Thin Films for Superconducting Cavities

HZB

2

Outline

• Introduction to Superconducting Cavities• The Quadrupole Resonator• Commissioning• Outlook

3

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

4

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

5

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

8

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?