The Terahertz Intensity Mapper (TIM) is a NASA-funded balloon-borne telescope that aims to measure the [CII] emission from star forming galaxies over an enormous cosmic volume. TIM’s cryogenic receiver, based on the BLAST-TNG design, utilizes a three-stage He sorption refrigerator backed by a 280-liter liquid helium tank to achieve a base temperature of 250 mK, which enables photon noise-limited performance for its MKID detectors. Two low-impedance multi-channel heat exchangers enhance cooling efficiency, contributing to a designed hold time of 20 days under ground conditions. Preliminary simulations and assembly tests showcase the cryostat's reliability, and data validation is anticipated by the summer of 2024. We will present the design and current status of the TIM cryogenic receiver and our ongoing characterization effort toward an Antarctic flight in 2026.
We present preparation for fabrication and deployment of science-grade kilo-pixel Kinetic Inductance Detector (KID) based arrays for the Terahertz Intensity Mapper (TIM). TIM is a NASA-funded balloon-borne experiment planning its Antarctic flight for 2026. TIM employs two focal planes, each with four subarrays of ~900 hexagonal-packed, horn-coupled aluminum KIDs. Fabrication yield is high, and we have successfully mapped KID resonant frequencies to spatial locations with our LED mapper. The spatial and frequency information associated with every yielded pixel allows a study of spatial coincidences as cosmic rays interact with the array, as well as interpretation of a covariance analyses performed on the noise timestreams. We also describe the improvement on the science-usable yield of our 864-pixel array achieved by (1) the lithographic trimming that de-collides resonators, and (2) our characterization of interpixel crosstalk. This pioneering work on the postprocessing will pave the way for science with our large KID arrays.
The Terahertz Intensity Mapper (TIM) is designed to probe the star formation history in dust-obscured star-forming galaxies around the peak of cosmic star formation. This will be done via measurements of the redshifted 157.7 µm line of singly ionized carbon ([CII]). TIM employs two R~250 long-slit grating spectrometers covering 240 to 420 µm. Each is equipped with a focal plane unit containing four wafer-sized subarrays of horn-coupled aluminum kinetic inductance detectors (KIDs). We present the design and performance of a prototype focal plane assembly for one of TIM’s KID-based subarrays. The overall detector package must satisfy thermal and mechanical requirements, while maintaining high optical efficiency and a suitable electromagnetic environment for the KIDs. In particular, our design manages to strictly maintain a 50 µm air gap between the array and the horn block. The prototype detector housing in combination with the first flight-like quadrant were tested at 250 mK. A frequency scan using a vector network analyzer shows 823 resonance features, which represents ⪆90% yield, indicating a good performance of our TIM detector wafer and the whole focal plane unit. Initial measurements also showed that many resonances were affected by collisions and/or very shallow transmission dips as a result of a degraded internal quality factor. This is attributed to the presence of an external magnetic field during cooldown. We report on a study of magnetic field dependence of the quality factor of our quadrant array. We implemented a Helmholtz coil to vary the magnetic field at the detectors by (partially) nulling earth’s. Our investigation shows that the earth magnetic field can significantly affect our KIDs’ performance by degrading the quality factor by a factor of two to five, well below those expected from the operational temperature or optical loading. We find that we can sufficiently recover our detectors’ quality factor by tuning the current in the coils to generate a field that matches earth’s magnetic field in magnitude to within a few µT. We emphasize that it is impractical to fly a Helmholtz coil on TIM and dynamically “null” earth’s. Therefore, it is necessary to employ a properly designed magnetic shield enclosing the TIM focal plane unit. Based on the results presented in this paper, we set a shielding requirement of |B| ⪅3 µT.
The Terahertz Intensity Mapper (TIM) is a balloon-borne far-infrared imaging spectrometer designed to characterize the star formation history of the universe. In its Antarctic science flight, TIM will map the redshifted 158um line of ionized carbon over the redshift range 0.5-1.7 (lookback times of 5-10 Gyr). TIM will spectroscopically detect ~100 galaxies, determine the star formation rate history over this time interval through line intensity mapping, and measure the stacked CII emission from galaxies in its well-studied target fields (GOODS-S, SPT Deep Field). TIM consists of a 2-meter telescope feeding two grating spectrometers that that cover 240-420um at R~250 across a 1.3deg field of view, detected with 7200 kinetic inductance detectors and sampled through a novel RF system-on-chip readout. TIM will serve as an important scientific instrument, accessing wavelengths that cannot easily be studied from the ground, and as a testbed for future FIR space technology.
