Operational Experience with Steady-state Accelerator Backgrounds at PEP-II presented by W. Kozanecki (CEA-Saclay) for the BaBar - PEPII MDI group Introduction: PEP-II Layout & Operational Parameters Background sources & their impact Background characterization experiments Monitoring tools Topics not covered today... Summary

PEP-II Layout Head-on collisions HER (e-) collimators LER (e+) collimators Head-on collisions (almost) no Xing angle - but bend only at the last moment! Magnetic separation & in-detector focussing Permanent-magnet dipole + off-center quads (QD1 shared between e- & e+)

PEP-II Beam-current & Luminosity History (2000-2008) LER current (mA) 3000 Luminosity (1033 cm-2 s-1) 12 PEP-II Beam-current & Luminosity History (2000-2008) HER current (mA) 2000 Drift chamber current (A) 1500

Background: the issues SVT radiation damage (  < 5 MRad) monitored by pin diodes + diamonds Background: the issues Drift chamber current (total + spikes!) + accumulated charge [+ rad. damage: < 20 kRad] L-1 trigger rate < 2  7 kHz CsI calorimeter: occupancy (> 1 MeV) =>  (E) # Xtls > 10 MeV => # clusters [radiation damage:  < 10 kRad] DIRC PM counting rate (<=> dead time)

Radiation abort & diagnostics (early running) RadFets & dosimeters Beam loss monitors DIRC PM counting rate + occupancy Drift chamber current [+ occupancy] Trigger rates Pin diode & diamond currents [SVT occupancy] Calorimeter: pin diodes DCH electronics: pin diodes

Background sources Synchrotron radiation (this background negligible in PEP-II) Beam-gas (bremsstrahlung + Coulomb) HEB only: BHbg ~ IH * (pH0 + PHDyn * IH) Note: p0 = f(T) ! LEB only: BLbg ~ IL * (pL0 + PLDyn * IL) Note: p0 = f(T) ! beam-gas x- term: BLHbg ~ cLH * IL * IH (LEB+HEB, out of collision) (?) Luminosity (radiative-Bhabha debris) – rising concern as L  BP ~ dP * L (strictly linear with L) Beam-beam tails (severity highly variable, mitigation largely empirical) Injection losses (before trickle injection started in Apr 04) Trickle background: BLi , BHi (injected-beam quality, orbit, beam-beam) Touschek: BLT (background signature ~ bremsstrahlung; impact very small ) see M. Sullivan's talk

Bremsstrahlung e- trajectories (x) in the HER (zscat > -26 m) QF Dipole (or offset Q) QD IP Nominal e- trajectory (curvilinear coordinates) Only those e- are shown that scattered between –26m & –4 m of the IR hit an aperture within +- 8.5 m of the IR Vacuum pipe / mask apertures

Spatial origin of beam-gas backgrounds IP Coulomb scattering in Arcs (y-plane) e- Brems-strahlung in last 26 m (x-plane) IP Normalized to: - uniform pressure profile of 1 nT - 1 A beam current

Luminosity background e+ e-  e+ e- g elm shower debris neutrons! no n from coasting HER or LER beam alone may dominate DCH, DIRC rate

Background impact (1): trip rate, integrated dose, SVT pin holes SVT pin diodes & diamonds Radiation budget: 5 MRad Sensors: horizontal plane This plot: Jan-Jul 2000 Sensors: top 2008 typical, radiation aborts (some sympathetic)

Run-4 radiation dose-rate history HER trickle starts HER trickle starts Rad/day Rad/day e- sensitive Outgassing storms Rad/day e+ sensitive

Run-6 radiation dose-rate history BE Diamond (e+ sensitive) BW Diamond (e- sensitive) Dose rate (mrad/s) Incoming LER (e+) Vacuum Incoming HER (e-) Vacuum Pressure (nT)

