&DRIFT: INTEGRATION-PARAMETERS


accuracy

  Sets the integration accuracy. This parameter enters in the update
  of the stepsize used in drift line integration. The default value of
  this parameter is chosen for chambers with a reasonably complicated
  field. If the field is very simple, a smaller value should be chosen.

  The value of this parameter does not bear an immediate relation
  with the accuracy of the integration. Please consult the printed
  version of the manual for further details.

step

  When performing Monte Carlo drift line integration, one has the choice
  between 3 methods:

  - a constant time difference between 2 successive steps, selected
    with the MC-TIME-INTERVAL option, where the time difference between
    the steps has to be selected by the user
  - a constant spatial difference between 2 successive steps, selected
    with the MC-DISTANCE-INTERVAL option, where the distance in space
    between the steps has to be selected by the user
  - a simulation of the collision process with at each step a randomly
    selected distance between 2 collisions, based on the mean free
    path for the local electric and magnetic field, selected with the
    MC-STEPS option followed by the number of collisions over which to
    average.

  Default settings are:

  - time interval: 20 psec
  - space interval: 10 micron
  - steps to be skipped: 100

ntrap

  Sets the trap distance (in terms of wire radii). If an electron or
  ion can be attracted by a wire (this depends on the charge on the
  wire but also on the setting of the option CHECK-ALL-WIRES) and if
  the particle passes closer by the wire than a distance of

  ntrap*radius

  then the electron or ion is considered to be caught by the wire.

  From the moment a wire is considered caught by a wire, a dedicated
  integration algorithm takes over which is better at estimating the
  residual drift time than the default algorithm.

  [This parameter is preset to a value of the order of 2-5 (depending
  on program version). This can be too large if the wires are very
  thick but it may as well be too small for very thin wires.]

CHECK-ALL-WIRES

  Depending on their charge, wire can either attract a particle,
  repel it or have no effect. Wires can also have a multipole moment
  which makes them attractive from one side and not from another.

  If the CHECK-ALL-WIRES option is in effect, then all wires, no
  matter their charge, are considered able to catch a particle. As
  soon as a particle comes closer to any wire than the trap radius
  (see: ntrap) an attempt will be made to drift it to the wire.

  This is meaningful if you have e.g. dipole (q=0) type wires, but
  this is harmful if you particles pass near repelling wires, such
  as gating grids. When not needed, this option also wastes a lot
  of CPU time.

  [This option is on by default.]

CHECK-ATTRACTING-WIRES

  When the CHECK-ATTRACTING-WIRES option is in effect, a particle
  will not be considered caught when it comes closer than the trap
  radius to a wire that is charged such that it can not attract
  the particle that is drifting.

  This is usually the recommended mode but there are cases, such
  as the presence of wires with almost no net charge, but with a
  multipole moment, where the alternative is better suited.

  [This option is NOT default.]

REJECT-KINKS

  This option requests drift line calculation to be aborted if
  the drift line makes a bend sharper than 90 degrees. Such bends
  rarely occur in smooth fields, the most common case is a drift
  line that tries to cross a saddle point. The REJECT-KINKS option
  will ensure that the drift line doesn't repeatedly go back and
  forth across the saddle point.

  Since fields obtained with finite element methods occasionally
  have areas with very uneven fields, it may be advisable in such
  cases to switch the option off.

  [The option is on by default.]

ncloud

  Sets the distance from the wire (in multiples of the wire radii)
  at which the integration routine for combined longitudinal and
  transverse diffusion changes from accumulating the diffusion
  covariance matrix to projecting the accumulated probability
  distribution onto the target wire.

  This parameter is preset to a value of the order of 2-5.

method

  When both transverse and longitudinal diffusion have been entered
  in the gas section, the diffusion is calculated by propagating a
  cloud along the drift line, adjusting the dimensions at each step
  according to the following phenomena:

  - longitudinal stretch due to acceleration and decelaration
    of the electrons along the central drift line
  - transverse compression due to convergence of drift lines
    neighbouring to the central drift line
  - additional transverse and longitudinal diffusion at each
    step, according to the local diffusion coefficients.

  The cloud is considered Gaussian far from the wires.

  When the drift line approaches the wire, the cloud as a whole
  is projected onto the wire. For this phase, various algorithms
  are available put at your disposal:

  NO-PROJECTION
       No special treatment when approaching the wire, hence
       the value of ncloud is not relevant.

  INTEGRATION
       As soon as the cloud enters the 'ncloud' zone,
       the following is done:

       Complete integration over the cloud of the local
       distance to the wire divided by the local drift
       velocity.

       The longitudinal diffusion over the remaining
       distance to the wire is added to the estimate.

  CENTRAL-VELOCITY-INTEGRATION
       As soon as the cloud enters the 'ncloud' zone, an
       integration similar to INTEGRATION is carried out,
       but the drift velocity is always taken to be the
       drift velocity at the centre of the cloud.

       The longitudinal diffusion over the remaining
       distance to the wire is added to the estimate.

       This is currently the default method.

  LONGITUDINAL-DIMENSION
       When the cloud center enters the 'ncloud' zone,
       the dimension of the cloud over a line through
       cloud center and wire center is taken as measure
       of diffusion spread.

       The longitudinal diffusion over the remaining
       distance to the wire is added to the estimate.

