The name tokamak (derived from the Russian for'toroidal-chamber-magnetic') is applied to axiallysymmetric toroidal systems in which the plasma isconfined by a strong toroidal magnetic field Bt ,produced by an external toroidal solenoid, togetherwith a weaker poloidal field Bp, produced mainly bya toroidal current Ip flowing in the plasma itself. Thecombination of the two fields produces nested toroidalmagnetic surfaces composed of helical field lines.Equilibrium of the plasma is produced by the poloidalfield, whilst the toroidal field serves to suppress themain magnetohydrodynamic instabilities, providedthe 'safety factor' q = aBt/RBp is sufficiently large,where a and R are the plasma minor and major radii,respectively. The toroidal plasma current also provides'Ohmic' heating of the plasma—the only heatingmechanism operative in almost all early tokamakexperiments. Particle orbits in tokamaks fall into twoclasses: there are 'passing' particles that travelcompletely around the torus, following helical fieldlines quite closely; and there are magnetically'trapped' particles that travel back and forth alongthose parts of field lines that lie on the outer-major-radiusside of the torus, undergoing reflection in theregion of higher toroidal field on the inner side. Inboth cases, conservation of canonical toroidal angularmomentum, which follows from the axisymmetry ofthe ideal tokamak configuration, implies that aparticle's excursion away from a magnetic surfacecannot exceed its gyroradius evaluated with thepoloidal field Bp. In this sense, the ideal tokamakconfiguration confines 'all' particles; in a tokamakreactor, the 3.5-MeV alpha-particles will be confinedprovided the plasma current exceeds about 3 MA.
Some theoretical problems of the toroidal plasma equilibrium