Tokamak disruptions and runaway electrons

5 problems

• Sideways forces from asymmetric vertical displacement events in ITER

  Open

  When a tokamak loses vertical position control, the plasma drifts into the wall and drives large currents through the vessel. If this happens unevenly around the machine, it pushes the vessel sideways, and the size of that push for ITER is uncertain.

• Scaling of benign runaway beam termination to ITER

  Partially resolved

  Experiments found that puffing light gas into an existing runaway beam can make it end harmlessly, spreading its energy over a large wall area. Whether this works at ITER's much larger beam energy is unknown.

• Can a disruption predictor trained elsewhere protect a new tokamak?

  Open

  Machine-learning programs can warn of an oncoming disruption on the tokamak they were trained on. A new large machine like ITER cannot afford many disruptions to learn from, so the predictor must work almost from the start.

• Runaway electron current after mitigated disruptions in ITER

  Open

  ITER plans to shoot frozen pellets of deuterium and neon into a disrupting plasma to radiate its energy safely. Whether this also prevents a dangerous runaway electron beam, which grows by an avalanche of knock-on collisions, is uncertain.

• Mechanism and duration of the disruption thermal quench

  Open

  In a disruption the plasma loses nearly all its heat in about a millisecond, much faster than normal leakage. How the magnetic field becomes tangled to allow this, and where the heat goes, is not predictable.