High Efficiency X-band Klystron Design Study Rich Kowalczyk, Brandon Weatherford and Jeff Neilson – SLAC Igor Syratchev, Jinchi Cai and Walter Wuench - CERN 2017-10-25

Overview Program description and goals Current status Related high efficiency programs at SLAC Conclusions

High Efficiency X-band Design Study - Overview SLAC/CERN collaboration funded by CERN to explore achievable efficiencies for high peak power X-band klystron Use recently developed techniques to improve existing SLAC XL series 50 MW, 45% efficient klystron Improve efficiency with high efficiency bunching techniques such as Core Oscillations Method (COM) Improve output cavity lifetime using design techniques learned from design and test of high gradient structures Performance targets: Frequency: 12 GHz Pulse Width: 2 µs Efficiency: >70 percent Peak RF Power: 50 MW minimum, 70MW goal

Design Challenges Minimizing radial stratification effects Limitation on output power due to radial dependence of electron deceleration in cavities Approach: Explore dependence of peak efficiency on beam fill factor and klystron rf circuit length. Generating optimal velocity distribution in fully saturated beam at the output How do we slow down all electrons to the same energy after the output gap? Given some energy spread, optimize the velocity profile of the incoming bunch Existing XL-5 design is already limited by high gradients in output cavity, even larger gradients will arise in a high-efficiency design Increase output circuit length Evaluate standing and traveling wave configurations Parallel rf extraction

Design Study Plan Preliminary COM bunching circuit designs in 1D AJDisk Effect of perveance Fill Factor Klystron length Intermediate design with 2D large signal codes (TESLA,Kly-C) Radial effects Benchmarking of Kly-C vs. MAGIC/VSIM vs. TESLA Final design on PIC codes (MAGIC,VSIM) Stability analysis Include calculations for depressed collector Output cavity designs in MAGIC-2D done in parallel with bunching circuit design Standing wave and travelling wave designs Evaluate for efficiency and peak surface fields

Preliminary Results Initial 1D COM circuit designs completed over three perveance values (0.9, 1.2, 1.5) with efficiencies of 82%, 76%, 73%, respectively Does not include 2D effects Expect up to ~10% degradation with 2D calculations Output cavity design studied using beam from existing XL buncher circuit For traveling wave, significant increase in efficiency (50% higher) possible through improved output circuit Efficiency increases come with increased peak surface fields Reduction of surface field being investigated through group velocity profiling and power extraction techniques

Example of COM HEX tube optimized using 1D code AJDisk Predicted efficiency 83.2%. In this simulation the output cavity is a simplification of actual design. It has been used to prove COM technology.

Simulations with 2D code KlyC About 10% efficiency degradation due to 2D effects. Predicted efficiency is 72.7%.

Complementary High Efficiency Programs at SLAC Two programs to upgrade efficiency and power of 65MW S-Band 5045 tubes used in LCLS Redesign of 5045 interaction circuit to increase rf output power via increase in efficiency using high efficiency bunching technique Bunch-Align-Collect (BAC) Increase system efficiency by recovery of energy in spent beam of klystron Both designs constrained to be “plug- compatible”

Status of BAC Klystron Efficiency Upgrade Program First version built and tested Oscillation at 6.65 GHz observed at standard pulse length of 3.5 microseconds Predicted efficiency increase of 20% observed in test at 100 ns pulse width MAGIC 2d simulations showed presence of 6.7 GHz trapped mode between cavity 7 and 8 Circuit redesigned to eliminate trapped mode; rebuild and test in early 2018

Retrofit of Existing Klystron RF Systems to Use Pulsed Energy Recovery Traditional Modulator (DC to Pulse Converter) RF Power Source RF Out SLAC Energy Recovery Modulator (Pulse to DC Converter) Spent Beam Energy Recovered Feed-forward energy recovery scheme enables traditionally wasted energy to be reused Existing infrastructure is re-used Energy recovery is completely passive Prototype Inverse Marx

Depressed Collector Testing at SLAC First test of multi-stage depressed collector with energy recovery between pulses will be started next week Collaboration between SLAC and CPI Four stage collector on modified VKS-8262 tube (2.856 GHz, 5.5 MW peak, 6 us, 180 Hz, 45% efficiency) Collector predicted to improve efficiency to 65% Depressed Collector on VKS-8262 “Inverse” Marx power recovery module

Depressed Collector on XL Tube At higher circuit efficiencies, diminishing gains from use of depressed collector. Conservative estimate of efficiency gains: 60% circuit efficiency increase to 75% 70% circuit efficiency increase to 80% Depending on financial factors, addition of depressed collector may not be cost effective

Conclusions Preliminary results indicate 60% efficiency realizable, good probability of achieving 70% Use of depressed collector could provide 75- 80% efficiency depending on interaction efficiency Study to be completed in early 2018