&GAS: MAGBOLTZ
The fraction of the gas mixture taken up by a component.
The sum of the fractions is normalised - in the examples,
fractions usually add up to 1 or to 100, but this is not
mandatory.
[Each fraction is 0 by default.]
The range in E/p for which gas tables should be prepared.
The electric field E is expressed in V/cm, p in Torr.
[By default 10/p to 100000/p.]
Number of E/p points in the gas tables.
[By default 20.]
Selects whether the spacing of the E/p points should be linear
or logarithmical.
[Logarithmic by default.]
Electron transport properties in the presence of a magnetic
field depend not only on the magnetic field strength but also
on the angle between the electric and the magnetic field.
Magboltz will, if B is non-zero, by default be asked to compute
tables of the transport properties for a 2-dimensional grid of
E/p vs the angle between E and B. The density and range of the
grid along the E/p axis is set with N-E/P and E/P-RANGE, while
the density and range along the angles is set with N-ANGLE and
ANGLE-RANGE.
If your chamber is such that E is always perpendicular to B,
then it is advisable to centre the ANGLE-RANGE on 90 degrees,
e.g. [80,100], and set N-ANGLE to 1. This will generate a
1-dimensional table for which you have greater control over
the interpolation and extrapolation methods (for further detail
on this, see INTERPOLATION and EXTRAPOLATION).
If E is not perpendicular to B, then it may in rare cases be
sufficient to compute the transport parameters for a limited
range of the angle between E and B. Since the dependence on the
angle tends to be smooth, this practice is not recommended.
This parameter is meaningful only if there is a magnetic field.
[The default range is 0 to 180 degrees.]
See ANGLE-RANGE for further information.
Sets the number of angles between the E and B field for
which Magboltz will compute an electron transport table.
A setting of 1 forces the table to be 1-dimensional even if
there is a magnetic field. This is useful if E and B are
perpendicular everywhere in the chamber. Settings other than
1 and the default are not recommended.
[By default: 7.]
Requests for each E, and if applicable each B and E-B orientation,
of the electron distribution functions F0, F1, F2 and F3. Such plots
are useful to understand the behaviour of the drift velocity, which
is dominated by the first anisotropic term F1 and the diffusion,
which depends mostly on the isotropic term F0.
Keep in mind that Magboltz only computes F2 and F3 on request:
to get F2 and F3 you need to request HIGH-PRECISION (or ORDER 2)
and for a reasonably accurate F3 you should specify ORDER 3.
Both are incompatible with the option ITERATE-ALPHA which enables
a more precise computation of the Townsend coefficient.
F0 is plotted with representation FUNCTION-1, F1 as FUNCTION-2,
F2 as FUNCTION-3 and F3 as FUNCTION-4.
This option potentially generates a lot of output.
The option is off by default.
Can be used to set the number of terms to be included in
energy distribution function, to ensure higher accuracy for
the transport properties.
Values larger than 1 are not compatible with ITERATE-ALPHA,
this setting os overruled by the SWITCH option.
[Default: n=2, equivalent to SECOND-ORDER-TERMS.]
Requests inclusion of zeroth, first and second order terms
in the calculation of the energy distribution function.
Selecting this option ensures a higher accuracy for the
transport properties, but is not compatible with the
ITERATE-ALPHA option which improves the accuracy of the
Townsend coefficients.
SWITCH overrules this selection.
[This is the default order, but is overruled by SWITCH.]
Only the zeroth and first order terms will be included in
the energy distribution function. This setting guarantees
a reasonable accuracy and is compatible with the ITERATE-ALPHA
option which improves the accuracy of the Townsend coefficients.
[The default is SECOND-ORDER-TERMS.]
This option enables a refinement of the calculation of the
Townsend coefficient. This is particularly useful if the
Townsend coefficients are large (larger than say 50). But
the option is not compatible with inclusion of higher order
terms.
[This is not default.]
The SWITCH option combines ITERATE-ALPHA and SECOND-ORDER-TERMS
in the following way:
If alpha/pressure is smaller than the threshold, the quantities
are computed with SECOND-ORDER-TERMS, NOITERATE-ALPHA
Otherwise, they are computed with FIRST-ORDER-TERMS, ITERATE-ALPHA.
This guarantees that the drift velocity, diffusion and Lorentz
angle are accurate at low field values, which is where they matter
most, whereas the Townsend coefficient is accurate at higher field
values, at the price of a somewhat reduced accuracy for the other
quantities.
[This option is default, alpha/pressure is set to 50/pressure.]
Magboltz offers several ways of computing the diffusion:
- The method from the textbook of Huxley and Crompton, for
both the transverse and the longitudinal diffusion, either
with the F0 or the H1 distribution functions.
These formulae are only valid when the approximation that
the longitudinal and transverse diffusion are equal is true.
This is of course never true, but one can get a correction
to the longitudinal diffusion from the ratio of the longitudinal
to transverse diffusion without the magnetic field in the
given gas at the E-field of interest. [from Steve Biagi]
These diffusion coefficients are obtained when you specify
F0-TRANSVERSE-DIFFUSION, H1-TRANSVERSE-DIFFUSION,
F0-LOGITUDINAL-DIFFUSION and H1-LOGITUDINAL-DIFFUSION. The
correction to the longitudinal diffusion is applied, using
the G0 estimate of the longitudinal diffusion.
F0-TRANSVERSE-DIFFUSION is default.
- For the transverse diffusion, an alternative estimate based
on the mean energy, computed with F0, is offered.
- For the longitudinal diffusion, Magboltz computes on request
a more accurate value using the so-called G0 functions. This
is the default.
Magboltz only computes the electron transport properties in
gasses. This keyword enables adding an ion mobility to the tables.
This format only allows for mobilities that are constant or depend
in a simple way on E/p. In the latter case, the argument of MOBILITY
should be a function with EP as variable.
If the mobility is available in tabular form, then using the ADD
instruction may turn out to be a more convenient way of adding the
mobility to the gas tables.
The unit of mobility in Garfield is cm2/microsec.V.
[By default: no mobility.]
Keyword index.
Formatted on 10/11/98.