Operating JET is like flying its namesake aeroplane. You need to take off and land safely – the part in the middle is comparatively easy. Ramping up the power too quickly can make for an unstable plasma, but too slow can allow contaminants to percolate into the plasma. And coming into land is tricky too, you can’t just turn off the system, you need to allow the power of the plasma to dissipate slowly and evenly. Getting the sequence of events just right is vital to the safe and productive operation of JET – at ITER with five times as much current in the plasma, even more will be at stake.

However, recent tests of JET operation have given the ITER team some encouraging results. To achieve ITER’s high level of fusion power, the proposal is to apply high-powered heating to the plasma a lot earlier in the pulse than is usually done at JET. This heating scenario was tested successfully four years ago at JET using carbon as first wall material.

However, carbon is not an ideal plasma-facing material, because it retains the fusion fuel. Recent experiments at JET have instead been testing tungsten as an alternative wall material for ITER – it is much less predisposed to bonding with hydrogen isotopes and has a very high melting point. However, because tungsten has many more electrons than carbon and therefore tends to radiate more, there were concerns that the tungsten impurities would lead to substantial energy loss from the plasma through radiation.

Hence, the experiments from four years ago were repeated with JET’s new tungsten divertor (area where the hot plasma interacts mostly with the wall).  Much to the relief of the ITER team, JET successfully demonstrated the feasibility of using early heating. In fact it seems the change from carbon to tungsten in the divertor may even give the operators a little more flexibility with their scenarios as the power required to obtain good plasma confinement is somewhat lower, an unexpected bonus!

Source: EFDA