Traveling-Wave Direct Energy Conversion (TWDEC)
TWDEC decelerates fusion ions by making them surf a radio-frequency wave that always pushes back, converting their kinetic energy into RF power.
Surfing a wave in reverse
A traveling-wave direct energy converter is, in spirit, a linear accelerator run backward. In an accelerator, a radio-frequency wave travels along a structure and pushes charged particles to higher speed. In a TWDEC, the wave is phased so that it continually pushes back on the incoming ions: they do work on the wave, giving up kinetic energy, and that energy appears as electrical power in the structure's electrodes.
Why a wave instead of a fixed grid
A fixed retarding grid applies one voltage to every particle regardless of its energy. A traveling wave, by contrast, acts on particles according to their phase and velocity. Fast particles and slow particles experience the field at different points in the cycle, so the structure can extract energy across a spread of ion energies rather than a single value. This is the key to handling the broad D-3He spectrum.
The two-step design
- Modulator (buncher): an upstream RF section velocity-modulates the beam so that ions gather into bunches by the time they reach the decelerator.
- Decelerator: a second RF structure, phased to retard the bunches, extracts their energy as RF power at the electrodes.
- Rectification: the recovered RF is rectified and conditioned into the DC bus feeding the grid interface.
TWDEC is the stage best matched to the fast 14.7 MeV protons. Its efficiency depends on how tightly the beam can be bunched and how well the wave phase velocity tracks the ions — the subject of the next several pages.