A gas jet from argon and tetramethylsilane mixtures is generated by supersonic expansion through a grounded tungsten capillary (inner diameter 600 µm). This jet expands into a low pressure region (p ∼ 10 Pa) formed as an open tube (diameter 3 mm, variable length between 1 and 8 cm) in a stainless steel block. The steel block is biased by a pulsed kilohertz-signal generating a low pressure plasma. Due to the low pressure, a hollow cathode effect is observed depending on process parameters. 2D compressible flow simulations show a strong pressure gradient of several hundred pascals inside the open tube, with a low pressure zone at the exit of the gas jet and a high pressure zone in the centre of the open tube. The deposition profiles, deduced from optical film thickness measurements for different tube lengths, are discussed. The dependence of the breakdown voltage and of the differential resistance on the process parameters can be consistently explained by Paschen's law. The effective pressure allowing plasma ignition is at least a factor of 3 smaller than the average pressure inside the open tube, suggesting that pre-ionization in the supersonic expansion zone facilitates plasma ignition in the high pressure zone.