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Rong-Li Geng SRF2005, July 10-15, 2005 1
Review of New Shapesfor Higher Gradients
RongRong--Li Li GengGengLEPP, Cornell UniversityLEPP, Cornell University
Rong-Li Geng SRF2005, July 10-15, 2005 2
• Rapid advances in single-cell cavities until 1995.• Single-cell gradient envelope saturated at 42 MV/m for last 10 years. • While multi-cell cavity performance advances rapidly.• Would it be possible for 45 MV/m and beyond?
500GeV
800GeV
1 TeV ILC(TESLA
type) energy reach
Rong-Li Geng SRF2005, July 10-15, 2005 3
Paths toward higher Eacc (I)The maximum feasible Eacc is determined by the RF critical magnetic field Hcrit,RF. When the surface magnetic field exceedsHcrit,RF, superconductivity breaks down into normal conductivity.
Eaccpk
H
RFcritH
Eacc
/
,max=
Eacc
Hpk• Eacc ↑, Hpk ↑• Eacc=Hpk/(Hpk/Eacc)• Hpk/Eacc determined by geometry• Hpk≤Hcrti,RF for superconductivity
Hcrit,RF,1
Hcrit,RF,2
Eacc max,1
Eacc max,2
Hcrit,RF ↑, Eaccmax ↑
Rong-Li Geng SRF2005, July 10-15, 2005 4
Paths toward higher Eacc (II)Reducing Hpk/Eacc delays breakdown of superconductivityand allows a higher Eacc to be tolerated.
Eaccpk
H
RFcritH
Eacc
/
,max=
Eacc
Hpk• Eacc ↑, Hpk ↑• Eacc=Hpk/(Hpk/Eacc)• Hpk/Eacc determined by geometry• Hpk≤Hcrti,RF for superconductivity Hcrit,RF
Eacc max,1
Eacc max,2
Hpk/Eacc↓, Eaccmax ↑
Hpk/Eacc,1
Hpk/Eacc,2
Rong-Li Geng SRF2005, July 10-15, 2005 5
Hcrit, RF intrinsic material property
0
500
1000
1500
2000
1970 1980 1990 2000 2010
TE011TM010Electropolished
HR
F[O
e]
Year
HPR
Courtesy: Kenji Saito
• Theoretical limit is ~ 2000 Oe for Nb.• Many cavities reached 1750±100 Oe.• Record Hpk in Nb cavity: 1850 Oe (Kneisel, 2005).
Rong-Li Geng SRF2005, July 10-15, 2005 6
Why new geometry• It seems that, at 90% of the theoretical limit level, a rather hard
magnetic barrier is encountered.
• To avoid this brick wall…
• New geometry is a possibility to boost Eacc.
• The trick is to alter cavity shape for a reduced Hpk/Eacc.
• With new geometry, 10-15% improvement in Eacc possible.
• Two leading approaches: Low-loss and re-entrant
Rong-Li Geng SRF2005, July 10-15, 2005 7
Re-entrant geometry
TTF1992
Re-entrant2002
• 2002, Cornell University.• 1300 MHz for ILC.• goal is to reduce Hpk/Eacc.• keeps large 70mm aperture.• for small HOM loss factor
• also a higher (R/Q)*G.• means lower cryogenic loss.
Shemelin, Padamsee, Geng, NIMA 496(2003)1-7.
Rong-Li Geng SRF2005, July 10-15, 2005 8
Low-loss geometry
TTF1992
Low-Loss2002/2004
• 2002, JLab/DESY.• 1500 MHz for CEBAF upgrade.• goal is to maximize (R/Q)*G.• so as to reduce cryogenic loss.• small aperture strategy.• also a reduced Hpk/Eacc.
• 2004, KEK/DESY/JLAB.• 1300 MHz for ILC.• highlight lower Hpk/Eacc feature.
Sekutowicz, Kneisel, Ciovati, Wang,JLAB TN-02-023,(2002).
Sekutowicz, Workshop on pushing the limits of RF superconductivity, September 22-24, 2004.Sekutowicz et. al., PAC2005, May 16-20, 2005.
Rong-Li Geng SRF2005, July 10-15, 2005 9
Cavity parameters
1.521.522.382.381.901.90%%
kk
37970 37970 (+23%)(+23%)
33762 33762 (+10%)(+10%)
30840 30840 (ref)(ref)
ΩΩ22
(R/Q)*G(R/Q)*G
60602.362.3636.1 36.1 ((--13%)13%)LLLL70702.402.4037.8 37.8 ((--9%)9%)R70R7070701.981.9841.5 41.5 (ref)(ref)TTFTTF
mmmm--Oe/(Oe/(MV/mMV/m))unitunit
Iris Iris diadia..Epk/EaccEpk/EaccHpk/EaccHpk/EaccShapeShape
• R70(70 mm aperture reentrant): 10% improvement in Hpk/Eacc; 10%improvement in (R/Q)*G; better cell-to-cell coupling.• LL(60mm aperture low-loss): 13% improvement in Hpk/Eacc; 23%improvement in (R/Q)*G; cell-to-cell coupling is weaker.• Both shapes have a higher Epk/Eacc.
