IST and IPP have developed a new method for controlling the plasma position and have successfully tested it on ASDEX Upgrade. EFDA supported the work as one of its high priority tasks.

Keeping the plasma in place is crucial for protecting the reactor wall of a fusion device. Today, real-time feedback systems use magnetic measurements to determine and control the plasma position. However, the fast neutrons and radiation emitted by deuterium-tritium (D-T) plasmas may affect the magnetic sensors. The measurements are likely to develop errors and these would grow with the duration of the plasma pulse. Because ITER and future fusion devices operate D-T plasmas with very long pulses, alternative methods to control the plasma position need to be developed.

We were only given two shots to test the system. And this was our only chance, as the campaign ended that month. But then it worked right away and the session leader gave us two more shots because he could not believe what he saw in the data.

Manfred Zilker, IPP

Reflecting microwaves off the plasma is a well-proven technique for determining the radial distribution of the plasma density. However, its application for measuring the plasma position without the issues associated with magnetic diagnostics has been started only recently. Portuguese scientists from IST and German scientists from IPP have a long standing collaboration in the area of microwave diagnostics. Within the framework of an EFDA priority support task, they now developed a method to control the plasma position using microwave reflectometry.

The new solution uses dedicated algorithms and estimations of the boundary density to determine the position of the plasma boundary in real time. IST experts conducted a feasibility study of the method for IPP’s tokamak ASDEX Upgrade and subsequently designed and assembled the reflectometry system and the real time data processing solutions. The whole system was then connected to the ASDEX Upgrade control system.

After a few years of intense work, I could not imagine a better reward for the team and for myself than the feeling of complete achievement that followed those tense few seconds when plasma position control was successfully handed to reflectometry for the first time ever.

Jorge Santos, project principal investigator, IST

First tests in July showed that the outer plasma radius could be determined with the required accuracy of one centimetre and the plasma position was successfully controlled. IST and IPP are now optimising the algorithm. Further upgrades under consideration include a second microwave reflectometer located opposite to the present one, to simultaneously control the inner and outer plasma radius.

Hartmut Zohm, IPP hartmut.zohm AT mgp.ipp DOT de
Maria Manso, IST mmanso AT DOT pt