Dynamic pumping and pulsed outgassing in waveguides and vacuum systems for plasma physics experiments

Some aspects of the vacuum problems raised by present plasma physics experiments, i.e., lower hybrid heating and current drive in tokamak discharges, are considered. Short time scale transient phenomena, such as a few milliseconds long outgassing pulses in waveguides used as antennas for plasma heating, may be observed. In these conditions the pressure distribution effects in long pipes cannot by neglected. Then the use of a one‐dimensional model for the pressure is required. In the molecular flow regime the pressure is found to be the solution of a heat‐conduction‐type equation. Most usual vacuum problems may be solved by including appropriate boundary conditions. A few examples are developed by using the Laplace transform technique. The particular case of a pumping step at one end of a tube is presented as it applies to a tokamak discharge. While pumping at a pipe inlet, the pulsed localized and distributed outgassings in the pipe are examined. An application to the specific case of waveguide conditioning by short radio frequency power pulses is presented. The gas flow rate being released and its location are deduced from pressure measurements.