## 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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### Equations (5)

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(1)
$${\nu}_{\text{res}}=\frac{1}{2\pi}{\left(\frac{\pi G{d}^{4}}{8l{I}_{R}}\right)}^{1/2},$$
(2)
$$\begin{array}{ll}\theta \left(t\right)=\hfill & \frac{{\mu}_{R}\times {B}_{S}/{I}_{R}}{4{\pi}^{2}{\left[{\left({{\nu}_{\text{res}}}^{2}-{\nu}^{2}\right)}^{2}+{\nu}^{2}{{\nu}_{\text{res}}}^{2}/{Q}^{2}\right]}^{1/2}}\text{Re}\left[\text{exp}\left(-i\delta \right)\text{exp}\left(i2\pi \nu \mathrm{t}\right)\right]\hfill \\ \hfill & \equiv {\Theta}_{\text{max}}\phantom{\rule{0.2em}{0ex}}\text{Re}\left[\text{exp}\left(-i\delta \right)\text{exp}\left(i2\pi \nu t\right)\right],\hfill \end{array}$$
(3)
$${P}_{h}=\Gamma \frac{{{B}_{S}}^{2}}{8\pi}\times {V}_{s}2{\nu}_{\text{chop}}=2.6\Gamma \text{mW},$$
(4)
$${P}_{e}\approx {V}_{c}{\sigma}_{c}{\left(\upsilon /c\right)}^{2}{{B}_{S}}^{2}=0.2\text{mW},$$
(5)
$${{\nu}_{\text{res}}}^{2}={{\nu}_{0}}^{2}+{\left(\delta \nu \right)}^{2}\text{cos}\left({\theta}_{c}+\varphi \right),$$