The all-tungsten wall of ASDEX Upgrade has raised the need for a differently designed antenna for its ICRF heating system. The current antenna design generates unwanted electric fields that sputter tungsten atoms from the wall. These impurities increase the radiation from the plasma and thus harm the plasma’s performance.

Yuntao Song und Jean-Marie Noterdaeme with the drawing of the new ICRH antenna for ASDEX Upgrade

Yuntao Song und Jean-Marie Noterdaeme with the drawing of the new ICRH antenna for ASDEX Upgrade

New antennas were designed to minimise these unwanted electrical fields. The first of two antennas, built by the Chinese partner, arrived this summer. “I am extremely happy with the way the antenna behaved in the test stand,” says Prof. Jean-Marie Noterdaeme of IPP. The Chinese team is lead by Dr. Song Yuntao of ASIPP and even though his team was able to draw on their experience from building the ICRF antennas for the institute’s tokamak EAST, the project is still quite challenging, he says. “The design and the manufacturing processes are very complex. We had to use special welding procedures, for instance, to meet the extreme precision requirements.”

Both project leaders agree that the collaboration went really well. “We had to work out the procedures first,” recalls Noterdaeme, “It was not sufficient to just exchange drawings, for instance. There are different standards in both countries and technical terms sometimes have different definitions”. “The good thing,” explains Song, was that “we all knew what we were talking about because both groups had built this type of antenna before”. His institute hopes to see further collaborations after this successful project, he adds. The collaboration agreement also envisages that ASIPP scientists will participate in the experiments on ASDEX Upgrade with the new antennas.

The second antenna will arrive in Garching in early 2014. Both antennas will be installed into ASDEX Upgrade during the next shutdown in 2014. ENEA will supply additional components like the Faraday shield, cooling structures and a microwave reflectometry diagnostic, which is a powerful system for measuring the plasma density at several locations in front of the antenna. The Portuguese Association IST will contribute the electronics for this system.

“Even though the antenna has been optimised for low impurity levels, it has some flexibility to operate around this optimum and thus could thus allow a solid confirmation of the optimisation procedure,” explains Noterdaeme. IPP is also building a test stand, IShTAR, which will complement the experiments on ASDEX Upgrade and allow dedicated experiments designed to provide us with a deeper understanding of the physical mechanisms. Jean-Marie Noterdaeme is looking forward to see the two antennas in action: “If we can prove our hypothesis that the antenna design solves the impurity problem, then we will have made a big step forwards,” he says.

Ion Cyclotron Range of Frequencies (ICRF) heating injects radio frequency waves into the plasma. The frequency matches the frequency at which ions gyrate around the magnetic field lines, thus causing resonance. The two new ICRF antennas for ASDEX Upgrade are produced in cooperation between IPP Garching, the Institute for Plasma Physics of the Chinese Academy of Sciences (ASIPP) and ENEA, Frascati. Jean-Marie Noterdaeme heads IPP’s Department for ICRF, where the engineer Helmut Fünfgelder coordinates the antenna project.