Electron Beam Simulation for GlueX

1. GlueX collaboration meeting April 27-29, 2006 Electron Beam Simulation for GlueX presented by Richard Jones, University of Connecticut

2. Photon Tagger Simulation • estimate background rates in focal plane counters • evaluate options for shielding • detailed field map of the tagger • design for the vacuum box downstream exit region • design for the electron beam dump Elements needed for tagger simulation (in addition to building dim.)

3. Simulation Overview electron beam + tagger photon beam + detector separate executables hall D “HALL” “CAVE” tagger area “HILL” common design and lots of shared code e- beam dump hdgeant gxtwist new since 1/2006 in use since 7/2001 • currently running with Geant3, Geant4 prototype under development

4. Photon Tagger Simulation original concept, shown at the Tagger Review in January, 2006 Tagger Building goniometer Hall D photon beam line quadrupole magnet tagger dipoles microscope fixed array hodoscope vacuum chamber to electron beam dump detailed magnetic field map (from TOSCA) 350 x 30 x 1600 = 17 M points areas of interest

5. Some Simulated Events concrete wall

6. New Concept (ca. 2 / 2006) • extend the length of the entire hall • bring the electron dump in closer to the tagger building • connect the electron beam dump to the tagger area by a corridor

7. New Concept (ca. 2 / 2006) • extend the length of the entire hall • bring the electron dump in closer to the tagger building • connect the electron beam dump to the tagger area by a corridor

8. New Concept (ca. 2 / 2006) • extend the length of the entire hall • bring the electron dump in closer to the tagger building • connect the electron beam dump to the tagger area by a corridor

9. New Concept (ca. 2 / 2006) • extend the length of the entire hall • bring the electron dump in closer to the tagger building • connect the electron beam dump to the tagger area by a corridor

10. New Concept (ca. 2 / 2006) Questions raised by the new design: • Is the electron dump a significant source of background in the tagger? • What is the minimum distance from the tagger to the dump? • What is the optimum beam pipe and shielding configuration?

11. Tagger + beam dump simulations • the problem of statistics • rate of electrons in beam (3 mA):2 x 1013/s • rate of electrons at focal plane (3 GeV e-):2 x 108 /s • rate of FP backgrounds we care about: 2 x 106 /s • 108 simulated dump events are needed to measure bg at the level of 10% error • required CPU time • 12 GeV electron shower simulation ~0.4s (AMD 2800+) • 4 x 107 s (1.2 GFlops-years) • usecascadingto enhance statistics

12. Monte Carlo cascading scheme x1000 I II x1000 III • Any particles passing through these virtual surfaces in the indicated direction are intensified, ie. replicated on the secondaries stack N times. • Once a particle has cascaded, it is marked so that it (or its descendants) cannot cascade from that same surface again. • Two cascade surfaces of x1000 each: 108 MC events ~ 1014 e-

13. Plan of beam dump simulations • track 108 12 GeV electrons from radiator (no cascades). • same as above, with x1000 at region II → I boundary. • same as above, with x1000 also at region III → II boundary. sample 1: inclusive spectrum (signal + bg) sample 2: sample 1 + (sources in region II) x 1000 sample 3: sample 2 + (sources in region III) x 1000,000 Simulations are still in progress… but preliminary results will be shown based on 1/5 statistics obtained so far.

14. Results of beam dump simulations The first 105 simulated events looked a lot like this: labyrinth is doing a good job

15. Results of beam dump simulations • focal plane microscope • fiber length 2 cm • fiber axis aligned with electron trajectories • mip peak at 4 MeV • bg is visible but small • only hits with energy deposition are recorded • gammas suppressed • nothing seen from dump • evidence is time of flight (not shown)

16. Results of beam dump simulations • fixed focal plane array • counter thickness 5 mm • counters aligned with focal plane (not ideal) • mip peak at 6 MeV for 8GeV electrons, less for lower energies. • bg is visible, larger • only hits with energy deposition are recorded • gammas must convert • nothing seen from dump

17. Results for cascaded simulations • focal plane microscope • direct electrons are still the primary source of hits over 2 MeV. • bg is visible from region II • rate ~ 0.5% of region I • comes from gammas • no excess visible from dump (100% stat. error)

18. Results for cascaded simulations • fixed focal plane array • direct electrons remain the dominant source of hits over 1 MeV. • bg is visible from region II • rate ~ 0.5% of region I • comes from gammas • small excess seen from dump (at dE < 1 MeV) • a few10-4of rate coming from sources in region II

19. Summary • Background rates in the Hall D tagger focal plane counters have been estimated using a detailed physics simulation, distinguishing three categories of sources: • region I – tagging spectrometer, vacuum box and exit flange • region II – electron exit beam pipe and labyrinth penetrations • region III – the electron beam dump • None of the backgrounds reach levels that threaten the performance of the tagging focal plane counters. • The present dump labyrinth design from the Radcon group leads to rates in the focal plane counters of that are a factor 10-4 smaller than backgrounds coming from sources in regions I and II.

20. Photon Tagger Resolution Resolution in focal plane microscope in energy and emission angle

21. Electron Beam Requirements • beam energy and energy spread • range of deliverable beam currents • beam emittance • beam position controls • upper limits on beam halo Specification of what electron beam properties are consistent with this design

22. Requirement: >12 GeV Electron Beam Energy effects of endpoint energy on figure of merit: rate (8-9 GeV) * p2 @ fixed hadronic rate • The polarization figure of merit for GlueX is very sensitive to the electron beam energy. • Decreasing the upgrade energy by only 500 MeV would have a substantial impact on GlueX.

