White noise performance of integrated DC SQUID susceptometers in high magnetic fields

We have characterized the white noise performance of integrated DC SQUID susceptometers in magnetic fields up to 400 G. The field is applied in the plane of the tunnel junctions and pickup loops. For 1 /spl mu/m diameter junctions, the noise does not increase significantly as the field is increased to 125 G. Further increase of the applied field results in higher noise until approximately 200 G where the critical current of the junctions is at a minimum. The noise then improves to approximately 4 times the zero field value at the maximum of the first lobe of the single junction interference pattern. We have also measured the noise of a SQUID with 0.6 /spl mu/m junctions and found no increase up to 250 G.<>