This could be vboom a phenomenon where a voltage V between a gap of length d span an electric force E=Vd that accelerates free ions to vboom and generates basic particles.
Intuitively, a higher voltage is needed for small gap to accelerate the particles to vboom before covering the length d,
v2=2ax
x≤d
from v2=u2+2ax,
where a is the acceleration under the constant field E=Vd and we assume that the particle has zero initial velocity.
F=qE
so,
a=Fmi=qEmi=qVmid
thus,
v2=2ax=2qVmixd
If v=vboom=3.4354∗densityZ and
PVol=nRT
P=nVolRT=MmiNAVolRT=densitymikT
where NA is the number per mole, M is total mass of the gas. So
v=vboom=3.4354∗miPZkT
and
(3.4354∗miZk)2(PT)2=2qVmixd
Vxd=3.435422m3iqZ2k2(PT)2
x≤d
V=Vmin when
x=d
Vmin=3.435422m3iqZ2k2(PT)2
Vmin=A(PT)2
where A=3.435422m3iqZ2k2
This assumes that after attaining vboom, collisions that generates particles is a certainty before reaching the other electrode, which may not be true for very low pressure and small gap length.
Strictly speaking, a discharge occurs,
V>Vmin
V>A(PT)2
where A=3.435422m3iqZ2k2
Maybe...