The Strategy for an Expansion of the ECRH System at W7-X.

After its completion, the W7-X stellarator was equipped with an ECRH system of 10 gyrotrons of the 1 MW class. However, the gyrotrons can only generate up to 0.9 MW of power with sufficient reliability. In addition, 4 gyrotrons can only be operated at a maximum of about 0.65 MW and one gyrotron, the first prototype of the TH1507 series from 1999, is only capable of pulse lengths of up to around 20 seconds. On the other hand, the experimental demands on the ECRH performance increase with each additional experimental campaign. It was therefore decided to expand the ECRH system in stages. In a first step, a new gyrotron of the 1.5 MW class was developed and 3 further gyrotrons of the same power class were ordered. At the same time, the 2 reserve gyrotron positions and transmission lines were equipped, so that the number of gyrotrons will initially be increased to 12. The next step is to develop 2 MW class gyrotrons from two suppliers. The plan is to replace the least powerful gyrotrons with the more modern high-performance gyrotrons. The freed-up gyrotrons will then be used for other applications such as CTS diagnostics and low field plasma start-up in new gyrotron positions. The target for 2030 is a combination of 4x1.0MW, 4x1.5MW and 4x2.0MW class gyrotrons, which will allow continuous wave operation with up to 10 MW absorbed power and 10 s pulse operation of up to 18 MW. In addition, the ECRH system is constantly being optimized to increase its reliability and make more efficient use of the high power infrastructure. In particular, the experience gained from the last W7-X experiment campaign was used to identify weak points in the microwave transmission line and coupling into the plasma, which are still being resolved.