       The longitudinal dimension is in principle the
       dimension that matters, but in the presence of
       a strong magnetic field, the cloud rapidly rotates
       near the wire. At the same time, the cloud stretches
       to the point of becoming almost one-dimensional.
       A small rounding error in the cloud alignment, can
       make the dimension along the axis pointing to the
       wire, very small.

       For this reason, this method is not recommended,
       unless the cloud trap radius is very large (in which
       case the velocity estimates are likely to be
       inaccurate).

  LARGEST-DIMENSION
       This is similar to LONGITUDINAL-DIMENSION but
       the cloud size is taken to be the largest cross
       section of the cloud.

       The longitudinal diffusion over the remaining
       distance to the wire is added to the estimate.

       For reasons explained under LONGITUDINAL-DIMENSION,
       this method must be considered superior, provided
       the cloud-trap radius is small.

eps

  A step is subdivided if the difference between the first and
  second order estimates differ more than a fraction epsilon of
  the total first order estimate without subdivisions.

  The default is 1.0E-3.

stack

  The stack depth is the maximum number of subdivisions allowed
  during the integration, in order to achieve the requested
  accuracy.

  For diffusion coefficients, the stack depth usually hardly matters
  since the coefficient does not make big jumps. For the Townsend
  coefficient, which suddenly grows from 0 to an appreciable value,
  the stack depth is a critical parameter in the accuracy of the
  computation. Although CPU time can go up rapidly with stack depth,
  it is a good idea to keep a large value: when not needed, no use
  of the stack is made.

  Default is MXSTCK, usually set to 20, which is also the maximum.
  The smallest permitted value is 1 and this setting will usually
  already give a reasonable accuracy. The default stack depth is
  large and may result in excessively lengthy computations.

DRAW-ISOCHRONES

  Requests isochrones to be drawn as lines, rather than marked.

  When this option is selected, you may also wish to inspect the
  settings of the other isochrone related options.

  [By default, isochrones are drawn as lines.]

MARK-ISOCHRONES

  Requests marking the points on the isochrones.

  If this option is active, no sorting needs to be done. Hence,
  the other isochrone options are ignored. Plotting isochrones
  is fast with this option switched on.

  [By default, isochrones are drawn as lines.]

SORT-ISOCHRONES

  Depending on the source of the points that serve to draw
  the isochrones, they can be in some definite order or not.

  By setting SORT-ISOCHRONES, an attempt is made to order
  the points in such a manner that the isochrones appear as
  reasonably smooth lines. Any attempt to do so is likely
  to fail for certain cases. Moreover, sorting can take a
  large amount of computing time - the problem is related
  to the notorious "traveling salesperson problem (TSP)".

  Garfield, for these reasons uses a simple algorithm to
  sort the points on a contour: each contour is classified
  as being either linear or arcs. Linear contours are
  sorted along the main axis, points on arcs by angle with
  respect to the centre of gravity. Arcs that appear to be
  nearly full loops are drawn as closed contours, otherwise
  as an open arc.

  The sorting algorithm by itself is fast - the check on
  intersects between isochrones and drift lines in contrast
  is fairly time consuming.

  Sorting is not useful (hence potentially harmful) when
  the drift lines come from a track on which the points are
  ordered - which is usually the case. The sort is useful
  on the other hand for drift lines starting from wires or
  other electrodes.

  The SORT-ISOCHRONES option is ignored when MARK-ISOCHRONES
  is in effect.

  [By default: sort done]

ISOCHRONE-CONNECTION-THRESHOLD

  Points on an isochrone are only joined if they are less than a
  fraction iso_thr away from each other on the screen. Points
  that can not be connected are shown by a marker.

  The fraction iso_thr can be set to any value between 0 (only
  markers) and 1 (isochrones are only interrupted by drift line
  crossings).

  Selecting NOISOCHRONE-CONNECTION-THRESHOLD is tantamount to
  setting iso_thr to 1.

  [Initial setting: 0.2]

ISOCHRONE-ASPECT-RATIO-SWITCH

  When an isochrone appears to be more or less circular, its
  points are ordered by increasing angle with respect to the
  centre of gravity. If the isochrone, on the other hand, seems
  to be more or less linear, the points are ordered along the
  longest principal axis of the distribution.

  Whether the set is circular or linear is decided by computing
  the RMS in the two principal axes of the point distribution.
  If the ratio of these two RMS's exceeds iso_aspect, then the
  isochrone is assumed to be linear, otherwise circular.

  [Initial setting: 3]

ISOCHRONE-LOOP-THRESHOLD

  Isochrones that appears to be circular (rather than linear,
  see ISOCHRONE-ASPECT-RATIO-SWITCH for the conditions under
  which this happens) are closed if the largest distance
  between 2 points doesn't exceed a fraction iso_loop of the
  total length of the isochrone.

  [Initial setting: 0.2]

CHECK-ISOCHRONE-CROSSINGS

  Requests ensuring that drift lines do not cross isochrones.

  Such crossings can for instance occur if the drift lines
  from a track flow left or right of an intermediate object,
  which itself also attracts some electrons, to a wire located
  behind the object.

  This check is fairly time consuming.

  [By default: check done]

Keyword index. Formatted on 10/11/98.