and connections to the pulse tubes and regenerators as will be described in greater detail below the first stage regenerator 13 is typically filled with a stack of screens which acts as a thermal sponge alternately absorbing heat from the working fluid and rejecting the absorbed heat back to the working fluid as the pressure oscillates first stage pulse tube 12 is a thin walled tube of a lower thermal conductivity material such as stainless steel if desired a heat exchanger can be included at the cold end of the first stage regenerator as is shown at 18 for the second stage regenerator first stage pulse tube 12 has heat exchangers 11 and 15 preferably of copper at its hot and cold ends respectively these are thermally coupled to the heat sink 25 and first stage cooling station 14 respectively and may also act as flow straighteners for the gas flow in the pulse tube gas passage 23 connects the cold end of the first stage pulse tube 12 to the cold end of the first stage regenerator 13 through heat exchanger 15 providing the gas expansion and refrigeration for this stage it should be noted here that while fig1 shows the second.