Proton beam characterisation for electron BPMs development

1. Proton beam characterisation for electron BPMs development Eugenio Senes for the AWAKE BI team…with many thanks for the support to the AWAKE teams !

2. Outline AWAKE Run 1 layout The laser, p+ and e- beams are together in the beamline portion between the electron merge point and the plasma cell E.Senes - AWAKE Collaboration Meeting Beam position sensing with two beams Proton beam shape Future developments

3. A photographer’s problem Credits: H.M. Larsen (REUTERS) E.Senes - AWAKE Collaboration Meeting • The position detection when two beams are copropagating it’s problematic • The proton beam is more intense and longer than the electrons

4. A photographer’s problem Credits: H.M. Larsen (REUTERS) ? It is similar to attempt to take a picture of the moon just before an eclipse! E.Senes - AWAKE Collaboration Meeting • The position detection when two beams are copropagating it’s problematic • The proton beam is more intense and longer than the electrons

5. Now back to Physics TODAY ! E.Senes - AWAKE Collaboration Meeting For a BPM system matters: Beam spectrum System response function

6. Now back to Physics TODAY ! For AWAKE run 1 operational setup: • Protons: 3e11 ppb, 0.25 ns • Electrons: 600 pC, 1 ns Expected 35-38 dB difference at zero frequency 400 MHz: present eBPM system Equal spectral power point at ~2 GHz At high frequencies the proton signal power drops and the electrons can be detected E.Senes - AWAKE Collaboration Meeting For a BPM system matters: Beam spectrum System response function

7. It’s a matter of shape A working system has to be designed at a high frequency to avoid the p+ beam bias E.Senes - AWAKE Collaboration Meeting Reality is more complicated: Non-Gaussian beams have tails that extend in frequency This also varies shot by shot

8. It’s a matter of shape A working system has to be designed at a high frequency to avoid the p+ beam bias Core problem: the proton beam longitudinal profile is measured via streak camera from OTR light. But what is the camera introducing in the spectrum and what is beam ? E.Senes - AWAKE Collaboration Meeting Reality is more complicated: Non-Gaussian beams have tails that extend in frequency This also varies shot by shot

9. Streak camera E.Senes - AWAKE Collaboration Meeting • Two classes of features in streak profiles • Tail modulations • Noise at the top of the profile • Streak cameras are operated in low light condition. It is key to understand the quantum noise impact on the profile.

10. Streak camera Laser reference Beam OTR • Reference measurementsto understand the streak noise impact using laser light • Same streak settings (Gain, slit aperture) • Same light intensity (Max intensity on the single pixel) • The laser pulse is rather smooth, but shorter Time E.Senes - AWAKE Collaboration Meeting • Two classes of features in streak profiles • Tail modulations • Noise at the top of the profile • Streak cameras are operated in low light condition. It is key to understand the quantum noise impact on the profile.

11. Understanding quantum noise Full sensorprofile Noise E.Senes - AWAKE Collaboration Meeting • The laser light illuminates the whole sensor • If the whole sensor is considered, the profile is up to x10 more intense than the beam profile and rather smooth (noise averages out) • A Region Of Interest can be selected to get the desired profile intensity (less light, so more noise). This is the OTR beam light equivalent. • Rescaling and subtracting the full sensor profile, we can estimate the expected noise induced by the low light operation • Now we look at the profile as the sum of the full sensor profile rescaled + the low light noise • Please note the factor ~10 in scale difference !

12. Lesson learned E.Senes - AWAKE Collaboration Meeting To measure the beam profile shot by shot via the streak camera, illuminating a large area of the sensor is beneficial. The noise amplitude is intensity dependent

13. Now the beam • Lowpassfiltering having in mind how much noises is generated by the camera itself at which intensity. Two cases: fcutoff too high (40 GHz) fcutoff too low (5 GHz) • No pattern, everything is still present even after the filtering • Large noise in the tails • No recognisable noise pattern E.Senes - AWAKE Collaboration Meeting Initial studies hint that the beam usually do not extends beyond ~30 GHz. Systematic studies will follow on this and two more datasets …to be continued

14. What comes next ? A. Curcio, CLIC workshop 2019 E.Senes - AWAKE Collaboration Meeting It is extremely challenging, if not impossible, having standard BPMs working at several tens of GHz Diffraction Cherenkov radiators are under study for beam position monitoring. An in-air PTFE device was tested last year.

15. A more engineered prototype E.Senes - AWAKE Collaboration Meeting Data of last year collected on a device to measure bunch length – i.e. not optimised for beam position measurement ! A BPM prototype was produced with AWAKE geometry, PTFE radiators. Radiation emitted at 45 deg. Still test in air, but more engineered model. Not yet vacuum compatible for ease of realisation, cost and time constraints EM simulation work in progress. Tests in CLEAR by the end of the year.

16. Conclusions E.Senes - AWAKE Collaboration Meeting A standard BPM is hardly suitable to measure the electron position when the protons are present The bunch length difference is the key to detect position The extension of the proton beam tails in the spectrum is the limiting factor for a working system. The frequency spectrum depends on the beam dynamics in the SPS. It is key to understand this on the beam dynamics side. A change on this side can invalidate all the work. Information on the p+ beam profile only from the streak camera. Difficult to interpret due to to the intrinsic noise. Quantitative work in progress. Tests for an high-frequency beam position monitor prototype will be carried out in CLEAR by the end of the year. Still an in-air device to gain experience and assess the performance of the technique. At the moment collecting RF parts and designing the acquisition system.

17. Thanks for your attention !