Further SR Studies for the Electron Polarimeter M. Sullivan for the JLEIC Collaboration Meeting Oct. 5-7, 2016

Outline Brief Summary of previous SR study – Window for the Compton scattered photon – Softer bend field needed and implemented – Things OK now for the Compton photon? Electron detector from the Compton scatter – Roman Pot idea (ala LHC Totem experiment) Background from scattered SR – From last upstream bend magnet – Straight beam pipe – Mask tip options JLEIC Collaboration Meeting Oct. 2016Oct. 5-7, 20162/33

Outline (2) Ante-chamber pipe – Like PEP-II LER – Strike surface of SR fan Summary Other places to worry – Where there are bend magnets and sensitive equipment downstream Conclusion JLEIC Collaboration Meeting Oct. 2016Oct. 5-7, 20163/33

Compton Polarimeter Chicane JLEIC Collaboration Meeting Oct cc Laser + Fabry Perot cavity e - beam from IP Low-Q 2 tagger for low-energy electrons Compton electron tracking detector Compton photon calorimeter Compton- and low-Q 2 electrons are kinematically separated! Photons from IP e - beam to spin rotator Luminosity monitor Thanks to Alexandre Camsonne Oct. 5-7, 20164/33

Anamorphic drawing of chicane JLEIC Collaboration Meeting Oct Assume beam pipe is 80 mm diameter Oct. 5-7, 20165/33

SR fans at Polarimeter Detectors JLEIC Collaboration Meeting Oct Electron detector Photon detector Soft bend SR fans 35 kW of SR power ~1   /bunch Window Oct. 5-7, 20166/33

Tough geometry to model Accurately tracking the large number of photons from this bend magnet through GEANT may be difficult – Need to track 5  to X-ray photons We can make some pretty good guesses to get a number at least in the ballpark (within a factor of 3) Further refinement with this procedure using better approximations is also possible JLEIC Collaboration Meeting Oct. 2016Oct. 5-7, 20167/33

Compton Photon Window The previous study suggested two soft bend magnets with a bend field of 2.3 kG that are 1 m long and a bend angle of 2x6.5 mrad and a critical energy of 18.5 keV for an 11 GeV beam The latest lattice has two soft bend magnets that are 0.5 m long and generate a bend angle of 2x2.5 mrad At 10 GeV we have a critical energy of 11.1 keV, significantly lower than the above value JLEIC Collaboration Meeting Oct. 2016Oct. 5-7, 20168/33

Compton Photon Window (2) In addition, the power levels are lower and the solid angle (SA) acceptance of the detector has been decreased to reduce the number of photons incident on the beam pipe and going through the pipe At a 5 GeV beam energy the critical photon energy is 1.39 keV a comfortably low number Simulations show that NO SR photons from the 5 GeV beam will penetrate a beam pipe composed of 25 um Au on 2 mm Be and 2 mm H 2 O JLEIC Collaboration Meeting Oct. 2016Oct. 5-7, 20169/33

Compton Photon Window (3) For 7 GeV we have K c = 3.8 keV and we have 1.8  incident and 1.1  transmitted so there are 198 getting through with average energy of 25.4 keV For the 10 GeV beam we get 9.6  10 9 photons incident on the beam pipe window and 1.9  getting through (1.82  10 6 ) with an average energy of 46.6 keV As before, we should be able to take credit for photons that scatter as they go through the beam pipe and hence miss the Compton photon detector (the number from the previous study was about 1% of the photons that go through the pipe are headed for the detector) JLEIC Collaboration Meeting Oct. 2016Oct. 5-7, /33

Compton Photon Window (4) It looks like the photon detector might be OK The bend magnets are softer The top electron energy is lower The 5 and 7 GeV cases have no background The 10 GeV case still has a large number of photons incident on the detector – probably will need a shield in front of the detector Need to repeat the more detailed study done earlier for the 10 GeV case The linear power density on the beam pipe wall is OK (~50 W/cm) JLEIC Collaboration Meeting Oct. 2016Oct. 5-7, /33

Compton Electron Detector JLEIC Collaboration Meeting Oct For small angle detection Two chambers Thin window (how thin?) Can be moved in and out from beam Typically 10 to 15 sigma As close as 4-5 sigma in optimal places Roman Pot design from TOTEM at LHC Thanks to Alexandre Camsonne Oct. 5-7, /33

SR fans from the upstream bend magnet JLEIC Collaboration Meeting Oct Electron detector Oct. 5-7, /33

SR photons will one-bounce to the electron detector The power and the total number of photons /bunch coming from the upstream bend and incident on the upstream beam pipe is: – 7 3A A – 35.7 kW 35.2 kW – 1.60x x10 10 This is a lot of power and a lot of photons Assuming a 3% reflection coefficient and… Calculating the average SA to the detector… JLEIC Collaboration Meeting Oct. 2016Oct. 5-7, /33

Straight beam pipe We find the following numbers for 6 segments of the SR fan from the bend magnet that hit the beam pipe – 7 GeV 10 GeV – Segment W/cm Inc. W/cm Inc. – – – – – – x x10 5 – Total 4.00x x10 5 JLEIC Collaboration Meeting Oct. 2016Oct. 5-7, /33

