Microwave heating - ECRH
Electron Cyclotron Resonance Heating (ECRH) launches microwave beams of the same frequency as the electrons move on corkskrew-like paths around the magnetic field lines. These resonant microwaves are absorbed by the electrons and the plasma heats up.
ECRH is a very precise and powerful heating tool: As the cyclotron frequency depends on the magnetic field, which itself is a function of the location, the heating energy may be deposited at selected locations, thus tailoring the temperature profile in the plasma. Moreover, if the microwave beams are launched under an oblique angle with respect to the field-lines – e. g. by means of mirrors – local currents may be driven modifying themselves the confining magnetic field.
ECRH is the main heating system of Wendelstein 7-X and the only one which is already by now capable to operate continuously. For Wendelstein 7-X high power microwave sources – so called gyrotrons – were developed under the auspices of the Karlsruhe Institute of Technology (KIT). Each gyrotron is capable to provide nearly 1 MW heating power over 30 minutes. The development of 1.5 MW units is being prepared. Ultimately, up to 12 of these mircrowave sources will be installed with their periphery in the ECRH-building providing a total power of up to 15 MW to heat the plasma.
For the second campaign (OP1.2) 10 gyrotrons were available and provided a maximum power of about 7.5 MW to the plasma.
The magnetic field of Wendelstein 7-X with 2.5 Tesla on the plasma axis requires a microwave frequency of 140 GHz corresponding to a wavelength of about 2 mm. Whereas most experiments use waveguide technology for these frequencies for Wendelstein 7-X a quasi-optical transmission line was developed together with the University of Stuttgart. This transmission line guides the beams over about 60 m from the gyrotrons to the plasma with copper mirrors. Due to the high steady state power these mirrors need to be cooled. The fact that on the other hand the microwave beams propagate in air with practically no losses allows to keep gyrotrons and their periphery distant from the torus building, which is advantageous in particular for later large devices.