[Paper Review] The Next Linear Collider Klystron Development Program*

E. Jongewaard, G. Caryotakis+4 more|ArXiv.org|Aug 19, 2000

Gyrotron and Vacuum Electronics Research1 references4 citations

TL;DR

This paper details the development of high-power X-band klystrons for the Next Linear Collider (NLC), focusing on solenoid- and periodic permanent magnet (PPM)-focused designs. It presents a 75 MW PPM klystron achieving 75 MW at 2.8 µs pulse length, with a low-cost, simplified 75XP-3 prototype under development to enable industrial production and reduce modulator costs via multi-beam klystron concepts.

ABSTRACT

Klystrons capable of 75 MW output power at 11.4 GHz have been under development at SLAC for the last decade. The work has been part of the program to realize all the components necessary for the construction of the Next Linear Collider (NLC). The effort has produced a family of solenoid-focused 50 MW klystrons, which are currently powering a 0.5 GeV test accelerator at SLAC and several test stands, where high power components are evaluated and fundamental research is performed studying rf breakdown and dark current production. Continuing development has resulted in a Periodic Permanent Magnet (PPM) focused 50 MW klystron, tested at SLAC and subsequently contracted for manufacture by industry in England and Japan. A 75 MW version of that PPM klystron was built at SLAC and reached 75 MW, with 2.8 microsecond pulses. Based on this design, a prototype 75 MW klystron, designed for low-cost manufacture, is currently under development at SLAC, and will eventually be procured from industry in modest quantities for advanced NLC tests. Beyond these developments, the design of Multiple Beam Klystrons (MBKs) is under study at SLAC. MBKs offer the possibility of considerably lower modulator costs by producing comparable power to the klystrons now available, at much lower voltages. * This work supported by the Department of Energy under contract DE-AC03-76SF00515

Motivation & Objective

• Develop high-power, high-efficiency klystrons for the Next Linear Collider (NLC) to enable future high-energy physics experiments.
• Overcome limitations of solenoid-focused klystrons, including high power consumption and complex magnetic systems.
• Design and test PPM-focused klystrons to eliminate solenoids, reducing power consumption by ~20 kW per tube.
• Enable industrial production of PPM klystrons through contracts with Marconi (UK) and Toshiba (Japan), and develop a low-cost 75XP-3 prototype.
• Explore multi-beam klystrons (MBKs) as a path to further reduce klystron operating voltage and modulator costs.

Proposed method

• Developed solenoid-focused XL-4 klystrons with a 4-cell traveling wave output structure, TE01 output window, and ion pump to reduce voltage breakdown.
• Designed the XL-PPM klystron using PPM focusing with samarium-cobalt magnets, a beam diode for testing, and TiN coating to suppress multipactor.
• Engineered the 75XP-1 klystron with Nd-Fe-B magnets and a fully permanent magnet structure, incorporating a loss ring and rf load to suppress gun and output oscillations.
• Designed the 75XP-3 klystron with a separable clamshell magnet structure, precision-machined electrodes, and reduced part count for simplified, low-cost manufacturing.
• Conducted beam diode testing and simulations to validate magnetic field alignment and transverse field control in the split magnet design.
• Collaborated with industry (Marconi, Toshiba, CPI) to produce prototype tubes and validate manufacturability for NLC-scale deployment.

Experimental results

Research questions

• RQ1 Can PPM focusing replace solenoid focusing in high-power X-band klystrons to reduce power consumption and improve efficiency?
• RQ2 What design modifications are required to achieve 75 MW output in a PPM-focused klystron while maintaining beam stability and high transmission?
• RQ3 How can multipactor and rf oscillations be suppressed in high-power PPM klystrons with complex internal structures?
• RQ4 Can a simplified, low-cost klystron design (75XP-3) be developed that maintains performance while enabling industrial production?
• RQ5 To what extent can multi-beam klystrons reduce operating voltage and modulator cost compared to single-beam designs?

Key findings

• The XL-4 solenoid-focused klystron achieved 75 MW output at 1.5 µs pulse length and has been used to power the NLC Test Accelerator.
• The XL-PPM beam diode demonstrated 99.9% beam transmission at 490 kV and 2.8 µs pulse length, validating the PPM focusing concept.
• The 75XP-1 PPM klystron successfully produced over 75 MW at 2.8 µs pulse length after installing a loss ring and rf load to suppress 1.4 GHz and 20 GHz oscillations.
• The 75XP-3 klystron prototype features a separable clamshell magnet structure, reduced part count, and precision-machined electrodes to simplify manufacturing and improve reliability.
• Industrial production of 50 MW XL-PPM klystrons is underway with Marconi (UK) and Toshiba (Japan), validating the path to large-scale NLC deployment.
• Simulations and test structures confirm that split magnet designs can achieve precise field control, enabling reuse of magnetic components across multiple klystrons.