&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 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.]

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 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.

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 os 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.

  [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.

  [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:

  - 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.

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.

  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.