Progress in low noise cooling performance of a pulse-tube cooler for HT-SQUID operation

Previously it was shown that Joule-Thomson- and pulse tube coolers (PTC) ar e attractive candidates for low noise cooling of HT-SQUIDs, Tests of a high ly sensitive MT-SQUID that was mounted directly on the cold tips of these c oolers revealed discrete peaks in the flux noise spectrum which were attrib uted to pressure wave-induced vibrations of the coolers. A first additional noise reduction for the PTC was achieved by using Ti-V-Al tubes instead of stainless steel for the regenerator and pulse tube. Further reduction was obtained by mounting the sensor on a separate platform that was thermally c onnected to the cold tip by use of a flexible copper Link. The remaining am plitude of axial vibrations at the cold platform was measured to be about 0 .5 mum as compared to 6.0 mum without vibration compensation. We report on the tests of different methods of vibration compensation for the PTC, using a sensitive rf HT-SQUID magnetometer with coplanar resonator showing an in trinsic noise of 45fT/root Hz at frequencies above 100 Hz and 110 fT/root H z at 10 Hz. Measurements of the flux noise spectrum of the MT-SQUID using t he new PTC cold head will be presented.