The Energy Recovery Fraction
The recovery fraction is the share of charged-particle energy the train turns into electricity; it is the honest figure of merit for DEC.
Defining the figure of merit
The energy recovery fraction is the electrical energy delivered to the bus divided by the charged-particle energy entering the DEC train. It is deliberately drawn at the charged-particle boundary, because that is what DEC can act on — the neutron fraction (~5.44%) and radiation are accounted separately. This fraction is the single number that says how well the whole train, not any one stage, is doing its job.
What it rolls up
- TWDEC capture and deceleration completeness.
- MHD conversion of the ordered flow, minus ohmic and end losses.
- Thermionic recovery of surface heat.
- Interception, secondary-electron, sheath, and rectification losses across every stage.
Why it beats any single-stage number
Quoting one stage's efficiency is misleading, because each stage is designed to hand its remainder to the next. A TWDEC stage that decelerates protons only partway is not ‘inefficient’ if the MHD and thermionic stages recover the rest. The recovery fraction is the only number that captures this cascade honestly, which is why it, not a stage figure, is the headline metric.
Status and honesty
The burner is a design-and-simulation study; the recovery fraction is a modeled quantity to be validated, not a measured result. The purpose of the ~2032 test unit is precisely to measure it on a real plasma. What can be stated firmly now is the structure of the number — what it includes, what it excludes, and why a staged train is expected to make it high enough that only a small residual needs cooling.