The Venetian-Blind Electrostatic Collector

The simplest direct converter is a set of angled, biased ribbons that let ions in and stop them against a retarding field, like a venetian blind.

The classic single-stage converter

Before traveling-wave methods, the canonical direct converter was the venetian-blind collector: a stack of thin, angled ribbon electrodes, biased so that ions entering along the field are decelerated and collected on the far ribbons while electrons are turned back. Ions climb the potential hill, deposit their charge at high voltage, and the current flows into the load. It is electrostatic, has no moving parts, and is conceptually the purest form of DEC.

Strengths

• Mechanically simple, no RF, no moving parts.
• Directly produces high-voltage DC.
• The angled ribbons present a large collecting area while staying transparent to the incoming beam.

The single-voltage limitation

A venetian-blind collector applies essentially one retarding potential to the whole beam. That is ideal for a narrow, monoenergetic spectrum but wasteful for the broad, two-species output of a D-3He plasma: set the voltage for the 14.7 MeV protons and the 3.6 MeV helium-4 is barely touched; set it lower and the protons arrive with large residual energy that heats the ribbons. Multi-stage electrostatic versions add several graded voltages to widen the accepted band.

Where it fits in the burner

The burner does not rely on a single venetian-blind stage as its main converter, but the concept underlies the electrostatic collectors used to catch residual ions after the traveling-wave and MHD stages, and it is the clearest way to understand what “retarding-field” conversion means. Its heat-handling limits — what to do with intercepted, un-decelerated ions — motivate the thermionic topping stage.