1. Energy recovery in depressed collector of klystron operating in pulsed mode
Mark Kemp, Aaron Jensen, Gordon Bowdon, Erik Jongewaard, Andy Haase and Jeff Neilson
Jan 21, 2016

2. Presentation Overview
Pulsed energy recovery concept
Summary of work to date
Next step and conclusions
SLAC/Industry collaboration

3. Energy Recovery Concept
Traditional Modulator
(DC to Pulse Converter)
Klystron
Power Source
RF Out
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

4. Ability to Recover Rise and Fall of Pulse
In addition to “flat top” efficiencies typically quoted, we can also partially recover the pulse rise and fall
Typically, these are completely wasted
For future systems, can have relaxed modulator specifications => less expensive

5. Summary of Work to-date
Generation 1: Transformer-based recovery circuit
Demonstrated concept. Used a few different cores. Model and experiment matched.
Generation 2: Inverse Marx (no recovery)
Demonstrated concept. Shown to be more efficient than transformer.
Generation 3: Inverse Marx (with recovery)
Used inductors as isolating elements. Recovered energy to external capacitor.

6. Gen 2: An “Inverse” Marx Energy Recovery Modulator
Capacitors charge in series, and discharge in parallel
A transformerless, solid-state topology
Using resonant recovery, can passively recover energy back to the modulator
During pulse
In-between pulses

7. Gen 2

8. Gen 2: Comparison of Simulation to Experiment
Good match between PIC/SPICE model and experiment
Additionally, we can pre-charge capacitors to produce a more-square pulse

9. Experiments on a 25 kW Klystron Demonstrate the Concept (Gen 3)
External
Capacitor
Voltage (V)
Voltage (V)
Collector
Stages
Time (µs)
Time (µs)
Cathode
Potential (/3)
RF Pulse
Two-stage depressed collector shown to:
Self-bias
Rise and fall quickly in potential
Recover energy passively to external storage capacitor

10. Geometry Optimization
Wrote Matlab scripts to:
Create “random” collector geometries and biasing potentials
Output geometries into Magic input deck and read in Magic results
Automatically calculate relevant statistics (efficiency, energy per stage, etc.)
Compile and view results from many simulations
Perform computational intelligence optimization routines

11. Geometry Optimization
Constraints added to improve heat conduction and ceramic shielding
Shown is 5045 result used for this presentation
This “optimal” geometry is not unique

12. DOE Accelerator Stewardship Funding Received: Bridge Gap From Concept To Commercialization
Two existing high power tubes will be modified to greatly increase efficiency
CPI VKS-8262S
5.5 MW S-band tube
Upgrade from 45% to 70% efficiency
SLAC 5045
65 MW S-band tube
Upgrade from 45% to 65% efficiency

13. Radiative Cooled Collector
Thin electrode model (.050” thick)

14. ANSYS model assumptions
Electrodes are solid molybdenum
Fixed properties (no temperature dependence)
ε = .35
Loaded on upper and inner surfaces
All surfaces radiate
Ground electrode copper
ε = .15
All inner surfaces radiate
Outer can SST
ε = .39
Fixed sink temperature at 473 K

15. Electrode temperatures (K)

16. Conclusions
Pulsed energy recovery concept has been experimentally proven with excellent agreement
The energy recovery technology is ideally suited for ultra- short pulse (<300ns) RF sources
Accelerator stewardship funding will make technology commercially available within two years

17. SLAC Tech Transfer to Industry (CPI) Demonstrated
Active RF source tech transfer to CPI
SLAC developed X-band RF source technology successfully transferred to CPI
S-band source technology to be transferred soon
Starting collaboration on development of pulsed depressed technology
Recently teamed on proposal to DOE to develop BAC version of CPI’s VKL-7967A 300 kW CW 1.3 GHz klystron

18. New Model for Source Development
Tube industry is risk adverse/profit driven
Reluctant to develop new sources unless there is clear market
Will stop production on source type with low production
SLAC has opposite motivations
Primarily interest in development activity, not production
Designs developed at SLAC can be transferred to any RF source vendor => multiple vendors possible
SLAC can act as backup production source in case industry no longer willing to produce device
Combination of SLAC with subsequent tech transfer to industry could be a good model for future source developments
Development of new BAC inspired X-band source, high rep rate, pulsed depressed collector?