Precise thermodynamic control of high pressure jet expansions.

We describe an experimental setup for supersonic jet expansions of supercritical fluids. It is characterized by well-defined thermodynamic values to allow systematic investigations of pressure and temperature effects on molecular beam parameters. The design permits stagnation temperatures T(0)=225-425 K with a thermal stability DeltaT(0)<30 mK and stagnation pressures p(0)=0.2-12 MPa that are measured with 0.05% precision. For optimum stability, gas reservoir, pressure transducer, and gauge amplifier are temperature-controlled, and a feedback loop permits active pressure stabilization using a pulseless syringe pump. With this approach stagnation pressures can be reproduced and kept constant to Deltap(0)<2.9 kPa. As a result, flow velocity and kinetic energy of molecular beams can be controlled with maximum accuracy.