Have you ever wondered what it’s like inside an operating tokamak?
If you know a little about tokamaks, you can probably picture their gleaming metal interiors and even imagine a cloud of spinning plasma. But what is it really like inside an operating tokamak? What might be heard, seen or felt in terms of human perception?
The honest truth is that we cannot know for sure about the inside of a working tokamak, because of course that could never be experienced first-hand. A burning plasma is no place for humans, or indeed any living thing.But someone who does know more than most is Michael Walsh, the head of ITER’s Fusion Technology – Instrumentation & Control Division. For many years, Walsh led ITER’s Diagnostic Program, so he is very much up to speed on all there is to know about the environment inside the plasma chamber. An intriguing chat with Walsh on a wintery afternoon at ITER Headquarters answered a lot of questions.We began with one of the most basic human senses, sound. Would it be possible to hear anything inside an operating tokamak like ITER? Is sound created during a plasma shot?Surprisingly, perhaps, it turns out that the answer is yes—but you would not actually be able to hear it. That is because ITER will be operating in a near-perfect vacuum, and sound needs some kind of medium to travel through such as air or, even more effectively, a solid. (This explains why children can have such a good time chatting at a distance with two tin cans and a length of string.)If you could hear anything at all in the tokamak, you would probably experience something like the effects of tinnitus—a high-pitched whine, as the burning plasma could sometimes emit a sound wave at around 10 kHz, which is right at the very top end of the human hearing range of around 20 Hz to 20 kHz. What about another human sense? Would we be able to see anything? Is there light inside the tokamak? That depends on how you define light, says Walsh. Plasma is effectively a very hot gas that emits electromagnetic waves over a very wide spectrum—including but also far beyond what is visible to the human eye. As humans, we typically only detect wavelengths from 380 to 700 nanometres, which means that, unlike bees, we cannot see ultraviolet light (300 nanometres). And we certainly cannot see the gamma rays that will also be produced during ITER operation, and that fall in the range of picometres and below.Walsh explains that the shell of the plasma will look reddish, but the core is much, much hotter, and will be emitting gamma rays, which can only be “seen†by the diagnostics team using a gamma ray camera. Not only is the core very hot, but its particles are also moving very, very quickly. Being relatively heavy particles, ions travel the slowest, with speeds ranging from 100 to 2,000 km/s. But even the most sluggish ions could travel from ITER to Marseille in a tenth of a second. Electrons, being much lighter, are also much faster, travelling at anything from 10,000 to 100,000 km/s. At the top of the range, the electrons in the plasma could circumnavigate Earth in around 0.4 seconds. The very high speeds of the electrons make measurement challenging, says Walsh. Since they are travelling at a significant fraction of the speed of light (up to a third), ITER’s diagnosticians need to take into account the relativistic effect when they are measuring particle speeds. It is like a much-exaggerated version of the Doppler frequency shift we experience when we hear a police siren approaching at a higher pitch and then receding at a lower pitch. For ITER’s electrons, it makes a huge difference whether the particles are coming towards you or moving away from you. With energy levels of 25 keV, there is almost no light behind the electrons and almost all light is in the forward direction. The clear parallel here, which ITER’s diagnostics team has to take into account, is the red shift which astronomers factor in: the universe is expanding, and moving away from observers so fast that the colours are no longer “true.â€What about air pressure inside a tokamak? Would it feel different, like being deep under water or in outer space?In fact, the pressure differential inside the tokamak is not something that would have much of a perceptible effect on humans—even if the required vacuum conditions on ITER are rare elsewhere on Earth. “It’s really important to remove as many impurities as we can from the vacuum vessel, because the impurities have lots of electrons, and we really don’t want to be wasting our time and precious input power energy heating them up,†says Walsh.“So initially at ITER we take out the oxygen, which also removes the impurities, and that brings the pressure down to around 10-6 Pascal—which is less than 100 billion times that of the air pressure we experience in the everyday world. We then put gas back in and that brings the pressure back up to around 100 Pascal. That’s still quite a powerful vacuum, but you wouldn’t actually feel it, because as humans we do not even notice drops in pressure, only dramatic increases, such as when you go deep-sea diving for example.†Holding the plasma in place, and stopping it from vaporizing everything around it instantaneously, are ITER’s powerful magnets, of which the strongest generates a magnetic field of 11.8 tesla —equivalent to around 260,000 times that of the Earth. The centre of the tokamak vessel has about 5.6 tesla. Would that magnetic field be perceptible to human senses?Again, the simple answer is, for a human, to a first approximation, no—it would not affect us even if we had a metal plate holding broken bones together, as metal plates and pins used in surgery are usually made of titanium, and hence non-magnetic. The same would be true of a pacemaker—as long as it was switched off, as is routinely the case before having an MRI. Indeed, with only the toroidal field on (no plasma), the centre of the ITER chamber would be virtually the same as sitting in a huge MRI scanner—only of course without air to breathe. It might affect other members of the animal kingdom, however. Increasingly, scientists are coming to the conclusion that large numbers of organisms—from bacteria and algae to honeybees, birds, many fish, and even dogs—find their way about, often over extraordinary distances, using their magnetic “sixth sense.†So it could be profoundly disorienting for them to be in the vicinity of the magnetic field inside the tokamak at ITER.Happily, that is not going to be a problem—either for the animals or for us.
