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&GAS: MAGBOLTZ


ingredient

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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.]


ELECTRIC-FIELD

Magboltz will compute the transport properties either for electric fields that are regularly distributed within a range, or for a set of fields that you specify.

Make sure that the all fields present in your chamber are covered by the table and do not rely on the EXTRAPOLATIONS. Pay particular attention to the region in the immediate vicinity of wires. Use e.g. the field section CHECK WIRES commands to determine the field there.

Try to be reasonable when selecting the number of electric fields at which you run Magboltz. On the one hand the number of points must be large enough so as to represent any fine structure that may be present in the transport properties. On the other hand, an excessively large number of points may lead to oscillations in the interpolation.

It is permissible to request transport properties for only a single field, for instance to study the dependence of the transverse diffusion on the magnetic field. The interpolation methods need not be specified in this case and only constant extrapolations can be used.

If you ask for a small number of points, e.g. 2, then you can only request low order INTERPOLATIONS.

[By default E ranges from 100 V/cm to 100000 V/cm in 20 logarithmically spaced steps.]


B-FIELD

A magnetic field can have a major impact on the direction of motion of electrons, as well as on the transverse diffusion. These effects depend not only on the magnetic field strength, but also on the ANGLE between the electric and magnetic fields.

You can either ask Magboltz to compute the transport properties at evenly spaced magnetic fields within a range or for a set of magnetic fields that you specify.

If you ask for only 1 magnetic field strength and only 1 angle between E and B, then Magboltz will generate a 1-dimensional table for which you have greater control over the INTERPOLATIONS and EXTRAPOLATIONS methods.

These parameters can not be used if there is no magnetic field.

[If the magnetic field can be determined to be constant or zero, then Magboltz by default only computes a table for the magnetic field that is present in the chamber. If the B field is not constant, but of known range, then Magboltz will by default be run for 6 magnetic fields ranging from the minimum field to the maximum field present in the chamber. In all other cases, it will compute tables for 0, 1, 2, 3, 4 and 5 T.]


ANGLE

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.

You can either ask Magboltz to compute the transport properties for angles between E and B that are evenly spaced over a range, or for a user-specified set of angles between the 2 fields:

If you ask for only 1 magnetic field strength and only 1 angle between E and B, then Magboltz will generate a 1-dimensional table for which you have greater control over the INTERPOLATIONS and EXTRAPOLATIONS methods.

These parameters can not be used if there is no magnetic field.

[Magboltz will, if B is non-zero, by default be asked to compute tables for an angle of 0, 30, 60 and 90 degrees between the E and B field.]


PLOT-DISTRIBUTION-FUNCTIONS

Requests for each E, and if applicable each B and E-B orientation, plots of the electron distribution functions. Use the KEEP option if you need more flexibility.

This option potentially generates a lot of output.

[The option is off by default.]


KEEP

Requests the cross section and the energy distribution function to be kept as global variables. This option is used if you need more flexibility in making graphs than can be obtained with the PLOT-DISTRIBUTION-FUNCTIONS option.

The energy range over which these quantities are stored is set by the Magboltz program so as to obtain optimal accuracy for the calculation of the transport parameters. This range can not be set directly from the command line. However, the maximum energy is strongly influenced by the electric field, which can therefore be used to select the range.

The names of the global variables are shown in the table below. In order not to overwrite parameters from earlier calculations, a sequential number "n" is appended to the name. The names are displayed while the command executes.

Name Quantity Unit
E_n Energy eV
F_n Energy distribution function Integral normalised to 1
CST_n Total cross section cm^2/molecule
CSE_n Elastic cross section cm^2/molecule
CSA_n Attachment cross section cm^2/molecule
CSI_n Ionisation cross section cm^2/molecule

Cross sections are given for the temperature and pressure that was specified in the gas section (or that was default).

For the conversion from cross section, given in cm2, to mean free path, one uses the relation (the constant is known as Loschmidt number):

lamba = 1 / (L x sigma)
L = 2.6867775 x 10^19 molecules / cm3

Example 1:

&GAS
magboltz argon 80 co2 1 coll 20 e-field 20000 keep
!opt log-x log-y
Call plot_frame(0.01,1e-18,100,2e-15, ...
   `Electron energy [eV]`, ...
   `Cross section [cm2]`, ...
   `Cross section in Argon 90 % CO2 20 %`)
Call plot_line(e_1,cst_1,`function-1`)       // Total
Call plot_line(e_1,cse_1,`function-2`)       // Elastic
Call plot_line(e_1,csa_1,`function-3`)       // Attachment
Call plot_line(e_1,csi_1,`function-4`)       // Ionisation
Call plot_end
This shows the various contributions to the cross section in an Argon/CO2 mixture.

Example 2:

&GAS
magboltz argon 100   co2   0   coll 20 e-field 200 keep
magboltz argon  99.5 co2   0.5 coll 20 e-field 200 keep
magboltz argon  99   co2   1   coll 20 e-field 200 keep
magboltz argon  98   co2   2   coll 20 e-field 200 keep
magboltz argon  95   co2   5   coll 20 e-field 200 keep
magboltz argon  90   co2  10   coll 20 e-field 200 keep
magboltz argon  50   co2  50   coll 20 e-field 200 keep
magboltz argon   0   co2 100   coll 20 e-field 200 keep
!opt lin-x lin-y
Call plot_frame(0,0,6,1, ...
   `Electron energy [eV]`, ...
   `Energy distribution [normalised]`, ...
   `Electron energy in Argon CO2 mixtures`)
Call plot_line(e_1,f_1,`function-1`)
Call plot_line(e_2,f_2,`function-2`)
Call plot_line(e_3,f_3,`function-3`)
Call plot_line(e_4,f_4,`function-4`)
Call plot_line(e_5,f_5,`function-5`)
Call plot_line(e_6,f_6,`function-6`)
Call plot_line(e_7,f_7,`function-7`)
Call plot_line(e_8,f_8,`function-1`)
Call plot_end
This shows the dramatic modification of the electron energy distribution that occurs when one adds tiny amounts of CO2 to pure Argon.

[This option is not on by default.]


ANALYTIC-INTEGRATION

Calls the analytic version of Magboltz, also known as Magboltz 1.

This option, which is not default, is provided mainly for backwards compatibility - Monte Carlo integration is currently believed to be superior for all gas mixtures. Analytic integration remains however the method of choice for a pure noble gas.

Several anisotropic gasss can not be used when analytic integration is requested.

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MONTE-CARLO-INTEGRATION

Calls the Monte Carlo version of Magboltz, also known as Magboltz 2.

This technique, which is believed to be more accurate than analytic integration for nearly all gas mixtures, is currently the default. For a pure noble gas, Magboltz 1, remains the preferred version. To obtain this version, specify ANALYTIC-INTEGRATION.

[This is default.]

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MOBILITY

Magboltz only computes the electron transport properties, not the ion transport properties. 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.]


PRINT

The original Magboltz output is shown when this option has been switched on.

[By default, this option is switched off.]


Go to the top level, to &GAS, to MAGBOLTZ, to the topic index, to the table of contents, or to the full text.

Formatted on 0108-03- at 19:47.