Low energy Beamstrahlung at CESR and the ILC

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Low energy Beamstrahlung at CESR and the ILC

• Giovanni Bonvicini

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With

• M. Dubrovin, M. Billings, E. Wisniewski, several
  REU students over the years (E. Luckwald, N.
  Detgen, N. Powell, M. West)
• and much help from many LEPP people
• I will discuss both visible (incoherent) and
  microwave (coherent) beamstrahlung (IB and CB)

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Outline

• Why develop low energy beamstrahlung
• Phenomenology of IB
• Phenomenology of CB
• ILC detector concepts
• Current status of CESR IB monitor
• Feasibility of CESR CB observation
• Summary

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What is beamstrahlung

• The radiation of the particles of one beam due to
  the bending force of the EM field of the other
  beam
• Many similarities with SR but
• Also some substantial differences due to very
  short magnet (L?z/2v2),very strong magnet
  (3000T at the ILC). Short magnets produce a much
  broader angular distribution and have different
  coherence properties

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Beam-beam collision (BBC) d.o.f. (gaussian
approximation)
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More realistic cartoon with asymmetric tails
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BBC d.o.f. counting at the ILC

• 7 gaussian transverse d.o.f.
• 2 beam lengths
• At least 4 wake field parameters, and possibly 2
  longitudinal
• currents well measured
• Beam energy spread not measurable by techniques
  described here but affected by properties of BBC
• Beam angle(s) and angular spread(s)?

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Other possible BBC detectors

• Beam-beam deflection via BPMs. Limited to 2
  quantities by Newtons 3rd law. Semi-passive
  device.
• Gamma ray beamstrahlung monitor. Almost certainly
  a powerful device if it can be built with enough
  pixels, interferes with the beam dump (340kW)
• Pairs spectrometer (105 per BBC)

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The rationale for developing CB and IB

• Very complex BBC phenomena in a large dimensional
  space
• Sensitivity to different variables than hard
  beamstrahlung, mainly through observation of
  polarization
• Simple, relatively inexpensive passive devices
  which can be located away from the beam line
• CB may provide imaging of the BBC
• CB so abundant (O(1kW)) so as to be a potential
  disruption for downstream sensors

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IB power (stiff beams)

• CB largely leaves the spectrum unaffected and
  adds a factor N1

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Large angle incoherent power

• Wider angular distribution (compared to
  quadrupole SR) provides main background rejection
• CESR regime exponent is about 10
• ILC regime exponent is very small

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IB power dependence in CESR configuration
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Some examples of IB pattern recognition
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Coherence vs incoherence
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Coherent beamstrahlung

• Coherent synchrotron radiation has been observed
  many times for very short beams
• Coherence condition is ?gt?z (there is also a
  transverse coherence condition, negligible here)
• A similar situation arises when beams are
  separated - coherent beamstrahlung
• Coherent enhancement always proportional to N

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Coherent enhancement at the ILC (dynamic beams,
complete coherence)
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CB coherent enhancement (vacuum, no angular
divergence)

• CP(CB)/P(IB)
• C(?,?)N exp(-(2??z / ?)2) (G. Bonvicini,
  unpublished)
• Angular effects reduce radiation by
• O ((?div/?rad)2) (not important at CESR,
  factor of 100 at the ILC). This gives a maximum
  CB power at the ILC in the neighborhood of 1kW

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Beam pipe shielding

• Beam pipe effects are important for long magnets
  (Heifets, Mikhailichenko, SLAC-AP-083)
• However in the case of beamstrahlung the magnet
  is shorter than the beam. This needs to be
  computed. ILC CB not in doubt

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Main low energy beamstrahlung observables

• Strong current dependence (N3 and N4
  respectively)
• Strong ?z dependence
• Observable dependence on beam-beam offset (very
  strong for CB)
• Correlated side-to-side radiation
• Strongly varying frequency spectrum which peaks
  at lower frequencies

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ILC CB detector concept
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ILC IB detector concept (1-2 mrad)
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Large Angle Beamstrahlung Monitor

• Giovanni Bonvicini,
• Mikhail Dubrovin

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¼ Set-up principal scheme

• Transverse view
• Optic channel
• Mirrors
• PBS
• Chromatic mirrors
• PMT numeration

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Azimuth angle dependence of radiated power

• Radiated power for
• horizontal and vertical polarizations
• Two optic ports are reserved for each direction
  (E and W)

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Set-up general view

• East side of CLEO
• Mirrors and optic port 6m apart from I.P.
• Optic channel with wide band mirrors
• Installed ¼ detector
• Prilim. experiments, VIS and IR PMTs

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On the top of set-up

• Input optics channel
• Radiation profile scanner
• Optics path extension volume

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The ¼ detector

• Input channel
• Polarizing Beam Splitter
• Dichroic filters
• PMTs assembly
• Cooling

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Sensitivity of R6095 vs R316-02
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CESR beam pipe profile
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Check for alignment _at_ 4.2GeV
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2D profile of radiation

• System is aligned
• Clear signal of
• radiation from
• dipoles and quads
• Radiation from IP is covered by this geometry

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Horizontal vertical projections
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Some late surprises
Parameter CESR-C proposal Current IB change
Curr.(mA) 185 80/60 0.15
?z (mm) 10 11.5 0.03
?obs(mr) 11 12-13 0.01-0.001
? (nm) 900-1000 600-700 0.01
Calc. err. N/A N/A 10
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PMT rate correlations with beam currents
RED VIS PMTs for this exp. are R6095 for
visible light
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Records selection

• For further analysis we exclude non-stable
  radiation periods at CESR currents re-fill
• In some cases we leave data for no-beam intervals

