GRAVITY is a second generation VLTI instrument for high-precision narrow-angle astrometry and phase-referenced
interferometric imaging in the astronomical K-band. The cryostat of the beam combiner instrument provides the required
temperatures for the various subunits ranging from 40K to 290K with a milli-Kelvin temperature stability for some
selected units. The bath cryostat is cooled with liquid nitrogen and makes use of the exhaust gas to cool the main optical
bench to an intermediate temperature of 240K. The fringe tracking detector will be cooled separately by a single-stage
pulse tube cooler to a temperature of 40K. The pulse tube cooler is optimized for minimum vibrations. In particular its
warm side is connected to the 80K reservoir of the LN2 cryostat to minimize the required input power. All temperature
levels are actively stabilized by electric heaters. The cold bench is supported separately from the vacuum vessel and the
liquid nitrogen reservoir to minimize the transfer of acoustic noise onto the instrument.
GRAVITY is a second generation VLTI instrument for high-precision narrow-angle astrometry and phase-referenced
interferometric imaging in the astronomical K-band. The cryostat of the beam combiner instrument provides the required
temperatures for the various subunits ranging from 40K to 290K with a milli-Kelvin temperature stability for some
selected units. The bath cryostat is cooled with liquid nitrogen and makes use of the exhaust gas to cool the main optical
bench to an intermediate temperature of 240K. The fringe tracking detector will be cooled separately by a single-stage
pulse tube cooler to a temperature of 40K. The pulse tube cooler is optimized for minimum vibrations. In particular its
warm side is connected to the 80K reservoir of the LN2 cryostat to minimize the required input power. All temperature
levels are actively stabilized by electric heaters. The cold bench is supported separately from the vacuum vessel and the
liquid nitrogen reservoir to minimize the transfer of acoustic noise onto the instrument.
After an introductory section on pulse tube cryocoolers (PTC) this paper reports the state of development of some medium-size PTCs for potential future replacement of commercial Stirling cold fingers. The coolers were designed for operation with the AIM SL200 compressor (nominal input power: 100 W) and Leybold Polar SC7 compressor (nominal input power: 200 W), respectively. Adjustment of phase shift between pressure and mass flow oscillation is accomplished by means of inertance tubes in combination with a reservoir and a second-inlet flow impedance that are attached to the warm end of the pulse tube. Two coolers with U-shaped and one with linear arrangement of regenerator and pulse tube have been built and optimized. Up to now, the smaller U-shaped PTC driven by the AIM compressor at 100 W input power reached a no-load temperature of 45 K, and a cooling capacity of 2.85 W at 80 K is achieved, corresponding to a coefficient of performance of COP = 2.85 %. For the two larger PTCs driven by the Leybold compressor at 200 W of input, the obtained no-load temperature and cooling power at 80 K are 38 K and 6 W for the U-shaped cooler and 44 K and 8.1 W for the linear cooler, corresponding to COPs of 3 % and 4 % at 80 K, respectively. Measurements of the refrigeration temperature as function of the cold head orientation with respect to gravity revealed a small convection-induced temperature variation of several percent. The minimum temperature is achieved with the pulse tube cold end facing downwards.
Pulse tube coolers operate without any moving solid parts inside the cold finger. This feature promises higher reliability, lower vibrations, and lower production cost, when compared to conventional Stirling coolers. We have designed and constructed a miniature-size pulse tube cooler for potential future replacement of Stirling coolers. To allow for easy access to the cold platform, a U-shaped configuration of pulse tube (diameter 4.5 mm, length 60 mm) and regenerator has been chosen. The pressure oscillation of the helium working fluid in the system is generated by means of a commercial linear compressor (AIM, model SL100) operated at a frequency of 50 Hz. A combination of capillary and buffer volume and a second-inlet capillary, which are connected to the warm end of the pulse tube, serve to adjust the phase shift between pressure and mass flow oscillation in the cooler. A maximum cooling power of about 0.3 W at 80 K and a slope of the load line of 40 mW/K have been achieved so far at a compressor input power of 90 W and a heat rejection temperature of about 330 K. The overall COP of 0.3% at 80 K is still appreciably lower than that of comparable Stirling coolers, which is related to enhanced regenerator losses and DC gas-flow in the pulse tube cold head. The experimental data are compared to a linear network model for the pulse tube cold head.
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