英语翻译Typically the coefficient ( B ) used to model the Bohm c

英语翻译
Typically the coefficient ( B ) used to model the Bohm collision frequency is 1/16.Obviously,this is a gross simplification of a complex problem and as it turns out enormously over-predicts the electron cross-field mobility in the acceleration zone.For this reason,this type of anomalous mobility is often only applied in specific regions of the Hall thruster model (“Mixed mobility” model)[140] and the typical value of 1/16 is modified.[165-167] Fife 166 found that a value of a coefficient of 1/107 yields the necessary amount of cross-field mobility and Ahedo 168 also found a value of 1/100 to be appropriate.
Another source of this anomalous cross-field electron transport is near-wall conductivity and was first proposed by Morozov.134,169-171 This theory proposes that electron collisions with the walls enhance electron cross-field mobility and in this way the walls act like a macro-particle.This effect is enhanced due to high secondary electron emission from the walls and particularly in the case of space-charge saturated wall sheaths.124,172 Due to the potential fall in the wall sheath and the low energy emitted electrons,the walls can act like gutters for electron cross-field mobility.There have been several investigations that suggest the importance of near-wall conductivity.124,172-174 Unfortunately,these experimental data presented in this thesis are not well suited to study near-wall conductivity since the closest measurements are 2 mm from the wall.
In reality,the cross-field mobility is likely a combination of these effects and many modelers account for all of these electron transport mechanisms.In the following sections,the classical mobility will be measured as well as an experimentally determined mobility.
5.5.2.1 Classical Analysis
In the classical concept of cross-field electron mobility,electrons can cross magnetic fields when they undergo a momentum exchange collision.By far the predominant momentum exchange collision that occurs is between electrons and neutrals; 140 however,to a lesser degree electron-ion collisions can enhance electron mobility.The total momentum exchange collision frequency is equal to the sum of the electron-ion and electron-neutral as shown Equ.5-48.Where in this equation,V e is the electron velocity given in Equ.5-49.The ion number density and neutral number density are known from the Langmuir probe measurements and the one-dimensional neutral number density calculation (Section 5.4.7.2).