Perveance
Encyclopedia
Perveance is a notion used in the description of charged particle beam
s. 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.
is determined as a coefficient
of proportionality between a space-charge limited current,
, and the gun anode
voltage,
, in three-half power in the Child- Langmuir law
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
is the potential difference between the emitter and the vacuum chamber, and the ratio of
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
defined as
,
where
= 17 kA is Budker current; 
and 
are the relativistic factors, and 
is the neutralization factor.
Charged particle beam
A charged particle beam is a spatially localized group of electrically charged particles that have approximately the same velocity . The kinetic energies of the particles are typically measured in keV or MeV, much larger than the energies of particles at ambient temperature...
s. The value of perveance indicates how significant the space charge effect is on the beam’s motion. The term is used primarily for electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
beams, in which motion is often dominated by the space charge.
Definition
For an electron gun, the gun perveance is determined as a coefficientCoefficient
In mathematics, a coefficient is a multiplicative factor in some term of an expression ; it is usually a number, but in any case does not involve any variables of the expression...
of proportionality between a space-charge limited current,
, and the gun anodeAnode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
voltage,
, in three-half power in the Child- Langmuir law The same notion is used for non-relativistic
Theory of relativity
The theory of relativity, or simply relativity, encompasses two theories of Albert Einstein: special relativity and general relativity. However, the word relativity is sometimes used in reference to Galilean invariance....
beams propagating through a vacuum chamber
Vacuum chamber
A vacuum chamber is a rigid enclosure from which air and other gases are removed by a vacuum pump. The resulting low pressure, commonly referred to as a vacuum, allows researchers to conduct physical experiments or to test mechanical devices which must operate in outer space...
. In this case, the beam is assumed to have been accelerated in a stationary electric field so that
is the potential difference between the emitter and the vacuum chamber, and the ratio of 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
defined as,where
= 17 kA is Budker current; and are the relativistic factors, and is the neutralization factor.