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

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

3. 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

4. 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

5. 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

6. 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

7. 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.

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

9. 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”

10. 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

11. 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

12. 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

13. 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

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