From: Flemming Videbaek (videbaek@sgs1.hirg.bnl.gov)
Date: Mon May 19 2003 - 09:48:58 EDT
Dear Claus,
I will ask Ramiro to correct me if I am wrong,. My understanding of this is
a) The parametrization in Brat comes from a measurement of the currents vs the central By ie.
on the mid-plane in the magnet.
b) The Hall probe field does NOT measure the central By, but only in a fringe region which has a different
satuation effect.
c) The Brat parametrization comes from the current, and the final values was worked out by Pawel as I recall.
That there aresome effects at high field is certainly true, and the saturation at edges indicates this.
The assumption in the reconstruction is of course that the effective edge is good i.e. that the central
measured Bdl can be appiled for all (relevant) paths through the magnet. This is probably not true, in particular for D4,
with the dispersed coils; The way to study these effects is as has at least been mentioned a couple of times over the
years to , implement a realistic field map e.g. from the Tosca calculations in BRAG and reconstruct with the effective
edge to see how important this is.
I would object at this point to change the parametrization since the By of the Hall probes is not the
proper central field. I suggest we have one discussion session at krakow on this issue
- Pawel can (re) tell the story how the fits were done.
- we can have the field map's and data at hand, and talked how to proceed.
Flemming
------------------------------------------------------
Flemming Videbaek
Physics Department
Brookhaven National Laboratory
tlf: 631-344-4106
fax 631-344-1334
e-mail: videbaek@bnl.gov
|
| Hi Ramiro,
|
| After a busy week I've looked at the fields again, and I think the
| parametrization in BRAT is not right on the data.
|
| > The value of By at high field deviates from a linear function of the
| > current because of saturation and hopefully that is well described by
| > the parametrisation in BRAT.
|
| The plots (www.nbi.dk/~ekman/fieldVsCurrent.gif) shows that the measured
| By and the parametrization in BRAT (solid lines) differs sligthly at the
| high fields. It's clear that a 2nd degree pol cannot reproduce the
| saturation effect that the Hall probes show, so the question is if we want
| to put the numbers in the DB (I can see that Kris started this project
| some time ago) or maybe we could just use a 3rd degree pol in BRAT?
|
| If there are no objections I would like to change the parametrization,
| and then we can always put the numbers in the DB later. I just want to be
| sure that the By values I got are the right ones. Could you please send
| me tables with the latest and greatest measurements?
|
| Cheers,
|
| Claus
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