30 W at 50 K Single-Stage Coaxial Pulse Tube Cooler with Tapered Buffer Tube
The performance of large-capacity single-stage pulse-tube coolers at temperatures below 70 K is often hampered by Rayleigh streaming, or the natural boundary-layer convection that occurs in the buffer tube (‘pulse tube’). In Olson and Swift’s landmark 1997 paper1, they explained how a slight taper in the buffer tube could suppress this streaming. They also showed how the streaming could be suppressed by the proper phasing of pressure vs. flow in the buffer tube, which is enforced by the proper choice of phase-shift mechanism (i.e. length and diameter of inertance tube, etc.). This is the approach usually taken because a straight buffer tube is simpler, especially when considering a coaxial construction (where a tapered buffer tube would imply a tapered regenerator). In addition, the phasing which suppresses Rayleigh streaming coincides with efficient cycle phasing for many applications. At temperatures below 70 K, however, this is less true, and for a 50 K machine a significant benefit may be realized by decoupling the streaming suppression from the cycle phasing. At the same time, we have found that coaxial coldheads can successfully use nonconductive buffer tubes, if the material with the right thermal expansion coefficient is selected. This enables the use of a tapered buffer tube in a coax design, as the material can be thick enough to have a constant outside diameter (OD) and a tapered inside diameter (ID). This paper will discuss the results obtained on a high-capacity cooler using a tapered buffer tube and includes some measurements showing the importance of having the right taper angle.