1. Cavity Diagnostic System for the Vertical Test of the STF Baseline 9-Cell Cavity at KEK
Y. Yamamoto, H. Hayano, E. Kako, S. Noguchi, M. Sato, T. Shishido, K. Umemori, K. Watanabe(KEK),
H. Sakai, K. Shinoe(ISSP, Univ. of Tokyo), S.I. Moon(POSTEC) and Q.J. Xu(IHEP)
Introduction
Four 9-cell TESLA-like cavities (STF Baseline Cavity) for STF (Superconducting rf Test Facility) Phase-1.0 in KEK were fabricated, surface-processed and measured from 2005 to They are named BL1, 2, 3 and 4, respectively. Their features are shown in Fig.1. They were measured totally 14 times in the vertical cryostat at KEK from Feb/2006 to Feb/2007. To search the heating spot on the cavity and detect the field emission, the simple monitoring system was introduced. It is composed of the carbon resistors (Allen-Bradley, 51Ω) and the PIN photo diodes (HAMAMATSU, S ). The number of them was gradually increased and the monitoring system was improved. The carbon resistors and the PIN photo diodes were fixed and attached on the equator of each cell every 90°, a few resistors were attached around the HOM couplers and a few diodes were attached on the top and bottom flanges of the cavity. MX-100(YOKOGAWA) and NR-1000(KEYENCE) were used as the data logger. The sampling time was 100 or 200msec. We observed successfully the heating spot for all the cavities, although the number of the carbon resistors is only 44. The appearance of the heating spot on the equator was coincident with the quench of the cavity. On the other hand, the diodes were useful for checking the presence or absence of the field emission.
The new optical inspection system was recently introduced to search the cause of the heating into KEK-STF from Kyoto University. This is the very useful tool and it is crucial for the evaluation of the cavity performance to check the correlation between the heating spot and the doubtful location. To shorten the survey time for a large number of the cavities, it is necessary to increase the number of the carbon resistor from 44 to several hundred. When the new vertical test facility is completed in June/2008, the new T-mapping system is also introduced. The paper reports the recent status of the new cavity diagnostic system (“Fish-Bone”) for the STF Baseline Cavity.
Fig.1 Features of STF Baseline Cavity
Fig.2 Previous system
Previous system
The carbon resistors were attached on the equator by the spring action to avoid the easy detachment in the previous system. The number of the resistors and diodes are 44 and 40, respectively. The seven pass-band modes from π to 3π/9 are normally measured in the vertical test. The result by the pass-band measurement was consistent with that of the T-mapping. This is very important.
During the test, the two data loggers for the T-mapping and the x-ray monitoring are continuously operated at the sampling time of 100 or 200msec, which is sufficiently fast for the response of the heating. The detail of the previous monitoring system is shown in Fig.2. It’s the very simple structure to attach to the cavity easily.
The results are shown in Fig.3-4. ΔT in the temperature monitor means the difference of the temperature between the liquid helium and the heating spot. A few resistors are attached to monitor the temperature of the liquid helium at the location except the cavity. A few
PIN diodes are attached on the top and bottom flanges of the beam pipe for the observation of the field emission.
Assign
Attachment to the cavity
Installation into the cryostat
Final state
DAQ System
Around the HOM coupler
Previous Results
The following table shows the previous results of all the vertical tests for STF Baseline Cavity. Totally, the 14 tests were done. After Exp #10, four carbon resistors and PIN diodes were attached on the equator at each cell every 90°and the limiting cell was identified at 100%. BL2 cavity was measured totally five times and eventually achieved 29.4MV/m. The other cavities (BL1, 2 and 4) were limited around 20MV/m. All the cavities eventually had the heating on the equator. It is conceivable that the quality of the electron beam welding was somewhat poor, when the dumbbells were connected. The two new cavities were recently fabricated by the improved EBW in the cleaner environment. They will be measured in the new vertical facility in STF from this autumn.
No field emission eventually occurred for the other three cavities (BL2, 3 and 4) except BL1. The radiation level in the table was measured by the radiation monitor set on the top flange of the cryostat. The PIN photo diodes were very useful for investigating the presence or absence of the field emission and monitoring the situation of the RF processing. For little field emission, we should pay attention to the cleanness of the clean-room, the careful assembly in the clean-room, the purity of the ultra pure water at HPR and etc.
