Perveance

Perveance is a notion used in the description of charged particle beams. The value of perveance indicates how significant the space charge effect is on the beam’s motion. The term is used primarily for electron beams, in which motion is often dominated by the space charge.

The word was probably created from Latin pervenio–to attain.

For an electron gun, the gun perveance ${\displaystyle P}$ is determined as a coefficient of proportionality between a space-charge limited current, ${\displaystyle I}$, and the gun anode voltage, ${\displaystyle U_{a}}$, in three-half power in the Child-Langmuir law

${\displaystyle {I}={P}\cdot U_{a}^{\frac {3}{2}}}$

...
Wikipedia