6/11/03R.L. Geng, NuFact031 200MHz SCRF cavity development for RLA Rong-Li Geng LEPP, Cornell University

6/11/03 R.L. Geng, NuFact03 1

200MHz SCRF cavitydevelopment for RLA

Rong-Li GengRong-Li Geng

LEPP, Cornell UniversityLEPP, Cornell University


H. PadamseeD. HartillP. BarnesJ. Sears

R. LositoE. ChiaveriH. PreisS. Calatroni


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


Muon-based neutrino source

Acceleration starts after coolingFast acceleration required since

muon has a short life time


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


200MHz SRF layout for Linac

Focusing Solenoid(2-4 T) 2-cell SRF cavity


200MHz SRF parameter list

300 high gradient 200MHz cavities needed


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$


First 200MHz Nb-Cu cavity

400mm BT

Cavity length: 2 m

Major dia.: 1.4 m


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


RF test at CornellCavity on test stand Cavity going into test pit

in Newman basement

Pit: 5m deep X 2.5m dia.


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


Two-point MP at 3 MV/m

MULTIPAC simulationconfirmed exp. observation

Resonant trajectory of MP electrons

It was possible to process through MP barrier


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


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


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)


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


Q-slope: impact angle effect

100mm

R67mm

• CERN explored low 350MHz cavities• With the same cathode geometry, lower low

Impact angle of Nb atom:


Q-slope: impact angle effect

Correlation: lower lower steeper Q-slope


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


Other techniques for Nb film deposition

Bias sputteringBias sputtering Energetic deposition in vacuumEnergetic deposition in vacuum Vacuum arc depositionVacuum arc deposition


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


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


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



Cavity stays intact up to Hext = 1200 Oe



• 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


Exploring new cavity shapes

• Smaller cavity size• Larger longitudinal acceptance

Spoke cavity


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


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

Documents

6/11/03R.L. Geng, NuFact031 200MHz SCRF cavity development for RLA Rong-Li Geng LEPP, Cornell University