Pump out the volume

Advanced cryopumps will help create the huge vacuum needed to operate the world’s biggest fusion reactor.

Coconuts aren’t usually the first thing that spring to mind when you think about clean energy sources. But it turns out they could be an important component of attempts to make nuclear  fusion a commercial reality.

Currently under construction in southern France, Iter will be the world’s largest experimental fusion reactor. It will replicate in a power station the sort of reactions that power the Sun and stars – the fusion of hydrogen isotopes into helium – in order to deliver safe, effectively limitless and environmentally clean energy on a commercial scale.

The main torus chamber, visible in this cutaway, will be served by six cryopumps

Success in controlling nuclear fusion  rests on using vacuum pumps to create large volumes of nothingness: any extraneous material would get in the way of the fusion reactions. Iter’s fusion reactions will happen inside a huge, 1,400m3 toroidal vacuum chamber sat inside a cryostat – a massive ‘vacuum flask’. Liquid helium will flow around this assembly, keeping the reaction chamber and cryostat at a temperature of 4.5K.

The latest design for the main cryopumps

The cryostat is one of Iter’s largest components, adding another 8,500m3 to the  total volume, and has to be operated at one millionth of normal atmospheric pressure. Its superconducting magnets will envelop  the reaction chamber and create the electromagnetic fields that will hold in place Iter’s hot plasma, the cloud of ionised fuel needed for the reaction. Together, these components will make up one of the largest vacuum systems ever built.

In order to create such a large vacuum, Iter requires very high pumping speeds and so will use cryopumps that work by trapping gases on a cold surface. The cooled surface is often partly covered with a porous material that can absorb the gases. In Iter’s case, tests on many different absorbing materials showed that activated charcoal made from a particular vintage of coconuts from a plantation in Indonesia gave the best results. Iter bought up all the supplies available and, 10 years on, there is no sign of degradation in the performance. However, the project has now moved on to the 2014 vintage of coconuts and holds a large stock in the basement of Iter’s headquarters.

’Activated coconut charcoal sounds  a bit unusual, but is a product that is used extensively where you need high purity,  high porosity and good sorption properties

Robert Pearce, Iter

The new cryogenic viscous flow compressors are due to be tested at the US Oak Ridge National Laboratory in Tennessee

“Activated coconut charcoal sounds  a bit unusual, but is a product that is used extensively where you need high purity,  high porosity and good sorption properties,” explained Robert Pearce, who leads Iter’s vacuum group. “The number-one market is drinking-water filters, but there are numerous other applications: Provence rosé wine is often filtered using active coconut charcoal. I even heard that munching on the coconut charcoal is a really good way to whiten your teeth [first they go black].”

Iter’s main pumping systems will include six eight-tonne pumps that exhaust the main torus. These are, said Pearce, “large complex pumps that have a combination  of mechanics and cryogenics”. Pearce runs a team of engineers responsible for the manufacture of the first of these cryopumps, which, he said, will allow them to sort out engineering issues that might crop up during the rest of the production run.

The power station will also include a further 12 cryopumps, taking their cost up to €50m (£36m), with as much again for the cryogenic plant and distribution and controls to run the pumps. Elsewhere, several hundred mechanical pumps will maintain the vacuum in peripheral systems and diagnostic instruments. While some of the cryopumps create the vacuum, others will ‘regenerate’ the absorbed gases to recover tritium, one of the hydrogen isotope ingredients for fusion. If, unlike earlier fusion projects, Iter is to run continuously, the system has to be able to regenerate tritium ‘online’, which means completing the regeneration within 600 seconds. “That is quite a task  for a pump,” said Pearce.

The regeneration comes about through ‘cryabsorption’ of the helium and the hydrogen isotopes from the fusion reaction into the panels of activated charcoal in the pumps. The pump’s surface is then heated to 100K to re-release the gases. “Then we pump it back, strangely with another cryogenic pump,” said Pearce.

This new arrangement, dubbed a cryogenic viscous flow compressor, is a joint development project with the Oak Ridge National Laboratory (ORNL) in the US. February saw completion of the first of this new type of pump, with tests about to begin in Oak Ridge, Tennessee.

At this stage of the process, the cryopumps also need to remove the helium ‘ash’ created by the fusion reaction through use of the ‘divertor’, an array of 54 remotely removable cassettes that sit at the bottom of the main chamber. Described as “like a giant ashtray into which the hot ashes and impurities settle”, the divertor is where the cryopumps face their toughest challenge.

An electro-polished radiation shield will seal the cryochamber

Helium is a hard gas to pump, said Pearce. Effectively you have to neutralise the ionised plasma particles in the divertor and create  a pressure in the bottom of the machine. “Then you have to pump that neutral gas. You are clearing out the helium, which is only about one per cent, but you are also clearing out all  of the fuel. That means that the pump has  to cope with a very large gas flow.”

Pearce, who previously ran the vacuum group on the Joint European Torus (JET) –  Iter’s predecessor at Culham in the UK – described his group’s current activity on Iter’s pumps as turning concepts into properly engineered and manufacturable designs.

The vacuum group, with teams in Oak Ridge, France and Spain, is also looking at  the engineering for fusion reactors post Iter.

“We have a concentration to get on and build Iter but we also have to do a lot of design and development work in order to have  a solution that really works  for the next step,” said Pearce. This is a reference to DEMO, a demonstration power plant that will show the viability of producing large-scale electricity  from fusion.

The new cryogenic viscous flow compressor could be a suitable candidate for future generations of reactor, said Pearce. It might eliminate what he described as the “slightly ironic” heating and cooling cycle in the current system that starts when fuel enters Iter as frozen pellets. “So the fuel gets cooled down; it gets heated up again in the plasma. Then we cool it down again and pump it. Then we heat it up again to get it out of the machine. Then we cool it down yet again to pump it. The gas goes through this cycle, which is obviously not very energy efficient.”

Fortunately, he added, “we have some ideas for how we could combine a lot of this heating and cooling in one device on the machine, which would save all of these cycles”. He continued: “You cool it once with your pump, which is also injecting it straight back in.”

This example illustrates the challenge  of Iter: turning science into real engineering.  In the past, said Pearce, there have been plenty of “nice ideas”. He added: “On Iter, we have actually got to turn all this into being  a machine that works reliably but that is also safe from a nuclear point of view. But before you can get to any of that it has to be buildable.”