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&GAS


overview

The set of useful instructions in the gas section depends of the origin of the gas data:

A couple of instructions can be used regardless of the origin.

Gas mixture prepared during the run:

Command Short description
ADD Adds/replaces elements of the transport table
CLUSTER Enters the cluster size distribution
EXTRAPOLATIONS Extrapolation of the gas tables
HEED Prepares cluster generation by Heed
INTERPOLATIONS Interpolation method in the gas tables
MAGBOLTZ Magboltz gas mixture (accurate)
MIX Schultz-Gresser gas mixing (approximate)
PARAMETERS Molecular parameters of the gas mixture
PRESSURE Sets the pressure
TEMPERATURE Sets the temperature
WRITE Stores the gas description

User specified gas mixture:

Command Short description
ADD Adds/replaces elements of the transport table
CLUSTER Enters the cluster size distribution
EXTRAPOLATIONS Extrapolation of the gas tables
GAS-IDENTIFIER Adds a label to the gas description
INTERPOLATIONS Interpolation method in the gas tables
PARAMETERS Molecular parameters of the gas mixture
PRESSURE Sets the pressure
RESET Erases gas data entered sofar
TABLE Enters the gas tables
TEMPERATURE Sets the temperature
WRITE Stores the gas description

Built-in gas mixtures with fixed proportions:

Command Short description
ARGON-20-ETHANE-80 Loads the mixture Argon 20 %, ethane 80 %
ARGON-50-ETHANE-50 Loads the mixture Argon 50 %, ethane 50 %
ARGON-80-ETHANE-20 Loads the mixture Argon 80 %, ethane 20 %
ARGON-73-METH-20-PROP-7 Loads Argon 73 %, CH4 20 %, propanol 7 %
CO2 Loads data for almost pure CO2
CO2-80-ETHANE-20 Loads the mixture CO2 80 %, ethane 20 %
CO2-90-ETHANE-10 Loads the mixture CO2 90 %, ethane 10 %
CO2-90-ISOBUTANE-10 Loads the mixture CO2 90 %, isobutane 10 %
ETHANE Loads data for pure ethane
ISOBUTANE Loads data for pure isobutane
METHANE Loads data for pure methane
PRESSURE Sets the pressure

Retrieval of a gas description previously stored:

Command Short description
GET Retrieves gas data from a dataset

General purpose instructions:

Command Short description
OPTIONS Plotting and printing of the gas tables
PLOT-OPTIONS Selects plots, sets ranges and log/lin axes


built_in

A set of gasses ready for use, supplied by users of the program, usually based on private or published measurements. These descriptions are provided mainly for rapid studies that do not require high accuracy.

Please ensure, with the GAS-PRINT and GAS-PLOT options, that the tables agree with what you think is reasonable.

These mixtures have fixed proportions, use MAGBOLTZ or MIX to obtain tables for arbitrary proportions.

Example:

ARG-50-ETH-50

(Until further notice, the program will use 50 % Argon, 50 % Ethane.)

Additional information on:
  
 


ADD

Adds/replaces one or more elements to/of the gas table. This instruction can be used to add for instance the ion mobility to transport tables that have been prepared by Magboltz, but can also be used to override the computed parameters with measured parameters.

The ADD command has 2 formats:

Beware that the WRITE command executes only when the section is left. Therefore, if you modify Magboltz computed gas tables with the ADD command, the modified tables will be written - not the original Magboltz data, no matter where you place the WRITE and ADD statements,

REPLACE is a synonym for ADD.

Format:

ADD  item_1 { function_1 | value_1 VS ep_1 [ORDER order] } ...

Example:

Global pbar = 3
magboltz argon 91 nitrogen 4 methane 5 ...
   e/p-range 0.05 135
Read-vector E_Ar_Ar K_Ar_Ar
   0  1.53
   8  1.53
  10  1.53
  12  1.53
  15  1.52
  20  1.51
  25  1.49
  30  1.47
  40  1.44
  50  1.41
  60  1.38
  80  1.32
 100  1.27
 120  1.22
 150  1.16
 200  1.06
 250  0.99
 300  0.95
 400  0.85
 500  0.78
 600  0.72
 800  0.63
1000  0.56
1200  0.51
1500  0.46
2000  0.40

Global E_Ar_Ar = E_Ar_Ar/(0.010354*300) Global K_Ar_Ar = K_Ar_Ar*1e-6/pbar add ion-mobility K_Ar_Ar vs E_Ar_Ar extrapolation low-ion-mobility constant high-ion-mobility linear

Magboltz is used to generate an electron transport table. This also sets the E/p scale.

