Saturday, 22 October 2016

Visit to the Alcator C-Mod Fusion Reactor at MIT

I recently became aware that MIT had an experimental fusion reactor on campus, so I decided to check it out. After a few back-and-forth emails, I ended up joining a tour that was being given to the Tuft's energy club (yes, Tufts) on Friday afternoon.

MIT's tokamak research began as an offshoot of its magnet research lab (where MRI was developed), that eventually became so large as to spawn a new center and envelop its host. Alcator is an elision of Alto Campo Toro (high field torus), and this is the third iteration of it. It was recently in the news because they had achieved a fusion reaction at a comparatively high pressure, immediately before it was shut off because the US government diverted all fusion research funds to ITER.

The tour began in the lecture room with an overview of the why's and what's of fusion. In discussing the when, the speaker showed the improvement towards the break-even point in fusion reactors over the past several decades, but said that because the tokamaks had gotten so much bigger, they timescale of the experiments had slowed down considerably and now they have to wait several decades for ITER to be ready before their next big learning experience. I talked more about the timeline to fusion here; it's a bit more pessimistic.

"Why use fusion to give plants energy to make sugar and then bury them for millions of years until they form fuel and burn the fuel to make electricity, when we can just use fusion to make electricity?"

Another amusing tidbit the speaker mentioned is that more money has been spent in recent years making movies about fusion (e.g. The Dark Knight Rises, Spiderman 2, etc) than on fusion research itself. He was also overjoyed when someone asked why tokamaks were shaped like donuts and not spheres and he got to cite the hairy ball theorem.

After the speech we went across the street to the facility. There was a big control room where a few people were watching a video and looking at data from the last experiment. There was also a neat demonstration where a plasma was established by running a high voltage across a mostly-evacuated tube, and then a magnetic field was activated that visibly pinched the plasma.

We then entered the room with the tokamak. Most of the room, however, was full of electrical equipment. One of the neatest things I learned was about the way the thing is actually powered. It requires very powerful short lived surges of electricity to power the magnetic coils, on the order of 200 megawatts. To achieve this, they integrate power from the grid and store it in a massive rotating flywheel. The infrastructure for transforming the grid electricity is actually much bigger than the tokamak and its other supporting infrastructure. I was informed that the flywheel is bigger than the tokamak and its shielding, but I wasn't allowed to see it. It is apparently 75 tons and spins up to 1800 RPM, storing 2 gigajoules of kinetic energy.

The flywheel at the Joint European Torus, bigger than Alcator's.

On the wall outside the reactor room was the floor plate from a previous version. It was large.

The tokamak itself is big enough for a person to crouch in, and is surrounded by the wires that generate the magnetic field, and a lot of shielding. The whole thing is about the size of four elephants. It is painted light blue on the outside. On top there is a tank of liquid nitrogen that they use to cool down the copper wires after each heating pulse (which lasts about two seconds). People took photos and asked our tour guide some questions.

Here I am with the reactor (light blue tank thing behind me). You can see the tank of liquid nitrogen on top. The black pipe behind me is the power supply cable for the magnets. Not sure what the cheese grater thingy is.

Afterwards in the lobby there were a few artefacts we could play with, including some of the copper wires used for the field and the superconductors that are replacing them, and some tungsten shielding plates that were visibly damaged from years of hot plasma abuse.

Overall, I didn't learn much that I didn't already know about fusion (except the flywheel!) but I'd beat myself up if I knew I worked down the street from a fusion reactor and never went to see it. I appreciate the Plasma Science and Fusion Center guys for putting on the tour, and the Tufts club for (perhaps unknowingly?) letting me crash their event.  Also, one of the safety signs had a typo.

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