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) are attractive candidates for low noise cooling of HT-SQUIDs. Tests of a highly sensitive HT-SQUID that was mounted directly on the cold tips of these coolers revealed discrete peaks in the flux noise spectrum which were attributed 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 connected to the cold tip by use of a flexible copper link. The remaining amplitude of axial vibrations at the cold platform was measured to be about 0.5 /spl mu/m as compared to 6.0 /spl mu/m 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 intrinsic noise of 45 fT//spl radic/Hz at frequencies above 100 Hz and 110 fT//spl radic/Hz at 10 Hz. Measurements of the flux noise spectrum of the HT-SQUID using the new PTC cold head will be presented.