Deep Dive: Fast-Ion Physics & Channeling

Energetic ions from fusion and heating must be confined, slowed usefully, and — ideally — channeled into the fuel. Kronos treats channeling as a bounded, gated upside.

Fast ions

From fusion products + beam heating

Slowing-down

Stix critical energy 0.34 MeV (p) / 1.36 MeV (α)

Channeling

Fisch–Rax; ≥ 15% credit target (P6)

Risk

TAE-driven transport at β_fast = 1.59%

Energetic ("fast") ions — from fusion reactions and from beam heating — are central to the Kronos physics case, for both good and challenging reasons.

Slowing down. As fast ions collide with the plasma they give up energy, splitting it between ions and electrons depending on their energy relative to the Stix critical energy (0.34 MeV for protons, 1.36 MeV for alphas at the design point; analyses S27/S28). Recent guiding-centre work (S75) finds a banana width of 0.10–0.19 m and negligible prompt loss.

Channeling. Ideally, the proton line's energy can be channeled into the bulk fuel ions (Fisch–Rax mechanism) to sustain the hot-ion posture — prediction P6 targets a ≥ 15% credit. But the redress is bounded at ~30% (S72), and the same proton line is contested by DEC.

The risk. Fast ions can also drive TAE instabilities (β_fast = 1.59%, S67) that expel them — a named open channel. Fast-ion behaviour is thus both a key upside and a genuine open question.