Very Low Resistance Al/Cu Joints for Use at Cryogenic Temperatures

Superconducting aluminum heat switch with 3 nΩ equivalent resistance

“…Electrical measurements of similar joints at 4 K yielded comparable resistances. 15 In order to quantify the eddy current heating due to the applied magnetic field, we ramped the field in a triangle pattern at a rate of 0.03 mT/s while maintaining the heat switch in the superconducting state. This ramp rate is adequate for controlling the heat switch state of a CNDR, even if it is necessary to ramp the field from zero to the 10 mT critical field of aluminum, since the CNDR cycle is expected to last more than one hour.…”

“…The joint is a gold-plated pressed contact as explained in detail in Ref. 15. The Al portion was cut from a 99.9999% pure rod 16 by spark erosion.…”

“…The designs and normal state conductances of several heat switches using different superconductors are tabulated in Ref. 15. Aluminum is an attractive material for constructing heat switches because of its relatively high Debye temperature ΘD = 394 which yields a relatively low phonon conductivity at a given temperature and phonon mean free path.…”

“…To our knowledge, this heat switch had the lowest normal state thermal resistance until now. 15 In this work, we report thermal measurements of an aluminum heat switch with a much lower normal state thermal resistance equivalent to 3 nΩ. The chemical gold plating procedure of Ref.…”

“…14 has been replaced with plasma etching of the aluminum followed by gold deposition without breaking vacuum. 15 Furthermore, an adequate superconducting state thermal resistance for use in a CNDR is achieved using a single piece of aluminum, as opposed to the several foils employed in Ref. 14.…”