1 gaskelld 1.1 tgt_field.txt	Glen A. Warren   Feb. 2000
  2              
  3                 This file documents some of the "features" of the code that handles
  4              the optical transport of particles through a target field.  Questions,
  5              complaints and praise should be send to me, Glen Warren, at
  6              gwarren@jlab.org.
  7              
  8              
  9              INTRODUCTION
 10              
 11                 Let me first say that the code is not truly a general purpose code.
 12              It is designed to handle the situation for the E93-026 experiment,
 13              GeN.  This means that the only configuration addressed is electrons in
 14              HMS and the proton going into open space (i.e. using_P_arm_montecarlo
 15              = .false.).  In addition, the target field input assumes cylindrical
 16              symmetry with the axis of symmetry arbitrarily oriented in the
 17              horizontal plane.  Despite thes restrictions, it would be relatively
 18              easy for a motivated individual to follow the existing examples to
 19              modify the code to handle different situations.
 20              
 21                 The second thing that I want to say is that Markus Muehlbauer
 22 gaskelld 1.1 should get much of the credit for this work.  I simply cobbled
 23              together his code written for the engine to work in SIMC.  This
 24              information, however, is of little use because Markus has since left
 25              physics.
 26              
 27                 The last introductory point that I want to make is that as of the
 28              time of writting this file I am maintaining a web page that discusses
 29              some of my findings.  The address is
 30              
 31              http://jazz.physik.unibas.ch/~warren/jlab/gen/targetfield.html
 32              
 33              I can make no promises about the future maintaince of this page, but
 34              it might provide some background info.
 35              
 36              
 37              USAGE
 38              
 39                 There are five input variables related to the target field code:
 40              
 41                targ.Bangle[real]: in beamandtargetinfo in the "extra" file.  This is the
 42                angle of the target field with respect to the beam.  Zero degrees
 43 gaskelld 1.1   means along the beam direction.  Positive angle points in the
 44                direction of the electron spectrometer.
 45              
 46                using_tgt_field[0,1]: in simulate in the "extra" file.  This turns
 47                on/off the target field code.  It is useful for doing checks on the
 48                code for zero field.  Clearly, if there is a target field, this
 49                parameter must be 1.
 50              
 51                tgt_field_file[string]: in simulate in the "extra" file.  This is
 52                the file name of the target field data file.  There are two special
 53                options.  If tgt_field_file.eq.' ' then a test field of 5 T is used
 54                over a -26<z<26cm and 0<r<16cm.  If tgt_field_file.eq.'0', then the
 55                field is set to zero.  This is very useful for trivial debugging.
 56                The current target field map file for GeN is 'trg_field_map.dat'.
 57                This file should reside in the same directory as the simc
 58                executable.
 59              
 60                debug(6)[0,1]: in debug in the input file.  This turns on/off
 61                debugging information related to the target field code.
 62              
 63                drift_to_ndet[real]: in simulate in the "extra" file.  This is the
 64 gaskelld 1.1   distance in cm from the target to the first plane of bars in the
 65                neutron detector.  It is used to reconstruct the proton "focal
 66                plane" coordinates.
 67              
 68              
 69                 I made some changes in the proton coordinates in the ntuple to
 70              accommodate aspects of the target field.  Since there is no
 71              spectrometer optics on the proton side, there is no need to worry
 72              about proper reconstruction of the event to the target.  The generated
 73              proton vertex positions in the ntuple remain unchanged.  The
 74              "reconstructed" proton coordinates, however, are the "vertex"
 75              coordinates seen outside of the target field region as if there was no
 76              target field.  That is, once the proton is outside of the target field
 77              region, it is reconstructed to the vertex plane (defined by the raster
 78              information) as if there was no target field.  I refer to this type of
 79              track as an image track.  For this reason, the generated and
 80              reconstructed vertical angles for the GeN measurement will be quite
 81              different.  The proton focal plane coordinates are the proton
 82              coordinates transported to "drift_to_ndet" cm from the target.
 83              
 84              
 85 gaskelld 1.1 ALGORITHMS
 86              
 87                 There are two distinct parts to the code: the tracking of the
 88              generated event from the vertex to outside of the target field and the
 89              reconstruction of the target coordinates from focal plane and raster
 90              information considering the target field.  The first part is relevant
 91              to both the electron and the proton tracking.  The second part is
 92              relevant only to the electron tracking.
 93              
 94                 The tracking of the generated event through the target field toward
 95              HMS is straight forward.  The generated event is mapped from the
 96              vertex through the target field to a field free region.  Once outside
 97              of the region, the track is traced back to z=0 as if there was target
 98              field.  This image track is then used as input in the HMS forward
 99              optics.
100              
101              The reconstruction of the target coordinates is not straight forward.
102              Here is an outline of the algorithm:
103              
104                1) As a first approximation, map the focal plane coordinates to the
105                   target (z=0), using the vertical beam position provided (this
106 gaskelld 1.1      could come from either the SEM or the slow raster) without
107                   considering the target field.  This is wrong for two reasons.
108                   First, and most obvious, it does not consider the target field.
109                   Second, the vertical position is wrong.  The electromagnetic
110                   interaction occurred somewhere along the beam line, which is
111                   given by the vertical and horizontal beam positions.  In general,
112                   the intersection of the beam line and the particle's trajectory
113                   do not occur at z=0.  But as I said, and as the code says, this
114                   is a first approximation.
115              
116                2) Next, drift the above track back 100 cm (the z=100 cm plane) in
117                   the direction of the spectrometer to get outside of the target
118                   field region.
119              
120                3) Map the above track through the target field to the "beam plane".
121                   By beam plane, I mean the vertical plane determined by the
122                   vertical and horizontal beam positions.  This plane contains the
123                   line of possible interaction vertices.  Note that this track will
124                   in general not have the proper vertical position even though we
125                   are looking at the correct plane.
126              
127 gaskelld 1.1   4A) Map the track from above to the z=0 plane.  
128                4B) Modify the track above so that it has the known vertical
129                    position at the beam plane and map to the z=0 plane.
130              
131                5) Adjust the vertical position used in step 1) by the differences
132                   of the final vertical position (x) in 4A) and 4B), and then
133                   repeat from step 1) using the new vertical position.  The process
134                   is repeated until the difference in vertical position from 4A)
135                   and 4B) is below a certain limit or a certain number of
136                   iterations have been performed.
137              
138                A fair question to ask is why is the code structured like this?  The
139              problem is complicated by the need to using the existing HMS optics
140              coupled with the target field coupled with the need to specify the
141              vertical position.  Actually, a similar process is used to determine
142              the target coordinates for the HMS without a target field when one
143              must specify the vertical position.
144              
145              I hope this has provided some useful background.  - GAW 
146