&FIELD

overview

• setting various parameters
• plotting and printing
• understanding the field
• service instructions

The use of this section is fairly straight-forward since it has only one frequently used instruction: PLOT-FIELD.

Setting parameters:

Plotting and printing:

Understanding:

Service instructions:

Notes:

1. There are several instructions in the optimisation section that perform related tasks.
2. There are also procedure calls that perform related tasks: ELECTRIC_FIELD, ELECTRIC_FIELD_3, MAGNETIC_FIELD, MAGNETIC_FIELD_3, INTEGRATE_CHARGE, INTEGRATE_FLUX, MAP_ELEMENT, MAP_INDEX and MAP_MATERIAL.

AREA

• a box which limits the part of the chamber at which you wish to look, e.g. particles are not allowed to drift outside the box
• a plane, which is perpendicular to the direction under which you look at the chamber and which also serves as surface over which the field is sampled for contours, histograms etc.
• a projection method to visualise the chamber, the light source, the reflective properties of the solids etc

AREA commands are found in several sections:

• field section: where the command establishes the plane on which e.g. contours are plotted
• drift sections: where the command limits the region over which particles are allowed to drift and sets the viewing direction for plots of drift-lines
• optimisation section: shares both the field and the drift area, respectively to set the optimisation plane and to limit drift-lines
• signal section: shares the drift area

The geometrical extent of the field and the drift area boxes are distinct, but the viewing plane is shared between the two.

Format:

AREA [xmin ymin xmax ymax | xmin ymin zmin xmax ymax zmax] ...
     [VIEW plane] ...
     [ROTATE angle] ...
     [X-Y | R-PHI | X-Z | Y-Z | 3D | CUT | NEBEM] ...
     [LIGHT-ORIGIN \φ_light \θ_light] ...
     [REFLECTED-FRACTION frac_refl] ...
     [ABSORBED-FRACTION frac_scat] ...
     [COLOURS ncol] ...
     [PARTIAL-BOX-TICKMARKS | FULL-BOX-TICKMARKS] ...
     [PARTIAL-TUBE | FULL-TUBE] ...
     [PARTIAL-PLANES | FULL-PLANES] ...
     [SPLIT-INTERSECTING-PLANES | NOSPLIT-INTERSECTING-PLANES] ...
     [NOSORT-PLANES | SORT-PLANES] ...
     [OUTLINE | NOOUTLINE] ...
     [PLOT-MAP | NOPLOT-MAP] ...
     [NOSTEP | STEP]

Example:

AREA -1 -1 1 1

Additional information on:

CHECK

• verifies that the potential near the wires agrees with the potentials specified with the ROWS statement and that the wire charges agree with the field integrals around the wires;
• verifies that the potential near the planes and the tube wall agrees with the potentials specified with the PLANE and TUBE statements;
• checks that the divergence of the electric field is zero and that the potential equals the gradient of the electric field.

CHECK is mainly a debugging instruction, but the WIRE option is a convenient means to to determine the field at the surface of the wires. This quantity is of interest since it gives a rough feeling for the avalanche amplification.

The CHECK command is not meant to be used with finite element field maps. Even the MAXWELL option is not particularly useful since finite element fields are, by construction, not gradient-free. The finite element programs that are most commonly used with Garfield represent their potential as a 2nd order polynomials. Such potentials are gradient-free only if one imposes constraints on the coefficients, something that is in general not done. The finite element programs themselves are much better equipped to evaluate the quality of their field maps.

The CHECK command works in the z=0 plane and ignores the VIEW part of the AREA command - this is justified since chambers that are made of wires, planes and periodicity do not have structure in the z-direction. This point will have to be revisited when 3D point charges are introduced.

Format:

CHECK [WIRES] ...
           [EPSILON-WIRE \ε_wire] ...
      [CHARGES] ...
      [PLANES] ...
      [TUBE] ...
      [MAXWELL] ...
           [BINS bins] ...
           [EPSILON-MAXWELL \ε_Maxwell] ...
           [PRINT | NOPRINT] ...
           [PLOT | NOPLOT] ...
      [FULL] ...
      [NOKEEP-RESULTS | KEEP-RESULTS]

Example:

CH WIRE BINS 50

(This would be used to find the field on the surface of the wires and you'll get a few checks on the wire-charges for free.)

