Low-level RF control of a klystron for medical linear accelerator applications

Sun-Hong Min,Gun-Sik Park,Bong Hwan Hong,Seontae Kim,Ilsung Cho,Chawon Park,Ranjan Kumar Barik,Anirban Bera,In Su Jung,Matlabjon Sattorov,Dongpyo Hong,Ohjoon Kwon,Won Taek Hwang

doi:10.1063/1.5084172

Abstract

A klystron is an important vacuum device which is useful in medical linear accelerators because it amplifies the radio-frequency (RF) signals required to produce high-energy particle beams. The properties of the nonlinear input and output dynamics in the klystron circuit arouse distortion in various types of parameters pertaining to the beam voltage, beam current, RF power, and operating frequency from unpredictable external factors. Therefore, low-level RF (LLRF) control systems can facilitate the regulation of the output of the klystron to a desired level. Low-level RF control system of the klystron is designed to stabilize the applied voltage and control the resonance frequency of the cavity of the klystron. In this work, an infallible low-level RF control system, the most suitable for a klystron, is designed to use a beam monitoring and an interlock system with components that include an isolator and a phase shifter. Moreover, with regard to pulse distortion at the flap top at the RF output power, the stability of the RF power pulse at the flap top part was improved somewhat by a variable inductor which controlled the inside of the pulse modulator. In this LLRF control experiment, the pulse modulator generated 140kV-95A and succeeded in generating RF output power of 2.856GHz-6MW level of an S-band klystron. During this process, the efficiency of the RF power was approximately 42∼45% and the pulse repetition rate was 10 Hz. The flattop pulse width was measured and found to be 1 µs.

Highlights

An RF linear accelerator (LINAC) for applications in the medical field is a device that uses electromagnetic waves, in the microwave range, to accelerate charged particles such as electrons
With regard to pulse distortion at the flap top at the RF output power, the stability of the RF power pulse at the flap top part was improved somewhat by a variable inductor which controlled the inside of the pulse modulator
To identify this normal operating flow, the most important considerations are to ensure that the pulse modulator is properly powered by the level RF (LLRF) PLC system through the base power supply and that it is synchronized with regard to the klystron and electromagnet power supplies

Summary

INTRODUCTION

Scitation.org/journal/adv generated by a low-power RF source commonly called a RF driver.. Klystrons are high-frequency, high-power RF amplifiers used to produce the driving force for particle accelerators They amplify RF signals by making use of two or more resonant cavities and of electrons flowing in a vacuum. As the velocity-modulated electron beam travels along the drift tube following the input cavity, the accelerated electrons from one RF cycle tend to catch up with decelerated electrons from the previous cycle This results in the formation of bunches of electrons at the frequency of the input cavity fields. A low-level RF (LLRF) control system of the klystron is used to maintain the phase and amplitude of LINAC-cavity fields given the thermal and mechanical perturbations that cause resonant frequency variations and the beam-current variations that directly perturb the cavity fields.

THE OPTIMAL DESIGN OF S-BAND KLYSTRON

SETUP OF EXPERIMENTAL LLRF CONTROL SYSTEM BASED ON KLYSTRON

EXPERIMENTAL RESULTS

CONCLUSION