Tuned noise thresholds and tightened time windows Background impact (2): Detector Occupancies (5-15 %) Tuned noise thresholds and tightened time windows % % SVT East: e+ sensitive SVT Layer 1 East SVT Layer 1 West SVT West: e- sensitive % % EMC Total DCH Total

Background impact (3): spikes and dead time

Disentangling background sources: experiments Collimation & steering (“Kill it all !”) Distant betatron & momentum collimators Steer around IP aperture restrictions & minimize beam losses near the Interaction Point Pressure bumps => calibrate response (“Can we make it worse?”) Localized pressure bumps in IR straight by heating NEG pumps Increase <ring pressure> by turning off ion pumps - or take advantage of multipactoring (LER)! Beam-current dependence (“What does it look like?”) Synchrotron radiation ( ~ IB) , or lost particles (~ P(IB ) * IB ~ Pbase IB + Pdynamic IB2 ) ? Identify source location by correlating with base & dynamic pressure in various sections of the ring Extrapolate to future machine performance => plan strategies

Background characterization measurements Data: Jan 04 (before therrmal outgassing crisis) Background characterization measurements Step 1: Beam-current scans  single-beam terms

Step 2: L & beam-beam terms EMC cluster multiplicity SVT occupancy (FL1 M01-f) Beam-beam term present in all subdetectors fluctuations, short - & long-term  parametrization optimistic ? Total occupancy minus HER single beam minus LER single beam These background parameterization & projections as of Jan 2004

LER contribution very small These background parameterization & projections as of Jan 2004 Step 3: Background Parametrizations DCH example: total current & occupancies Step 4: Background Extrapolations IDCH = 60 L Tracking efficiency drops by roughly 1% per 3% occupancy DCH PEP-II parameter projections (as of Jan 2004 !) LER contribution very small

# clusters (seed > 10 MeV EMC These background parameterization & projections as of Jan 2004 # of crystals used in cluster finding # crystals > 1 MeV Physics events have ~110 digis and 8 clusters Long term impact on physics analysis to be quantified # clusters (seed > 10 MeV

A major DCH electronics upgrade was launched to mitigate this DCH + TRG When combined with higher trigger rates, long read-out time was predicted (2004) to lead to unacceptable deadtime. A major DCH electronics upgrade was launched to mitigate this These background parameterization & projections as of Jan 2004

Background Monitoring Tools SVTRAD pin diodes + diamonds dose rates, dose / injection DCH high voltage current DIRC PMTs scaler rates IFR high voltage current Fast Control & Timing deadtime, L1 rates, time wrt injection Level 3 Trigger subdetector occupancies Neutron counters scaler rates CsI detectors in IR region (logarithmic response) Use background levels normalized to 2004 characterization whenever possible All update in small intervals (1-5 seconds)

Normalized background monitoring (measured/model) BE diamond (e+ sensitive) BW diamond (e- sensitive) Ratio Lots of vacuum work during pre-Run 6 downtime resulted in a long time to scrub. HER vacuum performance better than predictions from Jan’04 L1 Trigger Rate DCH Background Run 6 backgrounds consistent with characterization. Trigger rate somewhat larger than characterization  higher deadtime.

Stored-beam background history Monitored continously Reviewed weekly Stored-beam background history IDCH, msrd/pred DCH current normalized to Jan 04 background data Beam currents Limited by BaBar deadtime HER Q5 NEG outgassing L1 Deadtime (%)

Injection- & trickle- background history Monitor by integrating SVTRAD diode signals over 12 ms after each injection SVT electronics are sometimes “upset” by exposures greater than 50 mrad / injection. HER injection-quality monitor LER injection-quality monitor

Injection- & trickle- background history Monitor using triggers gated around the passing of the injected bunch (1 ms x 15 ms) Injection contaminates the BaBar physics data sample if backgrounds endure too long HER injection-quality monitor LER injection-quality monitor

