Recent Progress on High-Current SRF Cavities at Jlab Page: 2 of 3
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Proceedings of IPAC'10, Kyoto, Japan
mechanical process for material removal that has been
shown to be successful in other cavities. The 5-cell cavity
was polished using three grades of media resulting in a
smooth looking semi-glossy surface. Unfortunately one
cell was distorted when a fixture became loose during the
polishing. The damaged cell was retuned and the cavity
was processed and tested anyway. It recovered reasonably
well, reaching a quench limit of 20 MV/m but with a good
Q (>1010 at 2K), and no high-field Q-slope, see figure 3.
The damaged cell has now been cut out and will be
replaced by a fresh cell in the near future.
Large grain 1A 1497MHz 5-cell
Similarly concepts for high-power loads were
developed but at low current these can be greatly
simplified, however good HOM damping over a broad
frequency range is still required. Development has
concentrated on finding materials with broadband
absorption characteristics over a suitably wide range of
temperatures. Candidate materials have been identified
and prototype low-power broadband loads have been built
and tested, see figure 6.
** 4m*... ***. **.. *..* *..** * * .*.4.~
Quench@ 20 MVn
1 0DE-09 , i i i i i
0 4 8 12 16 20 24
Fig. 3. Test result of 5-cell 1.5 GHz cavity after CBP.
Low frequency cavity test
The 750 MHz 5-cell cavity never reached vertical
testing in the original program but was recently retrieved
from storage and processed. Due to the unusual shape and
large size it required special tooling for chemistry and
high-pressure rinse and a unique three-sided cage for
handling and manipulation, figure 4. The cavity is too
large to fit in any existing tuning bench at JLab but
fortunately it was acceptably field flat as fabricated.
Special tooling will be required to mount the cavity in the
vacuum furnace for hydrogen degassing so initial tests
were done without heat treatment but with rapid cool
down to avoid the risk of Q-disease. Initial processing
was ~100m of material removal by BCP followed by
one cycle of high pressure rinsing. Initial results are
encouraging with performance similar to the single-cell
750 MHz prototype . Maximum field was 22 MV/m,
figure 5, limited by field emission. Q0 was reasonable at
2K. Some multipacting was seen initially at about 3
MV/m but processed quickly. The in-cell multipacting
barrier was seen between about 10-14 MV/m but
processed away. Further tests are planned for this cavity.
Two prototype WR650 waveguide windows have been
successfully tested to over 60 kW, and several high power
coupler concepts have been developed. However the JLab
FEL program is moving in the direction of lower current,
higher energy machines so the development has shifted
towards simpler lower-power couplers.
Fig. 4. 750 MHz high-current cavity in preparation.
o 5 10
" 1.93 K
15 20 25
Fig. 5. First test result of 750 MHz five-cell cavity.
reflection response (d8)
-70 - simulated
- - -50 i
a - ___
'.0 3.0 4.0 5.0
Fig.6. Broadband low-power HOM load prototype.
A complete conceptual design of a test cryomodule has
been developed based on components from the original
CEBAF module, see figure 7. The concept uses a space-
07 Accelerator Technology
T07 Superconducting RF
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Robert Rimmer, William Clemens, James Henry, Peter Kneisel, Kurt Macha, Frank Marhauser, Larry Turlington, Haipeng Wang, Daniel Forehand. Recent Progress on High-Current SRF Cavities at Jlab, article, May 1, 2010; Newport News, Virginia. (digital.library.unt.edu/ark:/67531/metadc845278/m1/2/: accessed December 15, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.