Comment by binarycoffee

Absolutely, this is a passive compressor.

It kind of works like an aerodynamic diode: it is much easier for the incoming particles to go through the intake tubes (because they are oriented along the spacecraft velocity vector) than to exit the collector. This is because after a they collide with surfaces inside the collector, their velocity vector is randomized and no longer aligned with the tubes.

[Edit] Mass is not such an issue when you contrast it with the mass of propellant that you save...

[Edit2] I forgot to precise that my description only pertained to the intake part: there is of course a plasma thruster at the back (no moving part either)