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I(e) vs I(e-)

• Depending on shift the 2D plot area of CESR
  currents might be different
• It can be used to search for correlations with
  observed PMT rate

• P1 PMT dark noise
• P2 SR from dipoles and quads
• P3 SR reflected from masks
• P4 Luminosity term
• P5 Coherent beamstrahlung term

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Fit to the rate for one of PMTs

• Rate vs record
• Fit to the observed rate

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Residuals

• In most cases everything is fine with data for
  vertical polarization
• Data can be used for all day in the same fit
• Pull distribution is consistent with statistics

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Poor residuals for horizontal polarization

• Horizontal polarization is more sensitive to
  CESR tuning
• Data should be split for uniformly performing
  periods to get fit consistent with statistics

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Fitting strategies and results

• Separate fit for each PMT (no constrains for
  correlations)
• We find stable results for vertical polarization,
  use only PMT1,3
• Fix / float coefficients for correlating terms
• Good quality fits are achieved in folowing cases

• Fit1 w/o LUM and CBR terms (p4,p5 fixed to 0)
• fit returns 10-30 fraction of I term that
  does not look right
• Fit2 Const, SR, and CBR terms are float (p3,p4
  fixed to 0)
• CBR for RED 7-16 (3-5 sigma significance)
  !!!
• CBR for VIS always consistent with 0 !!!
• Fit3 Const, SR, and LUM terms are float (p3,p5
  fixed to 0)
• LUM term is 2 times larger than CBR in Fit2
  (arithmetic effect)

Term ltRategt, Hz Fraction,
const
I(e-)
I(e)
I(e-) I(e)
I(e-) I(e) I(e)
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Discussion of results

• Fit2 is the only indication on CBR so far
• But CBR term can be easily superimposed by the
  LUM term
• Const term is not a constant!
• It has 1hour time constant for relaxation
  after illumination depends on radiation
  background in the hall. Needs in special
  calibration.
• We may look at West-East and double-port
  correlations need to install full scale setup
• We need to improve cooling of IR PMTs

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The best try with IR R316-02

• Should be operating _at_ T-40C
• Use cold gas N2
• Cooling is not effective

• Noise rate varies with temperature
• No correlated signal have been observed

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Acknowledgments
We appreciate many people who were involved or
helped us to work on this project

• Mike Billing
• John Sikora
• Stu Peck
• Mike Comfort
• Yulin Li
• Sasha Temnykh
• Richard Ehrlich
• Steven Gray

• Scott Chapman
• John Dobbins
• John Galander
• Valera Mdjidzade
• Georg Trout
• Margee Carrier
• et al.

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Summary

• ¼ of setup is installed on CESR and aligned
• Parts for the full scale set-up are produced at
  WSU and we are working on their installation
• Preliminary measurements show that system can
  observe VIS and RED radiation from IP/magnets
  region
• Experiencing technical problems with observation
  of IR Need in better cooling system for IR PMTs
• Working on improvement of the cooling system for
  IR band

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CB Observability at CESR

• Radiated power is propagating essentially in
  waveguide mode
• A short beam is still crucial. Observability at
  KEK-B (?z 6mm) appears very promising
• Waves will probably propagate in TM mode (M.
  Billings). TM cutoff is 0.82d and TM maximum
  power (for ?z10mm) is 2 pJ per BBC (1.7d and 2nJ
  for TE mode)
• Observation possible at two BPM stations, located
  at 0.68m and 3.6m from the IP respectively(M.
  Billings). One can look at both time and
  frequency domain
• Beam pipe bottleneck at SR mask a potential
  problem
• E. Wisniewski, S. Belomestnykh, M. Billings,
  computing the magnetic wake fields at the BPMs

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CESR beam pipe profile
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Possible plan for CESR

• Compare vacuum CB expectation vs ABCI-computed
  wake fields at BPMs
• If comparison is favorable, try a test with
  highest beam population, shortest beam

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Future developments

• Design both detectors for the ILC
• Quadrupole radiation (A. Mikhailichenko)
• Detailed CB simulation
• Observables available in CB imaging
• Beam pipe effects and the possibility of
  waveguide CB at the ILC

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Conclusions

• Some progress in IB. We have hopes that it will
  be seen this fall
• CB at the ILC will certainly be present in large
  but not threatening amounts. Potentially
  extremely useful for BBC imaging
• CB observation at present accelerators would be
  most useful
• If both these techniques develop, there is a
  tremendous amount of work to do

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Spare slides
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PMT and HV divider

• Hamamatsu PMTs
• VIS R6095
• IR R316-02
• HV divider
• E990-07

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Hamamatsu PMT R6095
28mm head on PMT for 300-650nm
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Hamamatsu PMT R316-02
28mm head on PMT for Near IR Detection from
400-1200nm
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Current version of N2 cooling

• IR photocathods are blown by the cold N2
• Non-efficient cooling scheme, large termal flows
  through the metallic parts
• Working on improvement

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Glan-Taylor Polarizers




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Hot Mirrors



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Cold Mirrors



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45 Reflective Dichroic Color Filters



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Mirror coating



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Glass Specifications


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The best try with IR PMT R316-02(next day, March
31, 2005)
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Beam-beam vertical separation

• For beam-beam separation one would expect a
  camel-back dependence of CBR power vs
  separation distance

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Beam-beam vertical separation (cont.)

• 2004/10/14 dedicated shift
• John Sikora Mikhail Dubrovin
• VIS PMTs ok but does not show CBR
• IR PMTs does not show any correlations with beam
  currents, presumably are not sensitive to IR