Microwaves stabilise the fusion plasma

In December 2007, the control commands for the Japanese fusion plant JT-60 Upgrade at the Naka Fusion Institute originated from the faraway Max Planck Institute for Plasma Physics (IPP) in Garching, Germany. Working together, the teams aimed to overcome plasma turbulence. This is necessary because before atomic nuclei can be fused in a fusion power station to generate power, the fuel – a hydrogen plasma – first has to be heated up to ignition temperatures of more than 100 million degrees. To be able to retain the high temperature, the plasma has to be confined in thermal-insulating magnetic fields. The complex interplay between plasma particles and magnetic cage can cause a whole series of instabilities. Particularly unwanted effects include the so-called “neoclassical tearing modes”: In large plants, like the international test reactor ITER or a later power station, such effects could severely reduce the power output.

In fact, countermeasures had already been developed several years ago at the ASDEX Upgrade plant in Garching. Scientists there are combating the unwelcome plasma turbulence by means of microwaves that specifically heat up specially targeted critical areas in the plasma. The transcontinental Japanese-German test series now aims to directly compare the plasma‘s behaviour in the JT-60 Upgrade with that in the Garching plant. “In so doing, we wanted to determine how much microwave output is at least needed to break up the plasma instabilities,” explains Professor Hartmut Zohm from the IPP. In Garching, scientists, including doctoral student Laura Urso from Italy, had programmed some plasma discharges for the JT-60 Upgrade. On the day of the experiment, the physical parameters were sent to Japan via a secure data line and were fed into the plant‘s computer system in Naka. “We were immediately able to use the recorded gradients to judge the triggered discharges that were shown on the monitors in Garching, could then compare these with our expectations and make corrections for the next round of discharges,” describes Laura Urso.

Via video conference, the scientists were also personally in contact with the Japanese team. “The experiments worked as well as if we had been physically present on site. This kind of international collaboration will certainly become more important in the future, for example with ITER.” Parallel to these experiments, the scientists at the ASDEX Upgrade are working on improving the methods and on adapting these to the special conditions for ITER. To this end, the Russian company Gycom developed a microwave transmitter for the ASDEX Upgrade with two frequencies, and with three more multi-frequency tubes expected to be added soon. In contrast to the present transmitters with a fixed frequency, the frequency will now be variable. So, the frequency dependent resonance of the microwaves with the magnetic field will make it possible to reach any desired position in the plasma with the microwave beam. “Wherever a plasma disturbance occurs,” says Zohm, “it can be targeted and then broken up.” The results flow directly into the Advanced Microwave Heating for ITER (Advanced ECRH for ITER) at the Helmholtz Virtual Institute “Fortschrittliche Mikrowellenheizung für ITER” headed by the IPP. Partners are the Forschungszentrum Karlsruhe, the Universities of Stuttgart and Karlsruhe, the Russian Academy of Sciences in Nizhny Novgorod, and the Institute of Plasma Physics, Milan. “According to the current plan the microwaves for ITER will still come from single-frequency transmitters,” explains Hartmut Zohm, “while antennas that mechanically control the microwave beam will be used to transport these into the plasma.”

A development towards more flexibility – as in the case of the ASDEX Upgrade – would promise great benefits, however. This is why the Virtual Institute is working on a multi-frequency transmitter for ITER. A prototype ultra quick, non-mechanical switch for the powerful microwaves has already proven itself. “This then enables the German fusion programme to lead the way in advancing one of ITER‘s key auxiliary systems,” believes Zohm. Multi-frequency transmitters are already being developed for the ASDEX Upgrade; the experience with the microwave device for Wendelstein 7-X, which is comparable in size and frequency to the ITER system, will be valuable for ITER. And this, in turn, will strengthen the role of the German scientists at ITER – not only during the construction phase, but also once the plant is running. Because that is when the microwave experiments will be of decisive importance to the success of the whole project.