Possible effect of metallic dust on operation of rep-rate, high-power microwave devices

Summary form only given. There is a strong interest in studying the effect of absorption of strong RF fields by small metallic particulates on operation of high-power microwave (HPM) devices, such as HPM sources or high-gradient accelerating structures. For example, in X-band experiments at SLAC it was found that melting of small metallic debris in high-gradient accelerators takes place in the region of high RF magnetic fields. These observations initiated some theoretical studies of this effect; it was found that the maximum absorption takes place when the size of microparticles is on the order of the skin depth. In Ref. 2 the melting of microparticles in a single shot was analyzed. In principle, such a particle can also be melted in a sequence of shots when the energy absorbed in a single shot exceeds the energy radiated between pulses. This case is analyzed in a present study. Since the heating in a single shot was already studied, we focused on the cooling process. In a good vacuum, in the absence of direct contacts of microparticle with a circuit surface, the cooling of microparticles is caused by the black body radiation, which can be described by the Stefan Boltzmann law. The analysis of an example considering a copper sphere of the radius optimal for power absorption (2.4 times the skin depth) in the 11.424 GHz RF magnetic field of the amplitude 500 kA/m (such conditions correspond to recent high-gradient experiments at SLAC3) shows the gradual temperature evolution up to the steady-state in which the temperature rise during the RF pulse is compensated by the temperature fall between RF pulses. It is shown for the case of a 1 KHz rep-rate that in the case of short pulses (up to 700 ns) this steady-state regime occurs at the temperatures below the melting temperature. However, in the case of one microsecond pulses this temperature exceeds the melting one. Apparently, quite similar conditions leading to the RF breakdown may exist in HPM sources operating in rep-rate regimes, such as, for example, a nanosecond, gigawatt radar NAGIRA.