Machine Background Status & issues in BaBar/PEP-II. Background sources Characterization experiments Long-term projections & vulnerabilities. W. Kozanecki, CEA-Saclay. Background sources in P E P-II. Synchrotron radiation (this bkg negligible in PEP-II) Beam-gas (bremsstrahlung + Coulomb) - PowerPoint PPT Presentation
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beam-gas x- term: BLHbg ~ cLH * IL * IH (LEB+HEB, out of collision) (?)
Luminosity (radiative-Bhabha debris) – Luminosity (radiative-Bhabha debris) – major concern as L major concern as L BP ~ dP * L (strictly linear with L)
Beam-beam tailsBeam-beam tails
from LER tails: BL, bb ~ IL * fL(L,H +/-)
from HER tails: BH, bb ~ IH * fH(L,H +/-)
Trickle background: BTrickle background: BLi Li ,, BBHiHi (injected-beam quality/orbit + beam-beam)
Touschek: BTouschek: BLTLT (signature somewhat similar to bremsstrahlung; so far small)
A potential limitation A potential limitation for run 5!for run 5!
W. Kozanecki BBBTF meeting, 27 (27) May 04
HOM interference in IRHOM interference in IR
VGCC3027 (incoming
LEB)
VGCC2187 (HER sensitive)
BE diamond (LEB sensitive)
BW diamond (HEB
sensitive)
Collision phase = [t(e-) - t(e+)]IP
<ZIP> (BaBar)
Data: 13 Apr 04
W. Kozanecki BBBTF meeting, 27 (27) May 04
Thermal outgasssing now limits the beam currentThermal outgasssing now limits the beam current
VGCC3027 (nT, incoming LEB)
VGCC2187 (nT, HEB side)
HER current
LER current
BE diamond (mR/s)
Babar dead time (%)
W. Kozanecki BBBTF meeting, 27 (27) May 04
Summary (I)Summary (I)
TrickleTrickle injection injection is a major success in terms of improving
machine stability + abort frequency integrated L
overall injection quality
accumulated SVT dose
The associated detector backgrounds appear largely negligible (most – but not all – of the time)
Improved understandingImproved understanding of background & abort sources of background & abort sources genuine radiation aborts down to < 1 /day
clear & reproducible correlations between diamond dose rates, on-line SVT occupancies, dead time, and pressure measurements in incoming HER & LER straights
“lumi [background] is [really due to] lumi”! (except in the SVT – maybe)
Stored-beam backgroundsStored-beam backgrounds (dose rate, data quality, dead time) (dose rate, data quality, dead time) OK most of the time (& better w/ trickle) until recently
thermal outgassing now limits beam currents (primarily in the HER)
W. Kozanecki BBBTF meeting, 27 (27) May 04
Summary (II)Summary (II)
Background Background characterization experiments characterization experiments were highly valuable in identifying the origin, magnitude & impact of
single- & two-beam backgrounds.
On the long term, the dominant backgrounds are expected to be, in order of decreasing importance:
radiative-Bhabha debris (all subdetectors), incl. a significant neutron flux
HER beam-gas (SVT, TRG), especially that due to thermal outgassing
beam-beam tails & their fluctuations (DCH, EMC, TRG, IFR wall!)
In the medium term (In the medium term (2005-072005-07), the ), the main vulnerabilitiesmain vulnerabilities are are beam-gas backgrounds from HOM-related thermal outgassing as I+,- high dead time associated with growing data volume & trigger rates
[Mainly HER beam-gas (TRG, SVT) & radiative-Bhabha debris (DCH)]
high occupancy and radiation ageing in the mid-plane of the SVT local loss of tracking coverage (?)
closely monitor the HER single-beam background & keep it similar to 2002 levels
high n flux (~ 1 MeV) correlates with L, some spikes is it an issue?
• Trickle: only average shown. Must be able to accomodate large fluctuations.
• Beam-beam: only best case shown. Data taken since then show beam-beam can easily be 2 x larger – not counting short-term fluctuations.
• LER single beamLER single beam: small (mostly beam-gas), no fluctuations expected
W. Kozanecki BBBTF meeting, 27 (27) May 04
W. Kozanecki BBBTF meeting, 27 (27) May 04
Time evolution of the thermal outgassing backgroundTime evolution of the thermal outgassing background
The The different time dependenciesdifferent time dependencies of the of the pressure readings allow to fit the pressure readings allow to fit the measured background level (BE measured background level (BE diamond) as a linear combination of 4 diamond) as a linear combination of 4 LER Pumps, on a LER Pumps, on a fill by fill basisfill by fill basis
The 4 pumps are located in the The 4 pumps are located in the incoming LER straight and all exhibit incoming LER straight and all exhibit HOM-induced thermal outgassing (e.g. HOM-induced thermal outgassing (e.g. change of pressure associated with change of pressure associated with change of bunch length)change of bunch length)
A very satisfactory A very satisfactory description of the description of the background was obtained in all casesbackground was obtained in all cases
The The sensitivity coefficientssensitivity coefficients for each for each pump were then extracted. They pump were then extracted. They represent the Nrepresent the N22-equivalent pressure -equivalent pressure integral with the same time integral with the same time dependence as the pump reading.dependence as the pump reading.
Fill March 28, 12pm-3 pm
0 3 h1 2
Data points
VP3044
VGCC3027
Fit
VP3147
End of injection
mR
ad/s
W. Kozanecki BBBTF meeting, 27 (27) May 04
Evolution of the sensitivity coefficients (Apr 04 outgassing storms)Evolution of the sensitivity coefficients (Apr 04 outgassing storms)
The coefficients are The coefficients are normalizednormalized to to their pre-NEG activation values , their pre-NEG activation values , indicated by the red line (1 point per indicated by the red line (1 point per long fill)long fill)
The The backgroundbackground problem was problem was notnot related to an related to an increaseincrease in in locallocal pressure reading (at the pump) but pressure reading (at the pump) but to a to a huge increasehuge increase in background in background sensitivitysensitivity
The problem was solved The problem was solved (serendipitously) by:(serendipitously) by:
continued processing
opening collimator jaw(s)
changing in bunch pattern
These changes had different actions on the various background drivers
VP3044 VGCC3027
Days in March (April 1 = day 32)
10
200
W. Kozanecki BBBTF meeting, 27 (27) May 04
Mismatch (x 10-100) betw. time evolution of Mismatch (x 10-100) betw. time evolution of msrd pmsrd p and of and of bkgdbkgd
BE diamond (LEB
sensitive)
NE
G
actv
td
VGCC3027 (incoming
LEB)
NE
G
actv
td
BW diamond (HEB sensitive)
NE
G
actv
td
VGCC2187 (HER
sensitive)
NE
G
actv
td
demonstrated by detailed analysis of local pressure contributions to background signals
W. Kozanecki BBBTF meeting, 27 (27) May 04
NE
G a
ctvt
d
NE
G a
ctvt
d
NE
G a
ctvt
d
NE
G a
ctvt
d
Large variety of processing times, mechanisms, & bkg sensitivitiesLarge variety of processing times, mechanisms, & bkg sensitivities