Nov. 14, 2007 Lasers in hep 1
LASERS IN HIGH ENERGY PHYSICS
Adrian MelissinosUniversity of Rochester
Diagnostics for high energy electron beams Photoinjectors Generation of high energy photons Interaction with magnetic fields Laser “acceleration” of electrons and ions
Nov. 14, 2007 Lasers in hep 2
HIGH POWER PULSED LASERS Nd:glass λ =1064 nm
Ti:Sa λ = 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 τ = 30 – 1000 fsTransverse profile is Gaussian ; emittance = λ ~ 1 mm-mrDiffraction limited focus
w0 = 0.43[f/D] λ
z0 = 2.28[f/D] λ
Electric Field at the focus E ~ 1011 V/cm (for I = 1018 W/cm2)
2w0
2z0
Nov. 14, 2007 Lasers in hep 3
SCATTERING OF LASER BEAMSFROM HIGH ENERGY ELECTRONS
Electron γ = Ee/me
Backscattered photon angle θ < 1/γBackscattered photon energy
ω´ = (4γ2ω0) / (1 + 4γω0/me + γ2θ2)
Cross-section classical (Thomson)
σT = (8π/3) (e2/mec2)2 = 6.7×10-25 cm2
Compton σC = (σT/x) (ℓn x + ½ ) x = 4γω0/me
For protons σγ-p ~ 10-7 σT !!!
Photon density at focus (for I = 1018 W/cm2) ργ ~ 6×1028 / cm3 compare to N0 = 6×1023/cm3
Nov. 14, 2007 Lasers in hep 4
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.
Nov. 14, 2007 Lasers in hep 5
Transverse beam size measurement Shintake et al. SLAC 1995
Set up standing wave patternby interfering two arms of the laser beam
Nov. 14, 2007 Lasers in hep 6
For given grating spacing the depth of modulationdepends on the beam width along the direction of the scan.
The grating spacing is determined by the crossing angle
SLAC, FFTB 47 GeV beam, σY = 73 nm
Nov. 14, 2007 Lasers in hep 7
ELECTRO-OPTIC SAMPLING
Crystal
Electron bunch
Probe laser pulse
Detector
Nov. 14, 2007 Lasers in hep 8
First Electro-optic sampling signal 24 Aug. 1999 @ A0
Prompt signal
Frequency
spectrum of
wake fields
Nov. 14, 2007 Lasers in hep 9
Principle of single shot measurementUltra short laser pulse ~ 30 fs (10 μm) crosses a thin E/O crystalat an angle. This encodes the time of passage of the field onto the
spatial polarization profile of the laser pulse. It then suffices torecord with a ccd the image of the two orthogonal polarizations
laser pulse
E/O crystal
Nov. 14, 2007 Lasers in hep 10
Femtosecond pulse length measurement - SLACA.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.
Nov. 14, 2007 Lasers in hep 11
Scattering of circularly polarized laser light from transverselypolarized electrons introduces small asymmetry, ~ 10%.
Nov. 14, 2007 Lasers in hep 13
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
B. PHOTOINJECTORS
Nov. 14, 2007 Lasers in hep 15
RF PHOTOINJECTOR & BEAM LINE at A0
Capture cavity ~14 MeV
Rf gun and solenoids
Photocathode manipulator
Compression chicaneSpectrometer
~ 20 m
Laser path
Nov. 14, 2007 Lasers in hep 17
PERFORMANCE OF THE ZEUTHENPHOTOINJECTOR
1 ms
Pulse train top: output bottom: oscillator
Streak Camera measurement of single pulse
Nov. 14, 2007 Lasers in hep 18
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.
