WFC design session v15: reasoning

Design note — 2026-07-03

This sweep confirms something I suspected: once bulk geometry is locked at my adopted #1 cell (21.8/26.8mm×60mm), the ionic branch stops caring about surface_factor across a wide range. Candidates #1–#4 are numerically identical outside of C_dl (1.9µF→5.0µF as surface_factor runs 0.25→0.65) — same λ_D (227nm), same series R (11.1Ω), same tank Q (97.5), same cell voltage (1170V), same current (983.6mA). The double-layer capacitance is so much larger than C_eff (nF-scale) that it's effectively a short at 95.6kHz — the water capacitor's geometric term dominates displacement current (99.1%) regardless of how aggressively I scrub the electrode surface. Attacking Cdl/λ_D directly via surface treatment was the hypothesis this session; the result says the lever doesn't move anything downstream at this ppm/temp. What does move things is temperature: #5 at 5°C (vs 3°C) drops bulk R from 127.7kΩ to 119.2kΩ, drags tank Q down from 97.5 to 91.2, and current with it (983.6mA→915.8mA) — a real trade against the 1A ceiling, at the cost of ~76V less on the cell.

I'm keeping #1 as adopted: same performance as #2-4 with the least surface prep (×0.25) — no reason to spend more processing for zero return. Breakdown margin (×143) stays comfortable at 0.1ppm/3°C.

Next session: since surface_factor is saturated, sweep temperature more finely (0–10°C) to map the Q/current knee properly, and reintroduce ppm above 0.1 to see where the ionic branch actually starts contributing.