LASERS IN HIGH ENERGY PHYSICS

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• LASERS IN HIGH ENERGY PHYSICS
• Adrian Melissinos
• University of Rochester
• Diagnostics for high energy electron beams
• Photoinjectors
• Generation of high energy photons
• Interaction with magnetic fields
• Laser acceleration of electrons and ions

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HIGH POWER PULSED LASERS

• Ndglass ? 1064 nm
  
• TiSa ? 820 nm
  (tunable)
• Energy in pulse (after Chirped Pulse
  Amplification)
• 10 1000 mJ Table-top ( 10
  Hz)
• 10 10000 J Facility (
  10-3 Hz)
• Pulse length t 30 1000 fs
• Transverse profile is Gaussian emittance ?
  1 mm-mr
• Diffraction limited focus
• w0 0.43f/D ?
• z0 2.28f/D ?
• Electric Field at the focus
• E 1011 V/cm (for I 1018
  W/cm2)

2w0
2z0
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SCATTERING OF LASER BEAMSFROM HIGH ENERGY
ELECTRONS

• Electron
  ? Ee/me
• Backscattered photon angle ? lt 1/?
• Backscattered photon energy
• ? (4?2?0) / (1 4??0/me
  ?2?2)
• Cross-section classical (Thomson)
• sT (8p/3) (e2/mec2)2 6.710-25
  cm2
• Compton sC (sT/x) (ln x ½ ) x
  4??0/me
• For protons s?-p 10-7 sT
  !!!
• Photon density at focus (for I 1018
  W/cm2)
• ?? 61028 / cm3 compare to N0
  61023/cm3

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A. TYPICAL LASER DIAGNOSTICS

• 1. Transverse beam size
• Shintake monitor The electron beam is
  scanned across an optical grating.
• 2. Longitudinal beam size
• Electro-optic sampling The electric field
  of the passing bunch polarizes a bi-refringent
  crystal. The state of the crystal is probed by a
  short laser pulse.
• 3. Transverse polarization
• Polarized photon scattering Measure the
  (small) asymmetry in the backscattering of
  polarized photons from polarized electrons.
  Coupled with resonant depolarization provides an
  absolute calibration of the beam energy.

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Transverse beam size measurement Shintake et al.
SLAC 1995
Set up standing wave pattern by interfering two
arms of the laser beam
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For given grating spacing the depth of
modulation depends on the beam width along the
direction of the scan. The grating spacing is
determined by the crossing angle
SLAC, FFTB 47 GeV beam, sY 73 nm
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ELECTRO-OPTIC SAMPLING
Detector
Crystal
Electron bunch
Probe laser pulse
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First Electro-optic sampling signal 24 Aug.
1999 _at_ A0
Frequency spectrum of wake fields
Prompt signal
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Principle of single shot measurement Ultra short
laser pulse 30 fs (10 µm) crosses a thin E/O
crystal at an angle. This encodes the time of
passage of the field onto the spatial
polarization profile of the laser pulse. It then
suffices to record with a ccd the image of the
two orthogonal polarizations
E/O crystal
laser pulse
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Femtosecond pulse length measurement -
SLAC A.Cavalieri, D.Fritz, S.Lee, P.Bucksbaum,
D.Reis et al SPPS Collaboration
The electron beam pulse length is adjusted by
changing the compressor phase. A FWHM of 200 fs
is achieved. The synchronization jitter of laser
and beam is shown.
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Scattering of circularly polarized laser light
from transversely polarized electrons introduces
small asymmetry, 10.
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LEP
?p/p (Momentum compaction 5103)(strain
410-8) 210-4
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B. PHOTOINJECTORS

• 1. Polarized electron beams
• Strained GaAs cathode
• Circularly polarized tuned laser
  wavelength (TiSa laser)
• Achieve in excess of 90 polarization
• 2. RF photoinjectors (R. Sheffield) CsTe
  cathode
• FERMILAB?A0, DESY?TTF/FLASH, SLAC
  ?LCLS, etc.
• Charge per pulse
  Q 1 - 10 nC/pulse
• Pulse duration
  1 - 20 ps
• Frequency
  1 - 3 MHz
• Length of pulse train
  1 ms
• Repetition rate
  5 10 Hz