Nov. 14, 2007 Lasers in hep 19
Breakdown of the vacuum by a laser field(with help from a high energy electron beam)
SLAC E144
Ee = 47 GeV or γ = 9×104
Incident photon ω = 2.34 eVBackscattered photon ω´= 27 GeV Laser pulse U = 1 J, τ = 2 ps, A = 10 μm2
Laser Intensity I = 5×1018 W/cm2
Electric field at focus E = ( 2Z0 I )½ = 6×1010 V/cm
When a 47 Gev electron crosses the focus it sees (in it’srest frame) a field E* = 2 γ E ~ 1.2×1016 V/cm ~ Ecritical
This is also the basis for the ILC γ-γ option
Nov. 14, 2007 Lasers in hep 20
A virtual e+e- pair can get on the mass shell if eEλC = mec2
EC = me2c3/eħ = 1.3×1016 V/cm
Prob/V-T = [α E2/π2ħ] exp(-πEc/E*) J.Schwinger 1951
Photon-photon
Scattering Pair production
In the perturbative domain σ ~ [eE/ωme]2n n = number of photons
In strong fields the vacuum can spontaneously break down
Nov. 14, 2007 Lasers in hep 23
VIEW OF THE ELECTRON BEAM LINE AND OF THE LASER–e- INTERACTION CHAMBER
Nov. 14, 2007 Lasers in hep 26
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• Bextφa 1/M coupling constant (GeV-1)
Interaction depends on polarization of the laser field w.r.t. the external magnetic field directionIf ma < ω real particles can be produced; the laser field
is attenuated and retarded. If ma > ω 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 B=10 T, L=1 m induces ellipticity ψ ~ 10-15
Nov. 14, 2007 Lasers in hep 27
Graphs for photon interactions in a magnetic field
Production of real
particle
Production of
virtual particles
Regeneration (real particle)
Nov. 14, 2007 Lasers in hep 28
DETAILS1. Coherence of “axion” and laser field restricts the mass range that can be explored ma
2 ≤ 2πω/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.
Nov. 14, 2007 Lasers in hep 29
RESULTS : all are upper limits on coupling 1/M
Brookhaven-Rochester-Fermilab-Trieste (1993) gaγγ < 3.6×10-7 GeV-1 ma < 0.7×10-3 eV
“PVLAS” Trieste-Legnaro-Pisa-Ferrara (2007) gaγγ < 4.8×10-7 GeV-1 ma < 1.5×10-3 eV “GammeV” Fermilab (2007) Regeneration experiment gaγγ < 3.2×10-7 GeV-1 ma < 0.5×10-3 eV gaγγ < 5×10-6 GeV-1 ma < 2×10-3 eV
QED birefringence has not been measured as yet. An experiment had been approved at Fermilab in the 1990’s (F.Nezrick et al) using 2 SSC dipoles
Nov. 14, 2007 Lasers in hep 30
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
Nov. 14, 2007 Lasers in hep 31
Limits from the Fermilab regeneration expt (9/2007)
Regeneration limit
BRFT limit from
rotation
The excluded region is above the curves
Nov. 14, 2007 Lasers in hep 32
“Global” limits on light scalars/pseudoscalars
Note mass range allowed from “closure” arguments
Nov. 14, 2007 Lasers in hep 33
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
Nov. 14, 2007 Lasers in hep 34
EXAMPLES
(a)Self-modulated laser wake field (b)Forced laser wake field τLASER >> λPLASMA τLASER ~ λPLASMA
λPLASMA ~ 100 μm = 300 fs ( for ne = 1018/cm3 )
Nov. 14, 2007 Lasers in hep 35
TYPICAL RESULT V.Malka et al, Science 298, 1596 (2002)
Laser: TiSa λ = 820 nm, U =1 J, τ = 30 fs, A = 10 μm2, f = 10 Hz
Electron beam: Thermal spectrum, T = 18 MeVMax energy 200 MeV, Total charge 5 nC
When using solid targets
“thermal” protons and ions,
E < 10 MeV are produced
Nov. 14, 2007 Lasers in hep 36
ENERGETICS OF LASER ACCELERATIONConsider one of the 192 beams of NIF
(National Ignition Facility) at Livermore
20 kJ 10 ns long pulse, rep. rate 1 in 30 min.
Nov. 14, 2007 Lasers in hep 37
Energy stored/per pulse in the two ILC beams
U = 2e·[Ne = 1010 ]·[Ee = 250 GeV] = 800 J
Assuming that we can couple a significant part
of the laser’s 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
Nov. 14, 2007 Lasers in hep 38
PLASMA WAKEFIELD ACCELERATION SLAC-UCLA-USC I.Blumenfeld et al. Nature 445,741 (2007)
Lithium vapor, 10 cm long , ne = 2.7×1017 /cm3, Ee = 41 Gev
Nov. 14, 2007 Lasers in hep 39
Laser Parametric Converter Wish to measure the gravitational field of the Tevatron beam!
Modulate the proton beam to λ = 2L ~ 30 m. At some distance from the
beam line, install a high finesse Fabry-Perot cavity of length L ~ 15 m
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
Optical Cavity
15 m
30 mFilled beam buckets
The cavity has excited modes spaced at the “free spectral range”
f = c/2L = 10 MHz
Nov. 14, 2007 Lasers in hep 40
Metric perturbation induced at a distance b from the beam,
< h > ~ (4G/c2) γm (N/2πR) ln(2γ)
Bunch length cτB >> b, γ = 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 < MP/108 = 108 TeV
h > 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 > 106 TeV
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