Recent High Efficiency RF Source Developments at SLAC National Accelerator Laboratory

1. Recent High Efficiency RF Source Developments at SLAC National Accelerator Laboratory Presented by Jeff Neilson on behalf of Electrodynamics Dept members: Mark Kemp, Aaron Jensen, Erik Jongewaardand Sami Tantawi, Chief Scientist for RFARED division Work supported by the Department of Energy

2. Overview • RF and Accelerator Research and Engineering Division at SLAC • Energy Recovery for Pulsed RF Sources • Scalable High Efficiency Klystron

3. HPRF capability at SLAC is Highly Vertically Integrated • World’s only integrated capability to conceive, design, build test • Very high peak power sources (up to 150MW) and components (up to 500MW) • Associated modulators for sources • High gradient (170 MeV/m) normal conducting rfaccelerator structures • This capability all under one roof allows prototype and test in a tight, rapid development cycle • Unique capability to provide multivendor source of RF vacuum devices through licensing to industry

4. SLAC High Power RF Research & Engineering Has Spanned 0.3 -100 GHz and up to 150 MW Peak Power W-Band Sheet Beam Klystron 95 GHz B Factory Klystron 476 MHz 1.2 MW CW XP X-Band 11.4 GHz, 50 MW PPM focused XL4 & XL5 X-Band 11.4 GHz, 50 MW @SLAC CERN Sinc. Trieste PSI BNLLLNL • 5045 Klystron • S-Band • 2.856 GHz, 65 MW • > 800 produced since 1983 • MTBF > 90,000 hrs • Span wide range • 0.3 - 100 GHz • 1.2 MW CW – 150 MW Peak

5. Unique infrastructure geared towardfabrication and testing of specialized high power vacuum RF electron beam devices • Precision in-process machining • 8 Ultra-high vacuum bake-out stations • 12 Hydrogen Braze/Retort Furnaces and 3 vacuum furnaces • 5 vacuum cathode processing stations • Sputter and evaporative coating chambers B Factory klystron (MW-class cw) completing bake-out

6. Large Test Capability • 13 instrumented 150 MW pulse power modulators • Two MW-class CW test stands • Two shielded test bunkers 5045 klystron in final test Infrastructure is unique. Although originally sized for a higher klystron production rate, now utilized to build a much larger variety of high power RF devices and structures

7. High Power RF, Accelerator, and Pulsed Power Electronics Capability “Under One Roof” Enables Integrated System Design Klystron modulators • SLAC inductive adder topology is generally replacing line-type topologies • SLAC Marx topologies coming into use for long pulse SCRF applications Ultra-fast beam kicker drivers • Solid state nanosecond-switching pulse generators for transmission line beam deflectors Inductive adder modulator next to a SLAC 6575 standard modulator. 3x volume reduction.

8. RF Source Development Renaissance at SLAC • After nearly 10 years of continuous attrition of personnel devoted to RF source technology, recent changes underway: • Upper management decision to maintain as one of SLAC core competencies • All departments reorganized with new management • New funding being allocated for rf source R&D • Encouraged to actively seek outside funding sources • Two new hires in Electrodynamics department • New THz initiative – seeking to fill new staff scientist position to lead program Seeking to apply HPRF source and accelerator expertise to a broader set of problems, beyond DOE Office of Science

9. Spent Beam Energy Recovery for Pulsed RF Sources

10. Motivation for Energy Recovery in Pulsed RF Sources • Growing attention placed upon energy usage at laboratories • Potential for providing a time-phased way to improve an existing, aging facility • Future, high-rep rate (>kHz) applications place very tough demands on modulator

11. Depressed Collector Technology • Used to improve the effective efficiency of vacuum tubes • CW depressed collector technology is mature

12. CW Spent Beam Energy Recovery Methods not Suitable for Pulsed Systems Typical CW biasing methodology Parasitic elements Ringing on cathode from parasitic elements results in phase jitter – Need better solution

13. The SLAC Pulsed Depressed Collector • The collector stages self-biasas electrons impact the stage surfaces • The time-varying potentials of the stages are determined by the spent beam characteristics and the collector electrical impedance • Recovers energy for the next pulse Claims: This is the first demonstration of a pulsed depressed collector (using a single power supply) in a high power vacuum device 2)This is the first method to recover energy from the spent beam during the rise and fall time of the pulse

14. Self-Biasing Concept

15. Multiple Benefits of SLAC Self-Biasing Approach • Recovers energy during rise, fall, and flat-top • Modulators would have a relaxed requirement on rise and fall times • Existing systems can be retrofit • No extra power supplies. Cost no longer proportional to number of stages • Concept is not limited to a particular klystron or modulator topology • Bias tuning can be accomplished through adjustment of the storage capacitance. Can be done “automatically” if desired

16. Case for Economic Justification : A 5045 depressed collector • Average power consumption can potentially be reduced by over 25% • Implementation expense recover in ~10 years >$800k/year electricity savings if implemented on 80 LCLS stations.

