CERN Accelerating science

Towards 90% power conversion efficiency for klystrons
by Igor Syratchev (CERN)
General view of the 6MW, S-band MBK (40 beams) with PPM focusing with expected RF power production efficiency above 75%. Image credit: JSC ‘VDBT’, Moscow, Russia

Computer simulations confirm that RF production efficiency above 90% can be reached in klystrons with a new electrons bunching technique.  

The increase in efficiency of RF power generation for future large accelerators such as CLIC, ILC, ESS, FCC and others is considered a high priority issue. It can contribute to a significant reduction of the investment and operational costs of accelerators. The vast majority of existing commercial high power RF klystrons operate in the electronic efficiency range between 40% and 55%. Only a few klystrons available on the market are capable of operating with 65% efficiency or above.

It is commonly considered the high efficiency klystrons require a low perveance. Limited by technically accessible high voltage levels, this can only be obtained by operating with low currents, thus single beam klystrons are incompatible with the need for high power. The concept of multi-beam klystron (MBK) resolved this inconsistency by introducing many single beams which interact with common RF circuits of the klystron, increasing the power with the multiple beams, whilst preserving low perveance per beam. In the standard klystron optimization procedure the target is to minimize the bunch length prior to it entering the output cavity. Independent studies concluded that in this case the 80% efficiency can be achieved and this value remains as a relatively hard limit.

To reach higher efficiency, the intrinsic limits of the bunching processes and deceleration in the output cavity need to be understood at the level of the electron bunch dynamics.

The new method to increase efficiency is based on the concept of a fully saturated bunch formation using the new bunching technique with bunch core oscillations. The first simulations proved that klystron RF power production efficiency close to 90% can be achieved. The new technology demonstration prototypeis now in fabrication in industry, with expected efficiency above 75% (see image above). The testing of this tube is scheduled for November 2015.