Modelling of MBK with Parallel Version of Large-Signal Code TESLA

Multiple beam klystrons (MBKs) are high power and high efficiency amplifiers that use a set of well separated electron beamlets, each propagating in its own beam tunnel, but interacting with common fields in gaps near cavities. In this way the perveance of the individual beamlets can be kept low, reducing space-charge effects, while the total beam current can be high, facilitating high power operation of the device. The beamlets are highly separated from each other, except for their interaction with the common cavity fields. In the lowest approximation, the beamlets can be considered as identical and the response of a single beamlet can be multiplied by the total number of beamlets to account for their cumulative effect in the numerical simulation of a device. This approximation was used in the serial version of the code TESLA and is valid as long as the level of asymmetry is low enough. However, in a real MBK the level of asymmetry is not so small, and beamlets experience gap fields with different voltages due to the variation in R/Q values for different beam tunnels. This effect becomes pronounced for the input and output cavities which have ports that further disturb the symmetry. The approximation of identical beamlets still can be useful if the procedure of R/Q´s averaging was applied (Nguyen et al., 2004). In such way the overall power supplied by all beamlets to the cavity will be preserved and as a result total efficiency of the device will be close to the one in a real system. This approach was successfully used during the MBK modeling and design by help the serial code TESLA. Nevertheless, the procedure of averaging ignores the uniqueness of evolution of electron beam in every single beam tunnel with distinct working parameters. Then the ability to model beamlets taking in account their uniqueness is highly desirable.