[bezmiran]

I am a plasma researcher, though not in the fusion field. Containment and stability are required on tokamaks to keep a plasma burning. Losing either of these will quench the reaction. The best way to control a plasma - magnetic fields, also causes significant instabilities, which is why fusion is so difficult.

[slavik81]

Could you elaborate on how magnetic fields cause instabilities? As a layman, it's not immediately obvious to me why that would be the case.

[treyd]

Because the plasma itself is charged and moves within the field, generating eddy currents which self-interact in complex and unpredictable ways. At a much much larger scale, the twisting of magnetic fields from convection within the sun causes sunspots and other phenomena around the solar surface.