[Clas_offline] b-field parameterization/calculation

[Alexander Vlassov]

Mac Mestayer wrote:
> 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
>
>
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Hi,
I can understand, that making of a table B*r instead of B may improve 
accuracy,
but how can it affect the alculating time ?
- Alex.