-- Summary highlights --
- The 9-cell EP activity is in full operation as is the balance of ILC cavity tuning, surface preparation and vertical testing. Activities include specified "S0" cycles as well as additional trouble-shooting cycles.
- A significant number of single-cell tests have been underway to build the characterization of the performance of large-grain niobium material subjected only to BCP preparation.
- Fabrication of 9-cell cavities from both large-grain and polycrystalline material has progressed and the large grain cavities have received initial testing.
- Diagnostics for associating field-emitting contamination sources has begun, using samples exposed to elements of the cavity process and subsequently examined with the JLab DC field emission diagnostic systems.
- Basic Nb EP process characterization R&D continues, primarily as a graduate student project.
Nb Cavity Electropolishing R&D - 9-cell cavities
Defect(s) in AES3 located
AES3 had a limiting gradient of about 18 MV/m due to quench in past tests. The quench limit remains unchanged despite repeated surface removal by EP. Pass-band mode measurements consistently revealed that the quench source resides in cell#4 and/or #6. Additional RF test with 8 thermometers (4 each on cell #4 and #6) singled out cell #6 is the culprit. The final RF test with 16 thermometers attached in the suspected region in cell #6 (Fig. 1) was successfully completed after another 20 micron surface removal by EP. At about 20 MV/m, the cavity quench limit was reached with clearly correlated temperature responses in thermometers. We concluded that the defect is near but outside the equator weld of cell #6. The success of the last RF test also verified the hypothesis of “well-defined local defect” (such as a geometrically suppressed area). AES3 is now under preparation for additional HPR and will be shipped to FNAL under vacuum for additional RF test.
AES2 performance improved with more surface removal
AES2 was previously limited by quench in the gradient range of 18 – 20 MV/m. Pass-band mode measurements revealed that quench limit was reached equally in cell #5, cell pair #4/#6, and cell pair #2/#8. This “global” nature of quench limit in AES2 contrasts sharply with the “well-defined” nature of quench limit in AES1 and AES3. This leads to the hypothesis that the sheets used for AES2 have defects, which might be removed by further material removal. During the recent test after additional 20 micron surface removal by EP, AES2 reached a quench limit of 26 MV/m. This seems to verify the hypothesis and encourages more material removal for further performance improvement. AES2 has been processed with another 20 micron by EP and is waiting for a next RF test when the busy VTA dewars become available. Based on the behavior of AES2, it is likely that its limiting gradient can be further improved by applying the post-purification technology. Improving the limiting gradient in EP’ed cavities by post-purification has been not systematically explored but remains highly interesting. We plan to post-purify AES2 at JLab and test again after additional EP.
A8 reached 32 MV/m
A8 reached 32 MV/m without field emission after the 3rd 20 micron EP at JLab (Fig. 2).
Previously, A8 reached a quench limit of 31 MV/m at Cornell. Since then the cavity was processed and tested several times at Cornell before shipping to JLab. The last test at Cornell was limited by a Q-slope caused by off-normal parameters during the final EP. This result was confirmed during the initial baseline test at JLab. (The Q-slope bearing RF surface appears to be matted in contrast to the normal shiny surface.) After the first 20 micron EP at JLab, the Q-slope seemed to still exist. During the second test following the second 20 micron EP, the cavity had early field emission onset due to contamination. Because of this, it was impossible to evaluate the change in Q-slope. The success of the 3rd test following the 3rd 20 micron EP implies that 60 micron EP is the upper limit for removing the initial Q-slope. Although it is premature to speculate the mechanism of the Q-slope in A8, it is worthwhile to mention a similar Q-slope observed earlier in the cavity A6 EP processed at JLab. In that case, the RF surface also appeared to be matt due to off-normal EP parameters and the Q-slope was successfully removed by a 20 micron surface removal with nominal EP parameters. Exploration of the mechanism behind this apparent Q-slope is a possible topic for our FY08 single-cell cavity EP program.
