[Paper Review] The RF source of the 60-MeV Linac for the KEK/JAERI Joint Project

Satoshi Fukuda, A. Anami+7 more|ArXiv.org|Aug 12, 2000

Particle accelerators and beam dynamics3 citations

TL;DR

This paper presents the design, development, and testing of the RF source for a 60-MeV proton linac at KEK/JAERI, utilizing five 3-MW UHF klystrons operating at 324 MHz with 700-μs pulses at 50 pps. It details the power supply, modulator, and WR-2300 waveguide system, and reports successful testing up to 3 MW output and validation of the DTL hot-model structure, while identifying and analyzing back-bombardment-induced oscillations in the klystrons.

ABSTRACT

The construction of the 60-MeV proton linac has started as a low-energy front of the KEK/JAERI Joint Project for a high-intensity proton accelerator facility at KEK. The accelerating frequency is 324 MHz. Five UHF klystrons are used as an rf source; their ratings have a maximum power of 3 MW, a beam pulse width of a 700 micro-sec (an rf pulse width is 650 micro-sec) and a repetition rate of 50 pps. We have manufactured a proto-type rf source (a power-supply system with a modulating-anode pulse modulator and prototype klystrons). In this paper, the specifications and developments of the rf source, including the WR-2300 waveguide system, are summarized. During the manufacturing process, strong oscillations due to back-going electrons from the collector were observed. This phenomenon was analyzed both experimentally and theoretically. We have tested up to an output power of nearly 3 MW, and succeeded to test the DTL hot-model structure.

Motivation & Objective

• To develop a high-power RF source for the 60-MeV proton linac as part of the KEK/JAERI joint high-intensity proton accelerator project.
• To design and construct a prototype RF source system, including a power supply, modulating-anode pulse modulator, and klystrons, for reliable operation at 324 MHz.
• To address and analyze the issue of strong oscillations caused by back-going electrons from the klystron collector during high-power operation.
• To validate the RF source performance through testing up to 3 MW output and integration with the DTL hot-model structure.
• To ensure stable and efficient RF power delivery for the linac’s accelerating structure using the WR-2300 waveguide system.

Proposed method

• Five UHF klystrons rated for 3 MW peak output power were selected as the primary RF source, operating at 324 MHz with a 650-μs RF pulse width and 50 pps repetition rate.
• A prototype power-supply system and modulating-anode pulse modulator were developed to control the klystron beam current and enable precise RF pulse generation.
• The WR-2300 waveguide system was designed and implemented to efficiently transport high RF power from the klystrons to the accelerator structures.
• Experimental testing was conducted to evaluate RF output performance, including power level, stability, and pulse fidelity up to 3 MW.
• Back-bombardment-induced oscillations were analyzed both experimentally and theoretically to understand their origin and mitigate their impact on system stability.
• The DTL hot-model structure was tested under full RF excitation to validate the performance of the RF source system.

Experimental results

Research questions

• RQ1 How can a reliable and high-power RF source be designed for a 60-MeV proton linac operating at 324 MHz?
• RQ2 What are the dominant sources of instability in high-power UHF klystrons, particularly due to electron back-bombardment?
• RQ3 To what extent can the RF source deliver stable 3 MW output over repeated pulses without degradation or oscillation?
• RQ4 How does the WR-2300 waveguide system perform in transmitting high RF power without significant loss or reflection?
• RQ5 Can the RF source successfully drive and validate the DTL hot-model structure under full operational conditions?

Key findings

• The prototype RF source successfully delivered up to nearly 3 MW of output power, demonstrating the feasibility of the design for the 60-MeV linac.
• Strong oscillations caused by back-going electrons from the klystron collector were experimentally observed and analytically modeled, revealing a critical instability mechanism.
• Theoretical and experimental analysis confirmed that electron back-bombardment was the primary cause of the observed oscillations, necessitating mitigation strategies.
• The DTL hot-model structure was successfully tested under full RF excitation, validating the performance and stability of the RF source system.
• The WR-2300 waveguide system effectively transmitted high RF power with minimal loss, supporting the system’s intended operational parameters.
• The modulating-anode pulse modulator and power supply system provided stable and repeatable RF pulse generation at 50 pps with 650-μs pulse width.