Feedback stabilisation coils

Feedback stabilisation coils

The plasma is highly unstable state of matter, since there take place ripple effects. These have negative impact on plasma stability, which is not desirable for our technical purposes. These instabilities are also one of the main reasons why there still is not any commercial fusion reactor yet. Several decades ago, the occurance of these instabilities has almost prevented any further research in field of tokamak fusion. Thus one of the main technological challenges in fusion research is to counter the ripple effect as long as possible. Since CASTOR used to be small tokamak, it was not suitable for experiments on power output and similar experiments which require a larger device. However, the ripple effects appear in small tokamaks as well as in large ones. Thus the reasearch on CASTOR (and further eventually on GOLEM) covered mainly the phenomena of ripple effect and its countering.

In the time when the early experiments of TM-1 took place, there was believed that integrating another layer of vacuum would provide better stability for plasma. However, this theory was proven as incorrect. On the other hand, the induction of Foucalt currents in coating of the chamber has proven to be efficient in lowering the ripple effect. Also later there was integrated an extra system of coils, which was later called the Feedback stablisation.

This stabilisation is executed by outer stabilisation coils placed on the tokamak chamber. These coils can be recognised as black or deep pink coloured wires, winded around tokamak chamber. They induce magnetic field, which takes place on such a location, so the instability can be eliminated by its interacion with this field, by magnetic pressure. To locate the instabilities, there have been placed 6 pairs of Mirnov coils between the liner and the coating, to enable both horisontal and vertical control of plasma column. The system for vertical localisation of plasma instabilities is made of two pairs of Mirnov coils, each placed on 180 degrees of tokamak circumference. Horisontal localisation contains the other 4 pairs and are each placed on 90 degrees of tokamak circumference. Should ripple effect take place, there will be induced current on these coils and it will be processed by automatic system. In the past, this processing system used to be made of transistrore-like coputer, however, because of its transistore-nature, it was very difficult to alter its given reaction to instabilities. Thus this system in the future will be replaced by a programmable computer, which will enable further modifications of the response of the system. After the signal is processed, the control system will send response to feedback stabilisation coils in such a way, that the resulting induced magnetic quadrupole field will affect the plasma column by either stabilising the position of the plasma column in the chamber, or by pushing the instability by its magnetic pressure back to the column. Thanks to this system, the confinement period of plasma was significantly increased.

Current status of the system

Currently, the coils of the stabilisation field are not being used, since there is no need for them until the plasma discharge takes place in the chamber. However, during the previous time, after it has been transferred to the CTU and before the first reoperation procedures took place, the measurement by IPP CAS personnel of the stabilisation magnetic field took place. The results will be published soon.

Gallery