently based at: Differ Institute, Eindhoven (NL). (Picture: private)