Electron Suppression at the Collectors

Electrons must be turned back before the ion collectors, or they short-circuit the recovered voltage; a suppressor grid does this.

The polarity problem

Direct conversion recovers energy from positive ions climbing a positive potential. The plasma exhaust, however, carries electrons too, and electrons falling down that same potential would deliver current in the opposite sense, cancelling the recovered power. Any DEC collector therefore needs a way to separate electrons from ions and prevent the electrons from reaching the high-voltage ion collectors.

How suppression works

A suppressor grid placed ahead of the ion collectors is held at a negative potential. Ions, being positive and energetic, pass through the small negative dip with little effect; electrons, being negative and much lighter, are reflected back toward the plasma. The suppressor thus acts as a one-way filter, admitting the positive ions to be collected while turning electrons away.

Design considerations

• The suppressor voltage must be deep enough to reflect the electron population but shallow enough not to disturb the ions.
• It intercepts some ions on its own wires — another transparency-versus-function trade.
• Secondary electrons knocked off the suppressor itself must not become a new reverse current.

Why it is non-negotiable

Without electron suppression, a direct converter's net output collapses — electron leakage current can rival or exceed the ion current. Reliable suppression is therefore a precondition for any of the recovered voltage to survive to the bus. It works hand in hand with secondary-electron mitigation, since impacts everywhere in the collector region generate stray electrons that must also be controlled.