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In order to investigate the effect of pulse tube inclination on the performance of a pulse tube refrigerator (PTR), we have built a test rig in which the angle a between the pulse tube axis and the direction of gravity can be varied between 0 and ± 180°. a = 0°corresponds to the vertical orientation with the hot end up. The PTR was operated with orifice, reservoir and second inlet at the wann end using helium as working fluid. The pulse tube has a length of 250 mm and an inner diameter of 13.4 mm. Operating parameters are: average pressure 18 bar, peak to peak pressure variation 5.4 bar and frequency f= 1.6 -4 Hz. Optimum cooler performance is obtained for a = 0 and f= 2 Hz with a minimum no-load temperature ofT(OO) = 52.5 K and a net cooling power of Q(0°) '" 2 W at 80 K. Upon tilting the pulse tube, T(a) initially increases moderately up to T(70jff(00) '" 1.2. Further increase of a leads to a steep rise of T(a)ff(OO) attaining a maximum of", 3 for a", ±120°and finally a value ofT(±1800)ff(00) '" 2. The measured variation ofT(a) and Q(a) indicates that tilting results in excess heat loads of up to 6 W. These losses are ascribed to an enhanced heat transfer by natural convection of He-gas occurring for a~0°, which is superimposed on the oscillatory gas displacement in the empty pulse tube. This interpretation is supported by the calculated Nusselt number Nu(a) which can semi-quantitatively account for the observed inclination effect. At a frequency of 4 Hz the magnitude of T(a)ff(OO) is reduced with a most pronounced effect at a = ± 90°. The a-dependence from convection is considerably weakened by filling the pulse tube with a porous material, but this also leads to a degradation of the cooler perfonnance at a = 0°.

A new Ultra Precision Interferometer (UPI) was built at Physikalisch-Technische Bundesanstalt. As its precursor, the precision interferometer, it was designed for highly precise absolute length measurements of prismatic bodies, e.g. gauge blocks, under well-defined temperature conditions and pressure, making use of phase stepping imaging interferometry. The UPI enables a number of enhanced features, e.g. it is designed for a much better lateral resolution and better temperature stability. In addition to the original concept, the UPI is equipped with an external measurement pathway (EMP) in which a prismatic body can be placed alternatively. The temperature of the EMP can be controlled in a much wider range compared to the temperature of the interferometer's main chamber. An appropriate cryostat system, a precision temperature measurement system and improved imaging interferometry were established to permit absolute length measurements down to cryogenic temperature, demonstrated for the first time ever. Results of such measurements are important for studying thermal expansion of materials from room temperature towards less than 10 K.

Our measurements of the electrical resistivity p&(T) on high-quality copper whiskers reveal a T variation at very low temperatures. %ith the taking into account of appreciable corrections for surface scattering, the coefficient for electron-electron scattering in bulk copper could be determined, A =d, p/T2=27 fQ cmK, in good agreement with calculations assuming an isotropic relaxation time v(k). For the first time, a strong enhancement of A by a longitudinal magnetic field is observed, which is related to an anisotropy of r(k) A. t 4 T we find A = 110 fQ cm K 2 independent of an induced dislocation density of about 109 cm-'.

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A thermal heat switch has been developed intended for cryogenic space applications operating around 100 K. The switch was designed to separate two pulse tube cold heads that cool a common focal plane array. Two cold heads are used for redundancy reasons, while the switch is used to reduce the thermal heat loss of the stand-by cold head, thus limiting the required input power, weight and dimensions of the cooler assembly. After initial evaluation of possible switching technologies, a construction based on the difference in the linear thermal expansion coefficients (CTE) of different materials was chosen. A simple design is proposed based on thermoplastics which have one of the highest CTE known permitting a relative large gap width in the open state. Furthermore, the switch requires no power neither during normal operation nor for switching. This enhances reliability and allows for a simple mechanical design. After a single switch was successfully built, a second double-switch configuration was designed and tested. The long term performance of the chosen thermoplastic (ultra-high molecular weight polyethylene) under cryogenic load is also analysed.