Control of mechanisms for pulse shortening in the Reltron

Summary form only given. Pulse shortening in the Reltron has been correlated with the interaction of the electron beam with screens used in the output cavities (R.B. Miller, 1998). It appears that electron stimulated desorption (ESD) of gas from the screens leads to a high density gas cloud that is subsequently ionized by the beam and the high electric fields of the cavity. Although pulse shortening can be avoided by operation without screens in the output cavities at frequencies near 1 GHz, at higher frequencies, it is expected that the higher fields present in the modulation cavity will also lead to pulse shortening. Because elimination of the screen in the modulation cavity is not desirable, measures to reduce the gas loading in the screens are necessary. A conventional approach uses high temperature vacuum bakeout. However, high power Reltrons typically employ textile fiber cathodes, which have limited lifetimes. Thus, tubes with demountable vacuum joints are necessary, and extensive bakeout and hard tube processing techniques are not practical in this case. To avoid pulse shortening, alternative processing techniques are being developed and the mechanisms associated with the initiation of electron emission, ESD, and plasma formation are being investigated. Models have been developed and are being tested experimentally. The processing techniques include pulsed thermal desorption of screen, ESD cleaning of the fiber cathode, UV desorption of critical tube components, and a novel beam collector. These techniques can lead to improved vacuum conditions and reduced gas loading in tube components.