How to avoid excitation of parasitic modes in MW-class gyrotrons?

Megawatt-class, millimeter-wave gyrotrons for plasma heating in controlled fusion reactors operate either in very long pulses or in continuous-wave regimes. Therefore, to handle ohmic losses of microwave power in the cavity walls, these gyrotrons are designed for operation at high-order modes with a very dense frequency spectrum. In such conditions, to provide excitation of the desired mode and then its high-efficiency operation, it is necessary to carefully analyze the gyrotron start-up scenario. All gyrotrons operate either with diode or triode-type magnetron electron guns. Although it is simpler for users to operate with gyrotrons having diode-type guns, in such gyrotrons the excitation of a lower frequency parasitic mode prior to excitation of the desired mode is practically unavoidable. Therefore, even when the desired mode is excited later, when the beam voltage approaches its nominal value, there is a time interval during which the gyrotron generates MW-level radiation of the parasitic mode which can destroy many plasma diagnostics tools in tokamaks and stellarators. Our simulations show that when the voltage rise is short enough this excitation of parasitic modes can be avoided. It is also shown that in gyrotrons with triode-type electron guns it is even much simpler to avoid this parasitic mode excitation. To do this one can use one high-voltage supply with a slowly growing beam voltage but utilize for the mod-anode, which can be treated as a currentless electrode, another supply providing fast voltage rise.