Processing and Testing of PKU 3-1/2 Cell Cavity at JLab

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1 Processing and Testing of PKU 3-1/2 Cell Cavity at JLab Rongli Geng, Byron Golden August 7, 2009 Introduction The SRF group at Peking University has successfully built a 3-1/2 cell superconducting niobium cavity for the proposed PKU-ERL machine. This cavity was fabricated by using large-grain niobium from Ningxia. Initial post-purification, BCP etching and tuning for field flatness were also performed by the PKU SRF group. The cavity was then sent to JLab for performance evaluation. Ultimately, after further treatment at JLab, this cavity reached a maximum gradient of 23 MV/m at Q 0 = No field emission was present at the maximum gradient. The limit was due to repetitive quench. Two multipacting barriers were encountered at 8 MV/m and 20 MV/m, respectively. The later is consistent with the twopoint multipactor predicted by the code simulations. The origin of the multipacting barrier at 8MV/m is not yet fully understood. Nevertheless, both barriers can be processed through by using modest RF processing. Baseline RF test Initial baseline RF test was performed by high pressure water rinsing the cavity with the JLab production HPR machine. The 3-1/2 cell cavity has features different from the regular elliptical multi-cell cavities as shown in Fig. 1. Figure 1: Drawing of the PKU 3-1/2 cell cavity. The first cell on the left-hand side, 1/2 cell, has a squashed shape to match the low β value of electrons during the initial acceleration. A 14mm diameter hole allows electrons emitted from the photocathode (not shown) to drift into the acceleration space. Shown also in Fig. 1 is a stainless-steel adaptor piece with a CF16 Conflat flange, which is used for attachment of a RF pick-up antenna feedthrough.

2 Two indium wires (each inch diameter) are sandwiched between the adaptor and the flat wall of the cavity (made of Nb-Ti with threaded holes) to provide a sealed joint. Silver-plated bolts are successfully used repeatedly without difficulty despite the fact that the whole cavity (including the Nb-Ti flat wall with threaded holes) was post-purified previously. The incident power antenna feedthrough is attached to the TESLA type FPC port. The first attempt to test the cavity failed because of a rather large leak at one of the two HOM flanges (NW8). It appeared that the leak was caused by a soft grain on the sealing path of the NW8 flange (see Fig. 2 indicated by arrow). Finally, we decided to use an indium seal at this joint. Fig. 3 shows the PKU 3-1/2 cell cavity attached to a test stand under vacuum. Figure 2: Soft grain (indicated by arrow) on the sealing path of a NW8 flange. Figure 3: PKU 3-1/2 cell cavity attached to test stand and under vacuum.

3 A reasonably good low-field Q 0 of was measured during the first RF test. But field emission began at 5 MV/m followed by rapid Q decline. The cavity was then partially disassembled and high pressure water rinsed again for 3 passes. The standard HPR spray head (with 2 nozzles producing fanshaped jets perpendicular to the cavity axis) was replaced by a new one with nozzles at 45 degree angles with respect to the cavity axis. The new head provided improved cleaning over the wall surface between the cathode hole and the equator of the first cell. Nevertheless, the cavity performance was essentially unchanged by the additional HPR as can be seen in Fig. 4(a) and (b). Figure 4(a): Q 0 (E acc ) curves of the two baseline RF tests. Figure 4(b): X-ray dose rate measured at top plate outside of the Dewar for baseline tests.

4 RF surface Inspection and Bead-Pull Measurement Following the second RF test, the cavity was disassembled and visually inspected. Two regions near the photo-cathode hole were observed to have blemishes. These were suspected to be candidate source of field emission. The probable cause of these blemishes is insufficient material removal after the postpurification treatment. It should be mentioned that the PKU SRF group treated another cavity (1300 MHz, 2-cell, TESLA style) together with the 3-1/2 cell cavity before both cavities were shipped to JLab. The performance of the 2-cell cavity was already shown to be improved by additional removal by BCP etching at JLab [1]. This experimental fact supports the hypothesis of insufficient removal being responsible for the poor performance of the 3-1/2 cell cavity. The cavity was also bead-pull measured for checking the field flatness (PKU tuned the field flatness to 94% before the cavity was shipped to JLab). Fig. 5 shows the experimental arrangement and result of the field flatness measurements (92.5%). Figure 5(a): Field flatness measurement set-up.

