CW RF Systems of the Cornell ERL Injector

Two high power 1300 MHz RF systems have been developed for the Cornell University ERL Injector. The first system, based on a 16 kWCW IOT transmitter, is to provide RF power to a buncher cavity. The second system employs five 120 kWCW klystrons to feed 2-cell superconducting cavities of the injector cryomodule. The sixth, spare klystron is used to power a deflecting cavity in a pulsed mode for beam diagnostics. A digital LLRF control stem was designed and implemented for precise regulation of the cavities’ field amplitudes and phases. All components of these systems have been recently installed and commissioned. The first operational experience with the systems is discussed. INTRODUCTION A prototype of the ERL injector [1], under commissioning at Cornell University’s Laboratory for Accelerator based Sciences and Education (CLASSE), is the first step toward the future X-ray light source based on the Energy Recovery Linac (ERL) [2]. The injector faces a challenging task of producing high-current, ultra-lowemittance beam. This, in turn, imposes very stringent requirements on its RF systems [3]. There are three different types of cavities, all operating at 1300 MHz: buncher cavity [4], five 2-cell superconducting (SC) cavities [5], and deflecting cavity [6]. Due to different power requirements for buncher and SC cavities, two different RF systems have been developed. The buncher RF is based on a 16 kWCW IOT transmitter. The injector cryomodule (ICM) RF system employs five 120 kWCW klystrons. The sixth, spare klystron is used to power a deflecting cavity in a pulsed mode for beam diagnostics. A new generation of the Cornell low level RF (LLRF) controls is used for precise cavity field regulation. All components of the RF systems have been recently installed and commissioned. RF FOR BUNCHER CAVITY Specifications of the buncher RF system are listed in Table 1. As power requirements for this system are quite moderate, an IOT-based high power amplifier (HPA) was chosen. The HPA was manufactured by Thomson-BM. The system includes a 16 kWCW tube TH 713 (manufactured by Thales-ED) incorporated into a modified version of the DCX SIIA broadcast transmitter system. The high voltage power supply is manufactured by NWL. The block diagram of this system is shown in Figure 1. The HPA was tested at the factory and then at Cornell upon delivery [7]. A very small vacuum leaks were found in the buncher cavity tuners after the cavity installation. Vacuum dams were implemented to allow the injector operation while replacement tuners are being manufactured. After that the buncher was powered and commissioned up to 160 kV. A rather strong multipacting (MP) was observed during cavity processing. The multipactor exists at cavity voltages above 49 kV with the highest out-gassing between 60 and 70 kV. While the cavity body and input coupler are not susceptible to multipacting, it was found that the electric field in a small gap between the tuner plunger and the port is high enough to bring this area into the first order MP zone. It requires many hours to process this multipactor. Table 1: Buncher RF Specifications Number of cavities 1 Nominal accelerating voltage 120 kV Maximum accelerating voltage 200 kV Shunt impedance, R = V / 2P 2.1 MOhm Maximum dissipating power 9.6 kW Maximum transmitter output power 16 kW Amplitude stability 8×10 (rms) Phase stability 0.1° (rms)