JLAB, Newport News, Virginia, USA
Recent efforts in pushing the performance of superconducting RF niobium cavities for the International Linear Collider have resulted in two-fold progresses: reduced field emission and improved quench limit in real 9-cell cavities. RF testing at cryogenic temperatures assisted with quench-detection instrumentation reveals that quench happens often times at highly localized areas inside or near the equator weld of a cavity cell, which is also the high surface magnetic field region. High-resolution optical inspection of the identified quench location makes it possible to correlate the cavity quench limit with certain types of defects. Several sources of quench producing defects are being explored. We will discuss the experimental evidence in supporting each of these sources. We will also discuss the methods of curing or preventing these defects for improved gradient limit and reduced gradient spread.
JLAB, Newport News, Virginia, USA
We report on the progress of ILC high gradient cavity R&D at Jefferson Lab since the Berlin Workshop. Nine out of ten 9-cell cavities manufactured by ACCEL/RI achieved a gradient of more than 38 MV/m at Q0 of more than 8·109 up to a second-pass processing. Four out of six 9-cell second production batch cavities manufactured by AES achieved a gradient in the range of 36-41 MV/m. The cavity quench studies are further enhanced by adopting the Cornell OST’s and the KEK replica technique, in addition to the existing JLab fixed thermometry system and JLab high-resolution optical inspection machine. We have also processed and tested ILC alternate cavities including 9-cell large-grain niobium cavities and 9-cell low-loss shape (ICHIRO) cavities in collaboration with Peking University and KEK. To date, about 50 9-cell cavities have been processed and tested at JLab under the American Regional Team program in support of ILC. More than 110 ILC cavity EP cycles have been accumulated, corresponding to more than 320 hours of active EP time. More than 150 ILC cavity RF tests at cryogenic temperatures have been completed.
KEK, Ibaraki, Japan
JLAB, Newport News, Virginia, USA
JLAB, Newport News, Virginia, USA
CLASSE, Ithaca, New York, USA
We describe the JLab standard procedures of ILC cavity processing and handling for reproducible high gradient high Q0 results. The procedure begins with mixing fresh electrolyte with the molar ratio of HF:H2O:H2SO4 in the range that is compatible with that in the original Siemens recipe. Three key process parameters, namely the acid flow rate, the polish cell voltage and the cavity body temperature, are identified and in control. Our experience shows that optimal EP is achieved in the continuous current oscillation mode. The appearance of current oscillation also serves as a sensitive in-situ QA/QC indicator. The “auto polishing” procedure is introduced by continuing the acid flow and cavity rotation after the voltage is shut off. This effectively reduces sulfur-bearing niobium oxide granules, an inherent contaminant of the EP process. An elaborate post-EP cleaning procedure includes low-pressure water rinsing, HOM coupler brushing and ultrasonic cleaning with detergent. The vacuum furnace heat treatment procedure is updated. A no-touch bead-pull method is established. Slow pump down is routinely applied to prevent recontamination of the cavity surface.
JLAB, Newport News, Virginia, USA
AES, Medford, NY, USA
Fermilab, Batavia, USA
CLASSE, Ithaca, New York, USA
One of the major goals of ILC SRF cavity R&D is to develop industrial capabilities of cavity manufacture and processing in all three regions. In the past several years, Jefferson Lab, in collaboration with Fermi National Accelerator Laboratory, has processed and tested all the 9-cell cavities of the first batch (4 cavities) and second batch (6 cavities) production cavities manufactured by Advanced Energy Systems Inc. (AES). Over the course, close information feedback was maintained, resulting in changes in fabrication and processing procedures. A light buffered chemical polishing was introduced, removing the weld splatters that could not be effectively removed by heavy EP alone. An 800 Celsius 2 hour vacuum furnace heat treatment procedure replaced the original 600 Celsius 10 hour procedure. Four out of the six 9-cell cavities of the second production bath achieved a gradient of 36-41 MV/m at a Q0 of more than 8·109 at 35 MV/m. This result validated AES as the first “ILC certified” industrial vendor in the US for ILC cavity manufacture.
JLAB, Newport News, Virginia, USA
Fermilab, Batavia, USA
KEK, Ibaraki, Japan
Recent experience of ILC cavity processing and testing at Jefferson Lab has shown that some 9-cell cavities are quench limited at a gradient in the range of 15-25 MV/m. Further studies reveal that these quench limits are often correlated with sub-mm sized and highly localized geometrical defects at or near the equator weld. There are increasing evidence to show that these genetic defects have their origin in the material or in the electron beam welding process (for example due to weld irregularities or splatters on the RF surface and welding porosity underneath the surface). A local defect removal method has been proposed at Jefferson Lab by locally re-melting the niobium material. Several 1-cell cavities with known local defects have been treated by using the JLab local e-beam re-melting method, resulting in gradient and Q0 improvement. We also sent 9-cell cavities with known gradient limiting local defects to KEK for local grinding and to FNAL for global mechanical polishing. We report on the results of gradient improvements by removal of local defects in these cavities.