Additional information on:

GRID

In the field section, the grid determines the following:

• the density of vectors in PLOT-FIELD VECTOR plots;
• the density of the search grid when using PLOT-FIELD CONTOUR;
• the number of sampling points for PLOT-FIELD HISTOGRAM;
• the mesh in 3-dimensional plots made with PLOT-FIELD SURFACE;
• the density of the grid on which PRINT shows field values;
• the number of points in which the CHECK MAXWELL command verifies the divergence of the field and the compatibility of the potential and the field;

The grid is common to all sections.

Format:

GRID  number_of_steps_in_x  [number_of_steps_in_y]

Example:

GRID 50

Additional information on:

MULTIPOLE-MOMENTS

Dipole terms are available in Garfield for a few potential types via the DIPOLE-TERMS option. The absence of dipole terms for the remaining potential types, just like the absence of quadrupole etc. potentials in Garfield, is not a fundamental limitation. Such potentials can be added on request.

Format:

MULTIPOLE-MOMENTS ...
     WIRE wire ...
     [ORDER order] ...
     [RADIUS r] ...
     [NOPLOT | PLOT] ...
     [NOPRINT | PRINT] ...
     [EPSILON \ε] ...
     [ITERATE-MAXIMUM iter]

Example:

MULTIPOLE WIRE 1 PLOT

Will plot the multipole moments up to order 4 for wire 1.

Additional information on:

OPTIONS

Format:

OPTIONS [NOCHECK-MAP-INDICES | CHECK-MAP-INDICES] ...
        [CONTOUR-ALL-MEDIA | CONTOUR-DRIFT-MEDIUM] ...
        [NOWIRE-MARKERS | WIRE-MARKERS]

Example:

opt check-map contour-drift

Additional information on:

PLOT-FIELD

Similar instructions exist in the drift and signal sections.

CPU time can be saved if several plots are combined in a single command.

Format:

PLOT-FIELD [CONTOUR [f1]   [RANGE {cmin cmax | AUTOMATIC}] ...
                           [N n] ...
                           [LABELS | NOLABELS] ] ...
           [GRAPH [f2]]    [ON f_curve] ...
                           [N n]] ...
                           [SCALE min max] ...
                           [NOPRINT | PRINT] ...
           [HISTOGRAM [f3] [RANGE {hmin hmax | AUTOMATIC}] ...
                           [BINS nbin]] ...
           [SURFACE [f4]   [ANGLES \φ \θ]] ...
           [VECTOR [f5 f6 [f7]]]

If you don't manage to fit all this on a single line, remember that lines that end on an ellipsis are continued on the next.

Examples:

PLOT HIST VECTOR SURF CONT
PLOT CONTOUR RANGE 500 550
PLOT GRAPH 'SIN X'

(The first example makes most of the plots using default functions and ranges - useful as a first call. The second example makes a more detailed contour plot and the third one shows that you can use this program also to produce graphs of arbitrary functions.)

Additional information on:

PRINT

This procedure, contrary to e.g. PLOT-FIELD still operates only in the z=0 plane.

A large number of functions may be given as argument but the tables are always for at most 4\ functions at the time. This instruction tends to produce a lot of output: one page per 10\×10 block of points on the current GRID.

Format:

PRINT f1 f2 ...

Example:

PR EX, EY, E, V

Additional information on:

SAMPLE

This instruction was used for debugging, but is now superseded by the procedure calls ELECTRIC_FIELD and ELECTRIC_FIELD_3.

Format:

SAMPLE x y

Example:

SAMPLE 0.5 0.5

SELECT

In this section, the selection determines which wires are checked by CHECK. The grouping is of no importance. Selection of other electrodes than wires has no effect.

Format:

SELECT selection

Example:

SEL (1 S) 2 F

(Put wire 1 together with all S wires in one group, make wire 2 a group of its own and do the same for each of the F wires.)

Additional information on:

TIME

Format:

TIME [n]

Example:

TIME 10000

Additional information on:

TRACK

The TRACK command is shared between all sections and has therefore a rich format. In this section, only the geometrical aspects are used. Particle types and clustering models need not be specified.

Format:

See the TRACK command in the drift section.

Example:

TR -1 -1 -1 1

Formatted on 21/01/18 at 16:55.