Evaluation of South Carolina’s Reading First (SCRF) Initiative
6/11/03R.L. Geng, NuFact031 200MHz SCRF cavity development for RLA Rong-Li Geng LEPP, Cornell...
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Transcript of 6/11/03R.L. Geng, NuFact031 200MHz SCRF cavity development for RLA Rong-Li Geng LEPP, Cornell...
6/11/03 R.L. Geng, NuFact03 1
200MHz SCRF cavitydevelopment for RLA
Rong-Li GengRong-Li Geng
LEPP, Cornell UniversityLEPP, Cornell University
6/11/03 R.L. Geng, NuFact03 2
H. PadamseeD. HartillP. BarnesJ. Sears
R. LositoE. ChiaveriH. PreisS. Calatroni
6/11/03 R.L. Geng, NuFact03 3
Contents
Fabrication and RF testsFabrication and RF tests
Performance: Eacc and QPerformance: Eacc and Q
Q-slopeQ-slope
Performance when HPerformance when Hext ext 0 0
Future work plan and statusFuture work plan and status
ConclusionConclusion
6/11/03 R.L. Geng, NuFact03 4
Muon-based neutrino source
Acceleration starts after coolingFast acceleration required since
muon has a short life time
6/11/03 R.L. Geng, NuFact03 5
Requirements to acceleration
The highest possible Eacc to minimize muon decayThe highest possible Eacc to minimize muon decay
Very large transverse and longitudinal acceptancesVery large transverse and longitudinal acceptances
Both requirements favor the choice of SRF
SRF cavity has a high QSRF cavity has a high Q00
SRF can achieve high gradients with modest RF powerSRF can achieve high gradients with modest RF power
SRF cavities can afford a larger aperture without worrying SRF cavities can afford a larger aperture without worrying about a low R/Qabout a low R/Q
0
2
)( QQREacc
Pd
6/11/03 R.L. Geng, NuFact03 6
200MHz SRF layout for Linac
Focusing Solenoid(2-4 T) 2-cell SRF cavity
6/11/03 R.L. Geng, NuFact03 7
200MHz SRF parameter list
300 high gradient 200MHz cavities needed
6/11/03 R.L. Geng, NuFact03 8
Why Nb-Cu cavity? Save material costSave material cost Save cost on magnetic field shielding (Rs of Nb-Cu less Save cost on magnetic field shielding (Rs of Nb-Cu less
sensitive to residual mag. field)sensitive to residual mag. field) Save cost on LHe inventory by pipe cooling (Brazing Cu Save cost on LHe inventory by pipe cooling (Brazing Cu
pipe to Cu cavity)pipe to Cu cavity)
1.5GHz bulk Nb cavity (3mm) material cost: ~ $ 2k/cell200MHz: X (1500/200)2 = 56 $ 112k/cellThicker material (8mm) needed: X 2.7 $300k/cell
Nb Material cost for 600 cells: 180M$ Cu (OF) is X 40 cheaper: 5M$
6/11/03 R.L. Geng, NuFact03 9
First 200MHz Nb-Cu cavity
400mm BT
Cavity length: 2 m
Major dia.: 1.4 m
6/11/03 R.L. Geng, NuFact03 10
Fabrication at CERN
Electro-polished half cell
Magnetron Nb film (1-2 m) sputtering
• DC voltage: 400-650 V• Gas pressure: 2 mTorr• Substrate T: 100 °C• RRR = 11• Tc = 9.5 K
6/11/03 R.L. Geng, NuFact03 11
RF test at CornellCavity on test stand Cavity going into test pit
in Newman basement
Pit: 5m deep X 2.5m dia.