TIM, the Terahertz Intensity Mapper, is a NASA far-infrared balloon mission designed to perform [CII] intensity mapping of the peak of cosmic star formation. To achieve this goal, TIM will fly two grating spectrometers that together cover the 240 to 420 um wavelength range at an R~250. Each spectrometer will require large format arrays (4x~900 detectors) of dual-polarization sensitive detectors, which are photon noise limited at 100 fW of loading. We will present the design of a fully-aluminum lumped-element kinetic-inductance detector (KID) that incorporates a novel “chain-link” absorber design. Operating at 215 mK, we demonstrate that this detector achieves a photon noise limited performance at 80 fW of optical loading with a white noise spectrum down to 1 Hz. Informed by dark measurements, we except these KIDs to achieve a detector limited NEP of 2e-18 W/rt(Hz) at a loading <10 fW. In addition, we shall show our design of a kilopixel array and its initial performance measurements.
The third-generation South Pole Telescope camera (SPT-3G) improves upon its predecessor (SPTpol) by an order of magnitude increase in detectors on the focal plane. The technology used to read out and control these detectors, digital frequency-domain multiplexing (DfMUX), is conceptually the same as used for SPTpol, but extended to accommodate more detectors. A nearly 5× expansion in the readout operating bandwidth has enabled the use of this large focal plane, and SPT-3G performance meets the forecasting targets relevant to its science objectives. However, the electrical dynamics of the higher-bandwidth readout differ from predictions based on models of the SPTpol system due to the higher frequencies used and parasitic impedances associated with new cryogenic electronic architecture. To address this, we present an updated derivation for electrical crosstalk in higher-bandwidth DfMUX systems and identify two previously uncharacterized contributions to readout noise, which become dominant at high bias frequency. The updated crosstalk and noise models successfully describe the measured crosstalk and readout noise performance of SPT-3G. These results also suggest specific changes to warm electronics component values, wire-harness properties, and SQUID parameters, to improve the readout system for future experiments using DfMUX, such as the LiteBIRD space telescope.
TIM, the Terahertz Intensity Mapper, is a NASA balloon mission designed to perform [CII] intensity mapping of the peak of cosmic star formation. To achieve this, TIM has two longslit (1 degree slit length) grating spectrometers covering the 240-317 um and 317-420 um wavelength bands at R~250, respectively. We will present the design of the ~4000 pixel, horn-coupled kinetic inductance detector arrays servicing each of the spectrometer arms. Each pixel is a lumped-element superconducting resonator made from a 20 nm thick aluminum film, designed to achieve photon noise limited performance at 100 fW of loading. The inductor is a meandered narrow wire, designed to mimic a metal mesh grid at THz frequencies; it is optimized for absorption of both polarizations delivered by the circular waveguide. Each array will consist of four quadrants containing ~1000 pixels on a single microstrip readout line and will be mounted such that critical parameters of the absorber design are maintained.
The desire for higher sensitivity has driven ground-based cosmic microwave background (CMB) experiments to employ ever larger focal planes, which in turn require larger reimaging optics. Practical limits to the maximum size of these optics motivates the development of quasi-optically-coupled (lenslet-coupled), multi-chroic detectors. These detectors can be sensitive across a broader bandwidth compared to waveguide-coupled detectors. However, the increase in bandwidth comes at a cost: the lenses (up to ~700 mm diameter) and lenslets (~5 mm diameter, hemispherical lenses on the focal plane) used in these systems are made from high-refractive-index materials (such as silicon or amorphous aluminum oxide) that reflect nearly a third of the incident radiation. In order to maximize the faint CMB signal that reaches the detectors, the lenses and lenslets must be coated with an anti-reflective (AR) material. The AR coating must maximize radiation transmission in scientifically interesting bands and be cryogenically stable. Such a coating was developed for the third generation camera, SPT-3G, of the South Pole Telescope (SPT) experiment, but the materials and techniques used in the development are general to AR coatings for mm-wave optics. The three-layer polytetra uoroethylene-based AR coating is broadband, inexpensive, and can be manufactured with simple tools. The coating is field tested; AR coated focal plane elements were deployed in the 2016-2017 austral summer and AR coated reimaging optics were deployed in 2017-2018.
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