ICHIRO5 ready for first EP processing at JLab
Since ICHIRO5 was received by JLab, it was first tuned for field flatness and the baseline test was completed after HPR only. The low-field Q of the baseline test was higher than expected. This prompted rigorous measurement system investigation and error analysis. No smoking gun was fund to invalidate the original result. ICHIRO5 was retested in the same dewar after warming up to room temperature. The low-field Q during the retest reached a “nominal” value, but there was field emission activation near the highest gradient. Nevertheless, by scaling the original low-field Q to that of the retest, ICHIRO5 reached a gradient of 30 – 33 MV/m. This is corroborated by the independent analysis based on the Lorentz force detuning. Next step is to EP ICHIRO5 (Fig. 3) for a 20 micron removal followed by RF test.
Nb Cavity Electropolishing R&D - 1-cell cavities
No current activities on this topic - planning has begun
Large- and Single-grain Nb R&D
Fabrication of two 9-cell ILC cavities from large-grain Nb material
Both large grain cavities have been tested twice after the following treatments and a third test with cavity #1 is in progress:
- Pre-tuning100 micron bcp
- Hydrogen degassing at 600 C for 10 hrs
- Final tuning
- Final bcp, removing 40 –60 micron of material from the surface initially and app. 20 micron in subsequent preparation
z
The results from both tests with cavity #2 are plotted in figure 5.
As can be seen, neither cavity performed very well and both quenched early. We attribute this “inferior” performance to manufacturing problems during electron beam welding: both cavities developed holes during equator welding. Through mode analysis we have identified the cells, which cause the problem, but the quench locations have not yet been identified.
Re-tests of both cavities are planned after post-purification heat treatments at 1250 C for 3 hrs in the presence of Ti as a solid state getter material. We hope that an increase in the thermal conductivity of the niobium will stabilize the material and
permit a higher quench field.Return to top of page
Single Cell studies with Large Grain Niobium
- 5 – single cell cavities each from Heraeus RRR niobium (LL shape,one of the cavities is a single crystal cavity at 2.3 GHz) and CBMM Ingot “D” material (TESLA shape) have been fabricated in addition to the 5 cavities from Ningxia large grain niobium ( also TESLA shape) these cavities are being used for reproducibility tests;
- The 5 Ningxia cavities have been tested as well as the Hereaeus cavities (4 of them) after all of them received the same surface treatments.
- The results are shown in Figures 6 and 7 and can be summarized as following ( all cavities are limited by quenches):
Ningxia: 119 mT< Hq < 155mT Average: < 141mT>
Heraeus: 125 mT< Hq < 166mT Average: < 147 mT>
- The cavities from CBMM niobium are being prepared for testing: initial BCP, hydrogen degassing at 600 C for 10 hrs.
- The two additional single cell cavities of the TESLA and ILC_LL shape from CBMM ingot “E” have been tested; the tests on the TESLA-shaped cavity resulted in a quench field of Hpeak = 161 mT ( Eacc = 36 MV/m) before post-purification, meanwhile the cavity has been heat treated at 1250C in the presence of Ti and is awaiting further testing. The LL cavity was tested once, but the test failed because of deformation of the cavity under the vacuum load. Retesting with add. stiffening is scheduled for the near future after removal of the plastic deformation of the cavity.
- In collaboration with DESY, a TESLA-shaped single cell cavity ( f = 1300 MHz) from a single crystal was treated and tested several times at Jlab after its fabrication at DESY. After a post purification heat treatment the cavity reached a gradient of Eacc ~ 38.5 MV/m ( H ~ 164 mT)
- An additional single crystal cavity has been fabricated at Jlab from single crystal sheets produced by DESY; first tests are in progress.
- In collaboration with Peking University (PKU) a large grain single crystal cavity from Ningxia niobium – fabricated at PKU- has been treated and tested several times at Jlab and reached an exceptionally high gradient of Eacc = 43.5 MV/m, corresponding to a magnetic quench field of H ~ 185 mT, close to the fundamental critical field of niobium
- Because of the excellent performances of cavities made from Ningxia niobium, an order has been placed for 40 sheets of niobium with this company for the fabrication of two 9-cell cavities.
- The LL 7-cell cavity – after final tuning - is awaiting testing.
