High-gradient Experiments with Narrow Waveguides Kazue Yokoyama, Toshiyasu Higo, Yasuo Higashi, Noboru Kudo, Shuji Matsumoto, Shigeki Fukuda, Mitsuo Akemoto, Mitsuhiro Yoshida, Tetsuo Shidara, H iromitsu Nakajima Accelerator Laboratory, KEK WEPP106

2/18 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.

3/18 Rectangular Waveguide: WR90 Wavelength converter: Width 22.86( g  mm) → 14mm ( g  mm) Cosine taper (  1 g ) : Height mm → 1 mm Calculated to get a low VSWR by an HFSS. Narrow Waveguide Design field gradient of 200 MV/m at an RF power of 100 MW group velocity of around 0.3c

4/18 Fabrication  4 pieces for a narrow waveguide  braze bonding in hydrogen furnace #CU002#SUS003#CU004 materialOFCSUS316LOFC anneal500  C1020  C500  C processingmilling, WEDMmilling cleaningCPSUSpikaCP bondingCu/Au/Ni, hydrogen furnace Cu/Au, hydrogen furnace Cu/Au hydrogen furnace GHz Loss [dB] E-Field MW statusPrototype Tested at XTF Under Testing at Nextef plan to be tested  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

5/18 High power Dummy Load RF Narrow waveguide #CU002 Setup for High-Power  The first high-power test of copper(#CU002) was done at XTF (previous X- band Test Facility at KEK). Acoustic sensors PMT 1, 2, 3, 4 PMT 5

6/18 #SUS003 Setup for High-Power Nextef  We are on going high power testing of stainless-steel(#SUS003) at Klystron Test Stand. PPM Klystron Narrow waveguide in 5mm lead shield PMT 5, 6 Acoustic sensors Dummy Load PMT 1, 2, 3, 4  #CU002 tested at XTF (06.11 ～ 07.01) Moving to Nextef ( ～ 07.04) Using for system checking ( ～ 07.05)  #SUS003 Tested at Nextef ( ～ now)

7/18 Processing Interlock system -> a reflect power is large (vswr>1.4) and vacuum degrades for RF component protection.  Options: Controled fixed time and power step RF breakdowns caused deterioration of vacuum Daytime processing Power ~25 MW Time (~8h)

8/18  Control time step (flexible)  Control power step by limiting Vac. (flexible)  Processing time is almost 24 H.  We’re seeking for ways of processing. Processing Vac.< 1E-6 Pa Power~30MW Power step Vac. Time (4h) Pulse width 200ns -> 400ns

9/18 Processing history (Accumulated No. breakdown events vs. power  RF pulse ranged 50ns to 400ns with 50MW at repetition rate of 50pps.  #CU002 had more breakdown events than #SUS003.

10/18 Results of #CU002 and #SUS003 #SUS003 attained higher electric field than #CU002. The temperature related parameter, P*T 1/2, attained approximately 400 MW  ns1/2 of #CU002 and 900 MW  ns 1/2 of #SUS003. P*T^0.5 – the product of RF power and the square root of the pulse width

11/18 RF Power vs. number of BD events 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.

12/18 A rf pulse is detected with a crystal diode and a OSC that calculates a power, vswr and power loss. oscilloscope BD Measurement System

13/18 Data taking from OSC 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). Typical rf pulses at a breakdown events. Transmitted Forward Reflected Reflected upper stream

14/18 After processing, we’ve measured the rate of breakdown events (BDR) at a constant power for one day at 50Hz. BDR of #SUS003

15/18 #CU002 After high-power processing top body  Many breakdown damages were seen on the E-plane surface.  The surface is intensively damaged, and it could also melt due to breakdown. Observation area

16/18 Observation of Breakdown surface (top) by SEM and Laser Microscope  m

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18/18 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.