CCAT: Magnetic Sensitivity Measurements of Kinetic Inductance Detectors

TL;DR

This work tackles how external magnetic fields and flux trapping affect CCAT's kinetic inductance detectors in Prime-Cam. It measures three witness KID designs (Al 280 GHz, TiN 280 GHz, and Al EoR-Spec) at $T ≈ 100\,\mathrm{mK}$ using a dilution refrigerator with Helmholtz coils to apply DC fields up to $B ≈ 500 μT$ in orientations normal and parallel to the detector plane, extracting resonant frequency and quality factor via $S_{21}$ fits. The results reveal pronounced hysteresis and field-direction–dependent losses, likely due to trapped flux, but demonstrate that Earth-field–driven changes during observing should be negligible thanks to shielding and common-mode effects. These findings inform shielding and field-management strategies for Prime-Cam operations and guide future studies of magnetic-field effects in superconducting KIDs.

Abstract

The CCAT Observatory is a ground-based submillimeter to millimeter experiment located on Cerro Chajnantor in the Atacama Desert, at an altitude of 5,600 meters. CCAT features the 6-meter Fred Young Submillimeter Telescope (FYST), which will cover frequency bands from 210 GHz to 850 GHz using its first-generation science instrument, Prime-Cam. The detectors used in Prime-Cam are feedhorn-coupled, lumped-element superconducting microwave kinetic inductance detectors (KIDs). The telescope will perform wide-area surveys at speeds on the order of degrees per second. During telescope operation, the KIDs are exposed to changes in the magnetic field caused by the telescope's movement through Earth's magnetic field and internal sources within the telescope. We present and compare measurements of the magnetic sensitivity of three different CCAT KID designs at 100 mK. The measurements are conducted in a dilution refrigerator (DR) with a set of room temperature Helmholtz coils positioned around the DR. We discuss the implications of these results for CCAT field operations.

Figures (8)

• Figure 1: Al 280 GHz, TiN 280 GHz, and EoR-Spec pixel designs (bottom row) with close-ups of the respective absorbers (top row), shown from left to right. Figure adapted from Ref. duell2024ccatcomparisons280ghz, with the EoR-Spec pixel design added.
• Figure 2: EoR-Spec test chip mounted in a copper test box and wire-bonded to microstrip pieces on the sides. The microstrip pieces contain copper traces and are soldered to SMP connectors on each side, not visible from this angle. The lid of the box is not shown.
• Figure 3: Helmholtz coils mounted outside the DR in two different configurations. Left: field applied perpendicular to the detector absorber plane. Right: parallel. The test chips are mounted on the DR mixing chamber near the coil center. The mu-metal magnetic shield used during cooldowns and the Lakeshore 460 gaussmeter are not shown.
• Figure 4: An example of how the Al $280$ GHz resonator's frequency shifts in an external magnetic field. Each of the four panels represents a different sweep in magnetic field. In the top left we increase the magnetic from $0$-$500$$\mu$T with the field pointing vertically upward (perpendicular to the plane of the pixels). After reaching $500$$\mu$T we ramp back down to $0$$\mu$T, as shown in the top right panel. The bottom two panels follow the same pattern, ramping up and down in magnetic field directly after the first sweep. The effect of the external magnetic on both the quality factor and the resonant frequency are clearly visible, as well as a persistent degradation in the quality factor following the each ramp, likely due to remnant magnetization.
• Figure 5: The fractional change in frequency shift for CCAT Al $280$ GHz TiN $280$ GHz and EoR-Spec detectors are plotted as a function of the applied external magnetic field. Different markers represent different detectors on a given test chips. In the top panel, the external magnetic field strength is increased from $0$ to $500$$\mu$T in both field directions, whereas, in the bottom panel the samples start at $500$$\mu$T and the external field is brought back to $0$$\mu$T. Shown here is that the resonators experience a hysteresis where the return curve has a different functional form than the initial ramp up in external magnetic field. Error bars, defined as the measurement standard deviation, are plotted but are too small to be visible.