Na converter — radiating across the vacuum gap
fuel in the core · no conduction to the PV · energy crosses the gap as matched line/band (useful) + broadband thermal (parasitic)
—
—gas temperature (K) · blue band = cooled silicon across vacuum gap
Drive & geometry
fuel power core deposition2.0 kW
secondary pressure annulus density40 kPa
emitter radius R₂ envelope15 mm
length Lz50 mm
core R₁ = 5 mm · primary 1 kPa · silicon 400 K across the gap
model coefficients — assumptions, not derived
envelope emissivity ε transparent → low loss0.10
conductivity ×k_eff sets core ΔT1.0×
Steady state
useful fraction
—
matched / total crossing the gap
peak core T
—
on axis, mid-length
envelope T
—
gas-edge, sets thermal loss
819 / NIR share
—
of matched output
power crossing the gap — fuel ——
589 (matched) 819/NIR (matched) broadband thermal (parasitic)
what this is — and what's next

Now the silicon sits across a vacuum gap, so nothing conducts to it — the gas sheds its fuel power entirely as radiation, and closure still reads 1.00. Two channels cross the gap: the gas line/band emission (matched, what the PV converts) and the envelope's broadband thermal (parasitic, what heats the PV). The useful fraction is the spectral-efficiency knob — it climbs with drive here because line emission outpaces the T⁴ thermal as the gas heats.

Two coefficients are mine, not derived, and now exposed: the broadband ε (envelope + continuum greybody — lower = transparent = the vacuum-gap payoff, but the envelope runs hotter to compensate, and at ε≈0.05 it hits the sapphire ceiling) and the ×k_eff scale (which alone sets the peak core T — raise it and the core flattens). Transport is still escape-factor + radiative-diffusion, not short-characteristics; and the Na₂ bands (∝ n², favoring the cool skin) are the missing matched-NIR — still the next physics brick.