Perveance

Origin of the word

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

Definition

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

I = P ⋅ U a 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 U a is the potential difference between the emitter and the vacuum chamber, and the ratio of I U a 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 K defined as

K = I I 0 ⋅ 2 β 3 γ 3 ⋅ ( 1 − γ 2 f e ) ,

where I 0 = 4 π ε 0 ⋅ m c 3 e ≈ 17 k A (for electrons) is the Budker (or Alfven) current; β and γ are the relativistic factors, and f e is the neutralization factor.

Examples

The 6S4A 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. Each section of a 6AL5 is a high-perveance diode as opposed to a 1J3 which requires over 100V to reach only 2mA.

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.