Not sure about KSTAR specifically, but a lot of these devices use the field both for containment of the plasma and for starting/maintaining the fusion reaction

Not sure about KSTAR specifically, but a lot of these devices use the field both for containment of the plasma and for starting/maintaining the fusion reaction in the first place. If the field fails, the reaction stops. If it didn't (hypothetically), I imagine it would do some pretty severe damage to the reactor parts specifically. At some point, the vacuum would be broken, air would rush in, and the reaction would _definitely_ stop. Most of the larger fusors (as far as I'm aware) seem to be refinements of the basic Farnsworth-Hirsch fusor design (https://en.wikipedia.org/wiki/Fusor https://en.wikipedia.org/wiki/Fusor), which is a relatively simple device. If you have the motivation, you could build one at home with common-ish tools.