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 μm as compared to 6.0 μm 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/√Hz at frequencies above 100 Hz and 110 fT/√Hz at 10 Hz. Measurements of the flux noise spectrum of the HT-SQUID using the new PTC cold head will be presented