1. Improvements of the A2 beamline for the linear polarised beam Patrik Ott Institut für Kernphysik Johannes-Gutenberg Universität Mainz 28.02.2011

2. Content Motivation –Characteristics of linear polarised photon beam –Why do we need these two devices Beam Stabilisation –Recent Status –Possible automation? Pair-Spectrometer

3. Therefore collimation is a simple way to increase the relative amount of coherent linear polarised photons in the beam Coherent peak Ken Livingsten Characteristics of linear polarised photon beam Roman Leukel, Diss. 2001 Large incoherent background Coherent part is strongly collimated Colli at 7.5 m pol. 70% Colli at 5.0 m pol. 58% Colli at 2.5 m pol. 44%

4. Problems of the collimation So far we have no possibility to measure the coherent peak without analysing the data Cause of the small emission angle, a small deviation of the electron beam can cause the complete loss of the linear polarised photons

5. Position control

6. A2-Beamline TM 110 Scanners The stabilisation converges best with the correction magnets SFA2WEDL06 and SFA2WEDL08.

7. Measuring the position in both cavities as a function of the current in both correction magnets With these gradients one can calculate the necessary change of current. The Hysteresis of the correction magnets cause an unpredictable error. Due to this only 90% of the aberration is compensated. After a few iterations the nominal position is reached without oscillation.

8. Measuring the position in both cavities as a function of the current in both correction magnets Cavity1 Cavity2 Magnet 1 Magnet 2 Necessity for convergence: Different slopes for both magnets in at least one cavity

9. Lock-in amplifier Cavity output 2.45 GHz MAMI main oscillator 2.45 GHz – 100 kHz 2.45 GHz ∆φ∆φ Add a reference Signal of 2.45 GHz – 100 kHz to reduce the frequency Measure the output signal at a time defined by the MAMI main oscillator To adjust the phase ∆φ one needs to steer the beam highly. This produces a large output signal so that the Lock-in amplifier can find a reproducible reference phase. 100 kHz Wavelength : 12.2 cm A difference of the electron path through MAMI of 2 cm reduces the signal output to one half.

10. Measured horizontal beam position Position is measured 25 times before switching to the other orientation. This is done to conserve the HF-Relay.

11. Measured vertical beam position Position is measured 25 times before switching to the other orientation. This is done to conserve the HF-Relay.

12. MAMI-Optimisation After a MAMI-optimisation the beam position often has a large offset, but the operator can bring back the beam with one button.

13. After the first 2 days of beam time, the accelerator is in thermal equilibrium and the position is quite stable. If an aberration is detected, it is not clear if the reference phase or the beam position is not right. Due to this, there is no automation so far. The operator gets an acoustic signal and adjusts the phase again. If an aberration is still detected he can correct the beam. During the phase adjustment we need the blank radiator. After a MAMI-optimisation the operator can bring back the beam with one button. Position Control

14. Pair-Spectrometer

15. Experimental setup Measure the tagger channels in coincident with the Pair-Spectrometer to get the coherent Peak.

16. Hamamatsu APD Dark currentSignal No need of HV Is not disturbed by magnetic fields easy connection with scintillator

17. First tests Does the setup work with the APD? Measure the influence of the current and the foil-thickness Can we find the coincident of both scintillators? Find the timing between Pair-Spectrometer and Tagger (Moeller-FPGA)

18. Provide an excellent grounding of the electronic to suppress noise. Find the correct timing in the FPGA Open the beam line to build in longer scintillators. Pair-Spectrometer To do list: