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&GAS: MAGBOLTZ


frac

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.]


E/P-RANGE

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.]


N-E/P

Number of E/p points in the gas tables.

[By default 20.]


scale

Selects whether the spacing of the E/p points should be linear or logarithmical.

[Logarithmic by default.]


ANGLE-RANGE

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 INTERPOLATIONS and EXTRAPOLATIONS methods.

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.]


N-ANGLE

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.]


PLOT-DISTRIBUTION-FUNCTIONS

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 SECOND-ORDER-TERMS (or ORDERS 2) and for a reasonably accurate F3 you should specify ORDERS 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.


ORDERS

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 is overruled by the SWITCH option.

[Default: n=2, equivalent to SECOND-ORDER-TERMS.]


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.

HIGH-PRECISION is synonymous to SECOND-ORDER-TERMS.

[This is the default order, but is overruled by SWITCH.]


FIRST-ORDER-TERMS

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.

LOW-PRECISION is synonymous to FIRST-ORDER-TERMS.

[The default is SECOND-ORDER-TERMS.]


ITERATE-ALPHA

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.]


SWITCH

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.]


DIFFUSION

Magboltz offers several ways of computing the diffusion:


MOBILITY

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.

ADD provides a similar functionality, and can in addition be used if the mobility is available in tabular form.

The unit of mobility in Garfield is cm2/microsec.V.

[By default: no mobility.]


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Formatted on 0099-12-08 at 15:52.