Scientists at Portugal’s Instituto de Plasmas e Fusao Nuclear (IPFN) have made a dramatic twenty-fold improvement to the operation of their tokamak ISTTOK, by repeatedly reversing its plasma current to achieve stable AC operation – swirling it back and forth like Corridinho folk dancers.

“If we scale this achievement to JET, it represents a plasma discharge duration equivalent to the expected ITER run-time.” says IPFN physicist Ivo Carvalho. “We’ve moved our time-axis from milliseconds to seconds!”

Most tokamaks, such as JET, operate by propelling the plasma particles in a single direction around the donut-shaped vessel using the principle of electrical transformers, driven by the tokamak’s central (poloidal) electromagnet. Because this principle relies on the central or primary electromagnet having a changing magnetic field, it limits the pulse length; when the electromagnet reaches maximum current, the pulse has to stop, because the confinement of the plasma relies on this changing field.

However the Portuguese team have succeeded in switching the direction of the current quickly enough to allow the pulse to be continued, with the central electromagnet now driving current in the opposite direction. Of course the central magnet will eventually reach its maximum current – now in the reverse direction – at which stage the polarity is switched back to the original direction again. In all twenty half-cycles each of 25 milliseconds were achieved, driving the current back and forth, perhaps a little like the Portuguese Corridinho folk dancing pictured above.

ISTTOK has been experimenting with AC discharges for over a decade, but with a lack of control. The development by IFPN electronic engineers of new control and data acquision electronics with a lightning-fast control cycle of only 100 microseconds has provided the breakthrough that was needed. The new system is based on the ATCA standard (Advanced Telecommunications Computing Architecture) – the same hardware in use at JET in the vertical stabilisation system. The addition of a new magnetising power supply completed the setup which allows switching of the current direction quickly enough to prevent the plasma dissipating.

Studies of AC pulse operation in JET in the early nineties were limited by the requirement to restart the plasma at each reversal of the current direction. The ISTTOK results show that the plasma can be maintained for multiple AC cycles – when new power supplies are commissioned soon the Portuguese team expect to be able to improve the results further, well beyond the twenty cycles exhibited to date.

Source: EFDA