Construction of the tokamak (that might kill us all!)

In some very specific conditions, electrons (you know, those tiny negatively charged subatomic particles) can run away. But how does this happen? Where do they go? Are they dangerous? We sent Senior Staffer Betsy Ladyzhets to the Plasma Physics Colloquium yesterday afternoon to find some answers.

Shortly before 1pm yesterday, I ventured into the depths of Mudd to find my way to room 214 – a small lecture hall in the physics department, full of old wooden chairs and pictures of people whose accomplishments I would need at least two more physics classes to understand. The room was about half-full, mostly with students in the physics department, a couple of other professors, and alumni.

After a brief skirmish with the projector, physics professor Allen Boozer (the presenter and a well-known theorist in the field of particle physics) launched immediately into his presentation. He described ITER, an international project to build the world’s largest tokamak, a magnetic fusion device that theoretically may be able to prove that fusion can be used as a large-scale energy source. 35 nations and thousands of scientists are involved in ITER, and it is the most expensive scientific device ever built.

But, as Prof. Boozer explained and as over 150 papers in the past twenty years have examined, this enormous project has an enormous potential flaw. Tokamaks (such as the one built in ITER) require a plasma current to produce energy. If the electrons in this plasma current are transformed into relativistic electron carriers which can escape the current – and, essentially, “run away.” These relativistic electrons can be dumped into the wall of the device, creating what Prof. Boozer called a “very unpleasant situation.”

So, where does all of this relativity come from? Relativity in a plasma will increase significantly if the plasma is suddenly cooled, which can happen for two reasons: “magnetic islands” can grow inside the plasma, causing temperature spikes that throw the normal temperature off balance, or the plasma may need to be “quenched” with cold gas to stabilize the magnetic field around it. In either case, the plasma cooling results in a disconnection between the plasma and some of the electrons powering it.

The plasma can undergo reconnection, meaning all of the unstable electrons go into the walls of the tokamak device and electron runaway is prevented. But if something goes wrong and reconnection is not completed, electron runaway is actually catalyzed. This can be very dangerous, especially if it occurs multiple times over the course of a tokamak’s function, each instance of runaway releasing more relativistic electrons to the surrounding area. The relativistic electrons can form, slam into the wall, return to the plasma, reform, slam into the wall again – like an electromagnetic battering ram.

From this theoretical discussion, Prof. Boozer transitioned into demonstrating his points about relativistic electrons using Maxwell’s equations (a set of partial-differential equations that form the foundation of electrodynamics) and “simple” vector calculus. I got a bit lost between Faraday’s Law and Hamiltonian Mechanics – it’s been two years since I took AP Physics or AP Calculus – but I was able to follow Prof. Boozer’s derivations well enough to understand the gist of his main points. In a tokamak, it’s impossible to locate individual plasma particles, and we cannot guarantee that plasma is conserved beyond a very small scale. Plus, when the plasma is located in 3D (which ITER’s is, being that we exist in a 3D world), electromagnetic field lines separate exponentially, and are as a result broken with exponential ease.

“You have to fail, whether you like it or not,” Prof. Boozer said.

As though all of that information isn’t disheartening enough, Prof. Boozer then went on to explain that the characteristic flux charged required for an electron to reach a relativistic state is about 0.0664 Volt*seconds. ITER has thousands of times this much Volt*seconds. So, essentially, any electron present has more than enough potential charge to run away. This runaway effect can be increased by pitch angle scattering and other conditions of the plasma.

Another professor attending the lecture pointed out that in actually looking at prototypes of the tokamak ITER will use, no relativistic electrons have been observed. But then, it’s impossible for people to examine the tokamak too closely, because of the radioactivity generated by the fusion device. And Prof. Boozer’s explanations and equations were fairly convincing: there’s a significant possibility that ITER will cause electron runaway, and if it does, the radioactivity produced will be incredibly dangerous.

One sidenote of Prof. Boozer’s presentation that I found interesting was how little actual data he was able to provide about ITER – not because none exist, but because the company in charge of the project refused to give it to him. At one point, he told us that he had to “weasel information out” of the researchers he talked to because “they don’t tell you things like plasma density and field strength.” This secrecy may be profitable for ITER now, but in the long run, it could be dangerous; surely if they release more technical details of the project to physicists like Prof. Boozer, someone may be able to find a solution to the runaway electron problem.

As the problem currently stands, a couple of possible solutions exist, but ITER has shown no interest in any. So, will any theoretical physicists be able to convince those runaway electrons to stay at home and listen to their parents? Or will the entirety of Southern France be drenched in radioactivity? We’ll just have to wait until 2025 (when the first plasma is developed) to find out.

Source of the radioactivity via ITER’s website