Background impact matrix L = LER, H = HER Background impact matrix Bgd source Beam-gas (brems) Beam-gas (Coulomb) Luminosity Beam-beam + tune space Tunnel halo Injection problems Trickle bgd Thermal outgassing Bgd bursts, L+T instab'lty Rad. abort SVT dose DCH spikes DCH current DIRC PM rate L1 rate: dead t EMC dose IFR rate / I L+H H L ? L ? X masked see M. Sullivan's talk

Background cure matrix Bgd source Beam-gas (brems) Beam-gas (Coulomb) Luminosity Beam-beam + tune space Tunnel halo Injection losses Trickle bgd Thermal outgassing Bgd bursts, L+T instab'lty Rad. abort SVT dose DCH spikes DCH current DIRC PM rate L1 rate: dead t CsI rad dose IFR rate / I TSP's + NEG's TSP's TSP's+ NEG's Scrub Collims : ( Pb shield Trickle, tune Trickle, collim Trickle, collims Steel wall Trickle, ops Ops tuning mostly masked see M. Sullivan's talk

What I did not talk about (or barely alluded to) … Trickle-injection issues, trickle backgrounds & diagnostics Dust (aka 'trapped') events Detector upgrades to mitigate impact of machine backgrounds on dead time: front-end electronics (SVT, DCH, DIRC) dataflow diagnostics & throughput L1 trigger Beam-beam collimation Neutrons Background simulations Impact of backgrounds on physics analysis

Conclusion: the present background "climate" at PEP-II Monitor, understand, measure, monitor - all you can, all the time! Those backgrounds which are strictly steady-state. no longer are an issue Coulombs & brems under control by scrubbing & vacuum upgrades luminosity backgrounds (DIRC, DCH) somewhat mitigated by Pb shielding around Q2 septum & FD beam pipe would have become more of an issue if we could have doubled L ! beam-beam bacgrounds greatly helped by running at constant current (trickle  stability) some mitigation from LER collimators - but HOM limited their use very susbtantial front-end electronics, dataflow and trigger upgrades eliminated dead-time bottlenecks Injection background: a nightmare of the past (trickle!) With ever-increasing currents, today's bgd issues = HOM-induced thermal outgassing by NEG's (solved) background bursts & beam instabilities from arcing heating-triggered vacuum leaks (BPM buttons, vacuum pipe fatigue…) …as will be now detailed by M. Sullivan

Backup slides

Global issues: BaBar vs. Belle Task force (Summer 2004…) Relative importance of the various background sources? only common one seems to be beam-gas (before outgasing storms) Belle important: SR, beam-gas, Touschek. What is the main long-term limitation? no concern (?): luminosity, beam-beam BaBar limiting (in 2004; now solved): thermal outgassing (beam-gas) limiting (or so we thought): luminosity backgrounds, HER b-g, beam-beam no concern: SR, Touschek Can we understand quantitatively how different / similar our backgrounds are? excluding or subtracting (for Babar) beam-gas backgrounds in the mid-plane (magnetic separation) the thermal outgassing problem why is the L background so small in Belle (and did it stay that way...) ? why is the Touschek background so large in Belle…or is it? where (and when) did the main limitations appear?

Steady-state backgrounds: physical processes Synchrotron-radiation X-rays Power (~ 70 kW in 1012 ’s within +- 5 m..): mostly separation dipoles Background: mostly HER IP quadrupoles Duck it if you can! otherwise mask it, but watch out for multiple bounces ( Belle- at first!) Masking very effective: SR backgrounds not a problem in BaBar Cool it well - or else! Lost-particle backgrounds Bremsstrahlung: e + gas -> e’ +  (E’ < E) Almost exclusively from the last few (tens of) m ==> vacuum! Coulomb scattering: e + gas -> e’ (E’ = E, but  ) Potentially from the whole ring, depending on limiting apertures and on pressure profile ==> collimation, especially in LER Beam-beam tails ~ Coulomb-like signature Luminosity (e+ e- => e+’ e-’ ) Elm shower debris (radiation + occupancy) + beam-wall p’s (trigger)

Synchrotron-radiation backgrounds SVT relative occupancy Clear synchrotron-radiation signature in Silicon Vertex Detector, traced to a ~ 2 mm cutoff in coverage by a 150 m thick Ta shield...

