TWDEC Electrode Structure & Geometry

The decelerator is a graded array of electrodes whose spacing, aperture, and voltage grading set both the deceleration profile and the beam optics.

A graded ladder of electrodes

Physically, a TWDEC decelerator is a series of ring or grid electrodes strung along the beam axis, each held at a stepped potential and driven at the working RF phase. The spacing between electrodes increases or decreases along the length to taper the wave phase velocity; the aperture of each electrode must clear the beam envelope, which grows as the beam slows and space charge spreads it.

Voltage grading and standoff

Each electrode sits at a higher retarding potential than the last, so the total structure holds a large voltage difference end to end. Because the fast protons carry 14.7 MeV, the collector end operates at very high potential, and the insulators and gaps must stand off that voltage without breakdown in vacuum. Grading rings and field-shaping are used to keep local fields below the breakdown threshold.

Beam optics through the array

• Apertures act as electrostatic lenses — they focus or defocus the beam as it passes.
• As the beam slows, space charge pushes it outward, so later apertures are larger.
• Losing particles to the electrodes both wastes their energy and heats the electrode — a heat load handed to the thermionic and cooling systems.

Materials

Electrodes and grids see particle impact, secondary electron emission, and radiation. They are built from refractory, low-sputter materials and actively cooled where impact heating is significant. Grid transparency — the open fraction the beam passes through — is traded against mechanical strength and heat handling, a recurring theme across the collector pages.