Rong-Li Geng SRF2005, July 10-15, 2005 10
Down side of a higher Epk/Eacc• Both new shapes have a higher Epk/Eacc compared to TTF.• This means higher Epk for the same Eacc.• Eacc=40 MV/m, Epk=80 MV/m (TTF).• Eacc=40 MV/m, Epk= 96 MV/m (new shapes). • Field emission is a practical challenge because of exponentialdependence of surface electric field.
⎟⎠⎞
⎜⎝⎛−=
ECECjFN
221 exp
• However, electric field has no intrinsic limit…
Fowler-Nordheim
Rong-Li Geng SRF2005, July 10-15, 2005 11
No intrinsic limit to Epk
113 MV/m
3GHz Nb
210 MV/m
Nb
Delayen, Shepard, 1990 Graber et. al., 1990
• Particulate contamination is a main cause of field emission.• Effective methods exist to remove particulate field emitters.• High-Pressure water Rinsing (HPR).• High-Peak-Power RF processing (HPP).
Rong-Li Geng SRF2005, July 10-15, 2005 12
Multipacting analysis• Simulations show no hard multipacting(MP) barrier.• For re-entrant geometry and low-loss geometry.
• Simulations predict the existence of two-point MP.• Similar two-point MP barrier exists in TTF shape.
• Two-point MP occurs at cavity equator region.• The electron impact energy typically 30-50 eV.• Two-point MP is usually surpassed by modest RF processing.
Rong-Li Geng SRF2005, July 10-15, 2005 13
Performance of single-cell Re-entrant and Low-loss
cavities
Rong-Li Geng SRF2005, July 10-15, 2005 14
46 MV/m reached in 70mm aperture single-cell reentrant cavity at Cornell
1.3 GHz
Q0=1E10@ 46MV/m47 MV/m pulsed
A soft MP barrier near 20MV/m,as predicted
Hpk=1755 OeEpk=101 MV/m
Rong-Li Geng SRF2005, July 10-15, 2005 15
45 MV/m reached in a scaled low-loss single-cell cavity at JLab
1E+09
1E+10
1E+11
0 5 10 15 20 25 30 35 40 45 50
Eacc [MV/m]
Q0
BaselineAfter 120 C, 24 h bake T = 2 K
2.2 GHz
Courtesy:Peter Kneisel
Scaled at 1.3GHz
Hpk=1602 OeEpk=93 MV/m
Rong-Li Geng SRF2005, July 10-15, 2005 16
500GeV
800GeV
1 TeV ILC(TESLA
type) energy reach
• CW 45-46 MV/m with little field emission demonstrated in Low-loss cavity and in re-entrant cavity.• Re-entrant cavity reached 47 MV/m in long pulsed mode.• Unloaded Q > 1×1010 at 45 MV/m.
Rong-Li Geng SRF2005, July 10-15, 2005 17
Cavity fabrication and processing• “Standard” niobium cavity fabrication and processing.• RRR250 high-purity sheet Nb (JLab 2.2GHz Low-loss cavityuses large grain Nb disks sliced directly from ingot).• Deep drawing cups and trimming half-cells.• Electron beam welding at iris and equator.• Post-purification (Ti or Y) boosts thermal conductivity.• Buffered chemical polishing HNO3:HF:H3PO4=1:1:2, orHNO3:HF:H3PO4=1:1:1, or electropolishing HF:H2SO4.• High-pressure water rinsing (HPR).• Cleaning room drying and assembly.• Slow pump-down.• 100°C bake-out under vacuum.
Rigorous HPR is required and re-contamination must be avoidedto keep field emission at bay.
Rong-Li Geng SRF2005, July 10-15, 2005 18
Multi-cell cavities of new geometry
Rong-Li Geng SRF2005, July 10-15, 2005 19
JLab Low-loss cavity
Courtesy: Charlie Reece
• 1.5 GHz, 7-cell.• vertical test results shown.• tested to 25 MV/m.• installed in cryomodule.• CEBAF 12 GeV upgrade.• Hpk/Eacc=37.4 Oe/(MV/m).• Talk MoA04(C. Reece).
