Confusion about the longitudinal diffusion coefficient in Garfield

Garfield++
1

@hschindl
Dear hschindl,

I try to understand the longitudinal diffusion coefficient in the Garfield++.
I compared two simulation results (drift distance is 1 cm), one code is the read.C in the ”GasFile“ director, another is [Examples/GainFluctuations/arco2.C]. In the file "read.C:, the longitudinal diffusion coefficient can be obtained directly and the value is 0.0085sqrt(cm)@2kV/cm (Figure 1). In the file “arco2.C”(just consider drift process), I obtained the drift path by using “GetElectronDriftLength()”，the length distribution is strange (Figure 2). However, the STD obtained from the distribution (Figure 3) of end points in the x-axis direction (transverse diffusion) is consistent with that value from Figure 1. Could you help me understand the results? Thanks very much.

diffusion coefficients
diffusion coefficients588×545 65.4 KB

drift length
drift length834×409 42.6 KB

Xposition
Xposition832×372 35.5 KB

Best regards,
Jiechen

2

Hi,
thanks for your message! In what sense is the distribution in fig. 2 strange?

To verify that the longitudinal diffusion in your simulation is consistent with the longitudinal diffusion coefficient in the gas file, it’s probably more instructive to plot the distribution of drift times.

3

hschindl
Dear hschindl,
Thanks for your reply, as you suggested, if plot the distribution of drift times, the result is consistent with the longitudinal diffusion coefficient.
Figure.2 is the distribution of drift length obtained from the function “GetElectronDriftLength()”, the strange point is that the longitudinal diffusion coefficient does not reflect from the distribution, and the sigma is very much smaller than longitudinal diffusion coefficient. Could you help me understand it?

Best regards,
Jiechen