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THE SLAC POLARIZED ELECTRON SOURCE
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RF PHOTOINJECTOR BEAM LINE at A0
Photocathode manipulator
20 m
Capture cavity 14 MeV
Laser path
Rf gun and solenoids
Spectrometer
Compression chicane
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LASER SYSTEM FOR THE ZEUTHEN (DESY-BERLIN)
PHOTOINJECTOR
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PERFORMANCE OF THE ZEUTHEN PHOTOINJECTOR
Streak Camera measurement of single pulse
1 ms
Pulse train top output
bottom oscillator
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C. HIGH ENERGY PHOTONS
Backscattering produces quasi-monochromatic high
energy photons 1963 R.Milburn
(proposal) 1969 J.Ballam et al
SLAC photoproduction expts. 1995
SLAC/E-144 Critical field expts.
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Breakdown of the vacuum by a laser field (with
help from a high energy electron beam) SLAC E144
Ee 47 GeV or ? 9104
Incident photon ?
2.34 eV Backscattered photon ?
27 GeV Laser pulse U 1 J, t
2 ps, A 10 µm2 Laser Intensity
I 51018 W/cm2 Electric field at
focus E ( 2Z0 I )½ 61010 V/cm
When a 47 Gev electron crosses the focus it
sees (in its rest frame) a field E
2 ? E 1.21016 V/cm Ecritical
This is also the basis for the ILC ?-? option
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Photon-photon Scattering
Pair production
In the perturbative domain s eE/?me2n n
number of photons
In strong fields the vacuum can spontaneously
break down
A virtual ee- pair can get on the mass shell if
eE?C mec2 EC
me2c3/eh 1.31016 V/cm Prob/V-T a
E2/p2h exp(-pEc/E) J.Schwinger 1951
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E-144 Physical layout of the beams and detectors
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The Final Focus Test Beam in the SLAC Switchyard
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VIEW OF THE ELECTRON BEAM LINE AND OF THE
LASERe- INTERACTION CHAMBER
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POSITRON YIELD vs. LASER INTENSITY
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POSITRON YIELD vs. 1/Y
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D. LASERS IN STRONG MAGNETIC
FIELDS

• The magnetic field is a source of virtual photons
  (of zero energy)
• Consider (axion-like) particles that couple to
  two photons
• Lint (1/M) EL Bextfa 1/M
  coupling constant (GeV-1)
• Interaction depends on polarization of the laser
  field w.r.t.
• the external magnetic field
  direction
• If ma lt ? real particles can be
  produced the laser field
• is attenuated and
  retarded.
• If ma gt ? only virtual particles can
  be produced the
• laser field is only
  retarded.
• First predicted by V.Weisskopf (1936) for photons
  traversing a magnetic field (involves electron
  box diagram).
• QED for B10 T, L1 m induces ellipticity
  ? 10-15

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Graphs for photon interactions in a magnetic
field
Production of real particle
Production of virtual particles
Regeneration (real particle)
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DETAILS 1. Coherence of axion and laser field
restricts the mass range that can be
explored ma2 2p?/l 2. With the laser
linearly polarized at 450 to the magnetic field
(a) Rotation of polarization (dichroism)
(b) Polarization becomes elliptical
(birefringence) (c) QED birefringence 3.
Detection sensitivity needs modulation of laser
polarization and modulation of
magnetic field. 4. Multiple traversals, N,
through magnetic field Optical delay line
or Fabry-Perot cavity. Signal increases linearly
with N.