Next, a file is read in that contains mobilities as function of E/N for Ar+ ions in Ar at a pressure of 1 atm. The data is taken from the literature, in this case Hornbeck '51 and Beaty '68 (for an extensive compilation consult the H. W. Ellis et al. papers). The E/N values are stored in the matrix E_Ar_Ar, while the mobilities are kept in K_Ar_Ar.

The E/N vector is transformed to E/p. The mobility is divided by the pressure, and its units is changed from cm2/sec.V to cm2/microsec.V.

Finally, the mobility is added to the gas tables using the ADD statement.

References:

[1]
H. W. Ellis, R. Y. Pal and E. W. McDaniel, Transport properties of gaseous ions over a wide energy range, At. Data and Nucl. Data Tables 17 (1976) 177-210.
[2]
H. W. Ellis et al., Transport properties of gaseous ions over a wide energy range, part II, At. Data and Nucl. Data Tables 22 (1978) 179-217.
[3]
H. W. Ellis et al., Transport properties of gaseous ions over a wide energy range, part III, At. Data and Nucl. Data Tables 31 (1984) 113-151.
[4]
J. A. Hornbeck, The drift velocities of molecular and atomic ions in Helium, Neon and Argon, Phys. Rev. 84 (1951) 615-620.
[5]
E. C. Beaty, Proc 5th International conference on ionisation phenomena in gasses, München (1961), Vol 1, p 183, North Holland. Phys. Rev. 170 (1968) 116.

Additional information on:
  


CLUSTER

Enters the cluster-size distribution. You may choose between entering a parametrisation, listing of all probabilities and a mixture of the two formats.

The cluster size distribution is not by itself enough to generate clusters along a track. The clustering model based on the distribution entered here, also need to know the cluster spacing, which can be set with the MEAN keyword of the PARAMETERS command.

If you use the HEED interface, then you probably neither need to enter a cluster size distribution nor the cluster spacing. Entering a cluster size distribution and initialising Heed is however allowed. It is only at the TRACK level that you decide which clustering model you are going to use.

Additional information on:
  
 


EXTRAPOLATIONS

Indicates how the gas-tables have to be extrapolated to E/p values smaller and larger than those present in the table.

The EXTRAPOLATIONS command has no effect on extrapolation in 2-dimensional tables, such as those produced by Magboltz when the B field is non-zero. For such tables, polynomial extrapolation is performed with the order set with the INTERPOLATIONS command.

Format:

EXTRAPOLATIONS item1 method1 item2 method2 ...

Examples:

EXTR DRIFT: LINEAR, DIFF: CONST, TOWN: CONST
EXTR DRIFT EXP

Additional information on:
  


GAS-IDENTIFIER

Assigns an identification string to the gas. This string is placed on plots when gas data has been used to make the plot.

Format:

GAS-IDENTIFIER string

Example:

GAS-ID "Some gas"

GET

Retrieves a compact format gas description written by WRITE. This command clears gas information you may have entered already. It is executed immediately and you may, with caution, replace some of the elements of the description after issuing the command.

The compact gas description contains electron transport property tables, the ion mobility, cluster size and cluster spacing data, Heed initialisation information, the pressure and the temperature. GET overwrites all of these.

Format:

GET dsname [member]

Example:

GET gas_data.dat gas2

Additional information on:
  


HEED

Invokes the gas definition part of the Heed program, which simulates the energy loss through ionisation of a particle that traverses the gas. This initialisation, which is very fast, is mandatory if you intend to use the HEED clustering model of the TRACK command. Heed initialisation is performed automatically when reloading (GET) a gas for which an Heed initialisation has been performed.

The temperature and the pressure should be specified before issuing the HEED command. See TEMPERATURE and PRESSURE for default settings.

Neither temperature nor pressure scaling is applied to the cluster information provided by HEED.

The author of Heed, Igor Smirnov, should be contacted for further information about this program.

