Nov. 14, 2007Lasers in hep1 LASERS IN HIGH ENERGY PHYSICS Adrian Melissinos University of Rochester...
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Nov. 14, 2007Lasers in hep1 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 Slide 2 Nov. 14, 2007Lasers in hep2 HIGH POWER PULSED LASERS Nd:glass =1064 nm Ti:Sa = 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 = 30 1000 fs Transverse profile is Gaussian ; emittance = ~ 1 mm-mr Diffraction limited focus w 0 = 0.43[f/D] z 0 = 2.28[f/D] Electric Field at the focus E ~ 10 11 V/cm (for I = 10 18 W/cm 2 ) 2w 0 2z 0 Slide 3 Nov. 14, 2007Lasers in hep3 SCATTERING OF LASER BEAMS FROM HIGH ENERGY ELECTRONS Electron = E e /m e Backscattered photon angle < 1/ Backscattered photon energy = (4 2 0 ) / (1 + 4 0 /m e + 2 2 ) Cross-section classical (Thomson) T = (8/3) (e 2 /m e c 2 ) 2 = 6.710 -25 cm 2 Compton C = ( T /x) (n x + ) x = 4 0 /m e For protons -p ~ 10 -7 T !!! Photon density at focus ( for I = 10 18 W/cm 2 ) ~ 610 28 / cm 3 compare to N 0 = 610 23 /cm 3 Slide 4 Nov. 14, 2007Lasers in hep4 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. Slide 5 Nov. 14, 2007Lasers in hep5 Transverse beam size measurement Shintake et al. SLAC 1995 Set up standing wave pattern by interfering two arms of the laser beam Slide 6 Nov. 14, 2007Lasers in hep6 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, Y = 73 nm Slide 7 Nov. 14, 2007Lasers in hep7 ELECTRO-OPTIC SAMPLING Crystal Electron bunch Probe laser pulse Detector Slide 8 Nov. 14, 2007Lasers in hep8 First Electro-optic sampling signal 24 Aug. 1999 @ A0 Prompt signal Frequency spectrum of wake fields Slide 9 Nov. 14, 2007Lasers in hep9 Principle of single shot measurement Ultra short laser pulse ~ 30 fs (10 m) 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 laser pulse E/O crystal Slide 10 Nov. 14, 2007Lasers in hep10 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. Slide 11 Nov. 14, 2007Lasers in hep11 Scattering of circularly polarized laser light from transversely polarized electrons introduces small asymmetry, ~ 10%. Slide 12 Nov. 14, 2007Lasers in hep12 p/p = (Momentum compaction ~510 3 )(strain ~410 -8 ) ~ 210 -4 LEP Slide 13 Nov. 14, 2007Lasers in hep13 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 FERMILABA0, DESYTTF/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 Slide 14 Nov. 14, 2007Lasers in hep14 THE SLAC POLARIZED ELECTRON SOURCE Slide 15 Nov. 14, 2007Lasers in hep15 RF PHOTOINJECTOR & BEAM LINE at A0 Capture cavity ~14 MeV Rf gun and solenoids Photocathode manipulator Compression chicane Spectrometer ~ 20 m Laser path Slide 16 Nov. 14, 2007Lasers in hep16 LASER SYSTEM FOR THE ZEUTHEN (DESY-BERLIN) PHOTOINJECTOR Slide 17 Nov. 14, 2007Lasers in hep17 PERFORMANCE OF THE ZEUTHEN PHOTOINJECTOR 1 ms Pulse train top: output bottom: oscillator Streak Camera measurement of single pulse Slide 18 Nov. 14, 2007Lasers in hep18 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. Slide 19 Nov. 14, 2007Lasers in hep19 Breakdown of the vacuum by a laser field (with help from a high energy electron beam) SLAC E144 E e = 47 GeV or = 910 4 Incident photon = 2.34 eV Backscattered photon = 27 GeV Laser pulse U = 1 J, = 2 ps, A = 10 m 2 Laser Intensity I = 510 18 W/cm 2 Electric field at focus E = ( 2Z 0 I ) = 610 10 V/cm When a 47 Gev electron crosses the focus it sees (in its rest frame) a field E * = 2 E ~ 1.210 16 V/cm ~ E critical This is also the basis for the ILC - option Slide 20 Nov. 14, 2007Lasers in hep20 A virtual e+e- pair can get on the mass shell if eE C = m e c 2 E C = m e 2 c 3 /e = 1.310 16 V/cm Prob/V-T = [ E 2 / 2 ] exp(-E c /E * ) J.Schwinger 1951 Photon-photon Scattering Pair production In the perturbative domain ~ [eE/m e ] 2n n = number of photons In strong fields the vacuum can spontaneously break down Slide 21 Nov. 14, 2007Lasers in hep21 E-144 Physical layout of the beams and detectors Slide 22 Nov. 14, 2007Lasers in hep22 The Final Focus Test Beam in the SLAC Switchyard Slide 23 Nov. 14, 2007Lasers in hep23 VIEW OF THE ELECTRON BEAM LINE AND OF THE LASERe - INTERACTION