- Material studies (mechanical formability, oxidation behavior of single crystals of different orientation, interstitial interactions and internal friction, magnetic field studies as function of surface treatment) are on going.
- The study of Q vs. Eacc (Hpeak) behavior at high fields (“Q-drop”) have continued with temperature mapping and the influence of the oxygen distribution and hydrogen concentration in the penetration depth has been explored on a large grain cavity from Ningxia material.
A paper about the results of these studies has been published. Return to top of page
Superconducting Joint
- A cavity from Nb1%Zr has been fabricated and tested; the results indicated that NbZr is a better material for a superconducting joint, since it can sustain fields > 30 mT ( the actual cavity quenched at H ~ 42 mT) needed for a superconducting joint in a super-structure ( paper WEPMS062 at PAC2007)
- Based on this result a “double single cell” cavity with NbZr flanges has been designed and fabricated and will be tested in the near future with Nb gaskets.
- As a further alternative for flange material for the sc joint we have tested a NbN cavity and measured a quench field of H ~ 60 mT. If a thin layer of the very hard NbN could be made on a conflat flange made from Nb; this would be the preferable choice for a superconducting joint because of the superior properties of NbN in comparison to NbZr. W.C. Heraeus is helping with the development.
Thermometry
Two sets of single-cell thermometry for Tesla-style cells are in fabrication. They will be available for use in localizing performance-limiting defects in future ILC R&D cavities.
Polycrystaline Nb 9-cell Cavity Fabrication
Two 9-cell ILC cavities using standard polycrystaline Nb are under fabrication at JLab
Both cavities are app. 75 % complete. Problems with the welding of the helium vessel end dishes and the HOM couplers need to be resolved before the cavities can be completed.
It is anticipated that the fabrication of both cavities will be completed by March 2008.
Contamination Control
Contamination and other studies of Nb surfaces EP’ed together with 9-cell cavity getting first results
Two aspects of EP’ed surface are of interest. One is surface contaminants that become field emitters under a high surface electric field. The other is variation of surface properties (geometrical or compositional) that becomes a source of early quench under a high surface magnetic field. Fundamental surface studies are necessary to address these issues. Currently, we are focused on contamination studies by investigating Nb samples electropolished together with 9-cell real cavities. The advantage of this study is that the sample surface experiences the same EP process of a real 9-cell cavity. This gives the possibility to correlate the surface contamination properties of the sample with the field emission behavior of the cavity.
Two Nb samples have been produced by using the above mentioned method. To characterize the field emitters on the sample surface, we use the existing Scanning Field Emission Microscope (SFEM) at the Surface Science Lab of JLab’s SRF Institute. Figure 9 gives an example result, showing the field emitters on the original post-EP surface. Individual field emitter can be analyzed with the integrated SEM in the SFEM system. The sample surface will go through ultrasonic cleaning with Micro-90 and HPR (same procedure for a real cavity) and SFEM analysis will be repeated to characterize changes in field emitters. Through these surface studies, we expect to establish an understanding of the effectiveness of these post-EP cleaning procedure (initial JLab ILC 9-cell experience has shown impressive success with ultrasonic cleaning with Micro-90 solution). Ultimately we expect to establish an optimal post-EP cleaning procedure, for example the optimal concentration of micro-90 and optimal HPR duration etc.
Electropolishing Process R&D
Improving the characterization of the basic electrochemical process, surface effects, and their association to techniques applied to cavities
- How is the niobium EP process to be characterized in contemporary electochemistry terms?
- What range of local parameters are represented in multi-cell EP as presently practiced?
- How can the evolving surface topology during EP be quantitatively described?
- How does the evolving surface topology depend on local process conditions (polarization potential, temperature, flow, electrolyte composition)?
- How does the maximum sustainable H-field depend on local surface profile?
- What range of EP process conditions consistently produces the surface profile necessary for highest-performance cavities?
Talk from SRF2007 summarizing recent R&D progress:
"Improved Characterization of the Electropolishing of Niobium with Sulfuric and Hydrofluoric Acid Mixtures, Hui Tian (College of William and Mary, and JLab)" download
Manuscript in preparation.