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THERMONUCLEAR IGNITION 9Figure 4. Representation of magnetic fields present in a tokamak configuration. Theplasma current I p , the toroidal magnetic field B T , the poloidal magnetic field B p generatedby the current l p , and the resulting total magnetic field B are indicated in the figure. Thefield lines generated by the transformer in the surrounding air are also traced.the safety factor q s it is necessary to use the highest value possible forthe magnetic field that is compatible with the smallest value possible forthe major radius of the plasma. The minimum value of the major radiusR is limited by the necessity to install the windings for the transformerin the center of the torus, coaxial to the axis of symmetry. Moreover, thetoroidal magnetic coil must have a cross section sufficiently large tocarry the current I c and be designed so as to resist the relevant magneticforces that come into play. Recall that the typical values considered for