Format:

HEED  [ ARGON  fraction ]      [ HELIUM-4  fraction ] ...
      [ NEON  fraction ]       [ XENON  fraction ] ...
      [ KRYPTON  fraction ]    [ METHANE   fraction ] ...
      [ ETHANE  fraction ]     [ ETHENE  fraction ] ...
      [ ACETYLENE  fraction ]  [ ISOBUTANE  fraction ] ...
      [ PROPANE  fraction ]    [ NEOPENTANE  fraction ] ...
      [ HYDROGEN  fraction ]   [ NITROGEN  fraction ] ...
      [ WATER  fraction ]      [ CO2  fraction ] ...
      [ CF4  fraction ]        [ OXYGEN  fraction ] ...
      [ DME  fraction ]        [ NITROUS-OXIDE  fraction ] ...
      [ AMMONIA  fraction ]

Example:

pressure {3*760}
Heed argon 50 ethane 50

(If you have a 3 atm 50/50 Argon-ethane mixture in your chamber.)


INTERPOLATIONS

The gas tables are usually interpolated with cubic splines. Sometimes splines oscillate too much and then you may prefer to use polynomial interpolation instead.

Format:

INTERPOLATIONS item1 method1 item2 method2 ...

Examples:

INTERP DRIFT-VELOCITY NEWTON 2,  LONG-DIFFUSION NEWTON 1
INT TOWNSEND SPLINE

Additional information on:
  


MAGBOLTZ

Invokes the Magboltz program to compute for electrons, the drift velocity, the longitudinal and transverse diffusion coefficients, the Townsend and attachment coefficients and the Lorentz angles if there is a magnetic field.

Since Magboltz takes the magnetic field into account to compute the transport properties, the &MAGNETIC section should precede the gas section.

Likewise, TEMPERATURE and PRESSURE statements should be issued before invoking Magboltz. If the temperature has not been specified when Magboltz runs, then a default temperature of 300 K will be assumed. No scaling will be applied if the temperature is changed later on. The default pressure is 760 Torr. The transport properties will be scaled according to simple scaling laws if the pressure is modified after the transport properties have been computed. It is not recommended, however, to rely on these scaling laws since these are very approximate.

The author of Magboltz, Steve Biagi, should be contacted for further information about this program.

Format:

MAGBOLTZ [ ARGON  frac ]            [ HELIUM-3  frac ] ...
         [ HELIUM-4  frac ]         [ NEON  frac ] ...
         [ XENON  frac ]            [ KRYPTON  frac ] ...
         [ METHANE  frac ]          [ ETHANE  frac ] ...
         [ ETHENE  frac ]           [ ACETYLENE  frac ] ...
         [ NITROGEN-A  frac ]       [ NITROGEN-B  frac ] ...
         [ AMMONIA  frac ]          [ OXYGEN  frac ] ...
         [ ISOBUTANE  frac ]        [ WATER  frac ] ...
         [ CO2  frac ]              [ METHYLAL-HOT  frac ] ...
         [ METHYLAL-COLD  frac ]    [ PROPANE  frac ] ...
         [ NEOPENTANE frac ]        [ CF4  frac ] ...
         [ DME  frac ]              [ NITRIC-OXIDE  frac ] ...
         [ NITROUS-OXIDE  frac ] ...

[ E/P-RANGE epmin epmax ] [ N-E/P nep ] ... [ LINEAR-E/P-SCALE | LOGARITHMIC-E/P-SCALE ] ... [ ANGLE-RANGE amin amax ] [ N-ANGLE nangle ] ... [ NOPLOT-DISTRIBUTION-FUNCTIONS | ... PLOT-DISTRIBUTION-FUNCTIONS ] ...

[ SECOND-ORDER-TERMS | FIRST-ORDER-TERMS | ORDERS n ] ... [ NOITERATE-ALPHA | ITERATE-ALPHA ] ... [ SWITCH [alpha/pressure] | NOSWITCH ] ... [ F0-TRANSVERSE-DIFFUSION | H1-TRANSVERSE-DIFFUSION | ... MEAN-ENERGY-TRANSVERSE-DIFFUSION ] ... [ F0-LONGITUDINAL-DIFFUSION | H1-LONGITUDINAL-DIFFUSION | ... G0-LONGITUDINAL-DIFFUSION ] ...

[ MOBILITY mob ]

Example:

magboltz argon 50 ethane 50

(The gas in your chamber will be 50 % Argon and 50 % Ethane.)

Additional information on:
  


MIX

Computes the drift velocity and diffusion for a mixture of gasses. The calculations are based on the work of G. Schultz and J. Gresser NIM 151 (1978) 413-431 and use parametrised cross section and energy loss data provided by Fabio Sauli and Anna Peisert.

The main limitation of this method is that it neglects ionisation effects and that it treats excitation inaccurately. This implies that the results are not valid for large E/p values, i.e. close to the wires.

