The Bicep2telescope sits within a large cylindrical vacuum vessel. At the center of the vessel is the telescope itself, surrounded by a toroidal liquid helium (LHe) bath with a capacity of 100 L
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and a boiloff rate of ⇠20 L/day. As the LHe boils off, cold He vapor is forced through a series of small diameter tubes. These are heat sunk to two large cylindrical heat shields that surround the 4 K environment. These vapor-cooled shields remove the need for a liquid nitrogen bath, as was required for Bicep1. Owing to the low ambient pressure, at the South Pole the LHe bath temperature equilibrates to roughly 3.9 K. The inner and outer vapor-cooled shields equilibrate to 40 and 100 K, respectively. The vapor-cooled stages additionally act as heat sinks for the two Teflon filters described in Section 2.2.3.
The telescope is heat sunk to the bottom of the LHe bath by a large copper “camera plate.”
This is also the heat sink used for the sorption fridge, which is used to cool the focal plane assembly to sub-Kelvin temperatures. When cycled, the fridge generates a substantial amount of heat. As a result, conductivity exceeding 1 W/K is required between the fridge and the He bath, which is achieved through the thick camera plate.
2.5.1 Helium sorption fridge
A three-stage helium sorption fridge provides the cooling power to bring the focal plane assembly to its operating temperature of 280 mK. The fridge comes from Lionel Duband’s group atCommissariat à l’Énergie Atomique, and similar designs have been used in a number of similar experiments, including Z-SPEC and Bicep1. The closed-cycle fridge consists of three boot-strapped sorption coolers, one of which uses high-purity 4He as the condensate, while the other two use 3He (Figure 2.12). The coldest point of each stage (the evaporator) serves as the heat intercept for the subsequent stage. The4He stage serves to bring the temperature below the3He condensation temperature. The intermediate 3He stage serves as a heat intercept, reducing the parasitic load on the 3He ultra- cool stage. Using this boot-strapping scheme, the 4He/3He/3He stages are able to equilibrate to 1.4/0.35/0.23 K, respectively.
The fridge hold time is > 72 hours, requiring the fridge to be re-cycled every 3 days. This is well-matched to the boiloffrate of the LHe tank, which needs to be refilled at similar intervals. The fridge cycle, together with the time taken to reach equilibrium, takes 4-5 hours. Once the focal plane reaches base temperature (⇠230 mK), an active thermal feedback loop elevates the focal plane temperature to 280 mK.
2.5.2 Sub-Kelvin architecture
The sub-Kelvin mechanical structure is designed to make the focal plane maximally mechanically rigid, while reducing the parasitic heat load on the fridge. The sub-Kelvin structure consists pri- marily of two components. The first structure, colloquially known as the “truss,” serves to thermally and mechanically isolate the focal plane. The second component consists of a bundle of heat straps
3He Ultra-cool evaporator
3He Intercooler evaporator/
ultra-cool intercept
4He evaporator/
intercooler intercept
4He intercept
(4 K mounting bracket) Heat shield
4He pump
3He pump
Figure 2.12: TheBicep2helium sorption fridge. The fridge is heat-sunk to the 4 K bath at the4He intercept. The4He evaporator reaches⇠1.2 K and acts as the3He intercept. The3He intercooler evaporator, in turn, acts as the heat intercept for the ultra-cool stage.
Stage CF conductivity (mW/m) Total load (µW)
4!2K 46.5 37.2
2!0.35K 15.9 8.97
0.35!0.25K 0.18 0.06
Table 2.2: Parasitic heat loads from the Bicep2 sub-Kelvin carbon fiber (CF) truss structure.
Conductivities are calculated using material properties from Runyan and Jones 2008.
used to connect the focal plane and other structures to the evaporator heat sink locations on the fridge.
The sub-Kelvin truss consists of a rigid structure connecting three thermally isolated stages.
The design was based on the Bicep1 structure, and modified to improve the mechanical rigidity and thermal isolation. Each stage in the truss is separated by 12 carbon fiber legs, which serve to thermally isolate each subsequent stage (Figure 2.13). The choice of carbon fiber over the Vespel legs used byBicep1was largely motivated by the work of Runyan and Jones, which demonstrated the favorable sub-Kelvin properties of carbon fiber rods (Runyan and Jones 2008). The parasitic heat loads from each of the stages from the carbon fiber truss structure are summarized in Table 2.2.
In the first season ofBicep1, it was discovered that mechanical vibrations of the telescope caused temperature spikes on the focal plane. There are two potential sources of thermal excitation at the
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4 K 2 K
350 mK
Carbon fiber legs
Figure 2.13: Photograph of the sub-Kelvin mechanical support structure. The three concentric rings are heat sunk to 4, 2, and 0.35 K, as labeled. The stages are separated by carbon fiber legs epoxied into aluminum feet. Carbon fiber is chosen for its excellent thermal and mechanical properties.
focal plane: The first is mechanical vibrations that are thermalized at the ultra-cool evaporator point on the fridge. In this scenario, it is predicted that vibrations cause boiling in the superfluid
3He, which, in turn, cause temperature fluctuations that propagate up the thermal heat strap to the focal plane. In the second scenario, mechanical vibrations are thermalized at the focal plane, rather than the fridge. TheBicep2thermal design was intended to address both potential scenarios.
The Bicep2focal plane is thermally sunk to the fridge via electron-beam welded flexible heat straps. The straps are constructed from 60 layers of 1 mil oxygen-free high-conductivity (OFHC) copper. The layered copper foil is then welded into copper fixtures at either end, which, in turn, are fastened to the fridge and focal plane. The measured conductivity of the thermal strap is 3 mW/K.
To prevent thermal touches of the flexible strap to other thermal stages, the flexible straps are loosely tied to a carbon fiber “spine,” preventing the thermal strap from thermally shorting to nearby components.
By design, any mechanical excitation originating at the bottom of the thermal heat strap will be attenuated by the flexible foil, effectively thermalizing any vibration before it reaches the focal plane. For this reason, as well as to attenuate thermal fluctuations at the fridge, we implemented a passive thermal filter, consisting of a large chunk of stainless steel.
The passive thermal filter is placed between the upper section of the heat strap and the focal plane. Because of stainless steel’s high heat capacity, the thermal time constant is large (of order seconds). Any high-frequency thermal excitation originating at the fridge or in the heat strap itself is heavily attenuated by this filter. Stainless steel has moderate conductivity, measured to be
0.3 mW/K, roughly an order of magnitude worse than the thermal strap. At equilibrium, however, the temperature gradient is only 10 mK. SinceBicep2operates 40 mK above the base temperature of the fridge, this thermal gradient is tolerable.
The passive thermal filter is effective at attenuating high to mid-frequency thermal excitations.
The thermal transfer function has been measured, achieving > 40 dB attenuation at frequencies above 0.1 Hz (Kaufman et al. 2013a). Low frequency thermal fluctuations are filtered with an active thermal filter byBicep1-style temperature control modules. The module is a PID-controlled thermometer and resistive heater. Together, the active and passive thermal control achieve a focal plane temperature stability of better than±0.2mK rms during typical telescope scans. The thermal performance of the instrument will be discussed in more detail in later sections.