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Schematic drawing of Pilot-PSI. A long diameter vacuum vessel (pumped to ) is placed inside five coils that produce an axial magnetic field up to . The cascaded arc exhausts plasma into the vessel along the magnetic field axis. It consisted of three tungsten cathodes in a cathode chamber, a stack of five insulated copper plates with a hole that forms the length discharge channel, and a copper tungsten nozzle that also serves as anode. The inner diameter of the leading was varied between and and followed by a preexpansion section of . The source was operated on hydrogen with a gas flow of and a discharge current of . This sets the cathode chamber pressure to . Due to the high collisionality the Saha equilibrium fixes the temperature at . (Ref. 7). At the exit of the plasma channel the plasma flows at sound speed ( boundary condition) and expands into the vessel.
and (upper panel) and discharge power and (lower panel) vs nozzle diameter at . The plasma output (proportional to ) increases much more than the input power, hence the source becomes more efficient at larger nozzle diameter.
Effect of the nozzle geometry on the electron temperature for a scan of the magnetic field. The arrows are drawn to guide the eye and to point to the two regimes that are crossed by the nozzle.
(squares, left axis) and ionization efficiency (triangles, right axis) measured for the nozzle.
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