[Clas_offline] b-field parameterization/calculation

[Mac Mestayer]

Hello folks;

Here is my suggestion for a useful, self-contained software
project for Sebouh in the context of his Java-ization of SOCRAT.

I understand that a fair fraction of tracking computing time is
devoted to estimating the magnetic field along a trajectory.
One way to estimate the field is to fill a table with pre-calculated
values of the B field components on a grid of space points, and
then to interpolate between grid points to estimate the field value
at any arbitrary space point.  The speed of such a process varies
with the grid size and with the order of interpolation (linear, 2nd
order, etc.).

In the summer, Peter Bosted made a good suggestion.  Since the
dominant kinematic trend of the B-field for a toroidal magnet is
a 1/r dependence, he suggested that we tabulate r*B instead of
B itself.

If folks agree, I'd like to see Sebouh investigate this.
The project would have several stages.  First, simply isolate
and modularize the existing B-field estimation part of SOCRAT into
an independent module.  Secondly, measure how much time the
B-field estimation is taking for a wide variety of typical tracks.
Thirdly, create a new table with r*B as the tabulated values instead
of B, and time this (it shouldn't be any faster if we don't change
anything else).  Now we can play around with reducing the number of
grid points by making a coarser binning and see how this affects
the computing time.  Likewise, we can investigate using a lower-order
interpolation scheme.  As well as measuring computing time, we need
a measure of the loss of resolution due to coarsening the grid, so
we'll also have to keep track of the momentum and angular resolution
of the reconstructed tracks.

Any comments on the importance of such a project or on its implementation?

 				- Mac

"mestayer at jlab.org", (757)-269-7252