to·ka·mak 
(tôkə-măk′, tŏkə-)
(tôkə-măk′, tŏkə-)n.
A doughnut-shaped chamber used in fusion research in which a plasma is heated and confined by magnetic fields.
[Russian, from to(roidal'naya) kam(era s) ak(sial'nym magnitnym polem), toroidal chamber with axial magnetic field.]
tokamak