Download - Nov. 14, 2007Lasers in hep1 LASERS IN HIGH ENERGY PHYSICS Adrian Melissinos University of Rochester Diagnostics for high energy electron beams Photoinjectors.

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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 12

Δp/p = (Momentum compaction ~5×103)·(strain ~4×10-8) ~ 2×10-4

LEP

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 14

THE SLAC POLARIZED ELECTRON SOURCE

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 16

LASER SYSTEM FOR THEZEUTHEN (DESY-BERLIN) PHOTOINJECTOR

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 21

E-144 Physical layout of the beams and detectors

Nov. 14, 2007 Lasers in hep 22

The Final Focus Test Beam in the SLAC Switchyard

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 24

POSITRON YIELD vs. LASER INTENSITY

Nov. 14, 2007 Lasers in hep 25

POSITRON YIELD vs. 1/Y

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

Nov. 14, 2007 Lasers in hep 41

END

Nov. 14, 2007 Lasers in hep 42

BRFT limit (rotation)

BRFT limit (ellipticity)

PVLAS signal reported in 2006 (rotation)

PVLAS signal reported in 2006 (ellipticity)