Physicists are not content with things that just look pretty; they need to be useful as well. For example, a ruby such as the one physicist Mark Kempenaars is holding in this picture might be worth £7000, but it is not just for use as jewellery, it can also produce a very short, high-powered pulse of red laser light. It is part of the LIDAR system on JET, one of the ninety-odd diagnostic systems that scrutinize the plasma’s every twist and turn inside the vessel.
Unfortunately this ruby rod suffered an accident some years ago, when the cooling system for the laser leaked. “A water droplet got onto the front of the rod, and acted as a lens.” says Mark. ” It focussed the laser power into the middle of the rod… phhutt!”
The LIDAR system provides information about the temperature and density of the plasma, using a technique called Thomson scattering. In this technique a pulse of laser light is directed into the plasma and the light that is scattered back by the plasma particles is measured.
“The problem is that the efficiency of Thomson scattering is really low.” says Mark. “For every about 1014 photons we send in we get only one back! So the laser needs to have a pretty enormous power output. These ruby lasers gives out 3 joules of energy, but in a very short pulse, only 350 picoseconds long [350 trillionths of a second] which equals 3 gigawatts; that is 50 percent more than the local power station at Didcot. And what we get back is only tens to hundreds of nanowatts [billionths of a watt].”
As well as detecting an extremely faint signal, the LIDAR system is remarkable for its short pulse. The two ruby laser systems at JET use a pulse shortening technique called mode-locking, in which a vibrating crystal in the heart of the laser blocks the light for all but the shortest instant. “Our laser pulse is only about 9 cm long.” says Mark. “This gives you the spatial resolution to build up a detailed electron temperature and density profile.”
As to the future of the damaged ruby rod, Mark says it is totally unusable, even as jewellery. “It’s not a natural ruby crystal but lab grown. And I don’t think my girlfriend would like the colour, not red enough, it’s a bit too pink!”