6/11/03R.L. Geng, NuFact MHz SCRF cavity development for RLA Rong-Li Geng LEPP, Cornell University

6/11/03R.L. Geng, NuFact032 H. Padamsee D. Hartill P. Barnes J. Sears R. Losito E. Chiaveri H. Preis S. Calatroni

6/11/03R.L. Geng, NuFact033 Contents  Fabrication and RF tests  Performance: Eacc and Q  Q-slope  Performance when H ext  0  Future work plan and status  Conclusion

6/11/03R.L. Geng, NuFact034 Muon-based neutrino source Acceleration starts after cooling Fast acceleration required since muon has a short life time

6/11/03R.L. Geng, NuFact035 Requirements to acceleration  The highest possible Eacc to minimize muon decay  Very large transverse and longitudinal acceptances Both requirements favor the choice of SRF  SRF cavity has a high Q 0  SRF can achieve high gradients with modest RF power  SRF cavities can afford a larger aperture without worrying about a low R/Q

6/11/03R.L. Geng, NuFact MHz SRF layout for Linac Focusing Solenoid (2-4 T) 2-cell SRF cavity

6/11/03R.L. Geng, NuFact MHz SRF parameter list 300 high gradient 200MHz cavities needed

6/11/03R.L. Geng, NuFact038 Why Nb-Cu cavity?  Save material cost  Save cost on magnetic field shielding (Rs of Nb-Cu less sensitive to residual mag. field)  Save cost on LHe inventory by pipe cooling (Brazing Cu pipe to Cu cavity) 1.5GHz bulk Nb cavity (3mm) material cost: ~ $ 2k/cell 200MHz: X (1500/200) 2 = 56  $ 112k/cell Thicker material (8mm) needed: X 2.7  $300k/cell Nb Material cost for 600 cells: 180M$ Cu (OF) is X 40 cheaper: 5M$

6/11/03R.L. Geng, NuFact039 First 200MHz Nb-Cu cavity 400mm BT Cavity length: 2 m Major dia.: 1.4 m

6/11/03R.L. Geng, NuFact0310 Fabrication at CERN Electro-polished half cell Magnetron Nb film (1-2  m) sputtering DC voltage: V Gas pressure: 2 mTorr Substrate T: 100 °C RRR = 11 Tc = 9.5 K

6/11/03R.L. Geng, NuFact0311 RF test at Cornell Cavity on test standCavity going into test pit in Newman basement Pit: 5m deep X 2.5m dia.

6/11/03R.L. Geng, NuFact0312 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

6/11/03R.L. Geng, NuFact0313 Two-point MP at 3 MV/m MULTIPAC simulation confirmed exp. observation Resonant trajectory of MP electrons It was possible to process through MP barrier

6/11/03R.L. Geng, NuFact0314 Performance of the cavity Eacc = 11MV/m Low field Q = 2E10 Limited by RF coupler 75% goal E acc achieved Q-slope is out of expectation Q(Eacc) after combined RF and Helium processing Q improves at lower T  FE not dominating

6/11/03R.L. Geng, NuFact0315 Q-slope of sputtered film Nb cavities  Q-slope is a result of material properties of film Nb  It also has to do with Cu substrate  The exact Q-slope mechanism is not fully understood yet Sputtered Nb Bulk Nb

6/11/03R.L. Geng, NuFact0316 Nb-Cu cavities before 200MHz 350MHz LEP cavities 400MHz LHC cavities Despite Q-slope, sputtered Nb-Cu cavities have achieved a 15MV/m Eacc at 400MHz Q0(X1E9)

6/11/03R.L. Geng, NuFact0317 Expected performance Projecting LHC 400MHz to 200MHz Empirical frequency dependence of Q-slope 200MHz Measured Q-slope of 200MHz cavity is 10 times too steep than projected

6/11/03R.L. Geng, NuFact0318 Q-slope: impact angle effect 100mm R67mm CERN explored low  350MHz cavities With the same cathode geometry, lower   low  Impact angle of Nb atom: 

6/11/03R.L. Geng, NuFact0319 Q-slope: impact angle effect Correlation: lower   lower   steeper Q-slope

6/11/03R.L. Geng, NuFact0320 Q-slope: impact angle effect  A smaller impact angle results in pronounced shadowing effect and poor film quality (open boundaries, voids, dislocations)  The cathode used to sputter 200MHz cavity was recycled from sputtering system for LEP2 cavities  Due to an increase in equator radius, a smaller impact angle is evident for 200MHz cavity  First thing to do next: re-coat using a new cathode with optimal impact angle

6/11/03R.L. Geng, NuFact0321 Other techniques for Nb film deposition  Bias sputtering  Energetic deposition in vacuum  Vacuum arc deposition

6/11/03R.L. Geng, NuFact0322 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

6/11/03R.L. Geng, NuFact0323 Approach to tackle Q-slope: improve film property  Study Nb film with 500MHz cavities (save LHe) with existing LEPP infrastructure developed for CESR SRF  Seamless Cu cavities to simplify fabrication

6/11/03R.L. Geng, NuFact0324 H ext 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 H ext > 0 ? Layout of Linear Accelerator for source Cavity test in the presence of an H ext

6/11/03R.L. Geng, NuFact0325 H ext effect on cavity Cavity stays intact up to Hext = 1200 Oe

6/11/03R.L. Geng, NuFact0326 Hext effect on cavity Nb is a type-II SC Mixed state above Hc1 Magnetic flux penetration Normal core causes Rs  Onset H ext for loss increase consistent with Hc1 of Nb Msmts at higher Eacc needed: H ext + H RF ; resistive flux flow A cavity test with a 2T solenoid is desirable

6/11/03R.L. Geng, NuFact0327 Exploring new cavity shapes Smaller cavity size Larger longitudinal acceptance Spoke cavity

6/11/03R.L. Geng, NuFact0328 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

6/11/03R.L. Geng, NuFact0329 Conclusion  First ever 200MHz cavity completed successfully  First results achieved Eacc = 11 MV/m and Q 0 = 2E10 at low field  MP barriers can be processed through  Cavity not affected by Hext < 1200 Oe  Further work needed to reduce Q-slope: re-coat with a new cathode; bias sputtering 500MHz spun cavities  Measurements on Hext effect at higher Eacc  Explore new cavity shapes