Flux dynamics in ultrasensitive superconducting focal planes

The performance of superconducting focal planes will drive the achievable specifications of ultrasensitive instruments for NASA astrophysics missions, yet they have serious challenges with magnetic shielding in achieving their optimal performance.  We will undertake a study to demonstrate suppression of flux dynamics that hinder detector performance for future NASA mission focal planes.  We will further investigate capabilities relevant to NASA missions including lithographed or ion beam patterned designs for improving detector performance and field stitching for increasing array formats for future ultrasensitive imaging arrays.

GSFC has aggressive programs in superconducting focal planes including transition edge sensors (TES) for x-rays (ATHENA X-IFU) and infrared (HIRMES, PIPER), magnetic calorimeters, or MagCal, (X-ray surveyor) and kinetic inductance detectors (KIDs) for infrared spectroscopy and imaging (Microspec, Far-IR Surveyor).  The TESs are susceptible to flux trapping and other effects of stray magnetic fields that cause degraded performance through “kinks” in the superconducting transition creating noisy bias spots.  While a single detector can be shown to have good performance, large arrays will have degraded performance on average as some devices are forced to be biased on or near a kink.   While subtle, the effect can be measured in high fidelity testbeds uniquely available at GSFC.  KID detectors consisting of superconducting microwave resonators experience a more direct degradation through reduction of the quality factor Q that defines the width of the resonance and correlates to the noise performance of the device.

1. We will identify length scales pertinent to flux trapping in superconducting leads – the so-called “Clem length” below which it is energetically unfavorable for the lead to contain magnetic field.  We will show transition edge sensor performance in instrument relevant magnetic fields and confirm improved performance with sub-Clem length lead structures. [1]
2. We will demonstrate that other techniques for control of flux or “fluxonics” in superconducting structures can be beneficial to TES performance. [2] This effort will encompass the fabrication of graded interface structures for the elimination of vortex pinning. Focused ion beam will be employed as necessary for rapid prototyping and metrology at the nanoscale.
3. We will structure resonators lithographically to improve performance in the presence of magnetic fields for future KID designs including higher Q. [3]
4. We will further demonstrate enhanced capabilities critical for deployment of focal plane technologies (resonators, TES, and possibly others) including field stitching and ultrafine lithography. For these demonstrations, we will focus on mission enabling focal planes suitable for future proposals in both the x-ray and IR.

References:

1. JE Sadleir, “Superconducting Transition-Edge Sensor Physics”, Ph.D. dissertation at the University of Illinois Urbana-Champaign, 2010.
2. Silhanek, Van de Vondel, and Moshchalkov, “Guided Vortex Motion and Vortex Ratchets in Nanostructured Superconductors”, in Moshchalkov, Woerdenweber, and Lang (eds.), Nanoscience and Engineering in Superconductivity, Springer-Verlag, Berlin, 2010.
3. Song, DeFeo, Yu, and Plourde, “Reducing microwave loss in superconducting resonators due to trapped vortices”, App. Phys. Lett. 95, 232501 (2009); doi: 10.1063/1.3271523

Large focal planes of ultrasensitive detectors for future missions will benefit from reduced magnetic sensitivity.  Benefits to the FPA performance include increased uniformity.  Benefits to the instrument design include lower mass dedicated to magnetic  shielding and electromagnietic  environment requirements.

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