rature required, about 100 million Kelvins, is several times that of the Sun. The product of the density and energy confinement time of the plasma (the time it takes the plasma to lose its energy if not replaced) must exceed a critical value. There are two main approaches to controlled fusion – namely, magnetic confinement and inertial confinement. Magnetic confinement of plasmas is the most highly developed approach to controlled fusion. The hot plasma is contained by magnetic forces exerted on the charged particles. A large part of the problem of fusion has been the attainment of magnetic field configurations that effectively confine the plasma. A successful configuration must meet three criteria: (1) the plasma must be in a time-independent equilibrium state, (2) the equilibrium must be macroscopically stable, and (3) the leakage of plasma energy to the bounding wall must be small. A single charged particle tends to spiral about a magnetic line of force. It is necessary that the single particle trajectories do not intersect the wall. Moreover, the pressure force, arising from the thermal energy of all the particles, is in a direction to expand the plasma. For the plasma to be in equilibrium, the magnetic force acting on the electric current within the plasma must balance the pressure force at every point in the plasma. The equilibrium thus obtained has to be stable. A plasma is stable if after a small perturbation it returns to its original state. A plasma is continually perturbed by random thermal "noise" fluctuations. If unstable, it might depart from its equilibrium state and rapidly escape the confines of the magnetic field (perhaps in less than one-thousandth of a second). A plasma in stable equilibrium can be maintained indefinitely if the leakage of energy from the plasma is balanced by energy input. If the plasma energy loss is too large, then ignition cannot be achieved. An unavoidable diffusion of energy across the ma...
