Thin Films for Superconducting Cavities
HZB
2
Outline
• Introduction to Superconducting Cavities• The Quadrupole Resonator• Commissioning• Outlook
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Basics of RF Cavities
• Acceleration of charged particles using a radio frequency field
• There are normal conducting and superconducting (sc) cavities
• Qnc ≈ 105 vs. Qsc ≈ 1010
• The needed power for operation is about a factor of 200 less than for superconducting cavities
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Materials for sc Cavities
• Niobium (from sheet)– Critical temperature Tc = 9.2K– Accelerating gradients reaching the theoretical limit
(≈45MV/m) due to improved treatment techniques– Expensive
• Niobium on copper– 1-2μm niobium on copper cavity– Less need of niobium– Accelerating gradients up to 10MV/m
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Where can we improve?
Understanding the dominant loss mechanisms in niobium
Reducing losses
Improving the performance of niobium films
Reducing material costs
Finding new materials ?
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How can we improve?
• Power consumption in a superconducting cavity is proportional to its surface resistance RS
• RS shows a complex behavior on external parameters, such as temperature, frequency, magnetic and electric field
),,,(Sc EBTfRP
Systematic studies on cavities are no option
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The Quadrupole Resonator
361
mm
• The Quadrupole Resonator enables RF characterization of small, flat samples over a wide parameter range
• Samples of 75mm diameter are welded to a niobium cylinder with flange so that they can be mounted to the host cavity
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Field Configuration & Features
• Resonant frequencies: 400MHz, 800MHz, 1.2 GHz
• Almost identical magnetic field configuration
• Ratio between peak magnetic and electric field proportional to frequency
B ׀׀
0
max
50 mm1 E. Mahner et al.
Rev. Sci. Instrum., Vol. 74, No. 7, July 20032 T. Junginger et. al
Rev. Sci. Instrum., Vol. 83, No. 6, June 2012
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DC Heater
Quadrupole Resonator Thermometry Chamber
Heat Flow
The Calorimetric Technique
• Measuring the temperature on the sample surface
• Precise Calorimetric measurements over wide temperature range
TemperatureSensorsSample Surface
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time
Temperature
Bath Temperature
Temperatureof Interest
P DC,
1
P DC,
2P RF
Power
DC on RF on≈60 s ≈40 s
dSHRPPPSample
SurfaceDCDCRF 22,1, 21
dSH
PPR
Sample
DCDCSurface
2
2,1, )(2
TemperatureSensors
DC Heater
Heat Flow
The Calorimetric Technique
Measured directly
• Measurement of transmitted power Pt
• Pt=c∫H2ds, c from computer code
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Imperfect Meissner Effect
Meissner effect
Trapped magnetic flux
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Flux Trapping in the Quadrupole Resonator
Sample
DC Coil
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Flux Trapping in the Quadrupole Resonator
DC Coil�⃗�
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First test with trapped flux
• Bulk niobium sample• Reactor grade, RRR • Standard BCP, no bake out
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RS(B) at 400MHz, 2-4K
• Convex curve for • Concave curve for • Different loss
mechanisms dominant
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Trapped Flux at 400MHz and 4K
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Outlook
• MgB2 from Superconductor Technologies, Inc.– Tc = 39K, Bc up to 1T
– expected: Eacc > 75MV/m, RBCS(4K,500MHz) = 2.5nΩ
• HiPIMS: Nb/Cu from CERN and Berkeley– Dense film, low cost, but competitive to bulk Nb?
• ECR: Nb/Cu from Jlab– High RRR, low cost, but competitive to bulk Nb?