![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
This parameter enters in the update of the step size used in drift line integration. The initial value of this parameter is suitable for chambers with a size of several cm's which have a reasonably complicated field structure. If the field is very simple, or if the chamber is very small, then a smaller value should be chosen.
This parameter has no effect on Monte_Carlo integration.
[This parameter is initially set to 1E-8.]
The Runge_Kutta_Fehlberg drift line integration method automatically updates the step length at each step. When traversing a large area with a very smooth field, the step size becomes large. If this is not desired, for instance because there is a fine structure behind the smooth area, then one should limit the step size with this option.
Recommended value: a maximum step length of order 1/10th-1/20th of the distance to be traversed.
This parameter has no effect on Monte_Carlo integration.
[By default, there is no limit to the step size.]
Default: 20 psec
This is the most commonly used method since the same settings can be used for both electrons and ions.
Default: 10 micron
Default: 100 steps
These parameters have no effect on Runge_Kutta_Fehlberg integration.
Excessively small and large scaling factors are indicative of a locally too large step_size. This is commonly the case in the vicinity of small electrodes such as wires. Garfield therefore reduces the step size locally if the scaling is found to lie outside the range given.
The minimum of the scaling range has to be larger than or equal to 0. The maximum has to be larger than 1.
Using the default range, one effectively uses the expressions for diffusion spread in the limit of infinitely small steps. This is not appropriate in case the mean free path is substantially larger than a few microns.
[Default: 0.95 as minimum and 1.05 as maximum.]
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.]
For the lower orders, you may also specify the keywords LINEAR, QUADRATIC or PARABOLIC and CUBIC.
Although values between 1 and 10 are accepted, orders larger than 2 are not recommended since they tend to lead to oscillation.
[Default: 2, parabolic interpolation.]
Reasons why a track can't be interpolated for a point are:
Computation time can increase dramatically if many points need to be computed.
If you specify ABANDON-IF-INTERPOLATION-FAILS, then such points are assigned a status of "Abandoned", a drift time, diffusion, multiplication, loss and incidence angle of 0.
[By default, drift lines are computed when needed.]
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, 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. This option also interferes with calculation of signals due to ions drifting from the neighbourhood of the sense wires.
When not needed, this option also wastes a lot of CPU time.
[By default, only attracting wires are checked.]
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 on by default.]
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.
This option has effect when drift line integration is performed using the Runge_Kutta_Fehlberg method, but not when the Monte_Carlo method is used.
[The option is on by default.]
[This parameter is preset to a value of the order of 2-5.]
When the drift line approaches the wire, the cloud as a whole is projected onto the wire. For this phase, various algorithms are put at your disposal:
Additional information on:
[The default is 1.0E-3.]
This parameter should be set to a small value (1 or 2) if the field is interpolated in a finite element map because the electric field, and hence the quantity to be integrated, is usually discontinuous across element boundaries.
If the field is computed from analytic formulae, then the maximum stack depth has usually little impact on the integration of the diffusion coefficients, since the latter are normally fairly smoothly varying quantities.
For the integration of the Townsend coefficient however, 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 2, the maximum is given by MXSTCK, usually set to 5. 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.]
As a result, integrating quantities such as the Townsend and the attachment coefficient over Runge Kutta Fehlberg paths leads to smaller multiplications and larger losses than those obtained integrating over Monte Carlo paths, while the integral over the latter is ill defined since it depends on the step size.
When the PROJECTED-PATH-INTEGRATION integration parameter is active, the Townsend and attachment coefficients are integrated over path segments projected on the local drift velocity vector. This ensures that the path length integral doesn't depend on the step size used in Monte Carlo integration.
This option has effect on
[Default is TRUE-PATH-INTEGRATION.]
When this option is selected, you may also wish to inspect the settings of the other isochrone related options.
This action of this option can reversed with the MARK-ISOCHRONES option.
Isochrones are drawn using the ISOCHRONES polyline representation.
[By default, isochrones are drawn as lines.]
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.
This action of this option can reversed with the DRAW-ISOCHRONES option.
Isochrones are marked using the ISOCHRONES polymarker representation.
[By default, isochrones are drawn as lines.]
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 "travelling 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. Whether an isochrone is sorted as a line or as an arc depends on the setting of ISOCHRONE-ASPECT-RATIO-SWITCH. Whether 2 points on an isochrone are connected or not depends on the setting of ISOCHRONE-CONNECTION-THRESHOLD.
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]
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]
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.
This switch has effect only when SORT-ISOCHRONES has been switched on.
[Initial setting: 3]
[Initial setting: 0.2]
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]
Formatted on 0108-03- at 19:47.