The SLAC 75XP series klystron was developed in the late nineties / early 2000's as a 60% efficient, 75 MW permanent magnet-focused klystron at X-band for the Next Linear Collider (NLC). In 2023, SLAC is resurrecting the 75XP series for potential deployment in future science facilities like the Cool Copper Collider. Here, we discuss the history and lessons learned from the initial 75XP klystrons (75XP1 through 75XP3 series). Finally, an overview of SLAC's current efforts to finish the design, build, and test of an improved 75XP4 version will be presented.

Very low frequency communication systems (3 kHz-30 kHz) enable applications not feasible at higher frequencies. However, the highest radiation efficiency antennas require size at the scale of the wavelength (here, >1 km), making portable transmitters extremely challenging. Facilitating transmitters at the 10 cm scale, we demonstrate an ultra-low loss lithium niobate piezoelectric electric dipole driven at acoustic resonance that radiates with greater than 300x higher efficiency compared to the previous state of the art at a comparable electrical size. A piezoelectric radiating element eliminates the need for large impedance matching networks as it self-resonates at the acoustic wavelength. Temporal modulation of this resonance demonstrates a device bandwidth greater than 83x beyond the conventional Bode-Fano limit, thus increasing the transmitter bitrate while still minimizing losses. These results will open new applications for portable, electrically small antennas.

Design of a beam energy recovery system for application to the CPI VKS-8262S S-band klystron is presented. The multi stage pulsed depressed collector optics, mechanical and thermal design, energy recovery modulator, and experimental program will be highlighted.

The Core Oscillation Method (COM) and Bunch-Align-Compress (BAC) cavity tuning schemes are discussed and applied to the re-design of the 5045 klystron towards achieving the highest efficiency micro-perveance 2.0 tube ever built.

The 5045 klystron has been in production and accelerating particles at SLAC National Accelerator Laboratory for over 25 years [1]. Although the design has undergone some changes there are still significant opportunities for improvement in performance. Retrofitting the 5045 for higher efficiencies and a more mono-energetic spent beam profile is presented.

X-band klystron research and development has been underway at SLAC for over two decades culminating in the current workhorse X-band RF source, the XL-4. The XL-4 traveling wave output structure consists of 4-cells and will be extended to 6-cells to reduce breakdown and further improve upon the klystron's robustness. Simulations show the gradient can be reduced by roughly 25% and that the cavity is stable. The new output cavity will be cold tested and implemented on an existing XL-4 in the near future.

Both solenoid and periodic permanent magnet (PPM) focusing schemes were evaluated for their ability to transport the beam and maintain beam stability. Final results from this study show beam stability in a solenoid field at all values above the Brillouin field (the minimum field required to transport the beam, i.e. to balance the electric field forces in the beam which would otherwise cause the beam to scallop or grow in size). However, the PPM focusing scheme was unstable at all achievable field strengths. For this reason the solenoid focusing scheme will be used as the baseline for the ONR sheet beam klystron. Given the weight and size advantages of a focusing scheme based on permanent magnets, further theoretical studies and simulations of the PPM design will be conducted to evaluate methods of stabilizing PPM transport of the beam in a future revision of the tube.

The SLAC 5045 S-band klystron has proven to be a remarkably reliable high peak power tube. Originally developed in the 1980’s as an upgraded RF power source for the Stanford Linear Collider, it has continually powered the SLAC linac in support of numerous programs in particle physics and photon science. The large number of tubes built and operated (more than 800) coupled with accumulated running statistics over the last 25+ years represents an unprecedented wealth of operational experience for high pulse power klystrons in accelerator applications. Mean time between failures has continued to rise during this period and is frequently in excess of 100,000 hours during the last several years. Lifetime statistics as well as some important failure modes are presented and examined here.

The TTF3-style coupler is typically used to power 1.3 GHz TESLA-type superconducting cavities. For the US ILC program, parts purchased in industry for such couplers are received at SLAC where they are inspected, cleaned, assembled as pairs in a Class 10 cleanroom, pumped down, baked at 150 C and rf processed. The pairs are then shipped to FNAL and installed in cavities that are tested at input power levels up to 300 kW. This paper describes the coupler results to date, including improvements to the preparation procedures and efforts to understand problems that have been encountered.

A new facility to clean, assemble, bake and rf process TTF3 power couplers is in operation at SLAC. This facility includes a class-10 cleanroom, bake station and an L-band source capable of producing up to 4 MW pulses. This paper describes the facility, test results from processing a pair of couplers that will be used in cryomodules at FNAL, and efforts to simplify the manufacturing of the couplers for large scale production for ILC.

SLAC is developing a 10 MW, 5 Hz, 1.6 ms, L-band (1.3 GHz) Sheet-Beam Klystron as a less expensive and more compact alternative to the ILC baseline Multiple-Beam Klystron. The Klystron is intended as a plug-compatible device of the same beam current and operating voltage as existing Multiple-Beam Klystrons. At this time, a beam tester has been constructed and currently is in test. The beam tester includes an intercepting cup for making beam quality measurements of the 130 A, 40-to-1 aspect ratio beam. Measurements will be made of the electrostatic beam and of the beam after transporting through a drift tube and magnetic focusing system. General theory of operation, design trade-offs, and manufacturing considerations of both the beam tester and klystron will be discussed.

The International Linear Collider (ILC) is a 200-500 GeV center-of-mass high-luminosity linear electron-positron collider, based on 1.3 GHz superconducting radio-frequency (SCRF) accelerating cavities. The ILC has a total footprint of about 31 km and is designed for a peak luminosity of 2x10^34 cm^-2 s^-1. The complex includes a polarized electron source, an undulator-based positron source, two 6.7 km circumference damping rings, two-stage bunch compressors, two 11 km long main linacs and a 4.5 km long beam delivery system. This report is Volume III (Accelerator) of the four volume Reference Design Report, which describes the design and cost of the ILC.

X-band klystron work began at SLAC in the mid to late 1980's to develop high frequency (4 times the SLAC Sband klystron), high power RF sources for the linear collider designs under consideration at that time. This work culminated in the current workhorse X-band RF source, the XL4. To date 26 XL4 tubes have been built. The XL4 4-cell disk loaded traveling wave output structure has a high operating gradient. A new 6-cell structure has been designed to reduce breakdown and to further improve the klystron's robustness. Initial simulations show the 6-cell design reduces the gradient roughly 25% and that the structure is stable. A physical XL4 will be retrofitted with the new output cavity and hot tested in the near future.