KEK, Ibaraki, Japan
JLAB, Newport News, Virginia, USA
Sokendai, Ibaraki, Japan
JLAB, Newport News, Virginia, USA
PKU/IHIP, Beijing, People's Republic of China
As reported previously at the Berlin workshop, applying the JLab standard ILC EP recipe on previously BCP etched fine-grain multi-cell cavities results in improvement both in gradient and Q0. We recently had the opportunity to experiment with two 1300 MHz 9-cell large-gain niobium cavities manufacture by JLab and Peking University. Both cavities were initially BCP etched and further processed by using JLab’s standard ILC EP recipe. Due to fabrication defects, these two cavities only reached a gradient in the range of 20-30 MV/m. Interestingly both cavities demonstrated significant Q0 improvement in the gradient range of 15-20 MV/m. At 2K, a Q0 value of 2·1010 is achieved at 20 MV/m. At a reduced temperature of 1.8K, a Q0 value of 3·1010 is achieved at 20 MV/m. These results suggest that a possible path for obtaining higher Q0 in the medium gradient range is to use the large-grain material for cavity fabrication and EP and low temperature bake for cavity processing.
JLAB, Newport News, Virginia, USA
We have studied thermal breakdown in several multicell superconducting radiofrequency cavity by simultaneous excitation of two TM010 passband modes. Unlike measurements done in the past, which indicated a clear thermal nature of the breakdown, our measurements present a more complex picture with interplay of both thermal and magnetic effects. JLab LG-1 that we studied was limited at 40.5 MV/m, corresponding to Bpeak = 173 mT, in 8pi9 mode. Dual mode measurements on this quench indicate that this quench is not purely magnetic, and so we conclude that this field is not the fundamental limit in SRF cavities.
JLAB, Newport News, Virginia, USA
PKU/IHIP, Beijing, People's Republic of China
Quench limits accelerating gradient in SRF cavities to a gradient lower than theoretically expected for superconducting niobium. Identification of the quenching site with thermometry and OST, optical inspection, and replica of the culprit is an ongoing effort at Jefferson Lab aimed at better understanding of this limiting phenomenon. In this contribution we present our finding with several SRF cavities that were limited by quench.
JLAB, Newport News, Virginia, USA
We designed and built two high resolution (0.6-0.55mm special resolution [1.1-1.2mm separation]) thermometry arrays prototypes out of the Allen Bradley 90-120 Ω 1/8 watt resistor to measure surface temperature profiles on SRF cavities. One array was designed to be physically flexible and conform to any location on a SRF cavity; the other was modeled after the common G-10/stycast 2850 thermometer and designed to fit on the equator of an ILC (Tesla 1.3GHz) SRF cavity. We will discuss the advantages and disadvantages of each array and their construction. In addition we will present a case study of the arrays performance on a real SRF cavity TB9NR001. TB9NR001 presented a unique opportunity to test the performance of each array as it contained a dual (4mm separation) cat eye defect which conventional methods such as OST (Oscillating Superleak second-sound Transducers) and full coverage thermometry mapping were unable to distinguish between. We will discuss the new arrays ability to distinguish between the two defects and their preheating performance.
PKU/IHIP, Beijing, People's Republic of China
JLAB, Newport News, Virginia, USA
A computer program which was used to simulate and analyze the thermal behaviors of SRF cavities has been developed at Jefferson Lab using C++ code. This code was also used to verify the quench initiation due to geometrical defects in high magnetic field region of SRF cavities. We built a CEBAF single cell cavity with 4 artificial defects near equator, and this cavity has been tested with T-mapping. The preheating behavior and quench initiation analysis of this cavity will be presented here using the computer program.
daijin@pku.edu.cn
PKU/IHIP, Beijing, People's Republic of China
JLAB, Newport News, Virginia, USA
KEK, Ibaraki, Japan
Three kinds of geometrical features analysis techniques were adopted in association with cavity gradient R&D at Jefferson Lab: (1) Feature shape analysis by Kyoto camera system; (2) 3D shape analysis using a HIROX KH-7700 High Resolution Digital-Video Microscopy System; (3) Replica technique plus surface profiler for profile measurement of geometrical features. Up to now, many features were found in two nine cell SRF cavities: PKU2 from Peking University in China and NR1 from Niowave-Roark in America. Both of them have been RF tested at 2K. The shape analysis of geometrical surface features and correlation with RF test results using a thermal analysis code will be presented here.
daijin@pku.edu.cn
JLAB, Newport News, Virginia, USA
A new high-resolution cavity inspection machine has been built at JLab, integrating a KEK/Kyoto camera system with a JLab built system based on a long-distance microscope. This system has been a working horse in support the on-going ILC cavity gradient R&D. More recently, this machine has been also used to inspect small aperture cavities including some 1497 MHz 7-cell CEBAF upgrade cavities and S-band single-cell crab cavities, demonstrating the capability and flexibility of the machine. We will describe the detailed features of the inspection machine along with some exemplary inspection results.