23. Electron Beam Energy Resolution • beam energy spread dE/E requirement: 0.1 % r.m.s. • compares favorably with best estimate: 0.06 % Typical channel where one of the particles might escape detection • tied to the energy resolution requirement for the tagger • derived from maximizing the ability to reject events with a missing final-state particle. gp K+K-p+p- p [p0] The reviewers challenged the validity of our requirement on dE/E of 0.1% based on a missing-mass plot we showed for this reaction. My view: this is a soft requirement (0.2% is also OK) but we get it essentially for free based on other requirements. post-review note

24. Range of Required Beam Currents • upper bound of 3 mA projected for GlueX at high intensity corresponding to 108g/s on the GlueX target. • with safety factor, translates to 5 mAfor the maximum current to be delivered to the Hall D electron beam dump • during running at a nominal rate of 107g/s : I =300 nA • lower bound of 0.1 nA is required to permit accurate measurement of the tagging efficiency using a in-beam total absorption counter during special low-current runs. Stability of 100pA beam was highlighted as a potential issue during the review process. My view: this is only needed for doing checks. Loss of beam orbit locks is an acceptable part of running at <1nA. post-review note

25. Beam emittance: key to effective collimation The argument for why a new experimental hall is required for GlueX • the short answer: because of beam emittance • a key concept: the virtual electron spot on the collimator face. It must be much smaller than the real photon spot size for collimation to be effective but the convergence angle a must remain small to preserve a sharp coherent peak. Putting in the numbers…

26. Optimization: radiator – collimator distance • With decreased collimator angle: • polarization grows • tagging efficiencydrops off • < 20 mr s0 < 1/3 c c/d = 1/2 (m/E) d > 70 m

27. Effects of emittance on photon beam quality plot from tagger review (constant beam current) same plot, matched hadronic rate post-review note

28. Effects of emittance on photon beam quality effects of collimation on figure of merit: rate (8-9 GeV) * p2 @ fixed hadronic rate Figure of merit expresses the degradation in beam quality in terms of the comparative loss of effective beam time for a run of fixed duration, or equivalent sacrifice in ultimate precision if experimental errors are systematic in nature. new slide, added for this presentation, follows from data shown in previous slide –rtj-

29. Electron Beam Position Controls • Must satisfy two criteria: • The virtual electron spot must be centered on the collimator. • A significant fraction of the real electron beam must pass through the diamond crystal. • criteria for “centering”: dx < s / 2  200 mm • controlled by steering magnets ~100 m upstream Using upstream BPM’s and a known tune, operators can “find the collimator”. Once it is approximately centered ( 5 mm ) an active collimator must provide feedback.

30. Integrated tail current is less than of the total beam current. 10-6 Electron Beam Halo origin of this number: What size of beam halo would be small enough to be safe, without needing a detailed model? post-review note • two important consequences of beam halo: • distortion of the active collimator response matrix • backgrounds in the tagging counters • Beam halo model: • central Gaussian • power-law tails • Requirement: • Further study is underway central Gaussian power-law tail central + tail log Intensity ~q-4 r / s 5 2 3 4 1

31. Results: summary of photon beam properties peak energy8 GeV 9 GeV 10 GeV 11 GeV N in peak185 M/s 100 M/s 45 M/s 15 M/s peak polarization0.54 0.41 0.27 0.11 (f.w.h.m.)(1140 MeV) (900 MeV) (600 MeV) (240 MeV) peak tagging eff.0.55 0.50 0.45 0.29 (f.w.h.m.)(720 MeV) (600 MeV) (420 MeV) (300 MeV) power on collimator5.3 W 4.7 W 4.2 W 3.8 W power on H2 target 810 mW 690 mW 600 mW 540 mW total hadronic rate385 K/s 365 K/s 350 K/s 345 K/s (in tagged peak) (26 K/s) (14 K/s) (6.3 K/s) (2.1 K/s) 1 1 1 2,3 • Rates reflect a beam current of 3mA which corresponds to 108g/s in the coherent peak, which is the maximum current foreseen to be used in Hall D. Normal GlueX running is planned to be at a factor of 10 lower intensity, at least during the initial running period. • Total hadronic rate is dominated by the nucleon resonance region. • For a given electron beam and collimator, background is almost • independent of coherent peak energy, comes mostly from incoherent part.

32. Photon Beam Position Controls • electron Beam Position Monitors provide coarse centering • position resolution 100 mm r.m.s. • a pair separated by 10 m : ~1 mm r.m.s. at the collimator • matches the collimator aperture: can find the collimator • primary beam collimator is instrumented • provides “active collimation” • position sensitivity out to 30 mm from beam axis • maximum sensitivity of 200 mm r.m.s. within 2 mm

33. Overview of Photon Beam Stabilization • Monitor alignment of both beams • BPM’s monitor electron beam position to control the spot on the radiator and point at the collimator • BPM precision in x is affected by the large beam size along this axis at the radiator • independent monitor of photon spot on the face of the collimator guarantees good alignment • photon monitor also provides a check of the focal properties of the electron beam that are not measured with BPMs. 3.5 mm 1s contour of electron beam at radiator 1.1 mm

34. High-level Parameters This slide was added in proof, to counter the suggestion that GlueX planners engaged in hopeful speculation in specifying their electron beam performance requirements. copy of slide 2, presentation to Lehmann Review 7/05, L. Harwood • Beam energy 12 GeV • Beam power 1 MW • Beam current (Hall D) 5 µA • Emittance @ 12 GeV 10 nm-rad • Energy spread @ 12 GeV 0.02% • Simultaneous beam delivery Up to 3 halls