The SPIDER CMB Polarimeter

He has been an important part of my education from the beginning of my work here. JPL's Matt Kenyon showed me how to fly by the seat of my pants and get a lot done.

Introduction

The Beginning of Physical Cosmology

The measured apparent locations of the stars were shifted in accordance with the predictions of general relativity [41]. In general, the curvature of the universe can depend on time, and the universe can expand or contract.

Discovery of the Cosmic Microwave Background

The small magnitude of the remaining observed temperature anisotropy (ΔT /T ≈10−5) ruled out models claiming that the CMB had a galactic source or a random distribution of sources. This was a solid confirmation of the Big Bang theory, to the exclusion of Steady State theories.

CMB Statistics

It is the most commonly observed parameter, and in this case is simply proportional to the temperature of the CMB. The cosmological information encoded in the CMB resides in the spatial correlations of the Stokes parameters.

Figure 1.1: Example of a polarization and temperature map, from the B2K experiment [90].

Inflation Theory

The height of inflation is usually quantified by the logarithmic growth of the volume factor, N = ln (a(tend)/a(tinitial)). However, the transition from the inflationary epoch would cause the scalar spectral index (ns) to deviate from unity (invariance).

CMB Temperature Anisotropies

The measured angular magnitude of the primary anisotropy was consistent with that expected for cold dark matter models in a flat (Euclidean) universe [34]. Evidence from various experiments supports the standard model of the universe, called the ΛCDM model.

CMB Polarizing Mechanisms

This peak is found on degree scales, approximately the angular size of the horizon at resolution. The amplitude of this peak is proportional to the square of τ, the optical depth of reionization.

Figure 1.3: Left : The scattering plane is a natural reference frame for a single scattering event

Polarized Foregrounds

Measurements of the lensing signal will be used to probe the matter distribution, the sum of neutrino masses, and the dark energy equation of state [ 36 , 112 ]. The theoretical BB power spectrum (solid line) is the sum of the inflationary and gravitational lensing components (dashed lines).

Figure 1.4: Currently published EE and BB power spectra measurements. Theoretical spectra from the standard model (ΛCDM) are shown for comparison

The S PIDER Balloon-Borne CMB Polarization Experiment

• Collaboration
• Frequency Coverage
• Sky Coverage
• Ballooning
• Cryogenics
• Optics
• Half-Wave Plates
• Magnetic Shielding
• Simulations

We then turn off the pump heater and apply 0.7 mW to the heater switch to cool the pump. The addition of the 280 GHz band in the second flight provides significant utilization for foreground subtraction.

Table 2.1: Spider collaborating institutions. William (Bill) Jones is the Principal Investi- Investi-gator

Detectors and Readout

• Bolometric Technology Development
• S PIDER Detector Architecture
• Fabrication Process
• TES Bolometer Basics
• Readout

The capacitance of the blocks depends on the thickness of the interlayer dielectric (SiO2 layer between the blocks and the niobium ground plane). The resistances of the series resistor (hundreds of ohms) and parallel shunt resistor (3 mΩ) are known. The normal state resistance of the TES is approximately ten times that of the shunt resistance.

When a small change in the optical charge δQ disturbs the temperature of the TES (TT ES = Tc+δT), a change in resistance is created (RT ES =R0+δR).

Figure 3.1: Thermally isolated metalized Si 3 N 4 absorbers coupled to Ge NTD thermistors.

Receiver Performance

Beam Profiles

The difference between the beams of two perpendicularly polarized detectors can be decomposed geometrically for any pixel. When scanning across the beam, the amplitude of the chopped signal was much larger than the change in unidirectional displacement, indicating that the two-fin blade was effectively chopping the source. The position of the source was not coded, but the forward and backward scan measurements were indistinguishable, indicating that the stage maintained a consistent scan rate (~0.5 cm/s) between each scan.

