Minutes of the special polarimeter meeting, 
        Tuesday, Feb 1, 2005, at DESY Hamburg





The purpose of this meeting was to discuss the status of the 
optimization of the polarisation in HERA, understand the 
state of the polarimeters, and discuss ways to improve the 
overall performance. 

These minutes attempt a very brief summary of the main points 
discussed. There is no attempt to summarise each talk; talks 
may be found on the pol2000 WEB page: http://www.desy.de/~pol2000

1) HERA polarisation status, M. Vogt

theoretical expectation: 
HERA I:  65-70% (no beam beam effects taken into account) 
HERA II: 57%    (                "                      )

actually achieved: 

HERA I:  routinely 50-60 %
HERA II: this year 35-40% for colliding bunches, 50% for non-colliding 

During January the HERA group has continuously worked on optimising 
the polarisation. The trends do look rather promising, and show 
a clear tendency towards large polarisation. 

A clear difference is observed between colling and non-colliding 
bunches. Work is under way to better understand the differences. 
One clear issues has to do with beam-beam interactions. An experiment 
done indicated that switching off the proton 
beam and thus the beam beam interactions brought both polarisations 
to the same value. 

Not understood at this point is 
a clear helicity dependence of the polarisation observed only with e- running.

Tools to further optimise the polarisation rely heavily on a reliable and 
fast measurement of the polarisation. 

Further steps to optimise the polarisation: 
- introduce a new optics which is specially computed for 
high polarisation in collindg bunches
- commission the fast kickers (they will be used primarily to 
measure the beam energy, important for a detailed understanding 
of the machine behaviour towards polarisation). 
- faster polarisation measurements to be able to react faster 
to tuning measures, and to be able to minimise the influence from 
drifts in the machine. 

2.) Requirements from physics: colliding beam experiments and 
HERMES (A. Tapper, E. Aschenauer)

Alex and Elke reported on the needs by the experiments for the 
polarisation measurement. FOr details please see their slides. 
The conclusion was: 
- an error of 2% relative is sufficient 
- the anticipated value of <P> = 55-60% is adequate (but of course 
the higher the polarisation, the better). 

3.) Status of polarimeters

TPOL: V Garibyan, S. Schmitt, R. Fabbri

Both TPOL and LPOL are operating with high reliability and 
availability. The TPOL has been in operation since HERA started again, 
the LPOL calorimeter was reinstalled in the tunnel in early January. 

The statistical errors of teh TPOL are 
1min   1.7%
5min   0.8%
10min  0.6%

The LPOL/TPOL ratio seems ok, and close to one througout this run. 
Note that the "focus" correction to TPOL data is applied online. 

The sampling calorimeter at the LPOL has been recommissioned 
and is working fine. It seems to agree with the LPOL at the few % level. 

A special trend display has been recently implemented in the 
TPOL omline software. This display seems to be working fine, and 
provides useful information to the operators. HERA requested 
that the display be extended to allow the operator to trigger 
the start of the averaging procdure. From the errors of the 
TPOL it is clear that a clear statement on trends can only 
be given after periods of a few minutes to 10th of minutes. 

Cavity: S. Baudrand

The LPOL cavity has been re-commissioned last fall. During the 
Christmas shutdown a temperature stabilisation system has been 
installed. After realignement in January the cavity is now 
working fine, and can be locked reliably. 

The work now is geared towards taking Brems-spectra in collaboration 
with the HERMSE group, and understanding the background 
situation at the cavity. After this it is planned to 
attempt to bring the laser beam and the electron beam 
in collision, and to observe first Compton spectra. It is estimated 
that this could be achieved within the next month. 

The new LPOL calorimeter has been extensivly simulated. A note 
on its design is currently being circulated and is waiting 
for comments. Once approved, the ordering and construction 
of the calorimeter can proceed. 

With the existing calorimeter it is estimated that the 
LPOL cavity could be operated a couple of weeks, before the 
calorimeter will suffer some damage.