Gun design criteria for the refurbishment of the first prototype of the EU 170GHz/2MW/CW coaxial cavity gyrotron for ITER

The first prototype of the EU 170 GHz/2 MW/CW coaxial cavity gyrotron for ITER has been tested in the frame of a collaboration between European Research Institutions (EGYC
¹
) supported by Fusion for Energy (F4E). The experiments showed (i) the limitation of the operating parameters range to a low efficiency domain, and (ii) a voltage stand-off problem. The first observation has been associated to a poor beam quality at the cavity. Possible explanations for the second observation are the existence of a potential well located at the rear part of the gun and/or the presence of an electron population trapped in the magnetic mirror between the cathode and the cavity. Both effects were simulated using Ariadne++ and a new gun design is proposed, were they are absent.