Advanced Klystron Development for High Peak Power and Variable Pulse Structure

Development of novel klystron concepts have been identified as a core technology needed for next generation accelerator applications in a recent Department of Energy (DOE) High Energy Physics (HEP) report [1]. As one example, we look toward intense X-ray pulses with variable separation from 100 ps to 100 μs are required for Dynamic Mesoscale Materials Science Capability (DMMSC) facilities [2]. The DMMSC requires a method for imaging dense, high Z materials of varying thickness over a wide range of time scales. Moreover, it requires extraordinary flexibility in terms of X-ray pulse delivery timing, as will any accelerator-based instrument (such as UED/UEM, compact backscatter light source, etc.) intended to study phenomena with similar timescales of interest. To meet this challenge, LANL is developing technologies to enable construction of several analytical instruments, including world’s highest photon energy free electron laser (FEL) to provide intense X-ray pulses with variable separation. Currently available particle accelerator technologies, particularly the klystrons, cannot provide the required pulse structure needed by this example and other systems. Improved accelerator structures are under development; however, corresponding advances in high-peak-power RF generators, such as klystrons, are required to power those structures. Concurrent, transformational improvements are required in three key areas of multiple beam klystron design: the electron beam source (order-of-magnitude cathode current density increase); the operating frequency (more than quadrupled, from 1.3 GHz to 5.7 GHz); and non-intercepting beam gating techniques (to reduce switching losses by two orders of magnitude). High gradient accelerators require very high peak RF power; existing linacs use dozens of 50 MW klystrons [3]. These tubes are, however, not capable of generating the high “burst rate” sequences of sub-microsecond pulses required by future facilities, such as an FEL for DMMSC. A fast-gateable electron gun using high current density cathodes, and a complementary circuit design, are needed to enable an RF source for these new accelerator imaging applications. Two technologies need to be developed to enable a high gradient option for the DMMSC-like FEL. The first is to demonstrate that (cryogenically cooled) copper cavities can reliably operate at gradients >50 MV/m. The second is the capability to rapidly modulate klystron output power to enable the required pulse structure. The Canon E37212 50 MW C-band klystron used at SACLA (Japan), SwissFEL and CERF (the new C-band Engineering Research Facility at the Los Alamos Neutron Science Center (LANSCE)) operates at 370 kV and 325 A. The nominal pulse repetition rate is 100 Hz; switching losses, proportional to CV 2/2, preclude pulse burst operation. Therefore, a new method for rapidly switching very high peak power klystrons is required. An electron gun design incorporating an isolated focus electrode (F.E.) provides a means to rapidly gate the klystron beam current, and therefore the RF output. The F.E. is non-intercepting,