Another limitation is that these calculations neglect the magnetic field - this is not an inherent limitation of the method, but there is no intention to invest further effort in this instruction. Garfield nowadays has an interface to the Magboltz program of Steve Biagi which is superior in accuracy to this instruction. See MAGBOLTZ.

Format:

MIX [ ARGON  frac ]          [ HELIUM  frac ] ...
    [ METHANE  frac ]        [ ETHANE  frac ] ...
    [ NEON  frac ]           [ NITROGEN  frac ] ...
    [ ISOBUTANE  frac ]      [ XENON  frac ] ...
    [ CO2  frac ]            [ METHYLAL  frac ] ...
    [ KRYPTON  frac ]        [ AMMONIA  frac ] ...

[ MINIMUM-ENERGY emin ] ... [ MAXIMUM-ENERGY emax ] ... [ STEPSIZE-ENERGY estep ] ...

[ CRITICAL-F0-FRACTION frcrit ] ...

[ E/P-RANGE epmin epmax ] ... [ N-E/P n ] ... [ LINEAR-E/P-SCALE | LOGARITHMIC-E/P-SCALE ] ...

[ PLOT-F0 | NOPLOT-F0 ] ... [ PLOT-ENERGY-LOSS | NOPLOT-ENERGY-LOSS ] ... [ PLOT-CROSS-SECTION | NOPLOT-CROSS-SECTION ] ... [ PLOT-PATH | NOPLOT-PATH ] ...

[ PRINT-TABLES | NOPRINT-TABLES ] ...

[ MOBILITY mob ] ... [ TOWNSEND-COEFFICIENT alpha/p ] ... [ ATTACHMENT-COEFFICIENT beta/p ]

Example:

mix argon 50 ethane 50

The gas in your chamber will be 50 % Argon and 50 % Ethane.

Additional information on:
  


OPTIONS

Enters global and local options. Only the local options are listed here. For global options, see the top level OPTIONS command.

Format:

OPTIONS [NOGAS-PLOT | GAS-PLOT] ...
        [NOGAS-PRINT | GAS-PRINT]

Example:

OPT NOG-PL G-PR

Additional information on:
  


PARAMETERS

Various parameters of the gas, such as the number of clusters per cm, are entered with this command.

The mean number of clusters is used if the clustering model you plan to choose with the TRACK command, is based on the cluster size distribution entered with CLUSTER.

The rest of the data is only used for a simple backup estimate of the cluster size distribution if neither a CLUSTER nor a HEED statement has been issued.

Format:

PARAMETERS [A a] [Z z] [RHO density] [E-PAIR epair] ...
           [E-MOST-PROBABLE emprob] ...
           [MEAN mean_number_of_clusters] ...
           [TRANSVERSE-ION-DIFFUSION sigma_T] ...
           [LONGITUDINAL-ION-DIFFUSION sigma_L]

As shown in the format description, several lines may be used although a single line is perfectly acceptable as well.

Example:

PARA MEAN 20

This format could be used if you wish to compute arrival time spectra and enter the cluster size distribution with CLUSTER.

Additional information on:
  
 


PLOT-OPTIONS

Selects the plots to be made in response to the GAS-PLOT option, when leaving the gas section. The PLOT-OPTIONS command also controls the range of the vertical axes of these plots and lets you choose linear or logarithmic scales for both axes.

Several plots may be modified in a single statement.

Use DRIFT_VELOCITY and related procedures to have full control over the presentation of the plot.

Format:

PLOT-OPTIONS [plot [options]] ...

Example:

plot-options drift lin-x log-y nodiff nocluster

(Requests a linear E/p axis and a logarithmic drift velocity axis, the opposite of the default. The diffusion coefficients and the cluster size distribution are not plotted.)

Additional information on:
  


PRESSURE

Sets the pressure of the gas.

The pressure is used by the gas mixing instructions (MIX and MAGBOLTZ) and also by HEED. Please be sure to specify the pressure before issuing these commands.

If you specify the pressure after a mixing command, then the tables will be prepared for standard atmospheric pressure and the conversion to the pressure you specify will be done by relying on the simple scaling laws.

Format:

PRESSURE pressure [unit]

Example:

pressure 2 bar

Additional information on:
  


REPLACE

REPLACE is a synomym of ADD.

RESET

Performs a reset of all gas-data entered so far. This instruction is only available in interactive mode.

