SQUID Time-Division Multiplexing (TDM) is a technique for the readout of arrays of Transition-Edge Sensors (TESs) for x-ray and gamma-ray science. TDM has been deployed in many recent 250-pixel-scale instruments including at synchrotron light sources and particle-accelerator facilities, as well as in table-top experiments. Two TES spectrometers employing TDM readout will soon be deployed to electron-beam ion-trap facilities.
TDM is also under development as a back-up readout option for the X-ray Integral Field Unit (X-IFU) of the Athena satellite mission. The 3,840 TES pixels of the X-IFU will enable efficient, high resolution spectroscopy (2.5 eV FWHM at 7 keV) of extended astrophysical sources. Multiplexing factors of 40 or more sensors per readout column are planned for the X-IFU. To advance the maturity of TDM readout for Athena, we are creating a focal-plane assembly for the readout of 960 TES pixels in a 24 column by 40 row configuration. We will describe the design and experimental progress on this technology demonstrator.
In a TDM system, each dc-biased TES has its own first-stage SQUID. Rows of these first-stage-SQUIDs are turned on and off sequentially such that the signal from only one TES at a time per readout column is passed to a series-array SQUID, to a room-temperature preamplifier, and to digital-feedback electronics. Recent implementations of TDM have a row period of 160 ns and non-multiplexed amplifier noise of 0.19 micro-Phi_0/sqrt(Hz) referred to the first-stage SQUID.
Some benchmark demonstrations of TDM with x-ray TES sensors include achievement of 2.55 eV FWHM energy resolution at 5.9 keV in a 32-row, 1-column configuration. Here, the fastest slew rates in the TES currents were similar to those of the X-IFU “LPA2” detector model. We have also achieved 2.72 eV FWHM resolution in a 32-row, 6-column configuration that contained 144 high-quality TESs that were similar to the much faster X-IFU “LPA1” pixels. We will describe on-going efforts to read out TDM arrays at the 6x32 scale and larger, as well as efforts to improve the performance of TDM system subcomponents. We will also describe system-level performance metrics such as cross-talk.
SQUID Code-Division Multiplexing (CDM) is closely related to TDM but has important performance advantages. CDM and TDM operation are similar with the main difference being that in CDM, all TESs are observed by the multiplexer at all times, with the polarity of the TES signals switched between rows. Because all TESs are observed by the multiplexer at all times, the sqrt(N_rows) noise-aliasing degradation inherent to TDM is eliminated.
We are developing flux-summing CDM to be drop-in compatible with existing TDM systems. The most recent CDM implementation has a nonmultiplexed noise level of 0.17 micro-Phi_0/sqrt(Hz) referred to the first-stage SQUID and a row period of 160 ns. We have demonstrated 2.77 eV FWM resolution at 5.9 keV in 32-row, 1-column CDM test.