IPP said installation of new water-cooled inner cladding of the plasma vessel will make the Wendelstein 7-X facility suitable for higher heating power and longer plasma pulses.
The new cladding’s centrepiece, the so-called divertor, was manufactured by the institute’s Garching branch. It was delivered to Greifswald on 17 March and installation work will last until well into next year.
Fusion systems of the stellarator type promise high-performance plasmas in continuous operation. Accordingly, heat and particles from the hot plasma permanently stress the vessel walls. It is the task of the divertor – a system of specially equipped baffle plates to which the particles from the edge of the plasma are magnetically directed – to regulate the interaction between plasma and wall.
Wendelstein 7-X is being used to investigate the suitability of such devices for power plants.
A t the end of 2018, experiments on Wendelstein 7-X were halted temporarily after two successful phases. Upgrading of the plasma vessel has been continuing since then.
“First of all, most of the old components had to be taken out. Installation of the new ones can now begin,” said Hans-Stephan Bosch, whose division is responsible for technical operation of the device.
“Whereas most of the wall protection components were previously operated uncooled, large sections of the wall will be water-cooled starting with the next round of experiments. This will then enable Wendelstein 7-X to generate plasma pulses lasting up to 30 minutes,” Mr Bosch said.
The aim of fusion research is to develop an environmentally sound and climate-friendly power plant. Similar to the sun, it will generate energy from the fusion of atomic nuclei. Because the fusion fire only ignites at temperatures above 100 million degrees, the fuel – a low-density hydrogen plasma – must not come into contact with the cold vessel walls. Confined by magnetic fields, it floats almost contact-free inside a vacuum chamber. The Wendelstein 7-X stellarator is intended to investigate the suitability of this type of device for a power plant.
Stellerator devices and tokamak-type fusion units like the International Thermonuclear Experimental Reactor under construction in France are both types of toroidal (doughnut-shaped) magnetic confinement devices. In order for the plasma to have good confinement in the doughnut-shaped chamber, the magnetic field needs to have a twist. In a tokamak, such as Iter, a large current flows in the plasma to generate the required twisted path. However, the large current can drive "kink" instabilities, which can cause the plasma to become disrupted.
In a stellarator, the twist in the magnetic field is obtained by twisting the entire machine itself. This removes the large toroidal current, and makes the plasma intrinsically more stable. The cost comes in the engineering complexity of the field coils and reduced confinement, meaning the plasma is less easily contained within the magnetic bubble.
While the Wendelstein 7-X and Iter use different approaches, most of the underlying technology is identical. They are both toroidal superconducting machines, and both use external heating systems such as radio frequency and neutral beam injection to heat the plasma, and much of the plasma diagnostic technology is in common.