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RESULTS all are upper limits on coupling 1/M
Brookhaven-Rochester-Fermilab-Trieste (1993)
ga?? lt 3.610-7 GeV-1 ma lt 0.710-3
eV PVLAS Trieste-Legnaro-Pisa-Ferrara
(2007) ga?? lt 4.810-7 GeV-1 ma lt
1.510-3 eV GammeV Fermilab (2007)
Regeneration experiment ga?? lt 3.210-7
GeV-1 ma lt 0.510-3 eV ga?? lt 510-6
GeV-1 ma lt 210-3 eV QED birefringence
has not been measured as yet. An experiment
had been approved at Fermilab in the 1990s
(F.Nezrick et al) using 2 SSC dipoles
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Most recent limits from PVLAS (9/2007) Similar
to the BRFT limits (1993), but extend the mass
range to 1 meV
The excluded region is below the curves
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Limits from the Fermilab regeneration expt
(9/2007)

• Regeneration limit

BRFT limit from rotation
The excluded region is above the curves
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Global limits on light scalars/pseudoscalars
Note mass range allowed from closure arguments
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E. LASER ACCELERATION
Tightly focused pulsed lasers achieve
ETRANSVERSE 104 GV/m Looks great .. (
compare to ILC 30 MV/m) , .. but (a)
Must create longitudinal field (factor of 10-2
) (b) Length of focal region (typically 100
µm to 1 mm) (c) Transverse dimensions of
focal region 10 µm (gives
rise to space charge issues) (d)
Woodward-Lawson theorem EM field in vacuum can
not lead to acceleration. Possible
structure damage BEST SOLUTION (so far) -
Blast a renewable target (gas jet) -
Excite a wave in a plasma (can not be damaged)
using a laser, or better, an
electron beam
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EXAMPLES
(a)Self-modulated laser wake field (b)Forced
laser wake field tLASER gtgt ?PLASMA
tLASER
?PLASMA ?PLASMA
100 µm 300 fs ( for ne 1018/cm3 )
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TYPICAL RESULT V.Malka et al, Science 298,
1596 (2002)

Laser TiSa ? 820 nm, U 1 J, t 30 fs, A
10 µm2, f 10 Hz
Electron beam Thermal spectrum, T 18 MeV Max
energy 200 MeV, Total charge 5 nC
When using solid targets thermal protons and
ions, E lt 10 MeV are produced
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ENERGETICS OF LASER ACCELERATION
Consider one of the 192 beams of NIF (National
Ignition Facility) at Livermore
20 kJ 10 ns long pulse, rep. rate 1 in
30 min.
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Energy stored/per pulse in the two ILC beams
U 2eNe 1010 Ee 250 GeV 800 J
Assuming that we can couple a significant
part of the lasers optical energy ( 5) to the
e-/e beams, the NIF laser would be
energetically OK for a single pulse. However to
have adequate luminosity we need a
repetition frequency f 104 Hz which is 107
times higher than what NIF-type lasers can
provide today
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PLASMA WAKEFIELD ACCELERATION SLAC-UCLA-USC
I.Blumenfeld et al. Nature 445,741 (2007)
Lithium vapor, 10 cm long , ne 2.71017 /cm3,
Ee 41 Gev
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Laser Parametric Converter
Wish to measure the gravitational field of
the Tevatron beam! Modulate the proton beam
to ? 2L 30 m. At some distance from the
beam line, install a high finesse Fabry-Perot
cavity of length L 15 m

15 m
Optical Cavity
30 m
Filled beam buckets
The cavity has excited modes spaced at the free
spectral range
f c/2L 10 MHz
Any perturbation at 10 MHz of dimensionless
amplitude h populates the excited modes and
gives rise to 10 MHz sidebands
Ps P0 (h
Q)2 For reasonable
values, Q 1014 , P0 10 W and recording one
photon per second, one can detect
h 10-24
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Metric perturbation induced at a distance b from
the beam, lt h gt (4G/c2) ?m
(N/2pR) ln(2?) Bunch length ctB gtgt b, ? E/m, R
Tevatron radius, N circulating protons If
G GN h 10-40
hopeless !! If gravity becomes strong at
this highly relativistic velocity
G GS GN(MP/MS)2 For Ms
lt MP/108 108 TeV
h gt 10-24 The effect is detectable in 100 s of
integration !

• Noise and false signal issues could be severe
• A 1986 Fermilab expt used a s.c. microwave
  parametric converter and set a limit MS gt 106
  TeV

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END
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BRFT limit (rotation) BRFT limit
(ellipticity)
PVLAS signal reported in 2006 (rotation)
PVLAS signal reported in 2006
(ellipticity)