1. 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

2. 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+)

3. 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 ( )
HER current (mA)
2000
Drift chamber current (A)
1500

4. 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)

5. 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

6. 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

7. 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 m of the IR
Vacuum pipe / mask apertures

8. Spatial origin of beam-gas backgroundsIP
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

9. 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

10. 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)

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

12. 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)

13. 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

14. Background impact (3): spikes and dead time

15. 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

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

17. 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

18. 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

19. # 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

20. 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

21. 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)

22. 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.

23. 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 (%)

24. 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

25. 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

26. 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

27. 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

28. 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

29. 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

30. Backup slides

32. 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?

33. 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)

34. 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...

35. 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

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

37. 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

38. 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

39. - 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

40. 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

41. 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, % 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?

43. 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%

44. 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