Gentle wall contact – the right scenario for a fusion power plant
A promising operating mode for the plasma of a future power plant has been developed at the ASDEX Upgrade fusion device at Max Planck Institute for Plasma Physics (IPP) in Garching. It allows to avoid instabilities in the plasma that place a heavy load on the vessel wall and to remove heat and particles from the plasma more gently than hitherto.
ASDEX Upgrade – a tokamak-type fusion plant – is intended to develop operating modes for a future power plant. In order for sufficient fusion reactions to take place, the ultra-thin plasma floating in a magnetic cage must be extremely hot in the centre: at least 100 million degrees Celsius. At the edge, however, it should be as cold as possible so as not to overload the walls of the vacuum vessel. This applies in particular to the so-called “divertor”, where the plasma has wall contact. A special magnetic field directs the outer edge of the ring-shaped plasma onto robust, water-cooled plates at the bottom of the vessel, the divertor plates. This allows disturbing impurities to be removed from the plasma. At the same time, the hot inner area of the plasma is effectively separated from the colder envelope. This is because the boundary layer formed by the divertor field envelops the central plasma like a warming jacket – the prerequisite for good thermal insulation.
To enable the divertor to perform these functions, additional loads should be avoided. Particularly undesirable are so-called edge-localized modes, or ELMs for short – one of the disturbances of the plasma confinement induced by the complex interaction between the charged plasma particles and the confining magnetic field. In this process, the edge plasma loses its confinement for a short time and periodically throws plasma particles and energies outwards onto the divertor plates. Typically, one-tenth of the total energy content can thus be ejected abruptly. While the current generation of medium-sized fusion plants can easily cope with this, the divertor in large-scale facilities such as ITER or a future power plant would not be able to withstand this strain.
Methods for attenuating or suppressing ELMs, for example by means of magnetic control coils, have already been successfully developed at ASDEX Upgrade (see PI 1/11). However, the plasma edge of the test reactor ITER or a future power plant cannot be simulated completely in smaller facilities such as ASDEX Upgrade. It is therefore not entirely certain that these techniques will work just as reliably in a large plant. Sophisticated experiments on ASDEX Upgrade recently showed, however, that this is different for an alternative, previously less considered method: This is because the new method applies at the very edge of the plasma. The plasma values found there, however, are the same for ASDEX Upgrade and a later power plant.
If it is possible to adjust the correct plasma shape here – via the magnetic field – and at the same time – by injecting hydrogen – to ensure a sufficiently high particle density at the plasma edge, ELMs cannot develop. Instead, many small particle bursts appear. They flatten the pressure profile at the plasma edge again and again before a too steep pressure increase can discharge in an ELM instability: In up to 500 small pulses per second, the power from the plasma arrives quasi-continuously at the divertor plates – without impairing the good thermal insulation of the central plasma.
The small, high-frequency pulses not only do not cause any damage, they are even useful. They throw unwanted impurities out of the plasma edge. The high particle density set at the plasma edge also protects the divertor plates. In addition, the widened plasma edge ensures that the power impinging on the divertor is distributed over an area four times as large. The method is even compatible with the established method of radiation cooling (see PI 12/12). Here, the plasma edge is actively cooled to further protect the divertor plates: by injecting small quantities of noble gases into the central plasma and nitrogen right in front of the divertor plates. These impurity particles are excited to glow on contact with the hot plasma. In this way, they gently create energy from the plasma as ultraviolet or X-ray light. Before the fast plasma particles collide with the divertor plates, they have already lost their energy to the impurity atoms.
In the meantime, ASDEX Upgrade has been able to verify experimentally that the new plasma scenario is compatible with all the important control methods for the plasma. These experiments were financially supported by the European fusion research programme EUROfusion. “We were able to show at ASDEX Upgrade that quasi-continuous power exhaust meets the essential requirements of a power plant scenario,” says project leader Professor Arne Kallenbach: “Among numerous approaches, this makes it the most interesting and promising mode of operation for a power plant plasma.”