Combined Magnetometric and Thermometric Characterization of a TESLA-Type Cavity in an External Magnetic Field

1. Combined magnetometric and thermometric characterization of a TESLA type cavity cooled down in an external magnetic field O. Kugeler, F. Kramer J. Köszegi, B. Schmitz, K. Alomari, J. Knobloch • https://aip.scitation.org/doi/10.1063/1.5030509

2. Aim: Measure and quantify magnetic field during the phase transition and correlate with dissipation Cavityandcard holder providedby DESY M. Caruso and C. Smith, “A new perspective on magnetic field sensing,” Sensors, vol. 15(12), pp. 34–46, 1998

3. Aim: Measure and quantify magnetic field during the phase transition and correlate with dissipation Behavior at cryogenic temperatures? • Fluxgate technology: • Precise, nT resolution, fast, known • Several cm in size, expensive • AMR technology: • Affordable, smaller Anisotropic MagnetoResistive effect • used as bridge: ± ΔR / R • 2% to 3% effect: R ± ΔR Easy axis Magnetization M H B Sensitive axis B ~ M + H B B B B B B B

4. Calibration and sensor testing • KMZ51 (Philips) • ZMY20M (Zetex) • AFF755 (Sensitec) • Calibrationusing internal testcoil Sensitivity: resolution ≈20 nT failedduring warmup S~DR/R sensorperformsbetter in cold ( lowerohmicresistance ! ) AMR bridge flipcoil • Radiation hardness: • Literature on similar AMRs: tested up to 10 kGy and in neutron radiation • AFF755 tested at HZB up to 12 kGy testcoil Flip coilpermanentlypulsed duringnormal operation R. Hahnet. al., in 14th Symposium for Magnetoresistive Sensors and Magnetic MagnetometicMeasurement Device Systems (2017) Sensitec: AFF755 data sheet

5. Using AMR Sensors with Magnetization coil Normal operation mode not applicable in cryogenic use Calibration Setup • Unpredictable behavior => no calibration possible • Accuracy not better than 2 µT • Should in principle work with higher flip current, however, sensor cannot cope with extra heat load • Setup to investigate reproducibility of change in output vs temperature curve

6. Problem at cryogenics: Using AMR Sensors without operation of magnetization coil Cycle Test-Board from room temperature to 2 K (tests done at DESY) • T shows system temperature, not Sensor temperature • Different cooldown speeds lead to different curves • Smooth curves => calibration possible relative accuracy at 0.02 µT comparable to fluxgates => Accuracy at 0.2 µT

7. Mapping setup: • 3 sensors in one group (r, z, ϕ) • 5 groups on one card (rz plane) • 4 cards around cavity (ϕ) • T mapping (from DESY) • Data acquisition hardware: 2 ms for complete cavity map • Helmholtz coils for x, y, z

8. B Mapping of sc cavity subjected to 10µT ambient field Relative field vectors visualized in 3d with python and blender software Presently offline visualization but plans to include into DAQ software

9. B-T maps for different applied Fields 1. Cycle with only background: Q0 = 3.1E+10 2. Cycle with 10 µT in X: Q0 = 6.1E+9 comparison to simulation of 100% trapping difference due to partial expulsionofflux • Magnetic field minus the background from 1st cycle • Maximum measured field: 3.9 µT 3. Cycle with 10 µT in Y-direction: Q0 = 6.3E+9 • Magnetic field minus the background from 1st cycle • Maximum measured field: 3.8 µT

10. T MAPS FOR DIFFERENT APPLIED FIELDS Moving hotspots for different fields Without Coil Coil in X direction Coil in Y direction

11. Hotspot also moves when polar angle is changed Without Coil; Q0 = 3.1E+10 Coil in X direction; Q0 = 6.1E+9 Coil in Y direction; Q0 = 6.3E+9 BX=BZ; Q0 = 5.3E+9

12. Q0 dependence on polar angle of applied magnetic field Sweep polar angle of ambient 10µT magnetic field from 0° - 90° in 15° steps • Normalization to same applied magnetic field • Calculate residual resistance • Normalizetomeanmagneticfieldduringcooldown • Recalculate Q0

13. Analysis ofdatarecordwith occuring 50 Hz fft-component • Discovery of a groundloopcausingcurrents in thecavitywalls in poloidaldirection Iris: smallercrosssection, large currentlarge field Equator: larger crosssection, smallcurrent, smallfield

14. SUMMARY • AMR sensors difficult to operate in cryogenic environmentAccuracy only 2 µT • Absolute calibration up to 0.2 µT (at 2K) achieved. • But: Large fields destroy calibration andshiftoffset • Relative calibration 0.02 µT (at 2K) demonstrated • Hotspots move according to trapped flux measured with AMR + T-map system • BT mapping useful to see local magnetic field and heating • Dependency of the quality factor on polar angle shown • Outlook • Incorporate AMR sensors into module foradditional diagnostics. • Want AMR withimprovedcooling / superconductingflipcoil

15. Thank you for your attention!

17. Anisotropic Magnetoresistive (AMR) SENSOR • Wheatstone bridge with four AMR elements • Set magnetization in each element • Magnetization coil to reset and flip magnetization 180°

18. Qualityfactor

19. Fluxgates during Calibration runs 26.11. 2018 F.Kramer