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High-Power Testing of Narrow Waveguides at KEK Accelerator Laboratory

This study explores the characteristics of materials in high-gradient RF breakdown at KEK's Nextef facility, examining prototype copper (Cu002) and stainless-steel (SUS003) waveguides. Experiments involve a narrow waveguide with a field of 200 MV/m at 100 MW RF power, focusing on design, fabrication, experimental setup, high-power testing results, and breakdown rate measurement system implementation. Analysis includes breakdown events data, processing schemes, power variations, and surface observations using SEM and Laser Microscope. Future plans involve further material testing and surface observation studies.

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High-Power Testing of Narrow Waveguides at KEK Accelerator Laboratory

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  1. Narrow waveguideexperiments Kazue Yokoyama, Toshiyasu Higo, Yasuo Higashi, Noboru Kudo, Shuji Matsumoto, Shigeki Fukuda, Mitsuo Akemoto, Mitsuhiro Yoshida, Tetsuo Shidara, Hiromitsu Nakajima Accelerator Laboratory, KEK

  2. Introduction • The study of characteristics of different materials on high-gradient RF breakdown at Nextef (New X-band Test Facility at KEK). • Experiments performed using a narrow waveguide with field of approximately 200 MV/m at RF power of 100 MW. • Status report on prototype copper (Cu002) and stainless-steel (SUS003)waveguides. • The measurement system for the breakdown rate is set up now.

  3. Contents 1. Design and Fabrication of Narrow Waveguide 2. Experimental Setup • CU002 at XTF (old X-band Test Facility at KEK) • SUS003 at Nextef (New X-band Test Facility at KEK) • Scheme of RF Processing 3. Results of High-Power Testing 4. Measurement system of BDR 5. Conclusion and Summary

  4. Narrow Waveguide Design field gradient of 200 MV/m at an RF power of 100 MW group velocity of around 0.3c Rectangular Waveguide: WR90 Wavelength converter: Width 22.86(g 32.15 mm) → 14mm (g 76.59 mm) Cosine taper( 1 g) : Height 10.16 mm → 1 mm Calculated to get a low VSWR by an HFSS.

  5. Fabrication • Annealing in a hydrogen furnace • Processing by milling and and wire electrical discharge machining (WEDM) • E-plane finished by milling. • 10 m chemical polishing by acid • 4 pieces for a narrow waveguide • braze bonding in hydrogen furnace

  6. Cu002 Setup for High-Power processing @XTF RF Narrow waveguide Acoustic sensors PMT5 PMT1, 2, 3, 4 High power Dummy Load • The first high-power test of copper(Cu002) was done at XTF (previous X-band Test Facility at KEK).

  7. SUS003 Setup for High-Power processing @ Nextef PPM Klystron • We are on going high power testing of stainless-steel(SUS003) at Klystron Test Stand. Narrow waveguide in 5mm lead shield • Cu002 • tested at XTF (06.11～07.01) • Moving to Nextef (～ 07.04) • Using for system checking (～ 07.05) • SUS003 • Tested at Nextef (～ now) PMT 5, 6 Acoustic sensors PMT1,2,3,4 High power Dummy Load

  8. Processing Scheme @XTF Power ~25 MW Time (~8h) • Interlock system -> a reflect power is large (vswr>1.4) and vacuum degrades for RF component protection. • Options: • Controlled fixed time and power step • RF breakdowns caused deterioration of vacuum • Daytime processing

  9. Processing Scheme @Nextef • Control time step (flexible) • Control power step by limiting Vac. (flexible) • Processing time is almost 24 H. Power~30MW Pulse width 200ns -> 400ns Vac.< 1E-6 Pa Power step Time (4h) • We’re seeking for ways of processing. Vac.

  10. Processing history (Accumulated No. breakdown events vs. power @XTF @Nextef • RF pulse ranged 50ns to 400ns with 50MW at repetition rate of 50pps. • Cu002 had more breakdown events than SUS003.

  11. Accumulated number of breakdown events vs. P*T^0.5 during processing @Nextef @XTF • The temperature related parameter, P*T1/2, attained approximately 400 MWns1/2 of Cu002 and 900 MWns1/2 of SUS003. P*T^0.5 – the product of RF power and the square root of the pulse width

  12. Results of Cu-002 and SUS-003 • SUS003 attained higher electric field than Cu002.

  13. RF Power vs. number of BD events @XTF @Nextef • Many breakdown events at pulse width > 100ns and power 20MW. • Few RF break down events at 50 ns and 100ns . • We had a guard window problem around 200ns.

  14. BD Measurement System oscilloscope • A rf pulse is detected with a crystal diode and a OSC that calculates a power, vswr and power loss. • After processing, we’ve measured the rate of breakdown events at a constant power for one day at 50 pps.

  15. Data taking from OSC Reflected • 10 pulses at a System down (HV, trigger and rf off). • Changed pulses with 5 % form a normal pulse (area and peak). • Estimated power, vswr and power loss • We are testing this system now. • Problem is pulse shape is unstable from pulse to pulse (rf jitter, rf power and noise). Forward Transmitted Reflected upper stream • Typical rf pulses at a breakdown events.

  16. Cu002 After high-power processing top body Observation area • Many breakdown damages were seen on the E-plane surface. • The surface is intensively damaged, and it could also melt due to breakdown.

  17. Observation of Breakdown surface (top) by SEM and Laser Microscope 27.70 m

  18. Conclusion • RF breakdown studies on different material has just begun. • Prototype Cu002 and SUS003 had been tested under different system conditions. • Number of break down events for SUS003 is less than that for Cu002 which may be a result of different systems . • We’re testing a processing scheme. • We’re going to observe the surface of SUS003 after measuring BDR. • We’re going to test Cu004, other stainless-steel waveguides and other materials.

  19. Thank you for your attention. !

  20. Summary

  21. Breakdown location PMT6 PMT5 Area of frequent breakdowns Cu-002 Acoustic sensors ?

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