Format:

RESET

TABLE

Enters the tables of electron transport properties and of the ion mobility, as function of E/p.

Each of the table entries (drift velocity, ion mobility, Lorentz angle, diffusion coefficients, Townsend and attachment coefficients) can either be tabulated after the TABLE line, or be given as a parametrisation on the TABLE line.

All tabulated entries must be specified at a common set of E/p values, which must itself be listed in the table if at least one transport property is tabulated. Use ADD if the data that you wish to use for one or more entries, is tabulated at a different set of E/p values.

The order of the tabulated entries, is indicated on the TABLE line by listing the names of the entries (such as DRIFT-VELOCITY) in the same sequence as in the table. The entry names should not be followed by parametrisations. The place of the E/p values should be indicated by 'E/P'. There is no prefered order of the entries.

The entries are interpolated, and if necessary extrapolated, using methods that can be set with the INTERPOLATIONS and EXTRAPOLATIONS statements.

Parametrisations should be functions of the symbolic parameter EP, which equals E/p. They should be entered by placing the name of the entry (e.g. LORENTZ-ANGLE) before the function.

The parametrisations that you enter are not stored as functions, rather they are evaluated at the E/p values in the table and the list thus obtained will be interpolated when transport properties are required, like for tabulated entries.

If all entries are entered in parametrised form, then you can either establish the list of E/p values by tabulating them after the TABLE statement, or use the RANGE and N keywords. These keywords are ignored if at least one entry is tabulated.

Quantities that obey simple pressure scaling laws, have to be entered multiplied by the appropriate power of the pressure.

Format:

TABLE [E/P] ...
      [DRIFT-VELOCITY [drift]] ...
      [ION-MOBILITY [mobility]] ...
      [LORENTZ-ANGLE [angle]] ...
      [LONGITUDINAL-DIFFUSION-COEFFICIENT [diff]] ...
      [TRANSVERSE-DIFFUSION-COEFFICIENT [diff]] ...
      [TOWNSEND-COEFFICIENT [townsend]] ...
      [ATTACHMENT-COEFFICIENT [attach]] ...
      [DUMMY [dummy]] ...
      [RANGE epmin epmax] [N nmax]

This line is followed by tables for those items that are not functions. The end of the table is signalled by a blank line.

Example:

TABLE  DRIFT=100*EP, DIFF, E/P
0.3 0.1
0.1 0.2
0.1 0.5
0.2 1.0
0.3 2.0

(The drift velocity is entered as the function 100*E/p which is evaluated at the E/p values listed in the second column. The longitudinal diffusion is listed in the first column. The ion mobility, the Lorentz angle and the Townsend and attachment coefficients are not specified. Note the blank line at the end of the table.)

Additional information on:
  
 


TEMPERATURE

Sets the temperature of the gas.

The temperature is used by the gas mixing instructions (MIX and MAGBOLTZ) and also by HEED. Please be sure to specify the temperature before issuing these commands.

The temperature is not needed if both the transport properties and the clustering properties have been entered via tables.

Garfield applies, if required, pressure scaling of the transport properties but does not apply temperature scaling laws.

Format:

TEMPERATURE temp [unit]

Example:

TEMPERATURE 300 K

For room temperature.

Additional information on:
  


WRITE

Writes a compact format gas dataset to be read by the GET instruction.

The use of this instruction is strongly recommended when you compute the electron transport properties with MAGBOLTZ or with MIX, both of which consume a lot of CPU time. WRITE is not of interest if you enter the transport parameters of your gas description with a TABLE statement.

The dataset contains initialisation information for Heed, which will automatically be performed when re-reading the file with GET.

The format of the compact dataset is subject to modification and backwards compatibility is not guaranteed. Compact datasets that can no longer be read and that are of value, can however be sent to the author of Garfield for recovery.

Files written with WRITE should in principle not be edited. These files are also not meant to be easily readable, use the GAS-PRINT option or procedures like DRIFT_VELOCITY to obtain legible tables.

Writing takes place while the section is being left, not when the WRITE command is issued. The statement can appear at any place in the gas section.

Format:

WRITE   DATASET dsname [member]   [REMARK remark]

Examples:

WR DATA 'vaxgarf"garfield lasagna"::[garfield]gas.dat' REM "Test gas"

(The keywords DATASET and REMARK are required in this example since the member is omitted. The single quotes are needed because of the double quotes in the remote login string and also because of the two colons.)

Additional information on:
  
 


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Formatted on 0100-08-27 at 04:53.