S. Keckert, R. Kleindienst, J. Knobloch, O. Kugeler

Status of the HZB Quadrupole Resonator*

* work partly funded by EuCARD2

Overview

RF characterization of superconducting thin films

• CW operation at high currents & turn-key systems• Detailed understanding of cavity loss mechanisms required

• Aim: Measurement of surface resistance• Parameters: Frequency, field strength, temperature

• Ideal tool: Quadrupole Resonator (QPR)• Wide phase space quickly available• sub-nΩ resolution ↔

• Comissioning of a QPR at HZB• Based on a system built at CERN

• E. Brigant, E. Haebel, E. Mahner, „The quadrupole resonator, design considerations and layout of a new instrument for the RF characterization of superconducting surface samples“, EPAC 98

• E. Chiaveri, E. Haebel, E. Mahner, J. M. Tessier, „The quadrupole resonator, construction, RF system, field calculations and first applications”, EPAC 98

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The Quadrupole Resonator (QPR)

• Resonator cavity andquadrupole rods madeof Nb RRR 300

• Pole shoes focusmagnetic fieldon sample

• Sample thermallydecoupled viacoaxial structure

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Pumping portsCoupler ports

Calorimetry chamber(large domain Nb)

Hollow quadrupolerods (Nb)

LHe

Resonator body (Nb)

Frame(SS, Ti)

Coaxial gap

Sample

Pole shoes

HeaterT-Sensor

Gap

Optimization of HZB design

• Optimization criteria• Phase space: Frequency, field strength, temperature• High resolution

• Full parameterization with CST• Maximizing figures of merit

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Baseline(CERN QPR)

Optimized

Operating frequencies 400 / 800 / 1200 MHz 433 / 866 / 1300 MHz (TESLA)

Focussing factor*

Risk of field emissionBSample/Epk

4.68 mT/(MV/m) 7.44 mT/(MV/m)

Operating rangeBSample/BPk

0.81 0.89

Microphonics1st mechanical mode 69 Hz 172 Hz

* fraction of field exposure between sample and resonator

[R. Kleindienst, „Developmentof an Optimized QuadrupoleResonator at HZB“, SRF 2013]

• Radius of rods increased8 mm → 13 mm

• Gap reduced(pole shoes ↔ sample)1 mm → 0.5 mm

Surface resistance measurement

RF-DC compensation technique

• High precision: calorimetric measurement• Resolution: sub-nΩ

• Wide temperature range: 1.8 K up to • Operating at low frequency (433 MHz)

• Low BCS resistance → sensitivity to Rres

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[S. Aull, „High Resolution Surface Resistance Studies“, SRF 2013]

QPR preparation at JLab

• QPR manufactured by Niowave• Resonator shipped to JLab for surface treatment and first RF tests

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BCP

600 °C bakeoutfor 12 hours

Ultrasonic rinsing

[Photos: A. Burrill, HZB]

Pole shoes – view from below

First RF test at Jlab (433 MHz)

Emitted power measurement

• Strongly coupled antenna• microphonics expected

• Forward power: step function

•

achieved on sample

Limited by quench

• Significant improvement

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Alternative calorimetry chamber

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Matching surfaces

M2.5 Helicoil

Double sided CF100 flange

Indium wire

Central heaterM4 threat

Cernox sensorsMotivation

Risks

• Cleanliness• Vacua possibly connected• Indium wire gaskets• Additional risk: low quench field

• Impact on RF

• Flat sample for coatingh = 12 mm

• No welding required• Height adjustment possible

→ sensitivity of resonator on

distance rods ↔ sample

RF simulations

• Impact of removable sample on RF?• Simulations using COMSOL 4.4

• Worst case: circumferential gap• Height: 0.5 mm• Position: 5 mm below sample

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• Color plot:• Magnetic field [mT]

(Different scales for sample and coax)• RF surface currents (red arrows)

→ Separated gaps of several millimeters are acceptable

Sample

5 mm

0.5 mm

Indium

Current status

• 433 MHz RF system ready• Data acquisition

• Hardware set up• Software in progress

• Heater with minimized magneticfield built and tested

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• Helium bath cryostat delivered andinstallation finished soon

• First RF test at JLab succesful• QPR is currently being shipped to HZB

• New sample holder in production• Indium seal tested successfully

(Al-prototype, room temperature)

Outlook

• QPR installation in October• First measurements with uncoated sample (Nb RRR 300)• Better understanding of unexpected microphonics

• New sample holder arrives next week• First test at CERN in November

• coating: ANR/DFG project (waiting for approval)• Universiät Siegen: coating, characterization of surface state• CEA-Irfu, Saclay: , RRR• Université Paris Sud, Orsay: thermal properties• HZB: RF properties, flux trapping

Always looking for collaboration partners whoare able to make films / prepare samples

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B A C K U P S L I D E S

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First RF test at JLab

Microphonics

• Frequency shift ≈ 4 kHz• Major contribution at 40 Hz• Mechanical oscillations

expected • Thicker rods included in design

to lower effect of microphonics

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First RF test at JLab

Quench observed

• Quick drop of transmitted power•

(on rods)

• Note • Significant improvement

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Antenna and Coupling

Power coupler @ 45 deg

microphonics expected

Field probe @ -90 deg

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CST simulation by Raphael Kleindienst

Electric and magnetic fields in QPR

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Electric Magnetic

CST simulation by Raphael Kleindienst