(κ-1)/κ saturation
UntestedSame V, d, A — swap dielectric material. Thrust scales with (κ−1)/κ which saturates near 1.
What this test isolates
Premise. The Maxwell stress on a dielectric scales with (κ−1)/κ. Air = 0, FR4 = 0.75, alumina = 0.9. There's diminishing return past κ ≈ 10.
Why it matters. Validates that the dielectric matters and matters in the right way. Going from FR4 → alumina should give ~20% more force, not 2× — and going alumina → exotic high-κ shouldn't help much.
Formula. F ∝ (κ−1) / κ
Variable. Dielectric constant κ
Hold constant
- Same emitter and ground plate
- Same thickness (matched sheets across materials, or matching to within ±20%)
- Same voltage and atmosphere
Prediction. Thrust ratio matches the ratio of (κ−1)/κ between materials.
How to run this test
- 1
Choose materials spanning κ
FR4 (κ ≈ 4.5), alumina-96 (κ ≈ 9.4), alumina-99.5 (κ ≈ 9.8). Add Stycast 2850FT (κ ≈ 4.0) for a low-κ epoxy comparison.
- 2
Match thickness across materials
Within ±20% — the (V/d)² term will dominate small thickness mismatches otherwise.
- 3
Same emitter, same V
Only the dielectric layer changes.
Pitfalls
- !Different breakdown limits — a thin alumina survives 28 kV that fries an FR4 of equal thickness. Pick a voltage that's safe everywhere.
- !Surface roughness changes between materials, biasing corona losses.