CHAMBER Slide 24 Nov. 14, 2007Lasers in hep24 POSITRON YIELD vs. LASER INTENSITY Slide 25 Nov. 14, 2007Lasers in hep25 POSITRON YIELD vs. 1/Y Slide 26 Nov. 14, 2007Lasers in hep26 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 L int = (1/M) E L B ext a 1/M coupling constant (GeV -1 ) Interaction depends on polarization of the laser field w.r.t. the external magnetic field direction If m a < real particles can be produced; the laser field is attenuated and retarded. If m a > 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 ~ 10 -15 Slide 27 Nov. 14, 2007Lasers in hep27 Graphs for photon interactions in a magnetic field Production of real particle Production of virtual particles Regeneration (real particle) Slide 28 Nov. 14, 2007Lasers in hep28 DETAILS 1. Coherence of axion and laser field restricts the mass range that can be explored m a 2 2 /l 2. With the laser linearly polarized at 45 0 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. Slide 29 Nov. 14, 2007Lasers in hep29 RESULTS : all are upper limits on coupling 1/M Brookhaven-Rochester-Fermilab-Trieste (1993) g a < 3.610 -7 GeV -1 m a < 0.710 -3 eV PVLAS Trieste-Legnaro-Pisa-Ferrara (2007) g a < 4.810 -7 GeV -1 m a < 1.510 -3 eV GammeV Fermilab (2007) Regeneration experiment g a < 3.210 -7 GeV -1 m a < 0.510 -3 eV g a < 510 -6 GeV -1 m a < 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 Slide 30 Nov. 14, 2007Lasers in hep30 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 Slide 31 Nov. 14, 2007Lasers in hep31 Limits from the Fermilab regeneration expt (9/2007) Regeneration limit BRFT limit from rotation The excluded region is above the curves Slide 32 Nov. 14, 2007Lasers in hep32 Global limits on light scalars/pseudoscalars Note mass range allowed from closure arguments Slide 33 Nov. 14, 2007Lasers in hep33 E. LASER ACCELERATION Tightly focused pulsed lasers achieve E TRANSVERSE ~ 10 4 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 m to 1 mm) (c) Transverse dimensions of focal region ~ 10 m (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 Slide 34 Nov. 14, 2007Lasers in hep34 EXAMPLES (a)Self-modulated laser wake field (b)Forced laser wake field LASER >> PLASMA LASER ~ PLASMA PLASMA ~ 100 m = 300 fs ( for n e = 10 18 /cm 3 ) Slide 35 Nov. 14, 2007Lasers in hep35 TYPICAL RESULT V.Malka et al, Science 298, 1596 (2002) Laser: TiSa = 820 nm, U =1 J, = 30 fs, A = 10 m 2, 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 < 10 MeV are produced Slide 36 Nov. 14, 2007Lasers in hep36 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. Slide 37 Nov. 14, 2007Lasers in hep37 Energy stored/per pulse in the two ILC beams U = 2e[N e = 10 10 ][E e = 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 ~ 10 4 Hz which is 10 7 times higher than what NIF-type lasers can provide today Slide 38 Nov. 14, 2007Lasers in hep38 PLASMA WAKEFIELD ACCELERATION SLAC-UCLA-USC I.Blumenfeld et al. Nature 445,741 (2007) Lithium vapor, 10 cm long, n e = 2.710 17 /cm 3, E e = 41 Gev Slide 39 Nov. 14, 2007Lasers in hep39 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 Any perturbation at 10 MHz of dimensionless amplitude h populates the excited modes and gives rise to 10 MHz sidebands P s = P 0 (h Q) 2 For reasonable values, Q = 10 14, P 0 = 10 W and recording one photon per second, one can detect h ~ 10 -24 Optical Cavity 15 m 30 m Filled beam buckets The cavity has excited modes spaced at the free spectral range f = c/2L = 10 MHz Slide 40 Nov. 14, 2007Lasers in hep40 Metric perturbation induced at a distance b from the beam, ~ (4G/c 2 ) m (N/2R) ln(2 ) Bunch length c B >> b, = E/m, R = Tevatron radius, N = circulating protons If G = G N h ~ 10 -40 hopeless !! If gravity becomes strong at this highly relativistic velocity G = G S = G N (M P /M S ) 2 For M s < M P /10 8 = 10 8 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 M S > 10 6 TeV Slide 41 Nov. 14, 2007Lasers in hep41 END Slide 42 Nov. 14, 2007Lasers in hep42 BRFT limit (rotation) BRFT limit (ellipticity) PVLAS signal reported in 2006 (rotation) PVLAS signal reported in 2006 (ellipticity)