Reversed Field Pinch Historical Review (2)

In the same years, at Moscow, Kurchatov and later Sacharov and Tamm began the design and construction of the first Tokamak, a stabilized pinch with particular peculiarities.

The broad conclusion of those ten years of work was that, even with a stabilizing toroidal field, although the pinch was globally stable and the column remained centred in the tube, there were nevertheless large energy losses, poor confinement, fluctuations and residual instabilities. Similar conclusions were reached in the ZETA experiment: large energy losses by vacuum ultraviolet radiation and unstable field configurations.

In 1958, during the last discharges of ZETA, a period of improved stability was observed. The period was called "Quiet Period" or "Quiescence" where the plasma current was approximately constant.
Some years later it was observed that, during the quiescence, the toroidal magnetic field at the wall of ZETA was reversed.

In the years 1958 - 60 it was proposed that the application of a reverse magnetic toroidal field at the wall would provide pinch stabilization; indeed Colgate in 1958 made the first attempt to explain that the presence of a reverse magnetic toroidal field at the wall is important for improving the stabilization of the plasma discharge. Other theoretical physicists proposed the same ideas to counteract the MHD instabilities m=0 and m = 1.

Once again in 1960 at General Atomic (S. Diego, USA) a little experiment in a quartz toroid, using fast programming, improved the stability of the plasma with a toroidal magnetic field reversed at the wall, but the resulting confinement was very poor.

The study of plasma confinement in Padova began in 1958 - 59 at the University Institute of Electrical Engineering with the collaboration of the Physics Institute. The first plasmas were made in linear devices, whose liners were made of quartz or glass.
The plasmas were discharges, with low gas pressure, between two electrodes (Z-Pinch) or annular induced discharges (Theta-Pinch).

During the 1960's fusion research moved away from the Z-Pinches to Mirror Machines, Stellarators and Cusps, where external conductors with no net plasma current defined the magnetic field and to Theta-Pinches.
In these machines it was possible to obtain a good MHD stability, but other sources of energy losses and instabilities were present.
In the early 1970's studies and experiments on Pinches began again and were investigated thoroughly.

The researchers used fast field programming for reaching a well-controlled configuration of the discharge, without excessive plasma wall interactions. They wished to reach high temperatures and high 'Beta' before being overtaken by instabilities. The fast programming entailed operation with insulating tubes, usually quartz. The discharges lived for tens of microseconds, the Beta's were high and the temperatures could reach high values but fell quickly.

These experiments were important because they demonstrated the theoretical expectations for the presence in plasmas of the ideal and resistive MHD instabilities, including early evidence of sustained reversed field generation (Dynamo).

In the same years at Padova more complex and promising experiments began on toroidal machines where the plasma heating was produced by the Pinch effect. The first Contract of Association between EURATOM and CNR had been established, so the research on pinches by the "Centro di Studio sui Gas Ionizzati del CNR" - a research institute of CNR and Padova University - became part of the European Fusion Program.

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