Perveance

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[1]

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

The same notion is used for non-relativistic beams propagating through a vacuum chamber. In this case, the beam is assumed to have been accelerated in a stationary electric field so that ${\displaystyle U_{a}}$ is the potential difference between the emitter and the vacuum chamber, and the ratio of ${\displaystyle {\frac {I}{U_{a}^{\frac {3}{2}}}}}$ is referred to as a beam perveance. In equations describing motion of relativistic beams, contribution of the space charge appears as a dimensionless parameter called the generalized perveance ${\displaystyle K}$[2][3] defined as

${\displaystyle {K}={\frac {I}{I_{0}}}\cdot {\frac {2}{{\beta }^{3}{\gamma }^{3}}}\cdot (1-\gamma ^{2}f_{e})}$,

where ${\displaystyle {I_{0}}=4\pi \varepsilon _{0}\cdot {\frac {mc^{3}}{e}}\approx 17\mathrm {kA} }$ (for electrons) is the Budker (or Alfven) current; ${\displaystyle \mathbf {\beta } }$ and ${\displaystyle \mathbf {\gamma } }$ are the relativistic factors, and ${\displaystyle f_{e}}$ is the neutralization factor.

The 6S4A[4] is an example of a high perveance triode. The triode section of a 6AU8A becomes a high-perveance diode when its control grid is employed as the anode.[5] Each section of a 6AL5 is a high-perveance diode[6] as opposed to a 1J3 which requires over 100 V to reach only 2 mA.[7]

Perveance does not relate directly to current handling. Another high-perveance diode, the diode section of a 33GY7, shows similar perveance to a 6AL5, but handles 15 times greater current, at almost 13 times maximum peak inverse voltage.[8]