[Halld-tagger] Fwd: Re: Tagger vacuum chamber design (from Alex)

[Elton Smith]

Hi,

As we have preliminary decided to use an NMR probe for the
tagger/ps magnets, I have talked to Jone LeRose from Hall-A;
the Hall-A people have a big experience using NMP probes for
the magnetic field monitoring.

- NMR probes are much more expensive compared with Hall
   probes (~$30 k for main module + ~ $4 k for each probe ).
   See www.gmw.com, for example.
   Several probes can be connected to the main module
   via a multiplexer, so that we can connect a pair spectrometer
   probe to the main module as well. The multiplexer cost is
    ~7 k. We need to check prices.


- NMP probes are much more accurate than Hall probes. The
   precision is on the level 0.1 uT. They provide an absolute
   measurement of the magnetic field. The Hall probe relative
   stability is rather good, on the level of 10-4, but the
   absolute value of the filed has to be periodically
   calibrated.

My understanding is that we need an accuracy on the magnetic
field on the level of 10-4 ( 10-5 ?) . CASA people claimed
that the electron beam energy would have uncertainties on
the level of  5 10-4.

- The NMR requires a uniform field. If we have a field
   gradient, we can use gradient compensation coils (see
   ranges/specification in  www.gmw.com).

We have to check field gradients in the probe area, my
understanding is that the field is very uniform there.

- The NMR probe is small. It should fit into a 2''
  hole even with most gradient compensator, in case we use
  them (check sizes at the same www page).

- The probe can work in vacuum. The Hall probe can work
in vacuum as well. I have checked with Stepan Stepanyan -
they are using Hall probes for the tagger and ps magnets
(in vacuum). The manufacture is 'lakeshore'.

We have to go through specs once again and decide what we need.

Cheers,
       Alex