The PLT in 1975. Toroidal coils are visible in green.

The Princeton Large Torus, or PLT, was an early tokamak built at the Princeton Plasma Physics Laboratory (PPPL). It was one of the first really large scale tokamak machines, and among the most powerful in terms of current and magnetic fields. A key feature was the use of external heating systems to raise the temperature of the plasma fuel, a requirement of any practical fusion power device.

The tokamak became a topic of serious discussion in 1968, and the PPPL was convinced to convert their Model C stellarator to the tokamak configuration. It immediately validated the Soviet results. These early machines had no effective way to heat the plasma, so PPPL built the Adiabatic Toroidal Compressor to test one method of heating, adiabatic compression, while the PLT was designed to test another, neutral beam injection.

The PLT was extremely successful. It was the first tokamak with a plasma current over 1 MA.[1]:214 In 1978 it heated its plasma to 60 million degrees C, just above the critical threshold for a "burning plasma" that is seen as key to practical devices. These results led to the effort to build a machine capable of reaching breakeven, the Tokamak Fusion Test Reactor.


Vacuum vessel of the PLT under construction

The PPPL website says:[2]

1975:The Princeton Large Torus (PLT) begins operation on December 20. PLT experiments are expected to give a clear indication whether the tokamak concept plus auxiliary heating can form a basis for a future fusion reactor.

1978: In July, PLT sets a world record for ion temperatures of 60 million degrees C using neutral-beam heating. For the first time, ion temperatures exceed the theoretical threshold for ignition in a tokamak device.

1981:PLT produces the first tokamak discharge in which the plasma current is driven entirely by lower-hybrid radio-frequency waves.

1984:PLT uses ion-cyclotron radio-frequency heating to produce ion temperatures of 60 million degrees C, a record for this technique.

Device detailsEdit

[3] says "Largely a copy of the Russian T-10, but with addition of NBI and LH systems. Demonstrated current drive from breakdown by LH, but that LH only effective in low density plasmas. Variable minor radius by adjusting limiter position. The first machine to achieve a plasma current of 1MA. Metal limiters replaced by carbon limiters ... about 1978."

Further readingEdit