Runaway Mobilities of Ions in Helium
The term “runaway mobility” of ions in a buffer gas like helium refers to a non-equilibrium condition: it is impossible for the ions to attain a defined drift velocity in an applied field, they are rather accelerated all the time. In contrast the usual mobility concept demands the condition of equilibrium where every momentum gain in the field is compensated by an average momentum loss over a long time. Microscopically the ions — even in the case of a well-established equilibrium — never have a constant speed: only on the average over many ions the whole group has a mean drift velocity and a certain velocity distribution. Mobility is defined as the ratio of the mean velocity attained in the buffer gas under a certain field strenght and the value of the field strenght. It is usually found to be a function of the reduced field strength E/N, where E is the field strength as measured in vcm-1 (or vm-1) and N is the gas number density in cm-3 (or m-3). The unit is Townsends (1 Towns-end = 10-17v m2 or 10-21V m2; its symbol is Td).
KeywordsDrift Velocity Drift Tube Momentum Transfer Cross Section Apparent Mobility Runaway Effect
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