Posted on: 20th August 2012
One of the great advances that ITER will make will be its ability to maintain hot plasma pulses for much longer than any previous large fusion experiment. ITER pulses will extend to around 480 seconds, an achievement made possible by superconducting electromagnets, which are able to carry extremely high current. On the other hand, JET, with its previous generation copper electromagnets, can only create plasma pulses around twenty seconds long. Nonetheless to test wall materials for ITER, a recent JET experiment emulated ITER operation by running 151 consecutive identical pulses, totalling around 900 seconds of stable ITER-Like operation.
The next stage of this experiment is to remove the tiles from the vessel and analyse how the materials have behaved – where has material been eroded from, and where has it ended up. This information will complement the measurements taken during pulses of how much gas is extracted from the vessel after a pulse compared with what was injected. These gas balance measurements indicated that the retention of fuel was around ten times lower than that observed with the prior carbon wall tiles – however, says E2 Task Force Leader Dr Sebastijan Brezinsek, the new measurements will be more accurate: “We think the fuel retention may be quite a lot lower than was measured by the gas balance. Also these measurements will show where the fuel is being retained, and which mechanism is responsible.”
In addition to information about retention, the prolonged campaign was a triumph for plasma stability with the new wall materials. “We have proved we can operate Type 1 ELMy H-mode with high reproduceability, low disruptivity, and no negative tungsten events at all, even though it is quite different to the carbon wall.” says Dr Brezinsek. “The operational window is quite narrow, but now we know how much fuelling and central heating is required to keep the divertor cool while still maintaining a minimum ELM frequency to flush the tungsten impurities.”
Among large tokamaks the current world record for the longest single pulse is six minutes and thirty seconds, held by Tore Supra in France*. It seems likely that record will be smashed by ITER, but any trophies associated with the record will not have to move far – ITER and Tore Supra share the same CEA site in Cadarache.
* The record for longest tokamak pulse ever is 5 hours 16 minutes, held by the 84 cm radius TRIAM-1M at Kyushu University in Japan