An additional criterion for gyrotron gun design

The first prototype of the EU 170 GHz/2 MW/CW coaxial cavity gyrotron for ITER has been tested¹ in the frame of a collaboration between European Research Institutions (EGYC∗) supported by Fusion for Energy (F4E). The experiments showed that the voltage stand-off properties were strongly affected by the magnetic field configuration. Although the voltage stand-off was excellent in absence of magnetic field, problems arose at voltages higher than 60 kV in presence of the nominal magnetic field. The tube was sent back to the manufacturer (TED) and damages were found when it was inspected. An hypothesis based on the existence of populations of electrons trapped by Penning effect and/or magnetic mirroring could explain the voltage stand-off problem and some of the damages. Observations supporting this hypothesis are: (i) the damage locations, (ii) a simple experiment which was carried out on the first prototype tube in Lausanne, and (iii) the previous experience with the 165 GHz/2 MW coaxial gyrotron at FZK.