The “Background Zones” reflect the combined effect of.... beam-line geometry (e.g. bends) optics at the source and at the detector aperture restrictions, both distant (good!) & close-by (bad!) X (mm) Zone 1 X (mm) Zone 2 Bremmsstrahlung Bremmsstrahlung in field-free region Zone 3 X (mm) IP Zone 4 Coulomb scattering in Arcs Bremmsstrahlung

EMC default digi map: luminosity background (N. Barlow) Fwd Bkwd q index E EMC default digi map: luminosity background (N. Barlow) f index W

Collimation LER collimators HER collimators PR04: 4 fixed: x,y,x,y (orbit bumps) horizontal collimators useful to reduce beam-beam background. vertical collimators sometimes help with the LER trickle backgrounds. PR02: 2 adjustables (x) did help with beam-beam tails in some subdetectors, but at the cost of swamping the IFR (and sometimes the DCH) with secondaries, so their usefulness was limited a major source of HOM heating  finally removed HER collimators PR12: 5 fixed: x,y,x,y,E occasionally useful  trickle, beam-beam PR02: 2 adjustable (x) not useful (removed) HER collimators LER collimators

Collimation experiments HER collimators LER collimators Betatron collimation in HER HEB lifetime Colls open Betatron collimation in LER DCH current Collimators closed Orbit bump amplitude at x-collimator (mm) LER abort diode (mR/s) x 2.5

- Turn all DIPs off/on in Arcs 3-11 Pressure-bump experiment I: how much of the HER background is due to distant Coulombs? - Turn all DIPs    off/on in Arcs 3-11 - lifetime (=> distant    backgrounds)    changes by ~ x 1.5 - IR12 BLMs vary by    ~ 40 % - I(DCH) varies by   ~ 4 % - BW diode totally   insensitive HEB lifetime OFF Pump on OFF PR12 BL8072 (-tron collimator) Drift chamber current BW pin diode

Pressure-bump experiment II: NEG heating in incoming e- straight section Create localized P-bumps NEG heating DIPS on/off Measure response of  background monitors Compare relative measured & simulated monitor response to validate Monte Carlo Vacuum gauge reading (nT) Abort diode signal (mR/s) Zone 3 (-40 m) Zone 1 (-8 m) Different regions ==> diff. patterns diff. abs. levels

Yearly dose predicted to exceed 1 Mrad/year by 2007 SVT Yearly dose predicted to exceed 1 Mrad/year by 2007 Backward: Forward: Top East West Bottom NOW 2004 2005 2006 2007 Background now ('04) is ~75% HEB [LEB negligible (!)] Prediction for 2007: 50% HER, 50% L Background strongly  - dependent By 2007 predict 80% occupancy right in MID-plane In layer 1, 10% will be above 20% occupancy It has recently been realized that in the SVT (but not in other subdetectors), a large fraction of the “Luminosity” background is most likely due to a HER-LER beam-gas X-term (but: similar extrap’ltn). the HER single-beam background in Jan 04 is about 2x what it was in 2002  improve?

Trickle-injection background Veto windows The background generated by the trickle injection is concentrated in a narrow time window corresponding to revolutions of the injected bunch. BABAR vetoes this window during data taking to avoid high dead time BABAR rejects a larger region at the analysis phase to guarantee good data quality. The total loss is around 1.5%

Monitor using injection-gated triggers (1 ms x 20 ms) Injection- & trickle- background history DCH trigs LER trickle EMC trigs (always on) LER trickle EMC trigs (always on) HER trickle DCH trigs HER trickle