Development of 325 MHz single spoke resonators at Fermilab Page: 4 of 5
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vacuum and evidence for multipacting predominately on the
bottom indicated a possible problem with condensates forming
during both the cool down and the multipacting. It was
decided to immediately try a second test if a better initial
vacuum could be obtained.1 E+09 -
1,E+0S 4-
-CAldteA1t, .,emi 2 and 44KE
Q 2003 02 28 d 4K
+200S 03 12 42
*20 03.12, 4
- - - - -- 200 21. -I
o x 2o08 21,44K
oo
a0 2 4 6 S 10 12 14 16 1S 20
Figure 6 Quality factor of the cavity .vs. accelerating gradient. Data
labeled 2008.02 are from Test 1, 2008.03 from Test2, and
2008.07 from Test 3. The X's are from the backward scan
noted in the text.
Test 2 began with a cavity vacuum of 1.2x10-7 Torr (again
there was no active pumping in the VTS). The Eac. scan of Qo
at 20K, shown in Figure 6, reached 13.5 MV/m before field
emission prevented further increase with our 200 W power
supply. The multipacting barriers shown in the figure were
also encountered upon subsequent Eac. scans in Test 2.
10 00 -Coldtet euQlt, x-iay. 10i11omtol in.stallemde shield
100 200S 03 1, 2K W
1.00 -- 2008 0- 14 2K - - - e--.1 LA'
, ~t#.A.
4'.
r)0 10
-P-il 61 211'
0 01 J
Figure 7 X-ray intensity .vs. accelerating gradient. The data labeled
2008.03 are from Test 2, and 2008.07 from Test 3. The X's
are from the backward scan noted in the text.
Figure 7 shows the X-ray intensity from a detector installed
just under the VTS Dewar's top plate. Field emission clearly
became a significant power drain above -8 MV/m in Test 2.
An Eac. scan of Qo at 4.4 K, shown in Figure 6, ended with a
thermal quench at 12.5 MV/m. Following the warm-up after
Test 2, the vacuum was measured to be 5.9x10-4 Torr and the
RGA again detected only hydrogen and water. As noted
earlier, the larger than expected degradation in vacuum may
be partly due to interstitial gasses released during
multipacting.
With the presence of field emission at high Eacc, we hoped
to do an improved HPR before the next VTS test, but the VTS
scheduling allowed for a third test before another HPR could
be arranged at ANL. Steps were taken to optimize the cavity2008 02143K
2008 0- 1, 4 3K
E, 1\fV m j1.E+10 1
'
4
20
1
6 1
-
vacuum throughout Test 3. A two day long 120 0C vacuum
bake of the SSR1-01 was performed shortly before the cavity
was installed in the VTS. In addition, the newly
commissioned cavity vacuum system of the VTS, which
allows a vacuum at the cavity of ~8x10-8 Torr, was used
during the test.
Test 3 started with an Eac. scan of Qo at 20K, and the result,
shown in Figure 6, was essentially identical to that from Test
2, with the important exception that after the initial scan, the
cavity no longer fell into multipacting barriers near the
operating gradient of 10 MV/m when raising the field. The
VTS was left to warm to -3.50K overnight, and the following
morning Eac. could not be raised above 15 k/in, indicating a
helium leak. After warming up to room temperature, helium
was clearly seen in an RGA, but efforts to isolate the source of
the leak at that time were unsuccessful.
After cooling back down to 4.4 K , the field could again be
raised, and the Eacc scan of Qo shown in Figure 6 was taken. In
this case, data taken during the initial increase in field
(including processing multipacting barriers) and data
subsequently taken working backward from the maximum
field are plotted with different symbols.
There are several interesting features of this scan. Qo at low
Eacc is nearly a factor of two higher than that recorded in the
4.4 0K scans in Tests 1 and 2. This corresponds to a reduction
in surface resistance from 70 nQ to 45 nQ, and we believe this
may be due to the 120 0C bake. Note that an improvement
was observed only in RS(4.4K), but not in RS(2K),~ R0.
Up to 10 MV/m, the cavity behaved similarly to Test 2, but
after 10 MV/m, the cavity properties started changing. The
intensity of X-rays dropped (figure 7) and Qo increased (figure
6). It appears that helium processing of field emitters had
occurred, allowing Eac. to reach 18 MV/m, well beyond the
earlier maxima of 12.5 MV/m at 4.40K and 13.6 MV/m at
20K. This scan could have continued to higher field levels,
since power was not yet limited by field emission.
A backward scan in field was started to record any change
in Qo after the initial processing. As evidenced by the reduced
X-ray intensity, shown in Figure 7, field emission was
considerably lower in the backward scan with a corresponding
increase in Qo, shown in figure 6. After recording the data
point at 14 MV/m, the cavity again became inoperable (the
field could not be raised past 20 kV/m) indicating further
problems with the helium leak. Test 3 ended, and the leak was
eventually isolated to the RF feed through for the power
coupler antenna. The helium leak was probably a two-edged
sword, sometimes not allowing operation (possibly due to
increased multipacting), but also allowing the processing of
field emitters to achieve higher fields.
VII. ACKNOWLEDGEMENTS
The authors wish to acknowledge the efforts of S. Gerbick,
M. Kedzie, and M. Kelly of ANL; T. Arkan, D. Arnold, D.
Assell, G. Romanov, and B. Smith of Fermilab; and T. Roark
and D. Osha of Roark.;.e
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Apollinari, G.; Gonin, I.V.; Khabiboulline, T.N.; Lanfranco, G.; Mukherjee, A.; Ozelis, J. et al. Development of 325 MHz single spoke resonators at Fermilab, article, August 1, 2008; Batavia, Illinois. (https://digital.library.unt.edu/ark:/67531/metadc894794/m1/4/: accessed March 28, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.