Abstract

We describe the design, construction, and performance of a resonant cryogenic chopper that operates at 4.2 K. The chopper is mechanically and thermally robust; it can occult a 2.54-cm aperture at 4.5 Hz while dissipating ~1 mW. Both the stator and rotor magnetic fields are controllable to allow for performance optimization and to help in measuring any possible interference effects. Data on long-term amplitude stability are presented.

© 1992 Optical Society of America

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References

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  1. L. A. Page, E. S. Cheng, S. S. Meyer, “A large-scale cosmic microwave background anisotropy measurement at millimeter and submillimeter wavelengths,” Astrophys. Lett. 355, L1–L4 (1990).
    [CrossRef]
  2. P. M. Downey, A. D. Jeffries, S. S. Meyer, R. Weiss, “Monolithic silicone bolometers,” Appl. Opt. 23, 910–000 (1984).
    [CrossRef] [PubMed]
  3. J. B. Peterson, “The spectrum of the cosmic background radiation from 2.3 cm−1 to 11 cm−1,” Ph.D. Thesis, Department of Physics, University of California at Berkeley (Apr.1985).
  4. H. Murakami, A. E. Lange, “A low frequency beam switch,” in A. E. Lange,” An Attempt to Measure the Diffuse Brightness of the Sky at Submillimeter Wavelengths,” Ph.D. dissertation Department of Physics, University of California, Berkeley, Berkeley (January1987).
  5. Frequency Control Products, Inc., Calif., 61-20 Woodside Ave., Woodside, N.Y. 11377; (212) 458-5811. This company also makes a taut band chopper for large apertures.
  6. Supercon, Inc., 830 Boston Turnpike, Shrewsbury, Mass. 01545; (617) 842-0174.
  7. Emerson & Cuming, Inc., Canton, Mass. 02021.
  8. Vacuumschmelze, 186 Wood Ave. South, Iselin, N.J. 08830; (201) 321-4791.
  9. M. N. Wilson, Superconducting Magnets (Clarendon, Oxford, 1983).

1990 (1)

L. A. Page, E. S. Cheng, S. S. Meyer, “A large-scale cosmic microwave background anisotropy measurement at millimeter and submillimeter wavelengths,” Astrophys. Lett. 355, L1–L4 (1990).
[CrossRef]

1984 (1)

Cheng, E. S.

L. A. Page, E. S. Cheng, S. S. Meyer, “A large-scale cosmic microwave background anisotropy measurement at millimeter and submillimeter wavelengths,” Astrophys. Lett. 355, L1–L4 (1990).
[CrossRef]

Downey, P. M.

Jeffries, A. D.

Lange, A. E.

H. Murakami, A. E. Lange, “A low frequency beam switch,” in A. E. Lange,” An Attempt to Measure the Diffuse Brightness of the Sky at Submillimeter Wavelengths,” Ph.D. dissertation Department of Physics, University of California, Berkeley, Berkeley (January1987).

Meyer, S. S.

L. A. Page, E. S. Cheng, S. S. Meyer, “A large-scale cosmic microwave background anisotropy measurement at millimeter and submillimeter wavelengths,” Astrophys. Lett. 355, L1–L4 (1990).
[CrossRef]

P. M. Downey, A. D. Jeffries, S. S. Meyer, R. Weiss, “Monolithic silicone bolometers,” Appl. Opt. 23, 910–000 (1984).
[CrossRef] [PubMed]

Murakami, H.

H. Murakami, A. E. Lange, “A low frequency beam switch,” in A. E. Lange,” An Attempt to Measure the Diffuse Brightness of the Sky at Submillimeter Wavelengths,” Ph.D. dissertation Department of Physics, University of California, Berkeley, Berkeley (January1987).

Page, L. A.

L. A. Page, E. S. Cheng, S. S. Meyer, “A large-scale cosmic microwave background anisotropy measurement at millimeter and submillimeter wavelengths,” Astrophys. Lett. 355, L1–L4 (1990).
[CrossRef]

Peterson, J. B.

J. B. Peterson, “The spectrum of the cosmic background radiation from 2.3 cm−1 to 11 cm−1,” Ph.D. Thesis, Department of Physics, University of California at Berkeley (Apr.1985).

Weiss, R.

Wilson, M. N.

M. N. Wilson, Superconducting Magnets (Clarendon, Oxford, 1983).

Appl. Opt. (1)

Astrophys. Lett. (1)

L. A. Page, E. S. Cheng, S. S. Meyer, “A large-scale cosmic microwave background anisotropy measurement at millimeter and submillimeter wavelengths,” Astrophys. Lett. 355, L1–L4 (1990).
[CrossRef]

Other (7)

J. B. Peterson, “The spectrum of the cosmic background radiation from 2.3 cm−1 to 11 cm−1,” Ph.D. Thesis, Department of Physics, University of California at Berkeley (Apr.1985).