Fig.3 Previous Example #1 (Exp #12)
4π/9 π π
4π/9 π π
3π/9
3π/9
8π/9
8π/9
These figures show the result of the fourth test (Exp #12) for BL2 cavity. This cavity was limited due to the heating of cell #2 at 20.6MV/m in π mode, as shown in the T-mapping display. The max. ΔT was 0.3K. The heating at cell #9 in 8π/9 mode was also observed. In this mode, cell #9 achieved at 22.7MV/m.
Although there were many heating at cell #1, 2 and 9 in 4π/9 mode, they were eventually processed out by the RF conditioning. The heating was not observed in 3π/9 mode.
On the other hand, the radiation level was high during the test (strong field emission). The x-ray emission was observed at every cell by the PIN photo diodes, as shown in the x-ray mapping display. The radiation monitor on the top flange of the cryostat recorded the radiation level of 14mSv/h. The PIN photo diodes on the top flange of the cavity observed the max. level in π mode, while the diode at cell #9 detected the higher radiation level in 3π/9 or 4π/9 than π mode.
After the vertical test, BL2 cavity was re-processed by EP2, because the field emission occurred. The result of the fifth test (Exp #14) is shown in Fig.4.
Fig.4 Previous Example #2 (Exp #14)
New vertical test facility and new T-mapping system
The new vertical test facility is completed on June/2008 in STF. The new T-mapping
system is also introduced for the vertical tests. It’s called “Fish-Bone” structure.
The first vertical test will be done at the beginning of July and is a pilot measurement
for the new T-mapping and DAQ system. Although the number of the carbon resistor
is only 48 now, it will be increased to several hundred in the future. The DAQ system
is composed of PXI-4071 (Digital multiplexer), PXI-2501
for the T-mapping and PXI-6225 for the x-ray monitoring
made by National Instruments. The sampling rate of
PXI-2501 and PXI-6225 are 10Hz and several 10Hz,
respectively. The data averaging will be used for PXI-6225
for the low noise effect. The DAQ program is based on
LabVIEW, which is same as that of the RF measurement.
They will be merged into the common one in the future.
From this autumn, a series of vertical tests will be done
for the new cavities (BL5&6) for S1-Global, and the other
cavities (AES#001, AC71, AC74, AC80) from DESY and
FNAL. The new T-mapping and the new optical inspection
system will be the standard and the strong tools through
these measurements in STF. It is crucial to check the
correlation between the heating spot by the T-mapping
and the doubtful location by the optical inspection system. π π π π π
8π/9
7π/9
6π/9
7π/9
4π/9 π
8π/9
4π/9 π
5π/9 π
6π/9
3π/9
5π/9 π
3π/9
The top and middle figures show the results measured in the first day of the fifth vertical test for BL2 cavity. Although, in the first measurement of π mode, the heating normally occurs around the HOM couplers, they are easily processed out. In this measurement, the both end cells had the heating at every mode except 3π/9 mode. While the heating at cell #1 disappeared in the end, that at cell #9 remained at Pos.1 and 5. This situation was same in the second day of the vertical test, as shown in the bottom figures. Therefore, the limiting cell was cell #9, when the accelerating field achieved the max. level of 29.4MV/m. The limiting cell was changed from cell #2 in Exp #12 to cell #9 in Exp #14. The heating at cell #2 was also observed during 3π/9 and 4π/9 in the first day, as not shown in the paper.
On the other hand, although the radiation level was low at the beginning of the first day, it became suddenly high in the end, as shown in the middle figures, and it became lower after the RF processing. The green scattered plot shows the exponential growth of the radiation level with the field. In the second day, the radiation level gradually became much lower during the RF processing, as shown in the bottom figures. No field emission was eventually observed. This BL2 cavity had the best performance within four STF Baseline cavities (BL1-4). π
7π/9 π
8π/9
6π/9
4π/9 π π
5π/9 π
3π/9 π
Fish-Bone
Fitting check
DAQ System π π π
Summary and Future Plan
The previous system was very useful for identifying the heating location and monitoring the radiation level during the vertical test. Although the number of the carbon resistor is only 44, it was possible to identify the location of the heating. It is necessary to increase to several hundred for a smaller location, because it is difficult to inspect the every region of the cavity inside using the optical inspection system for a large number of the cavities. The new T-mapping system is completed, which is based on LabVIEW. From this autumn, the vertical tests for many cavities will be done in STF.