Superconducting Cavities at 200 MHz

Fabrication and Performance

H. Padamsee

Rongli Geng, Enrico Chiaveri and Roberto Losito

Goals

• Extend SRF to 200 MHz ->large scale SC cavities• Aim for Nb-Cu to cut Nb material cost• Aim for.. Epk (at surface) = 25 - 30 MV/m • Develop large scale cavity fabrication methods

– Need ≈ 600 cells

200 MHz Cavity - Ready for Test

400 mm diameter beam pipe-> 300

First Test Results (Summer 02)

Need to • Reduce field emission• Improve processing

SEC =2.5

SEC=3.0

Multipacting Simulations

Re-Rinse at CERN, HPR, Fixed Probe

Epk=17 MV/m

X-Radiation Levels

3rd Test Jan 03, long time He processing

Radiation levelStill 30 R

Q-Slope = Material Property of FilmsDepends on Film Quality and Cu Substrate Also

Nb-Cu

Bulk Nb

1500 MHz

CERN 350 MHz LEP Cavities

Q-Slope Decreases with Temperature

Q-Slope Decreases With Frequency Measured Dependence of Q-Slope

What Next?

CERN has already built a second Nb-Cu cavity by the same method.This will test variability in film properties.Could re-coat with Krypton

Niobium sputteringNiobium sputtering

Sputtering parameters Sputtering parameters Discharge current stabilized at 3 A.

• Sputter gas pressure of 1.5x10-3 mbar, corresponding to ~ 360 V.

• Coating temperature is 150 °C.• Thickness: 1.5 µm

Film characteristics (same as for LEP):Film characteristics (same as for LEP):• RRR: 11.5 ± 0.1• Argon content: 435 ± 70 ppm • Grain size: 110 ± 20 nm• Tc: 9.51 ± 0.01 K

What Next?

AFM Results on Surface Structure of Nb/Cu

Compare Large and Small Grains

Standard films Oxide-free films

Better Films with Krypton

Large Scale FabricationINFN Spinning 500 MHz Cavity from Tube

Dimension Check

Several Cavities Received

Explore Bias Sputtering Using 500 MHz Cavities

Nb

Nb

Cu

Argon Ion bombardment of Nb film should increase film smoothness

Conclusions• First ever 200 MHz Nb-Cu Cavity completed successfully• Test facilities ready at Cornell• First results very encouraging (Eacc = 11 M/m CW),

surface E field = 19 MV/m..Goal 25-30 MV/m• Further work needed to reduce Nb-Cu Q-slope..

– Re-sputter with krypton gas

– Explore Bias magnetron sputtering

• Continue efforts to process to reduce field emission• Spinning of 500 MHz copper cavities from tube successful