LL Cavities for Renascence - VTA Performance
1.0E+09
1.0E+10
1.0E+11
0 5 10 15 20 25
Gradient (MV/m)
Q0
12 GeV Project SpecLL 29 WattsLL001LL002LL003LL004
T= 2.07
LL 29 W
3/28/05 cer
Rong-Li Geng SRF2005, July 10-15, 2005 20
KEK ICHIRO cavity
• 1.3 GHz, Low-loss shape.• Single-cell cavity tested to 40 MV/m.• Two 9-cell cavities built and test is on-going.• Many posters in this workshop.• TuP19 (Y. Morozumi), TuP20(T. Saeki), TuP21(K. Saito)TuP44(K. Saito), TuP45(K. Saito)
Courtesy: Kenji Saito
Rong-Li Geng SRF2005, July 10-15, 2005 21
Other important cavity parameters
• Lorentz force detuning• Wakefields and higher order modes
Rong-Li Geng SRF2005, July 10-15, 2005 22
Lorentz force detuningLorentz force detuning. Re-entrant vs Low Losses structures
-1.1
-1.05
-1
-0.95
-0.9
-0.85
-0.8
-0.75
40 45 50 55 60 65 70
Radius of stiffening ring (m m )
KL
[Hz/
(MV/
m)^
2)]
Re-entrant
Low Losses
tesla
• Wall thickness 2.8 mm.• Similar LFD sensitivity for Low-loss and re-entrant geometry.• Low-loss or re-entrant cavity with 3.1 mm wall thickness hasthe same LFD sensitivity as 2.8 mm wall TTF cavity.
Courstey: N. SolyakFermilab
Rong-Li Geng SRF2005, July 10-15, 2005 23
Wakefields and higher order modes
Courtesy: K. Ko, SLAC
• calculation started• SLAC/Fermi/DESY • low-loss geometry.• 9-cell with HOM coupler.• SLAC code Omega3P.• re-entrant geometry…• Talk ThA05 (K. Ko).
• Very important issue for beam quality and stability.• HOM requirements limits how small the aperture can be.
Rong-Li Geng SRF2005, July 10-15, 2005 24
MSU Half re-entrant cavityE
H
• MSU is exploring a half re-entrant geometry.• besides improvement in Hpk/Eacc, cell-to-cellcoupling and (R/Q)*G…• this geometry allows better fluid drainageduring chemistry and HPR.• MSU plans to fabricate and test single-cell.• Poster TuP15 (M. Meidlinger).
T. Grimm et al., Applied Superconductivity Conference,Jacksonville, FL, 2004
Rong-Li Geng SRF2005, July 10-15, 2005 25
60mm aperture re-entrant cavity
• Cornell’s next step in the re-entrant direction.• Improves Hpk/Eacc by 15% over that of TTF.• First single-cell cavity built and test in 2005.• Will use the cavity prep recipe tested for Hpk 1755 Oe.• It has potential of Eacc > 50 MV/m.• Poster TuP43 (R.L. Geng).
Rong-Li Geng SRF2005, July 10-15, 2005 26
Cavity parameters summary
60601.521.522.362.3637970 37970 (+23%)(+23%)36.1 36.1 ((--13%)13%)LLLL
60601.571.572.282.2841208 41208 (+34%)(+34%)35.4 35.4 ((--15%)15%)R60R60
66.866.82.092.092.402.4034673 34673 (+12%)(+12%)37.8 37.8 ((--9%)9%)HR,1HR,159.459.41.511.512.382.3838021 38021 (+23%)(+23%)36.0 36.0 ((--13%)13%)HR,1HR,1
2.382.381.901.90%%
kk
33762 33762 (+10%)(+10%)
30840 30840 (ref)(ref)
ΩΩ22
(R/Q)*G(R/Q)*G
70702.402.4037.8 37.8 ((--9%)9%)R70R7070701.981.9841.5 41.5 (ref)(ref)TTFTTF
mmmm--Oe/(Oe/(MV/mMV/m))unitunit
Iris Iris diadia..Epk/EaccEpk/EaccHpk/EaccHpk/EaccShapeShape
HR,1 and HR,2: MSU Half re-entrant geometry R60: 60 mm aperture re-entrant geometry.
Rong-Li Geng SRF2005, July 10-15, 2005 27
• Lowering Hpk/Eacc confirmed a right strategy for higer Eacc.
• Today’s record Eacc is 46 MV/m CW and 47 MV/m pulsed.
• New geometry allows lower cryogenic losses.
• No hard multipacting barrier found in neither low-loss norre-entrant geometry cavity.
• Epk=90-100 MV/m reached in new geometry cavities withlittle field emission.
• Cleaning and assembly of cavity for CW or long pulse Epk~100 MV/m is challenging, but it is proven possible.
Conclusions
Rong-Li Geng SRF2005, July 10-15, 2005 28
Conclusions (continued)• Unloaded Q of > 1×1010 at Eacc 45 MV/m is possible.
• Lorentz force detuning seems not a problem.
• Higher order modes need more study.
• Multi-cell low-loss cavity prototype being carried out aggressively in Japan.
• 50 MV/m demonstration seems to be within reach.
Rong-Li Geng SRF2005, July 10-15, 2005 29
Acknowledgement
I am grateful to the following colleagues for providinginformation for the preparation of this talk:
Cornell: Hasan Padamsee, Valery ShemelinDESY: Jacek SekutowiczJLab: Peter Kneisel, Charlie Reece, Bob RimmerKEK: Kenji SaitoMSU: Terry Grimm