Straight pipe (2) The total energy of the incident photons on the electron detector wall per beam bunch is close to 4 GeV for both cases (7 and 10 GeV) Estimate of the transmission coefficient through the electron detector wall – 7 GeV (K c = 9.51 keV) 500 um Al – um Al – – 10 GeV (K c = 27.7 keV) 500 um Al – um Al – JLEIC Collaboration Meeting Oct. 2016Oct. 5-7, /33

Straight beam pipe (3) The total SR photon energy incident on the detector for the 7 GeV beam then becomes 175 MeV per beam bunch which is probably not OK even though we are using 1 mm Al wall thickness The case for the 10 GeV beam is worse. Even though there are fewer photons the average photon energy is almost 2 times higher and therefore we find that 969 MeV of SR energy penetrates the wall and strikes the detector. JLEIC Collaboration Meeting Oct. 2016Oct. 5-7, /33

What to do? Try adding mask tips to the beam pipe on the strike side (slide coming up) We took the case of adding 6 tips (one for each fan segment) Now the SR photons have to ‘two-bounce’ before they can hit the detector This will lower the rate by at least another reflection coefficient (0.03) – Should also get some credit for another SA estimate JLEIC Collaboration Meeting Oct. 2016Oct. 5-7, /33

Mask tips First question to ask is whether the energy deposition or photon rate into the detector is now low enough – For the 10 GeV beam we now have 969  0.03 MeV or 29 MeV which might be OK What about power density on the mask surfaces? – We were already pretty high….. – We have steepened the slope of the strike surface w.r.t. the fan source JLEIC Collaboration Meeting Oct. 2016Oct. 5-7, /33

JLEIC Collaboration Meeting Oct Mask tip pictures Oct. 5-7, /33

JLEIC Collaboration Meeting Oct Mask tips picts (2) Oct. 5-7, /33

Mask tips picts (3) JLEIC Collaboration Meeting Oct. 2016Oct. 5-7, /33

Mask tip picts (4) JLEIC Collaboration Meeting Oct. 2016Oct. 5-7, /33

Surface Power with tips The steeper slopes reduce the length of the SR stripe on the surface – Fan Tip Surf. 7 GeV 10 GeV – segment mm cm W/cm W/cm – – – – – – NO GOOD (Strains the limit if not beyond that of GlidCop) JLEIC Collaboration Meeting Oct. 2016Oct. 5-7, /33

Ante-chamber beam pipe A beam pipe with masking tips has too much surface power on the masks An alternative is to make an ante-chamber for the SR photons This eliminates the problem of ‘one-bounce’ and even most ‘two-bounce’ photons to the electron detector Question of space – Too close to the next bend magnet? JLEIC Collaboration Meeting Oct. 2016Oct. 5-7, /33

Ante-chamber (2) JLEIC Collaboration Meeting Oct cm Surface power density on the photon stop is 62 W/cm Oct. 5-7, /33

Ante-chamber (3) JLEIC Collaboration Meeting Oct Need a vertically sloped (15:1) photon stop made of GlipCop to spread out the power (ala PEP-II LER) Power density is about 2400 W/cm 15 cm Oct. 5-7, /33

Summary There is a large number of SR photons coming from the bend magnet upstream of the electron detector for the polarimeter – 1  photons per beam bunch – The power is also significant (35 kW) A smooth beam pipe reflects too many of these SR photons into the detector – About 4 GeV of energy for each beam bunch JLEIC Collaboration Meeting Oct. 2016Oct. 5-7, /33

Summary (2) Adding mask tips to the beam pipe does not work – This forces any SR photon to have to ‘two-bounce’ in order to hit the electron detector which is good for the background levels – But the surface power density is too high. The beam pipe will probably either crack or melt. We need to add an ante-chamber to absorb the SR photons on a dedicated photon stop – The stop must be either horizontally sloped or vertically sloped in order to reduce the surface power density Based on the surface power density we also need ante-chambers for all of the pipes inside and downstream of the chicane – How this affects the central chamber needs study JLEIC Collaboration Meeting Oct. 2016Oct. 5-7, /33

Other places where we should worry Places in the ring where there is sensitive equipment downstream of any bend magnet – Lumi detector window – Spin rotator sections – Crab cavities – RF sections – Injection point – Beam abort point? JLEIC Collaboration Meeting Oct SC elements SC elements? Oct. 5-7, /33

Nota Bene The chicane is not too far from the IR and the bend magnets together deposit a significant amount of local SR power (>200 kW) into the nearby chambers (PEP-II had 120 kW) We found in PEP-II that this kind of SR power deposited locally produces a significant local source of thermal neutrons The general radiation level around this area in PEP-II (10-20 m downstream) was also quite high while the beams were running JLEIC Collaboration Meeting Oct. 2016Oct. 5-7, /33

Conclusions It looks like we can make the Polarimeter detectors work as far as SR backgrounds are concerned There are still issues of HOM power to evaluate There are several more places around the electron ring that need further study concerning SR power and photon scattering issues The JLEIC design with a large range of high-current electron beam energies (3-10 GeV) presents a unique machine where high numbers of SR soft photons and hard photons have to be controlled JLEIC Collaboration Meeting Oct. 2016Oct. 5-7, /33

JLEIC Collaboration Meeting Oct Thank you! Oct. 5-7, /33