TWDEC Deceleration Physics

An ion gives up energy to a traveling wave only if it stays in the wave's decelerating phase; keeping it there is the core physics of TWDEC.

Phase is everything

In a traveling-wave structure the axial electric field looks, to an ion moving with the wave, like a nearly static field whose sign depends on the ion's phase relative to the wave crest. An ion sitting on the decelerating side of the wave feels a steady backward push and loses energy continuously. If it drifts to the accelerating side, the wave gives energy back. The whole art is to keep ions parked in the decelerating bucket as they slow.

Synchronism and slip

As an ion decelerates, its velocity falls. If the wave's phase velocity stayed constant, the ion would slip backward through the wave and eventually leave the decelerating phase. TWDEC structures therefore taper the wave phase velocity along the length of the decelerator so that it tracks the slowing ions — a graded structure that keeps the particle synchronous while it is being milked of energy.

Bunching feeds deceleration

Deceleration is far more efficient when ions arrive in tight bunches all sharing nearly the same phase. A spread-out, continuous beam has ions on both sides of the wave, so accelerated and decelerated particles partly cancel. The upstream modulator exists to compress the beam into bunches; the tighter the bunch, the larger the net energy extracted per pass.

Energy accounting

• Energy extracted per ion scales with the integrated decelerating field it experiences while synchronous.
• Ions that slip out of phase early leave with residual energy — passed to the next stage, not lost.
• The proton's 14.7 MeV is the primary target here; the slower helium-4 is handled with different tuning or downstream stages.

The practical limits are how far the phase velocity can be tapered, how tight the bunches stay against space-charge spreading, and the standoff voltage the electrodes can hold — each covered in its own page.