At Textor experimental research is being done on nuclear fusion, a new, clean energy source. This is a process in which light nuclei fuse together to form heavier ones and is being done at a temperature of several million degrees. At such high tem-peratures the fuel is in the plasma state, the 4th state of matter. The electrons and nucleus move independently in a plasma, together they form a gas consisting of charged particles, in which electric and magnetic fields play an important role. The plasma is confined by magnetic field lines. Some places in the plasma are especially vulnerable to instability, so called magnetic islands can arise here. These islands can influence each other, by which the magnetic field can get chaotic. When current is driven into the islands by using microwaves, the islands can be suppressed. The microwaves are generated in a high power gyrotron and are transmitted with an optical transmission line to the plasma. The microwaves enter the vacuum vessel through a window and are being focussed with the launcher, a movable copper mir-ror, with 2 rotational degrees of freedom.