Mode competition in the 170 GHz coaxial gyrotron cavity for ITER

170 GHz coaxial cavity gyrotrons with 2 MW output power in continuous wave (CW) operation are regarded as potential ECRH sources in ITER. A first industrial prototype of such a gyrotron has already been fabricated and delivered to CRPP Lausanne, where a suitable test facility has been constructed. Experimental tests are expected for the second half of this year. In view of the further development work, it is important to improve our understanding of the complicated processes of mode competition occurring in oversized coaxial cavities. An example of the comparison between the results obtained by means of four independent codes is presented and discrepancies between the theoretical end experimental results are discussed.