TWDEC Efficiency & Its Limits
TWDEC efficiency is set by how completely bunched ions stay synchronous with the wave, minus RF, rectification, and interception losses.
What efficiency means here
TWDEC efficiency is the fraction of the incoming ion kinetic energy that leaves as usable electrical power. It is a product of several factors: the capture fraction (how much of the beam is bunched and synchronous), the deceleration completeness (how much of each ion's energy is extracted before it slips out of phase), and the circuit efficiency (RF collection and rectification).
The bunching bottleneck
The single biggest lever is bunching quality. A perfectly bunched, monoenergetic beam could in principle be decelerated to a small residual energy with high efficiency. Real beams have velocity spread from the plasma temperature and from scattering, so bunches smear as they travel and some ions inevitably fall out of phase. Space charge fights the bunching too. The efficiency you actually get reflects this competition.
Losses to account for
- Ions intercepted by electrodes — energy lost as heat, though partly recoverable thermionically.
- Ions that slip out of the decelerating phase and exit with residual energy — passed downstream, not lost.
- RF collection and impedance mismatch losses in the electrode circuit.
- Rectification and conditioning losses converting recovered RF to DC.
Why staging rescues efficiency
Because a single TWDEC stage cannot fully decelerate a broad two-species spectrum, its standalone efficiency is bounded. The system recovers what one stage leaves behind by handing residual energy to the MHD and thermionic stages. The honest figure of merit is the train efficiency, and demonstrating it on plasma is the job of the burner test program targeted for about 2032.