Thin Films for Superconducting Cavities HZB. Outline Introduction to Superconducting Cavities The Quadrupole Resonator Commissioning Outlook 2.

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 ?

6

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

7

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

9

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

10

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

11

Imperfect Meissner Effect

Meissner effect

Trapped magnetic flux

12

Flux Trapping in the Quadrupole Resonator

Sample

DC Coil

13

Flux Trapping in the Quadrupole Resonator

DC Coil�⃗�

14

First test with trapped flux

• Bulk niobium sample• Reactor grade, RRR • Standard BCP, no bake out

15

RS(B) at 400MHz, 2-4K

• Convex curve for • Concave curve for • Different loss

mechanisms dominant

16

Trapped Flux at 400MHz and 4K

17

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?