Remote Synchrotron Radiation-Based Longitudinal Beam Diagnostics

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Remote Synchrotron Radiation-Based Longitudinal
Beam Diagnostics

• J. Byrd, S. De Santis, A. Ratti, M. Zolotorev,
  Yian Yin

LBNL - november 1st, 2005
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Specifications for longitudinal measurements of
the LHC beam
LHC SPECIFICATIONS C. Fischer, LHC-B-ES-0005
- Abort Gap Monitor
- Debunched Beam - Bunch Tails - Ghost Bunches

• Bunch Core
• (real time)

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Fiberoptic-based Synchrotron Radiation Diagnostics
coupling the synchrotron radiation emitted by
the beam into an optical fiber
PROS
CONS
Synchrotron light can be taken to an easily
accessible location, at a distance from the
diagnostics port, inexpensively. Negligible
attenuation and dispersion(0.5 dB/Km, 0.1 ps/nm2
Km) Reliable components of common use in the
telecommunication industry No electronic
component has to be near the light port or in
other inaccessible areas.
Losses due to coupling of synchrotron
radiation into the optical fiber ( 3 dB ?)
Might need compensation of transverse beam
motion 10 bandwidth is easily achievable.
Larger bandwidths require special care (acromatic
lenses, dispersion compensation, etc.)
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Main Characteristics of Optical Fibers
commercially available optical fibers
Attenuation
Dispersion
850 nm 2.5 dB/Km 1310 nm
0.4 dB/Km 1550 nm 0.2 dB/Km
850 nm (multimode fiber) 6 GHz/100 m
bandwidth requires sampling electronics in
tunnel 1310 nm (non dispersion shifted fiber)
- nominal zero dispersion at 1310 nm -
total delay for 10 BW 2 ps/100 m 1550
nm(zero-dispersion shifted fibers are no more
manifactured) - D 17 ps/nm/Km - total
delay for 10 BW 260 ps/100 m
Dispersion at these wavelengths can be reduced by
compensating fibers (DCF, D -100 ps/nm/Km)
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Available photon fluxes from the LHC longitudinal
diagnostics light port
Shaded areas show 10 bandwidth around 850, 1310
and 1550 nm wavelengths.At top energy the three
bands are roughly equivalent (but 1310 nm is
intrinsically dispersion free).
At injection energy the emission intensity is
dominated by the SC undulator. The maximum flux
is at 850 nm, 5 times higher than 1310 nm and 30
times higher than 1550 nm. Maximum flux is anyway
a factor of 40 lower than at 7 TeV.
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Optical Fiber of Choice
commercially available 1310 nm single-mode, non
dispersion shifted fiber
This fiber, of standard use in telecommunications,
seems to be the optimal solution, due to its
excellent characteristics. It is available in a
radiation hard variety as well.
Dispersion
Attenuation
0.4 dB/Km
- nominal zero dispersion at 1310 nm- total
delay for 10 BW 2 ps/100 m
????? D(?) L
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Available photon fluxes from the LHC longitudinal
diagnostics light port
The shaded area shows a 10 bandwidth around the
1310 nm wavelength.At injection energy, the
intensity is only five times lower than its
maximum at 850 nm.There is a factor of 100
reduction in intensity between 7 TeV and 450 GeV.
(Facchini)
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Uses of synchrotron Radiation for Longitudinal
Diagnostics of the Beam

• Streak camera - Measurement of the beam
  longitudinal profile, bunch phase with picosecond
  resolution. (current ALS experiment)
• Optical sampling scope - Mapping of charge
  density around the machine.
• Photodiode - Accurate measurement of bunch charge
  (integrating over many turns, may need cooling).
• Gated Photomultiplier - Used as abort gap monitor
  at the Tevatron. (PAC 05)

By using the synchrotron light transported on an
optical fiber, it is possible to use many
different devices on the same port. Instruments
can be easily swapped and maintained, according
to the particular situation.
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Example ghost bunches measurement at 7 TeV
5 105 protons emit 30 photons/turn in a 10
bandwidth. The electro-optic modulator/fast
pulser combination can map the entire LHC ring,
with the required resolution, every 500
orbits. In the allowed integration time, every
single 50 ps-long region is sampled 200 times. A
70 QE photodiode would accumulate gt4000
counts. We can estimate a total of -6/8 dB from
the coupling into the optical fiber and the
various insertion losses. Main noise sources are
the modulator extintion ratio ( 3 10-3) and the
photodiode dark current ( nA)
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Present State of the ALS Experiments

• Two available beamlines
• 7.2 has a streak camera, but the optics are
  defective for our application. Phase variations
  on the wavefront which results in low coupling
  efficiency.
• 3.1 is good, we might temporarily move the streak
  camera there.
• Experiment run at cost zero (i.e. with the
  components we have).
• Main objective optimizing the coupling of
  synchrotron radiation into the fiber (10 BW).
• We are using 880 nm, rather than 1310.
• Preliminary results indicate 50 efficiency