17. Pulsed Depressed Collector Experiment • Purpose • Show that self-biasing concept works • Confirm models • Answer major concerns brought up in internal discussions • Approach • Get results as quickly as possible • Not about optimizing for best efficiency

18. Subbooster Klystron Depressed Collector Fabrication stage 1 stage 2 Existing Subbooster Klystron spent beam After Bakeout location of old collector

19. SLAC Klystron Test Lab Experimental Setup Energy dumped between pulses Depressed Collector Subbooster Klystron Energy recovered

20. Example Result of Self-Biasing Collector Stage

21. Klystron Test Lab Experimental Setup • Five separate circuit element parameters are available to tune N:1 ZL,leakage Collector Stage Zc,primary ZL,mag Zc,load

22. Comparison to PIC Simulations Resulting Stage Potentials • The stage potentials vary over time and depend on the biasing impedances and the collected currents • This problem is solved iteratively with a PIC and SPICE code 2D Magic PIC Simulation SPICE Circuit Simulation Collected Stage Currents

23. Comparison to PIC Simulations • To the left, simulated results are compared for two different circuit nodes • A good match is obtained over a fairly large range of different biasing conditions

24. Application to Long Pulse Systems • Modulator rise time less of an issue for long pulse (ms and longer) but potential interest for retrofit of existing systems for energy recovery during flat top • Unfortunately transformer approach does not scale well as loss and cost go up with pulse length Solution - An “Inverse” Marx Energy Recovery Modulator

25. 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

26. Repeat Test Using Inverse Marx Approach

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

28. Next-Generation Modulator Systems Both Provide and Recover Energy RF Out Traditional Modulator (DC to Pulse Converter) RF Power Source AC Power Spent Beam Energy SLAC Energy Recovery Modulator (Pulse to DC Converter) Recovered RF Power Next-generation modulator development completes holistic approach to RF system design

29. Summary and Next Steps • Summary • Basic concept has been experimentally demonstrated • Good matches with circuit and PIC simulations • Next step • Implement in a challenging application: • Scale up to the SLAC workhorse klystron, the 5045 (three orders of magnitude greater peak power than “subbooster” klystron) • Apply to ultra-short pulse, high repetition rate klystrons

30. Scalable High Efficiency Klystron

31. Unique Modular MBK Combining SchemeAs Method to Produce a Scalable Design • Goal is to generate a design which can be scaled to higher power levels using modular design • Each module to use low perveance beam for high efficiency • Reduced NRE costs for new designs • Economy of scale • Graceful power degradation Proposed scalable design where power scales as (2N)2

32. X- Band Multi-Beam KlystronDesign Specifications X-Band Multi-Beam Klystron

33. Gun Design Semi-automated gun design using R. Vaughan’s gun synthesis approach X-Band Multi-Beam Klystron

34. RF Cavity and PPM Design Magnet Iron Cavity RF Cavity Goes Here Drift Tube RF Cavity PPM Period To minimize size and the number of beamlets, the PPM iron will be plated with copper to act as a cavity wall. X-Band Multi-Beam Klystron

35. 65%+ Efficiency Predicted One-Dimensional AJDISK RF Design X-Band Multi-Beam Klystron

36. Rapid 2D Transport Simulation Pseudo Port Model (Using MAGIC2D) 1D Simulation voltages are set at the port boundary. Simulation Time: ~1minute I1/I0 (consistent with 1D simulation) X-Band Multi-Beam Klystron

37. Initial PPM Field Profile Based on Charge Density X-Band Multi-Beam Klystron

38. 2D PPM Transport98%+ Transmission Normalized Axial PPM Field Beam Transport Using Port Model X-Band Multi-Beam Klystron

39. Port Model Agrees with Cavity Model Port Model: Black Particles Cavity Model: Red Particles X-Band Multi-Beam Klystron

40. Two Energy Product PPM Stack Simple Script for Quickly Building the PPM Stack New PPM stack with optimized pole piece geometry to reduce the number of energy products required X-Band Multi-Beam Klystron

41. Beam Transport Including Collector PPM Stack Collector MAGIC Port Model X-Band Multi-Beam Klystron

42. Magnet Modification for the Input and Output Cavities Magnet Geometry for the Input and Output Cavities X-Band Multi-Beam Klystron

43. 3D PPM Stack Magnet Iron Cutout for input and output waveguide X-Band Multi-Beam Klystron

44. Magnet Cutouts Have Minimal Effect on Beam Transport 3D DC Transport X-Band Multi-Beam Klystron

45. X-Band MBK 3D Layout Input and Output Waveguide Gun Collector PPM Stack The outer diameter of the iron pole piece is used to shunt the field to match the current density as the beam bunches. The cathode to collector tip distance is ~30 cm (the maximum diameter is ~6.5cm) X-Band Multi-Beam Klystron

46. Summary • Simulations confirm the scalable MBK module will meet desired specifications • Mechanical design and drafting are underway • Modeling of the combining scheme is being finalized