Additional information on:
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.]
In either case, you can set the number of points with N-E or N-E/P (which are synonyms) and you have the choice between linear and logarithmic spacing of the points.
In either case, ensure that all fields are strictly positive and that they are in strictly increasing order.
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.]
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.
You can set the number of points with N-B.
In either case, ensure that the fields are expressed in Tesla, also be sure that they are strictly positive.
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.]
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:
Ensure that the range is entirely contained in [0\°,\ 90\°].
You can set the number of angles with N-ANGLES.
In either case, ensure that the angles are expressed in degrees, also be sure that they are in the range [0\°,\ 90\°].
It is advisable to use this format, with ANGLE=90, if your chamber is such that E is always perpendicular to B.
It is bad practice to set ANGLE=0 in TPCs where E is parallel with B in the drift volume. Other orientations occur in the read-out chambers, and these orientations have impact on the resolution of the TPC.
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 angles of 0\°, 30\°, 60\° and 90\° between the E and B field.]
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 function is plotted with representation FUNCTION-1,
[This option is off by default, but it is switched on automatically in case there are concerns about the validity of the energy range.]
Plotted is the quantity used inside Magboltz: the stp cross section scaled by pressure and temperature, multiplied with the Loschmidt number - a quantity that can be interpreted as the reciprocal of the mean free path.
This option is provided to enable a rapid inspection of the cross sections. The KEEP option is recommended to make better quality graphs.
The total cross section is plotted with representation FUNCTION-1, the elastic cross section with FUNCTION-2, the attachment cross section with FUNCTION-3 and the ionisation cross sections for the various components with FUNCTION-4.
[This option is off by default.]
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 cm\², to mean free path, one uses the relation (the constant is known as Loschmidt number):
\λ = 1 / (L \× \σ) L = 2.6867775 \× 10\<SUP\>19\</SUP\> molecules / cm\³
Example 1:
&GAS magboltz argon 80 co2 1 coll 20 e-field 20000 keep !options 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_endThis shows the various contributions to the cross section in an argon/CO\<SUB\>2\</SUB\> 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 !options 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_endThis shows the dramatic modification of the electron energy distribution that occurs when one adds tiny amounts of CO\<SUB\>2\</SUB\> to pure argon.
[This option is not on by default.]
This option, which is not default, is provided mainly for backwards compatibility, and only in Garfield up to version\ 8. Monte Carlo integration is currently believed to be superior for all gas mixtures. Analytic integration may still be the method of choice for a pure noble gas.
Several anisotropic gases can not be used when analytic integration is requested.
This option does not exist anymore in Garfield versions\ 9 and higher, which call Magboltz version\ 7 and higher. Magboltz version\ 7 can deal with pure noble gases - but it will be very slow.
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This technique, which is believed to be more accurate than analytic integration for nearly all gas mixtures, is default. Pure noble gases seem to be the only exception to this rule.
Garfield up to version\ 8 maintained an interface to both the analytic integration method from Magboltz version\ 1 and the Monte Carlo method from Magboltz version\ 2. Garfield version\ 9 is interfaced only with Magboltz version\ 7, which uses Monte Carlo techniques, since the format of the gas descriptions used by this Magboltz version is no longer compatible with Magboltz version\ 1.
[Monte Carlo integration is default.]
Additional information on:
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 cm\²/\μsec.V.
[By default: no mobility.]
This output contains amongst others the ionisation, attachment and excitation rates which are needed if one wishes to correct the Townsend coefficients for Penning transfers. One also finds there the energy distribution.
This output tends to be long and it may therefore be advisable to write it to a file:
&GAS pressure 3 bar temperature 25 C > magboltz.rates magboltz argon 90 co2 10 ... e-field 100 200 500 1000 2000 5000 10000 20000 50000 100000 ... coll 3 ... print >
[By default, this option is switched off.]
Formatted on 21/01/18 at 16:55.