Shaping the future

The design of a plasma vessel for a nuclear fusion reactor represents
a peculiar set of design challenges. Jon Excell reports.

When complete, the Wendelstein 7x will be the largest nuclear fusion reactor of its kind in the world. Its developers hope that once up and running it will take us closer than ever before towards the fusion scientists’ dream of a clean and abundant source of energy.

But reaching that goal has forced its designers to break new ground in laser-based measurement technology.

The $280m (£156m) experiment, overseen by the Max Planck Institute for plasma physics, is being put together at the site of a former nuclear power station near Greifswald in Germany. The device is expected to be switched on sometime around 2010.

There are essentially two types of fusion reactor: Tokomaks and Stellerators. Tokomaks are possibly the better known of the two devices. Oxfordshire’s JET experiment, the largest fusion experiment in the world, for instance, is a Tokomak, and so is the even larger, although as yet unbuilt international effort – ITER. While not quite attaining the mammoth proportions of either of these reactors, at 50ft across, 15ft high, and weighing 550 tonnes, Wendelstein 7x will be the largest Stellerator ever built.

While the superheated environment required for fusion means that both types of device work by confining a plasma (ionised gas made of atoms stripped of some electrons) in a magnetic field, Tokomaks and Stellerators differ greatly in their design.

In a Tokomak a magnetic field is created by transformer action.An additional magnetic field is superimposed on to this, which makes the magnetic field in the Tokomak a helical (or spiral) shape.

However, this field is only helical for as long as transformer action is taking place, meaning that Tokomaks operate in a pulse mode. The reason for this is that a current is being driven through the plasma and a current is needed to maintain stability. The JET reactor, for instance, has a pulse of 30 seconds.

In a stellerator, the helical field is produced by coils on the outside of the plasma vessel and no current is driven through the plasma. So it works continuously, rather than in a pulsed mode, leading to claims that it is a better solution for a nuclear plant.

However, the complicated arrangement of the coils means that the vessel itself must have a far more complex geometry than those found in Tokomaks. It is largely this degree of complexity that has led to the greater prevalence of Tokomakexperiments.

The plasma vessel of the Wendelstein 7x is a 3D torus, or doughnut-shaped, vessel made from free form sections of stainless steel.

Constructed and designed by German company Man Dwe, the bizarrely-shaped vessel is being constructed from 200 individual angled rings which when put together will form the characteristic doughnut shape. Each of these rings is made up of four segments which are multiply creased to reproduce the curved contours.

The design of the plasma vessel was an exceptionally tricky process. Each segment of each ring is subtly different, and with tolerances less than 3mm required to make the device work, the vessel represents a huge exercise in precision design.

Once fully constructed the vessel, which will later contain the hot plasma, will have a diameter of about 8m, enclosed in a 16m outer sheath. The complex coil system will be located in the gap between the vessel and the sheath.

Owing to the unusual free-form shape of the surfaces of the vessel, one of the key challenges in the design process has been checking that the geometry of the manufactured vessel rings conforms exactly to the original CAD model.

Traditionally, this kind of inspection would be carried out using theodolites or articulated measuring arms, but because of the vessel’s shape these methods were found to be unsuitable. Instead, Man Dwe worked alongside researchers from the nearby Deggendorf college of higher education, to develop a portable, laser-based measurement procedure based on equipment provided by laser expert Leica Geosystems.

Prof Rudolf Strohmayr, who led the development, explained that Leica laser tracker was deployed in the measurement of the single rings.

‘The procedure we have developed enables us to compare the actual geometry of the plasma vessel with the pre-determined 3D CAD model,’ he said. He added that one of the chief advantages of the tracker was its extremely high point density and therefore its ability to digitise all of the contours of the rings’ free form surfaces.

When the reactor is finally up and running, the plasma will be heated by ten microwave transmitters, each generating 1MW of power at a frequency of 140 GHz.

These transmitting tubes, called gyrotrons, have previously only been capable of heating pulses of just a few seconds and powers of a few hundred kilowatts. However, a prototype version of one of the ten microwave generators – manufactured by French company, Thales Electron Devices – has produced 1MW of power while running for several minutes.

The microwaves are directed into the plasma via water-cooled metal mirrors.

In the tests now being conducted all components are being checked for operation in conjunction with one another as well as the transmission system, the cooling system, high-voltage supply, measuring technique and data acquisition.

The start of operation, scheduled for 2010, now depends on the different industrial partners completing their various components on time. And as many of these are working in unchartered territory, timescales are still somewhat unpredictable.

‘With devices such as the Wendelstein 7X, industrial production and construction are themselves an experiment where new ground is being broken in numerous spheres’, explained Dr Manfred Wanner, of the Max Planck Institute.

However, while this does make it somewhat difficult to make exact predictions, lessons learnt on the project are also benefiting other fusion experiments. ‘The experience gained with Wendelstein 7X will also be of great benefit to the ITER international test reactor,’ he said.