H. Murakami, A. E. Lange, “A low frequency beam switch,” in A. E. Lange,” An Attempt to Measure the Diffuse Brightness of the Sky at Submillimeter Wavelengths,” Ph.D. dissertation Department of Physics, University of California, Berkeley, Berkeley (January1987).

Frequency Control Products, Inc., Calif., 61-20 Woodside Ave., Woodside, N.Y. 11377; (212) 458-5811. This company also makes a taut band chopper for large apertures.

Supercon, Inc., 830 Boston Turnpike, Shrewsbury, Mass. 01545; (617) 842-0174.

Emerson & Cuming, Inc., Canton, Mass. 02021.

Vacuumschmelze, 186 Wood Ave. South, Iselin, N.J. 08830; (201) 321-4791.

M. N. Wilson, Superconducting Magnets (Clarendon, Oxford, 1983).

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Figures (6)

Fig. 1
Fig. 1

Schematic view of the optical layout and chopper; the stator and rotor windings are not shown. The blade oscillates around the piano wire axle. When it is in (rotated 35° counterclockwise) radiation from the sky is reflected to detectors 3 and 4 and radiation from the reference load is reflected to detectors 1 and 2. When it is out (rotated 35° counterclockwise) radiation from the sky is transmitted to detectors 1 and 2 and radiation from the reference load is transmitted to detectors 3 and 4. The radiation is guided by 2.54-cm-diameter aperture Winston cones and light pipe. All the components shown are at 4.2 K; the detectors are at 0.24 K.

Fig. 2
Fig. 2

Stator assembly as viewed looking down the axle: A is the chopper blade. B is the stator yoke, which is made of laminated Cryoperm6 and thermally sunk to a LHe cold stage. C is the stator plug; it is also made of laminated Cryoperm. D and E are the potted superconducting wires.8 Each of the D sections has 1350 windings, each of the E sections has 150. The piano wire axle F runs through a hole in the center of G, the copper stator plug heat sink. A side view of the stator plug is shown in Fig. 3. H is the stainless-steel rotor winding substrate. I is an end view of the rotor windings. A magnetic field flux line following the path of minimum reluctance passes from the stator body, through the rotor windings, through the stator plug, again through the rotor windings, and back into the stator body.

Fig. 3
Fig. 3

Rotor assembly with top and side views. A regions are the copper axle clamping areas. Brass 2-56 screws and epoxy affix the tops of the clamps to the clamping area. B is a side view of the chopper blade. C is the stainless-steel rotor winding substrate; this is soldered to the copper clamp. D and E comprise a side view of the stator plug and copper stator plug heat sink. F is an example of a single-rotor winding. The windings are thermally sunk by passing over the end of the copper clamp. The winding path farthest from the blade must allow for the insertion of the stator plug. G is the piano wire axle. H is a perspective view of the copper rotor heat sink. One end is soldered to the copper clamp; the other is thermally sunk to a LHe cold stage.

Fig. 4
Fig. 4

Measurement of the chopper stability. The ordinate scale is obtained by assuming that the amplitude is proportional to the emf V E , which is the measured quantity. V E is sampled at 4.466 Hz with an integrating lock-in amplifier. The data were taken with a cryostat containing the chopper sitting on the laboratory floor. No special attempts at vibration isolation were made. As noted in the text the drift is an upper limit to chopper variations.

Fig. 5
Fig. 5

Power spectrum of the data in Fig. 4. During a balloon flight secondary modulation is provided by rotating the experiment at 0.004 Hz.

Fig. 6
Fig. 6

Detector output during the last 2 h of the measurement in Fig. 4. The ordinate is given in terms of the change in the temperature of a Planck emitter at 2.75 K. What is actually measured is the change in power from a 5 K Planck emitter. The absolute value is accurate to 15%. Just as with V E the detector output is sampled at 4.466 Hz with an integrating lock-in amplifier. The spikes are due to cosmic ray hits and local interference. During a balloon flight there is less noise.

Tables (1)

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Table I Chopper Construction and Operation

Equations (5)

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ν res = 1 2 π ( π G d 4 8 l I R ) 1 / 2 ,
θ ( t ) = μ R × B S / I R 4 π 2 [ ( ν res 2 ν 2 ) 2 + ν 2 ν res 2 / Q 2 ] 1 / 2 Re [ exp ( i δ ) exp ( i 2 π ν t ) ] Θ max Re [ exp ( i δ ) exp ( i 2 π ν t ) ] ,
P h = Γ B S 2 8 π × V s 2 ν chop = 2 . 6 Γ mW ,
P e V c σ c ( υ / c ) 2 B S 2 = 0 . 2 mW ,
ν res 2 = ν 0 2 + ( δ ν ) 2 cos ( θ c + ϕ ) ,

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