Pilot-PSI

The linear plasma generator Pilot-PSI

Supported by European Atomic Community (Euratom) as part of the European Fusion Programme

Pilot-PSI is the forerunner of Magnum-PSI and was constructed to study the production and transport of hydrogen plasma at flux densities that are required for Magnum-PSI. It consists of a 1 m long, 0.4 m diameter vacuum vessel. Two mechanical booster pumps are operated in parallel to yield an effective pumping speed of 7200 m³/hr. This is sufficient to keep the vessel pressure as low as 0.1-1 Pa during plasma operation. The vessel is placed inside five magnetic coils that produce an axial magnetic field (B) of up to 0.2 T in continuous wave and up to 1.6 T in pulses of 10 s. A cascaded arc is used to produce plasma. It consists of three tungsten cathodes in a cathode chamber, a stack of five electrically insulated water-cooled copper plates with a 4-10 mm hole that form a 30 mm length discharge channel, and a copper-tungsten nozzle that also serves as anode. Hydrogen is fed through this channel at a flow rate of 0.5-2.5 slm (1 standard-liter-per-minute = 4×10²⁰ H₂ s⁻¹). At the same time, an electric current of 100-1000 A is applied between the cathodes and the anode. This heats the gas to a temperature of typically 15000 ^oC and creates a pressure difference of 10⁴ Pa between the cathode chamber and the vessel. The plasma will therefore accelerate to sonic speed and expand into the vacuum vessel. The magnetic field confines the expanding plasma and guides it to the target at 0.5 m distance from the source. The plasma conditions are measured with Thomson scattering (see highlight), either near the source to monitor the source operation or near the target to characterize impinging plasma. Interaction between the plasma and the target is monitored with emission spectroscopy, Cavity Ringdown spectroscopy, IR-thermography, calorimetry, and probes.

Fig. 1 shows a photograph as well as a schematic overview of the experiment. Photographs of the experiment in operation are shown in Fig. 2.

Fig. 1: Photograph and schematic of the Pilot-PSI linear plasma generator. It uses a cascaded arc to produce the plasma that is exhausted into the vessel. Here, a magnetic field of up to 1.6 T is applied to confine the plasma and to guide it to a target at 0.5 m distance. The plasma is diagnosed with Thomson scattering. Interaction between the plasma and the target is monitored with emission spectroscopy, Cavity Ringdown spectroscopy, IR-thermography, calorimetry, and probes.

Fig. 2: Photograph of the Pilot-PSI experiment in operation. The photo shows the view on a hydrogen plasma column through the observational windows of the vacuum vessel. The plasma is exhausted from the nozzle of the cascaded arc on the left of the photo and hits the target on the right. In this case, the target was at 1 m distance from the source.