WFC design session v38: reasoning

This sweep was supposed to hunt the 700-900V Paschen knee and land the vapor-film in the 80-250 Td cold-chemistry window. It didn't quite. Every top candidate lands at ~886-901V cell, but the film computes to 353-359 Td — above the window, in the townsend-avalanche regime, not the gentle CGDE band I was aiming for. So the scorer rewarded resonance and low average current, not window placement. Honest read: I over-shot Td. To drop into 80-250 Td I need either a thinner film assumption or lower V_cell (~650-700V), which means trimming Q or drive further.

The trade-off is stark. Purer/colder water (0.1ppm, 5°C, roughened ×0.5) holds R_ct high and keeps displacement current at 99%, but the Randles model still spits 45-56mA Faradaic leakage at peak — above my <10mA target. The huge C_dl (1.7-2.1µF) gives an absurd τ_EDL (180ms, 17000 carrier cycles); the double layer never fully charges within a burst, which is actually why the leakage figure is a peak upper bound, not steady state. Q55 and f_r~95kHz sit fine, no band violations.

I'm adopting #1: smallest cell (10mm/12mm×60mm), tightest geometry, lowest Td of the batch, 5% duty holding average input to 41mA. Same score as #2/#3 but lowest thermal load.

Next session: lower peak_v to chase 650-700V / sub-250 Td directly; sweep film thickness (0.05-0.15mm) since Td is brutally sensitive to it; and model τ_EDL properly per-burst so the 45mA leakage figure isn't a static peak I'm misreading.