5 Figure 5(b): Bead-pull measurement result: field flatness 92.5%. BCP etching and RF test Following the visual inspection and the bead pull measurements, the PKU 3-1/2 cell cavity was ultrasonically cleaned and BCP (HNO 3 :HF:H 3 PO 4 =1:1:2 by volume) etched without further tuning for field flatness. The production BCP processing tool was used. The acid was chilled and a nominal acid temperature of 10 ºC was maintained in the acid tank. The cavity was vertically oriented with the ½ cell at the top. A special PVDF adaptor was used to allow continuous acid circulation (typically 4 GPM). A Viton O-ring was used to provide a hermetic seal between the adaptor and the flat surface of the Nb-Ti wall. Prior to the cavity etching, an experiment was conducted to estimate the accelerated material removal (due to the reduced aperture as compared to the large iris aperture) at the ID of the 14 mm photocathode hole. The removal rate near the photocathode hole was found to be about a factor of 4 higher than that at the equator regions. The total estimated material removal at equator regions was 30 µm. No adverse effect was observed on the Nb-Ti surface, part of which was necessarily exposed to the BCP acid. Fig. 6 shows a photograph of the BCP etching set-up. Figure 6: PKU 3-1/2 cell cavity chemical etching with the closed-loop BCP processing tool. Following the BCP etching, the cavity was processed and assembled as follows: HPR (head with nozzles at 45 degree angle) for 1 pass and drip dry over night. First assembly. HPR for 4 passes.

6 Final assembly. Pump down and leak check. RF test at 2K. Warm up to room temperature and cool down again. Re-test at 2K. Fig. 7 gives the RF test results. For comparison, the baseline RF test result is also shown in the figure. A low-field Q value of was measured, somehow lower than the baseline value. Nevertheless, it was possible to raise the maximum gradient beyond the baseline value, confirming the benefit of the light BCP etching in removal of filed emitters. At 8-10 MV/m, a strong multipacting barrier was encountered. Warming up to room temperature had no effect to the low-field Q value and the multipacting barrier at gradient range of 8-10 MV/m remained strong. With some RF processing, it was possible to raise the maximum gradient to 11 MV/m. Figure 7(a): Comparison of Q(E acc ) before and after BCP etching 30 µm.

7 Figure 7(b): Comparison of X-ray dose rate before and after BCP etching. Final test after 800 degree furnace treatment and second BCP The unexpected low Q value following the 30µm BCP etching suggests that the accumulated hydrogen in the penetration depth has exceeded the Q-disease threshold. It was decided to outgas hydrogen by high temperature anneal the cavity in a vacuum furnace (800 ºC for 2 hours). Fig. 8 shows profiles of the furnace temperature and major residual gas species. Figure 8: Profiles of temperature and major gas species for vacuum furnace treatment. The cavity was ultrasonically cleaned after the furnace treatment, followed by the second BCP etching for a 25 µm wall material removal and HPR and clean room assembly. The final RF test results are given in Fig. 9. The low-field Q value was successfully recovered to from the hydrogen removal. The multipacting barrier at 8-10 MV/m was still present, but it was possible to process through.

8 Figure 9: Final performance of the PKU 3-1/2 cell photo-injector cavity. A second multipacting barrier at MV/m (known barrier for standard TESLA shape) was also observed and was also processed through after modest RF processing. Finally, the cavity reached a maximum gradient of 23 MV/m with a Q 0 of , limited by repetitive quench. No field emission was present at the maximum gradient. After more liquid helium transfer, the cavity was tested again, reproducing the performance of the final power rise of the previous test. The processing effect was also found preserved and the multipactors at 8-10 MV/m and MV/m did not re-appear. Summary and discussion The PKU 3-1/2 cell photo-injector cavity reached a final maximum gradient of 23 MV/m with a Q 0 of The gradient was limited by repetitive quench. No field emission was present at the maximum gradient. An unexpected multipacting barrier at 8-10 MV/m was observed. The origin of this multipactor yet needs to be understood. It was suspected that the small volume formed between the Nb-Ti wall and the stainless-steel wall of the adaptor piece was involved. It is experimentally found that, depending on the surface conditions, this multipacting barrier may or may not be processed through. It is suggested to further explore the nature of this multipactor through simulation studies, especially given the fact that similar spatial configuration will be also present when this cavity is used with a real photo cathode attached to the cavity. The Lorentz force detuning coefficient of the PKU 3-1/2 cell cavity was measured to be -4.3 Hz/(MV/m) 2. The detuning sensitivity due to the helium bath pressure change was measured to be -179 Hz/Torr. Acknowledgement We want to thank many colleagues at Jefferson Lab and Peking University. Bob Mannus assisted in building and modifying necessary parts for the cavity processing and testing. Danny Forehand heat furnace heat treated the cavity. Haipeng Wang bead-pulled the cavity field flatness. Zhu Feng, Quan Shengwen and Hao Jiankui provided necessary information about the cavity. We also want to thank Peter Kneisel and Curtis Crawford for useful discussions. [1] P. Kneisel, private communication. References