Submitted by Glen Wurden on Thu, 04/17/2014 - 13:56



As you know, there are three problems caused by disruptions in a tokamak.

1). The large thermal quench heat loading on the armor (and the required uniformity to prevent melting), 2). the large electromagnetic forces (which can move structures, or rip apart items inside the vessel), and 3). the resulting runaway electron beam (which at 10 MA in ITER can carry 40-60 MJ of energy (more than the initial 20 MJ due to conversion of some of the poloidal field energy to more runaways as dI/dt occurs), which can basically e-beam weld holes in the armor, right down to the water cooling channels, resulting in water leaks.

Armor tiles in ITER have two conflicting design thin and water-cooled to take out the normal heat loads (at 5MW/m^2), and on the other hand, be thick (like ablative Space Shuttle tiles) to survive large transient loading. Unlike in the Large Hadron Collider (LHC), where there is a separate beam dump that its 200 MJ to which the beam can be directed, to protect its superconducting magnet bores, there is no special place in ITER to safely "land" a disrupting plasma.

Now the problem with a Disruption Mitigation System, is that it has to be able to stop all three problems, at the same time, every time. A "solution" for one of the three issues is not a solution if it makes one of the other two problems "worse". Furthermore, if the mitigation system is engaged when it isn't really needed, that has operational consequences too. Finally, for the case of dust/flakes falling into the plasma, in an otherwise "safe" operating regime, it can cause a density-limit/impurity bloom disruption.....there is no "safe" operating regime for a high-current, long-pulse large tokamak.…


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