1. Introduction Present Status Idea of optimisation Cavity Lpol
S. Baudrand Polarisation Optimisation
Baudrand Sylvestre

2. Cavity LPOL

3. Cavity fabry-perot: principle
Gain 9000
e beam L
Circular polar.
Linear polar
When nLaser  c/2L  resonance
But : Dn/nLaser =  feedback laser/cavity By tuning the laser frequency.
5kw reachable =>dP=1%/bunch/min (stat.)
Baudrand Sylvestre

5. Synchrotron isolation : 3 mm of lead Thermic isolation with aluminium
ZEUS
HERMES
Baudrand Sylvestre

6. New polarimeter using a fabry perot cavity
about 60 m
96 ns
Cavity
diffused g
Calorimeter
Laser
electron
bunch
driver
cable ~ 100 m
10 Mhz
calo signal
acqusition
PC PXI
database
PC PVSS
Alignment (Labview)
Locking (Saclay module)
MFCC cards (Lynx/Os)
ADC card
shaper
Control room
1 hz
alignment,
locking,
laser pola.
electron
beam
Infrared laser (l=1064mm)
Fabry-Perot cavity (2m)
Compton scatering (e-g angle=3.3 °)
Slow control: light polarisation, temp
Fast control: Laser frequency for the cavity
Baudrand Sylvestre

7. Present status

8. Effect of radations at the calorimeter location 2003/2004:
Hermes magnet if Off
Hermes magnet if On
Baudrand Sylvestre

9. Effect of radations at the calorimeter location 2003/2004:
Hermes magnet if Off
Hermes magnet if On
We are waiting for a new calorimeter. Absorbers in place should reduce the noise.
Cavity remain locked:
Time(h)
During complete sequence of fills the cavity is stable.
Baudrand Sylvestre

10. Ideas of optimisation
Fabian and Christian did some very rough estimation on simple model 3 years ago

11. Polarisation measurment
time(s)
T  few mins, depend on the estimator chosen
t  4 s t P
time(s)
The rise-up curve is fitted to: P
time(s)
3 possibles experiments :
Laser on Left or Right (2s each). Time to change polarity is only few ms.
Laser off (will not be implemented)
On line implemetation possible
Baudrand Sylvestre

12. Estimators 2 Estimation
The Polarimeter will be running continuously and the two (left and right) histograms will be issued every 4 seconds.
dP of about 1 per mill (stat.) for summing all histograms for colliding bunches.
dP of about 3 per mill (stat.) summing all histograms for non colliding bunches.
dP of about 3 per mill (stat.) for individuals bunches but summing during one min.
2 Estimation
Histograms are fitted on T in order to obtain a precise value of P and P . Change can be checked by redoing the same procedure again: Looking at the 2 probability or change in P.
Time in minuts needed to get Q=10
Baudrand Sylvestre

13. If then the change is ‘seen’ Otherwise it isn’t
Time in minits needed to get Q=1
Direct comparison:
If then the change is ‘seen’
Otherwise it isn’t
T depends on the scenario chosen. The largest it is, the more efficient is the optimisation on the plateau. But the smallest is T the earlier the polarisation changing can be ‘seen’
Baudrand Sylvestre

14. Conclusion A Fabry-Perot cavity will be used for a new LPOL
: Radiation damage. 2005: shielding.
NOW, very stable running condition (with temperature control in the cavity house)
Ready to perform the alignment with the e- beam
A likelihood program is used to extract the polarisation from a first principle fit (‘few photon’ mode) to the photon energy spectra
The expected precision is  1%/min/bunch (stat.)
10/00 min (stat.) for all bunch to optimize the polarisation
Effect of synch. rads. are under studies but don’t seems to have a dramatic effect on pol. measurement.
New calorimeter needed and should be installed soon
Baudrand Sylvestre