Recent upgrade of the klystron modulator at SLAC

Abstract

The SLAC National Accelerator Laboratory employs 244 klystron modulators on its two-mile-long linear accelerator that has been operational since the early days of the SLAC establishment in the sixties [1]. Each of these original modulators was designed to provide 250 kV, 262 A and 3.5 µS at up to 360 pps using an inductance-capacitance resonant charging system, a modified type-E pulse-forming network (PFN), and a pulse transformer. The modulator internal control comprised of large step-start resistor-contactors, vacuumtube amplifiers, and 120 Vac relays for logical signals. A major, power-component-only upgrade, which began in 1983 to accommodate the required beam energy of the SLAC Linear Collider (SLC) project, raised the modulator peak output capacity to 360 kV, 420 A and 5.0 µS at a reduced pulse repetition rate of 120 pps [2]. In an effort to improve safety, performance, reliability and maintainability of the modulator, this recent upgrade focuses on the remaining three-phase AC power input and modulator controls. The upgrade includes the utilization of primary SCR phase control rectifiers, integrated fault protection and voltage regulation circuitries, and programmable logic controllers (PLC) -with an emphasis on component physical layouts for safety and maintainability concerns. In this paper, we will describe the design and implementation of each upgraded component in the modulator control system. We will also report the testing and present status of the modified modulators. I. BACKGROUNDConsidered state-of-the-art engineering in the sixties, the SLAC-design 6575 klystron modulator -the number is designated for its original parameters of 65 MW and 75 kW of peak and average powers -has been operated continuously and reliably at SLAC National Accelerator Laboratory. This proven, line-type modulator technology was adapted and modified to provide drive power to a variety of klystron tubes in other laboratories both in the U.S. and abroad. The modulator structure consists of three compartments. The first one houses a thyratron, a charging transformer, and an end-of-line clipper circuit component. A short triaxial cable connects the PFN output to a step-up 1:15 pulse transformer residing in the klystron oil tank. The second compartment consists of two parallel PFNs containing eight sections each. Components in these two compartments have been upgraded in the 1980s to increase output powers to the current levels [2]. The third compartment, which houses a high voltage (HV) power supply, a modulator control system, and various auxiliary power sources for supporting thyratrons and klystrons, remains unchanged from its original form since creation almost 50 years ago.The current HV power supply is a conventional, unregulated 24 kVdc power source. It takes a three-phase AC input power from a variable voltage substation (VVS) and steps up through a rectifier transformer, which the secondary is then rectified, filtered and delivered to the PFN capacitors through a charging transformer. Each VVS supplies up to ...