STATUS AND PROSPECTS OF CONTROLLED THERMONUCLEAR FUSION

Of all approaches to controlled thermonuclear fusion the tokamak exper iments have been most successful. Over the last decade particularly th ree large devices have achieved plasma density, n, temperature, T, and energy confinement time, tau(E), in ranges necessary for a fusion rea ctor plasma. Such maximum values have, however, been obtained not yet simultaneously but only in separate pulses, although the crucial tripl e product, nTtau(E), has also been improved by several orders of magni tude. The high temperatures sufficient in a fusion reactor can be prod uced by injection of neutral atoms or by absorption of radio frequency waves in the ion cyclotron frequency range. The plasma confinement (t au(E) almost-equal-to 1s) is still not understood and is handled throu gh empirical ''scaling laws''. Particle densities have usually been on the low side (n less-than-or-equal-to 5 x 10(19) m-3) because increas ed fuelling rates can easily lead to violent current disruptions. Prog ress in obtaining peaked density profiles with pellet injection has le d to high density plasmas without disruptions. Serious unsolved proble ms concern the spoiling of the fusion rates by (nonhydrogenic) impurit ies, the plasma parameter control over longer periods of time and inde ed the plasma heating by fusion alpha-particles (''ignition, burning'' ). The most urgent technological question refers to the lifetime of th e first wall which is in direct contact with the plasma. An important step towards ignition has been made by the recent JET/DT experiments i n which, for the first time, the actual reactor fuel component tritium has been used to produce neutrons. The ''next generation'' tokamak IT ER is, at present, being planned and designed in a world-wide collabor ative effort. It should be operating before the year 2010 and is inten ded to investigate an ignited plasma burning for several minutes.