The measured beam slices can be well approximated by Gaussian fits, with a slight decrease visible in log-scale plots near the edges of the beam (>30 arc minutes from the center of the beam).

Far Field Beam Slices /LQHDU6FDOH

RJ6FDOH

By differentiating between the normalized beams from the two polarized detectors for each pixel, we find a shift in their beam centers (Figure 4.2). The large variation in measured differential ellipticity is believed to be due to beam distortions in some pixels caused by. Our far-field measurement of a single Spider pixel suggests that differential pointing may be a factor that is a few less inSpider than in BICEP2.

Differential pointing at the antenna level is not sufficient to produce differential pointing in the far field of the telescope, but it can generate effects created by a lens or filter.

Figure 4.2: Difference between the beams of the A and B polarization detectors for a single pixel

Near Field Maps

Frequency Band Definition

This factor ν2 is responsible for the slope of the noise floor of the semi-logarithmic plots. When we performed spectral measurements in version 2.1 of the Spider test cryostat, four noticeably different tiles were installed. The relative normalization of the spectra is chosen so that the integral of the difference spectrum disappears.

As with the 148 GHz band, the majority (~85%) of atmospheric loading is due to broadband water emissions.

Figure 4.7: Top: The average passband for devices from tile JAB090323.1. This passband was judged to be too low, and tiles are now made to target a higher passband

Polarization Efficiency

3RODUL]DWLRQ(IILFLHQF\

Efficiency of Optical Response

The back plate on the inside of the niobium cover is simply an absorber (Eccosorb LS-26/SS-3), not a quarter-wave reflective back short filament. Middle: On the opposite side of the silicon substrate, a suitable anti-reflective coating is painted. The transmission coefficient of a microstrip plate has an attenuation component (real) and a phase shift component (imaginary).

One is mounted on the cold side of the 20 K shield filter stack (VCS1), one at the input of the 4 K insert, and one at the spit.

Figure 4.14: Left : The cold load cryostat, about to be bolted to the front of the primary test cryostat

Internal Optical Loading of Devices

The sum of the transmitted power in the two polarizations also varies with the orientation of the waveplate relative to the detector axes. The internal strain is assumed to be the measured optical strain (Q) when the temperature of the cold strain black body is extrapolated to zero. Measurements of the internal loading of the 148 GHz band technical grade Spider receivers are listed in Appendix B.9.

The source of the (large) residual internal charge measured is not well understood.

Figure 4.17: Optical response, | dP J oule /dT RJ | , is calculated by a simple linear fit

Bolometer Properties

Bias selection thus requires a trade-off between the properties of the 32 TES devices in this column. The extent to which the desired total bias can be found depends on the uniformity of the device parameters. Multiplexer within-column standard deviation measurements of various device parameters are tabulated in Appendices B.4 and B.7.

Then, the Ti thickness varied by ~30% from the center to the edge of the wafer with a similar (~30%) variation in Rn and ~2% variation in Tc.

Figure 4.19 (left) illustrates the impact of parameter variation on effective (shared bias) device yield and on the spread in the operating R f rac of devices sharing a common bias

Noise Performance

Take the square root of the resulting PSD to set the results in pA/√ units. In Figure 4.21 I plot the science band noise level (median of . √PSD, 2–8 Hz) for the example device over a range of Vbias values ​​through its transition. Right, in gray: The squared sum of NETs of all devices at RT ES/Rn 0.8 divided by the number of devices.

Hz) produces 24 aW/√. Hz, in accordance with the measured noise of the optically loaded devices.

Figure 4.20: The square root of the noise power spectral density (PSD) for a noise trace taken during Run 5.1

Temperature Stability

The temperature of the stop will vary in flight due to changes in charge and changes in the temperature of the superfluid helium bath with altitude. Roger O'Brient has recently designed a Gaussian tapered pick-up boom that should reduce the amount of antenna beam power in the side lobes, loosening the temperature stability requirements of the stop. The SSA modules are now mounted from the liquid helium-cooled stage and self-heated to >5 K with significantly reduced resonance effects and improved temperature stability.

