抄録
An exact expression of the mean arrival time drift velocity Wm of electrons in swarms in gases is deduced by taking into account the third and higher order gradient terms in the continuity equation. Here, Wm is defined as the ratio of the difference between the two positions in the field direction to the difference between the mean arrival times of electrons at the positions. The deduced expression is:
Wm=Wr-2αTDL+3αT2D3-4αT3D4+…,
where Wr is the center-of-mass drift velocity, DL is the longitudinal diffusion coefficient and Dn (n_??_3) are the higher order coefficients. αT is the Townsend first ionization coefficient. The importance of the present expression lies in that despite that Wm is the most commonly measured drift velocity by shutter techniques, there has been no exact expression for it, and also in that recent exact treatment of electron and ion swarms has started to consider the higher order coefficients.
