Thursday, 4 September 2014

A Tale of Two Tokamaks - Post the First

By James Edwards

See Post the Second here.

It was the best of times, it was the worst of times. It was 1973. Design work officially began on the Joint European Torus (JET). Several of the pioneers of the first fusion devices had already been thinking about how to make much larger tokamaks than any that had existed before, and now they’d been given the permission to go ahead from the Council of the European Community.

The JET design team in 1977.

Design work for JET, headed by Paul-Henri Rebut, started off well and progress was quick, but there came to be disputes over where to build the machine. The two contenders to host the international experiment were Germany, which would host it at Garching, and the UK, which would host it at Culham. Neither country was willing to concede, so over the years following 1973, the design team became dissatisfied with the project’s political procrastination.

However, in October of 1977, a Lufthansa aeroplane had been hijacked by members of the Baader-Meinhof gang and Palestine Liberation Organisation. The GSG 9 (part of Germany’s police force) used stun grenades provided by the UK’s SAS to successfully rescue the passengers while the plane was at Mogadishu, in Somalia. After this event, it’s often said that West Germany and the UK came to an unofficial agreement that JET would be built at Culham. About a week after the incident, construction on machine components started.

The JET Torus Hall being built in 1980.

Final design work progressed well with the renewed vigour of the team, and on the 18th of May 1979, work started on constructing the JET buildings and infrastructure at Culham. As some of the machining for components had been started earlier than this, it meant that the initial functional tokamak was completed by 1983 on time and on budget and by 25th of June of that year, the first ever JET plasma popped into being. Allegedly, many bottles of champagne were quaffed in celebration.

First plasma in the (very) bare-bones control room.

The first few years involved experimenting with the plasma and how to operate such a large fusion device (and of course, this still goes on), but since then, there have been several other notable events at JET…

In 1991 on the 9th of November, JET was the first tokamak to use a deuterium and tritium fuel mixture (also known simply as DT), releasing 1.7MW of power for a short time. The deuterium and tritium fusion reactions occasionally used in JET and planned for more extensive use in ITER are the best balance between power output and ease of producing the right conditions in a tokamak for sustainable fusion.

1993 saw an almost entirely new machine setup, with the introduction of the divertor – a system initially thought to be useful to remove impurities and waste helium, but which also turned out to be extremely helpful in making more efficient fusion plasmas by having an effect on plasma density. JET wasn’t originally designed with a divertor, so the whole system had to be constructed inside the vessel, as it would have taken too long to take the tokamak apart to install it. Luckily, there was enough mechanical engineering expertise (and space!) to add this brand new system without dismantling the whole tokamak.

The JET Mark 1 divertor is the flat ring at the bottom of the vessel.

During September in 1997, JET achieved a record amount of fusion power using DT fuel: a peak output of 16.1MW, and a record JET still holds. This is enough to power just over 7400 UK homes*.

More recently, the 4000 or so carbon fibre composite wall tiles inside the vacuum vessel were replaced between 2009 and 2011 with a remote handling system (an industrial-strength version of the RIFT project). The new tiles, acting as a trial run for the planned wall type in ITER, are mainly beryllium and tungsten and have been very successful so far.


The ITER-Like Wall in JET, made of beryllium and tungsten.

What about now? Well, we’re preparing for the next set of DT experiments, planned for 2017 – twenty years after the record breakers of 1997. These DT “campaigns”, as they are known, will be gathering data for physicists and engineers to allow them to plan the DT operations for ITER, providing us working on JET with an exciting and ambitious set of goals ranging from materials analysis to plasma control.

And what about the other tokamak I implied in this post’s title? That would be the Mega-Ampère Spherical Tokamak (MAST), and it’ll be featured in a later post!

* Based on average UK household usage of 19MWh per year in 2013. Source: Department of Energy & Climate Change, 2014. Energy Consumption in the UK (2014). Link 

All images courtesy of the European Fusion Development Agreement.

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