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BICEP/Keck XIX: Extremely Thin Composite Polymer Vacuum Windows for BICEP and Other High Throughput Millimeter Wave Telescopes

BICEP/Keck Collaboration, :, P. A. R. Ade, Z. Ahmed, M. Amiri, D. Barkats, R. Basu Thakur, C. A. Bischoff, D. Beck, J. J. Bock, H. Boenish, V. Buza, K. Carter, J. R. Cheshire, J. Connors, J. Cornelison, L. Corrigan, M. Crumrine, S. Crystian, A. J. Cukierman, E. Denison, L. Duband, M. Echter, M. Eiben, B. D. Elwood, S. Fatigoni, J. P. Filippini, A. Fortes, M. Gao, C. Giannakopoulos, N. Goeckner-Wald, D. C. Goldfinger, J. A. Grayson, A. Greathouse, P. K. Grimes, G. Hall, G. Halal, M. Halpern, E. Hand, S. A. Harrison, S. Henderson, J. Hubmayr, H. Hui, K. D. Irwin, J. H. Kang, K. S. Karkare, S. Kefeli, J. M. Kovac, C. Kuo, K. Lau, M. Lautzenhiser, A. Lennox, T. Liu, K. G. Megerian, M. Miller, L. Minutolo, L. Moncelsi, Y. Nakato, H. T. Nguyen, R. O'brient, S. Paine, A. Patel, M. A. Petroff, A. R. Polish, T. Prouve, C. Pryke, C. D. Reintsema, T. Romand, D. Santalucia, A. Schillaci, B. Schmitt, E. Sheffield, B. Singari, K. Sjoberg, A. Soliman, T. St Germaine, A. Steiger, B. Steinbach, R. Sudiwala, K. L. Thompson, C. Tsai, C. Tucker, A. D. Turner, C. Vergès, A. G. Vieregg, A. Wandui, A. C. Weber, J. Willmert, W. L. K. Wu, H. Yang, C. Yu, L. Zeng, C. Zhang, S. Zhang

Abstract

Millimeter-wave refracting telescopes targeting the degree-scale structure of the cosmic microwave background (CMB) have recently grown to diffraction-limited apertures of over 0.5 meters. These instruments are entirely housed in vacuum cryostats to support their sub-kelvin bolometric detectors and to minimize radiative loading from thermal emission due to absorption loss in their transmissive optical elements. The large vacuum window is the only optical element in the system at ambient temperature, and therefore minimizing loss in the window is crucial for maximizing detector sensitivity. This motivates the use of low-loss polymer materials and a window as thin as practicable. However, the window must simultaneously meet the requirement to keep sufficient vacuum, and therefore must limit gas permeation and remain mechanically robust against catastrophic failure under pressure. We report on the development of extremely thin composite polyethylene window technology that meets these goals. Two windows have been deployed for two full observing seasons on the BICEP3 and BA150 CMB telescopes at the South Pole. On BICEP3, the window has demonstrated a 6% improvement in detector sensitivity.

BICEP/Keck XIX: Extremely Thin Composite Polymer Vacuum Windows for BICEP and Other High Throughput Millimeter Wave Telescopes

Abstract

Millimeter-wave refracting telescopes targeting the degree-scale structure of the cosmic microwave background (CMB) have recently grown to diffraction-limited apertures of over 0.5 meters. These instruments are entirely housed in vacuum cryostats to support their sub-kelvin bolometric detectors and to minimize radiative loading from thermal emission due to absorption loss in their transmissive optical elements. The large vacuum window is the only optical element in the system at ambient temperature, and therefore minimizing loss in the window is crucial for maximizing detector sensitivity. This motivates the use of low-loss polymer materials and a window as thin as practicable. However, the window must simultaneously meet the requirement to keep sufficient vacuum, and therefore must limit gas permeation and remain mechanically robust against catastrophic failure under pressure. We report on the development of extremely thin composite polyethylene window technology that meets these goals. Two windows have been deployed for two full observing seasons on the BICEP3 and BA150 CMB telescopes at the South Pole. On BICEP3, the window has demonstrated a 6% improvement in detector sensitivity.

Paper Structure

This paper contains 16 sections, 9 equations, 12 figures, 4 tables.

Table of Contents

  1. Introduction
  2. Design and Mechanical Tests
  3. Manufacturing
  4. Strength
  5. Safety Factor Measurement
  6. Creep
  7. Gas Permeation
  8. Optical Tests
  9. Anti-reflection
  10. Birefringence
  11. Optical Characterization on BICEP3
  12. Temperature-to-Polarization Leakage
  13. Measured Optical Load
  14. Noise Equivalent Temperature
  15. Conclusion
  16. ...and 1 more sections

Figures (12)

  • Figure 1: [Left] Modeled optical loading from a window with thicknesses of 1.4 mm, 25.4 mm, and 31.8 mm, at band centers from 30 to 270 GHz (fractional bandwidth of 0.25) through a BICEP-style instrument. Black points are measured optical loading on BICEP3 from a 31.8 mm (1.25") UHMWPE spare window and the 1.4 mm laminate window deployed on BICEP3 for the 2023 season. Shaded bands are uncertainty on the loss properties of the window, with $\tan\delta$ between 0.6$\times 10^{-4}$ to 2.4$\times 10^{-4}$. [Right] Modeled relative noise equivalent temperature per detector (NET$_{\text{det}}$) and relative survey time increase compared to an instrument without a window, for each of the BICEP Array observing band center frequencies.
  • Figure 2: A cutaway of the top portion of the BICEP3 cryostat. Displayed from top to bottom are the forebaffles, BOPP membrane (to contain warm circulated air above the window), thin HMPE window, HD30 IR foam filter stack, alumina IR filter, alumina objective lens, and first nylon IR filter. The rest of the cryostat is described in Figure 6 of bicep3. The thin HMPE window in this model is deflected 75 mm, with the foam filters remaining 45 mm away from the window.
  • Figure 3: Photo of several small scale laminated window samples, labeled with the pressure at which they were laminated in atmospheres (atm). The top two samples were used in the small-scale permeation tests in Figure \ref{['fig:perm_rates']}; the laminate recipes are identical apart from the difference in lamination pressure. The weave of the HMPE fibers remains visible in the 1 atm laminate, whereas the 10 atm laminate appears more homogeneous, like the HDPE sample below. he row of three smaller HMPE laminate samples at different pressures (homogeneity of the laminates requires pressures at or above 4 atm) and a solid extruded HDPE sheet have the same thickness. Note that the grid pattern on the mat below the samples laminated at lower pressure is blurred by light scattering associated with the incomplete filling of the HMPE weave with LDPE.
  • Figure 4: Tensile strength tests of three samples from a high pressure (10 atm, 135$^{\circ}$C) HMPE laminate. The stress is calculated by dividing the measured force by the initial cross sectional area of the sample, while the strain is the percent change in elongation. The ultimate tensile strength (UTS) and the failure strain (FS) for each sample are reported in the legend, and summarized and compared to other polyethylenes in Table \ref{['tab:strength']}.
  • Figure 5: Photo of the window at the highest pressure achieved during the hydrostatic test. The pressure gauge (center right) reads approximately 85 psi (5.7 atm, or 586 kPa), and there is water leaking out around the gasket seal on the left hand side.
  • ...and 7 more figures