6/11/03 R.L. Geng, NuFact03 12
Two-point Multipacting
• Two points symmetric about equator are involved• Spontaneously emitted electrons arrive at opposite point after T/2• Accelerated electrons impact surface and release secondary electrons• Secondary electrons are in turn accelerated by RF field and impact again• The process will go on until the number of electrons are saturated
MP electrons drain RF power A sharp Q drop
6/11/03 R.L. Geng, NuFact03 13
Two-point MP at 3 MV/m
MULTIPAC simulationconfirmed exp. observation
Resonant trajectory of MP electrons
It was possible to process through MP barrier
6/11/03 R.L. Geng, NuFact03 14
Performance of the cavity
• Eacc = 11MV/m• Low field Q = 2E10
Limited by RF coupler
• 75% goal Eacc achieved• Q-slope is out of expectation
Q(Eacc) after combined RF and Helium processing
Q improves at lower T
FE not dominating
6/11/03 R.L. Geng, NuFact03 15
Q-slope of sputtered film Nb cavities
Q-slope is a result of Q-slope is a result of material properties of film material properties of film NbNb
It also has to do with Cu It also has to do with Cu substratesubstrate
The exact Q-slope The exact Q-slope mechanism is not fully mechanism is not fully understood yetunderstood yet
Sputtered Nb
Bulk Nb
6/11/03 R.L. Geng, NuFact03 16
Nb-Cu cavities before 200MHz
350MHz LEP cavities 400MHz LHC cavities
Despite Q-slope, sputtered Nb-Cu cavitieshave achieved a 15MV/m Eacc at 400MHz
Q0(X
1E9)
6/11/03 R.L. Geng, NuFact03 17
Expected performanceProjecting LHC 400MHz to 200MHz
Empirical frequency dependence of Q-slope
200MHz
Measured Q-slope of 200MHz cavity is10 times too steep than projected
6/11/03 R.L. Geng, NuFact03 18
Q-slope: impact angle effect
100mm
R67mm
• CERN explored low 350MHz cavities• With the same cathode geometry, lower low
Impact angle of Nb atom:
6/11/03 R.L. Geng, NuFact03 19
Q-slope: impact angle effect
Correlation: lower lower steeper Q-slope
6/11/03 R.L. Geng, NuFact03 20
Q-slope: impact angle effect A smaller impact angle results in pronounced shadowing A smaller impact angle results in pronounced shadowing
effect and poor film quality (open boundaries, voids, effect and poor film quality (open boundaries, voids, dislocations)dislocations)
The cathode used to sputter 200MHz cavity was recycled The cathode used to sputter 200MHz cavity was recycled from sputtering system for LEP2 cavitiesfrom sputtering system for LEP2 cavities
Due to an increase in equator radius, a smaller impact Due to an increase in equator radius, a smaller impact angle is evident for 200MHz cavityangle is evident for 200MHz cavity
First thing to do next: re-coat using a new cathode with First thing to do next: re-coat using a new cathode with optimal impact angle optimal impact angle
6/11/03 R.L. Geng, NuFact03 21
Other techniques for Nb film deposition
Bias sputteringBias sputtering Energetic deposition in vacuumEnergetic deposition in vacuum Vacuum arc depositionVacuum arc deposition
6/11/03 R.L. Geng, NuFact03 22
Bias sputtering
With bias voltage
Without bias
• Apply a bias voltage to substrate• Induce substrate ion bombardment• Can achieve defect free film
Columnar grains
Dense film
6/11/03 R.L. Geng, NuFact03 23
Approach to tackle Q-slope:improve film property
Study Nb film with 500MHz cavities (save LHe) with Study Nb film with 500MHz cavities (save LHe) with existing LEPP infrastructure developed for CESR SRFexisting LEPP infrastructure developed for CESR SRF
Seamless Cu cavities to simplify fabricationSeamless Cu cavities to simplify fabrication
6/11/03 R.L. Geng, NuFact03 24
Hext effect on cavity
200MHz cavity
SC Nb/Ti coil
2T solenoid
• 2T solenoid needed for tight focusing• Solenoid and cavity fitted in one cryostat• Large aperture (460 mm)• Q: Will cavity still work Hext > 0 ?
Layout of Linear Accelerator for source
Cavity test in the presence of an Hext
6/11/03 R.L. Geng, NuFact03 25
Hext effect on cavity
Cavity stays intact up to Hext = 1200 Oe
6/11/03 R.L. Geng, NuFact03 26
Hext effect on cavity
• Nb is a type-II SC
• Mixed state above Hc1
• Magnetic flux penetration
• Normal core causes Rs
• Onset Hext for loss increase consistent with Hc1 of Nb• Msmts at higher Eacc needed: Hext + HRF; resistive flux flow• A cavity test with a 2T solenoid is desirable
6/11/03 R.L. Geng, NuFact03 27
Exploring new cavity shapes
• Smaller cavity size• Larger longitudinal acceptance
Spoke cavity
6/11/03 R.L. Geng, NuFact03 28
Exploring new cavity shapes
• Reduce Hpk/Eacc to mitigate Q-slope• Eliminate angle effect of magnetron sputtering over high-loss (cylinder) surface• Simplify fabrication• The catch: multipacting may limit (was a limit in 3GHz pill box cavities in the 70s)• Code (MULTIPAC) simulations will answer
Pill box cavity
6/11/03 R.L. Geng, NuFact03 29
Conclusion First ever 200MHz cavity completed successfullyFirst ever 200MHz cavity completed successfully
First results achieved Eacc = 11 MV/m and QFirst results achieved Eacc = 11 MV/m and Q00 = 2E10 at low = 2E10 at low fieldfield
MP barriers can be processed throughMP barriers can be processed through
Cavity not affected by Hext < 1200 OeCavity not affected by Hext < 1200 Oe
Further work needed to reduce Q-slope: re-coat with a new Further work needed to reduce Q-slope: re-coat with a new cathode; bias sputtering 500MHz spun cavitiescathode; bias sputtering 500MHz spun cavities
Measurements on Hext effect at higher EaccMeasurements on Hext effect at higher Eacc
Explore new cavity shapesExplore new cavity shapes