Wendelstein 7-x stellarator puts new twist on nuclear fusion power

In a large complex located at Greifswald in the north-east corner of Germany, sits a new and unusual nuclear fusion reactor awaiting a few final tests before being powered-up for the very first time. Dubbed the Wendelstein 7-x fusion stellarator, it has been more than 15 years in the making and is claimed to be so magnetically efficient that it will be able to continuously contain super-hot plasma in its enormous magnetic field for more than 30 minutes at a time. If successful, this new reactor may help realize the long-held goal of continuous operation essential for the success of nuclear fusion power generation.

Created by the Max Planck Institute for Plasma Physics (IPP) and designed with the aid of a supercomputer, the Wendelstein 7-x is the first large-scale optimized stellarator of its type ever to be commissioned. With a name like something out of Hitchhiker's Guide to the Galaxy and a containment vessel that literally provides a new twist on the doughnut shape we see in standard tokamak fusion reactors, the quirky stellarator design aims to provide an inherently more stable environment for plasma and a more promising route for nuclear fusion research in general.

Initially an American design conceived by Lyman Spitzer working at Princeton University in 1951, the stellarator was deemed too complex for the constraints of materials available in the middle of the 20th Century, and the more easily constructed toroid of the tokamak won out as the standard model for fusion research.

Though some stellarators have been constructed over the course of time – notably the predecessor to this latest iteration known as the Wendelstein 7-AS (Advanced Stellarator) – the calculations required to ensure ultimate plasma containment and control have only become possible with the advent of supercomputers.

As such, algorithms specifically created to fuse theory and practice have now been applied to the design of the Wendelstein 7-x, and its designers firmly believe that this latest version will have the stability required to be the precursor machine to full-blown, continuous nuclear fusion power generation.

For the eventual success of nuclear fusion power (essentially where two isotopes of hydrogen, deuterium and tritium, are subject to such energy that the strong nuclear force is overcome and they fuse to form helium and release copious amounts of neutron energy), stability is essential. This is because the enormous pressures and temperatures (around 100 million degrees Celsius (180 million °F)) used to create the plasma, and then accelerate the resulting ion and electron soup around the containment vessel, means that any instability in the magnetic containment field or the pressure vessel itself will result in degradation and ultimately the failure of the process.

To achieve a more stable environment, the stellarator eschews the method of inducing current through the plasma to drive electrons and ions around the inside of the vessel as found in tokamak designs, instead relying entirely on external magnetic fields to move the particles along. In this way, stellarator designs are basically immune to the sudden and unexpected disruptions of plasma and the enormous – and often destructive – magnetic field collapses that sometimes occur in tokamaks.

As such, a stellarator reactor is able to hold the plasma in a containment field that twists through a set of magnetic coils to continuously hold the plasma away from the walls of the device. This is because, in a normal tokamak, with its doughnut-shaped containment vessel and electromagnet windings that loop through the center of the toroid and around the outside, the magnetic field is stronger in the center than it is on the outer side. This means that plasma contained in a tokamak tends to drift to the outer walls where it then collapses.

The stellarator, on the other hand, avoids this situation by twisting the entire containment vessel into a shape that constantly forces the plasma stream into the center of the reactor vessel as it continuously encounters magnetic fields in opposing positions along its entire length.

The advantages of the stellarator over the tokamak come at a cost, however, as the many twists and turns that give the stellarator an advantage in magnetic containment also mean that many particles can simply be lost as they veer off course following the path of the containment vessel itself. To help avoid this, a great many more magnetic coils are required for the stellarator and must be set up at very close intervals around the structure and super-cooled with liquid helium for maximum efficiency.

In the case of the Wendelstein 7-x, the weight of the 50, 3.5-meter-tall (11.5-ft) non-planar super-conducting electromagnets alone is around 425 tonnes (468 tons) and their placement makes construction difficult and their assembly fraught with problems. Not to mention the fact that piping around vast quantities of liquid helium to ensure that the electromagnets superconduct at temperatures close to absolute zero makes the Wendelstein 7-x a plumber's nightmare, and a tricky addition to an already difficult balancing act.

As such, the physical design of the stellarator itself requires access ports for fuel ingress and egress, along with a myriad other entry points for instruments, sensors, and all the other necessary paraphernalia necessary to monitor the enormous pressures, voltages, and temperatures that it will be subject to in operation.

Despite all of these problems, tests on the completed stellarator to maintain the sub-millimeter accuracy for the plasma path are progressing and show promise. In one recent test, an electron beam was injected into the stellarator and progressed along a predetermined field line in the circular tracks through the evacuated plasma vessel. As it moved through the machine, the beam created a tracer in its wake created by collisions with electrons contained in the residual gas in the vessel.

Meanwhile, as the electron beam constantly circulated through the system, a fluorescent rod was pushed transversely through the vessel in cross section, and when the electron beam struck the rod, visible spots of light were created and the results recorded with a camera. In this way, the whole cross section of the magnetic field was gradually made visible.

"Once the flux surface diagnostics were placed in operation, we were immediately able to see the first magnetic surfaces," said Dr. Matthias Otte, the man responsible for this measurement process. "Our images clearly show how magnetic field lines create closed surfaces in many toroidal circulations."

Whilst in itself just another stepping stone toward the ultimate goal of practical fusion energy, the IPP stellarator is an important juncture in the field. With tokamak-based reactors still requiring more energy in than they actually produce, both the scientific and general public alike have grown wary of the long-held promises surrounding nuclear fusion. And, though many bodies, such as the University of Washington, Lockheed-Martin, and MIT, claim to be "close" to producing a working, sustainable, self-powering machine, nuclear fusion still remains a pipe dream.

This is where IPP's proving of the technology over the coming months leading to a full-blown commissioning of the machine may well provide the nexus between theory and practicality and, if not deliver on the promise of boundless energy, at least provide a proof of concept and renew flagging interest in a field that may, one day, solve all of our energy needs.

With approval to continue from nuclear regulators in Germany expected by the end of this month, the Wendelstein 7-x stellarator is slated for its first fully-operational tests in November this year. At a cost of more than €1 billion ($US 1.1 billion) and over one million man-hours of work committed so far, the hopes of Europe's future being a nuclear fusion-powered one may well rest on the ability of this machine to perform as expected. Watch this space.

Source: IPP