tokamak, n. The type of nuclear fusion reactor gets its name from the Russian toroidalnaya kamera s magnitym polem “toroidal chamber with magnetic field.”
You’ve taken this from the OED (but omitted the second n in magnitnym); the problem is that that expansion ignores the -ak of tokamak. Oddly (except that it was of course originally top-secret), there does not appear to be clear historical documentation of the origin of the word, but the two possibilities cited by Wikipedia are “тороидальная камера с магнитными катушками” (toroidal’naya kamera s magnitnymi katushkami)—toroidal chamber with magnetic coils—and “тороидальная камера с аксиальным магнитным полем” (toroidal’naya kamera s aksial’nym magnitnym polem)—toroidal chamber with axial magnetic field. The first is the only one cited by Russian Wikipedia; the second (the one I’m familiar with) is the only one given by M-W and AHD. The OED version is wrong on the face of it.