The amplitude of the relationship between SSA temperature and detector reading has not yet been quantified.

Magnetic Field Response

Anomalies in the temperature of SQUID series array (SSA) modules or first and second stage SQUIDs can produce patterns that are visible across an entire focal plane (including dark SQUIDs). Testing is performed along three perpendicular axes, including the primary axis of the telescope. Along the axes perpendicular to the primary telescope axis, the cryostat is too large to allow the spacing between the coils in the true Helmholtz configuration (half a diameter apart).

To calibrate our results, we calculate the expected field strength between the coils in the absence of shielding.

Summary and Recommendations for Future Work

Designing a summation tree for the 280 GHz band will be a difficult challenge, as reducing the antenna structure leaves very little room for a microstrip summation tree. The measured internal receiver loading in the 148 GHz band is currently twice the predicted CMB and atmospheric loading combined (§4.5). We know that most of the observed load varies linearly with the observed sky response, so it must connect to the bolometer through the antenna.

I recommend measurements of the response of this stage to microphone excitation and to changes in optical load.

Table 4.4: Summary of current measured receiver performance — 148 GHz band

Appendix A

Definitions and Methods

Optical Efficiency

A simple linear fit to the data collected at different loading temperatures determines the normalization prefactor “a” according to equation A.4. In the case where spectroscopy is not available, I isolate by defining the magnitude of the integrated optical response from the details of the spectral response. Therefore, to work in these units, we instead convert the incident power into a Rayleigh-Jeans “temperature”, given by.

It corresponds to the temperature of the idealized Rayleigh-Jeans source (hν kT) that would radiate the same amount of current in a 25% wide band around ν0 as our actual blackbody source.

Figure A.1: (Left ) F (ν) is measured by FTS, and arbitrarily normalized. (Center ) A simple linear fit to the data taken at different load temperatures determines the normalization pref-actor “a” according to Equation A.4

Alternative Methods for Calculating Internal Loading

The curve for the dark data intercepts the temperature axis atTsub =Tc, so the projected PJ for the fit to the cold load data is QatTsub=Tc. The optical power Q is the sum of the load of the cold load and the internal load. Neglecting this small charge, we can calculate Tc for the dark TES using only cold charge data.

For dark TES, the correction due to optical response to the cold load is small (~1 mK, see Appendix B.8), but we do it anyway.

Figure A.2: The optical load Q absorbed by devices in the cold load configuration introduces a simple DC offset in P J oule (independent of T sub )

Appendix B

Measured Detector Parameters

Row 2

• Aluminum TES T c and R n
• Titanium TES T c and R n
• Leg Thermal Conductance
• TES Uniformity
• Optical Response of 145 GHz Devices (No Telescope)
• Optical Response of 99 GHz Devices (No Telescope)
• Optical Response of 148 GHz Tiles Through Telescope
• Optical Response of Dark TES Through Telescope
• Internal Loading of Telescope (148 GHz Tiles)
• Internal Loading Measured By Dark TES
• Tile to Tile Band Center Uniformity

Starting in Run 5, the geometry of the Ti TES was deliberately redesigned to lower the normal resistance (to improve noise performance). The discrepancy between runs 1 and 2 in the measured optical response for this plate is not understood and may be the result of either improved optical train efficiency or a calibration error in one of these early runs (see B.2). A figure of merit determining the strength of this second (much larger) signal is cataloged in the last column of the table.

This loading component is believed to be related to direct radiation coupling to the TES.

Bibliography

In the beginning: the first light sources and the genionization of the universe. Large angular scale polarization of the cosmic microwave background radiation and the feasibility of its detection. A measurement of the attenuation tail of the cosmic microwave background power spectrum with the South Pole Telescope.

A preliminary measurement of the cosmic microwave background spectrum by the Cosmic Background Explorer (COBE) satellite.