# Presentation: 096017

## 
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

- 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**

- Lasers in hep

- Nov. 14, 2007

## HIGH POWER PULSED LASERS

-     Nd:glass                    ** ****λ** =1064 nm
-      Ti:Sa                         ** ****λ**** **=  820 nm  (tunable)
- Energy in pulse   (after **C**hirped **P**ulse **A**mplification)
-         10 –   1000  mJ       Table-top      ( 10 Hz)
-         10 – 10000     J        Facility      ( 10-3 Hz)
- Pulse length                    **τ** = 30 – 1000 fs
- Transverse profile is Gaussian ;  emittance = **λ** ~ 1 mm-mr
- Diffraction 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)

**2w****0**

**2z****0**

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## SCATTERING OF LASER BEAMSFROM HIGH ENERGY ELECTRONS

- Electron                                            **γ** = *E*e/me
- Backscattered photon angle            **θ** < 1/**γ**
- Backscattered photon energy
-                    **ω**** ́** = (4**γ**2**ω**0) / (1 + 4**γω**0/me + **γ**2**θ**2)
- Cross-section    classical  (Thomson)
-              **σ**T = (8**π**/3) (e2/mec2)2 = 6.7×10-25   cm2
- Compton       **σ**C =  (**σ**T/x) (**l**n x + 1⁄2 )     x = 4**γω**0/me
- For protons                **σ****γ**-p ~ 10-7 **σ**T              !!!
- Photon density at focus         (for I = 1018 W/cm2)
**  ****ρ**γ ~ 6×1028 / cm3      compare to N0 = 6×1023/cm3

- Lasers in hep

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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.

- Lasers in hep

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## Transverse beam size measurement 

- Transverse beam size measurement
- Shintake et al.     SLAC 1995

- Set up standing wave pattern
- by interfering two arms of the laser beam** **

- Lasers in hep

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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,   ****σ****Y**** = 73 nm**

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## ELECTRO-OPTIC SAMPLING

**Crystal**

**Electron bunch**

**Probe laser pulse**

**Detector**

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## First Electro-optic sampling signal 

- First Electro-optic sampling signal
-  24 Aug. 1999 @ A0

**Prompt signal**

- Frequency
- spectrum of
- wake fields

- Lasers in hep

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## Principle of single shot measurement

**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

**laser pulse**

**E/O crystal**

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##  The synchronization jitter of laser and beam is shown.  

**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.

- Lasers in hep

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## Scattering of circularly polarized laser light from transversely

- polarized electrons introduces small asymmetry,  ~ 10%.

- Lasers in hep

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## LEP

**Δ****p/p = (****Momentum compaction ~5****×10****3****)****·(strain ~4×10****-8****) ~ 2×10****-4**

**LEP**

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## 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

-           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

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## THE SLAC POLARIZED ELECTRON SOURCE

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## RF PHOTOINJECTOR & BEAM LINE at A0

**Capture cavity ~14 MeV**

**Rf gun and solenoids**

**Photocathode manipulator**

**Compression chicane**

**Spectrometer**

- ~ 20 m

**Laser path**

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## LASER SYSTEM FOR THE

- ZEUTHEN (DESY-BERLIN) PHOTOINJECTOR

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## PERFORMANCE OF THE  ZEUTHEN

- PHOTOINJECTOR

-   1 ms

- Pulse train   top:       output
-                     bottom: oscillator

- Streak Camera measurement
- of single pulse

- Lasers in hep

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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           **γ**** **= 9×104
- Incident photon                           **ω** = 2.34 eV
- Backscattered 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 = **(**** ****2Z****0**** I**** )**1⁄2 = 6×1010 V/cm
- When a 47 Gev electron crosses the focus it sees (in it’s
- rest frame) a field
-            E* = 2 **γ**** ****E ~ 1.2×10****16**** V/cm ~ E****critical**

- This is also the basis for the **ILC ****γ****-****γ** option

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## A virtual e+e- pair can get on the mass shell if    eEλC = mec2

- A virtual e+e-** **pair can get on the mass shell if    **eE****λ****C**** = m****e****c****2**
-                             **E****C ****= m****e****2****c****3****/eħ = 1.3×10****16**** V/cm **
-     **Prob/V-T = [****α**** E****2****/****π****2****ħ] exp(-****π****E****c****/E*********)**     J.Schwinger 1951

**Photon-photon****     **
**                 ****Scattering**

**Pair production**

- In the*** perturbative ***domain  **σ**** ~ [eE/****ω****m****e****]****2n**   n = number of photons

- In ***strong fields*** the vacuum can spontaneously break down

- Lasers in hep

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## E-144 Physical layout of the beams and detectors

**E-144 Physical layout of the beams and detectors**

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## The Final Focus Test Beam in the SLAC Switchyard

- Lasers in hep

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## VIEW OF THE ELECTRON BEAM LINE AND OF THE LASER–e- INTERACTION CHAMBER

- Lasers in hep

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## POSITRON YIELD vs. LASER INTENSITY

- Lasers in hep

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## POSITRON YIELD vs. 1/Y

- Lasers in hep

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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) **E****L****• B****ext**φa             1/M  coupling constant (GeV-1)
- Interaction depends on*** *****polarization** of the laser field w.r.t.
-               the external magnetic field direction
- If    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**

- Lasers in hep

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## Graphs for photon interactions in a magnetic field 

**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 ≤ 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.

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## 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

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## Most recent limits from PVLAS (9/2007)

**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

- Lasers in hep

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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

**“****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
-         τLASER >> λPLASMA                                               τLASER ~ λPLASMA
-                            λPLASMA ~ 100 μm = 300 fs  ( for ne = 1018/cm3 )

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## 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 MeV
- Max energy   200 MeV,
- Total charge 5 nC

- When using solid targets
- “thermal” protons and ions,
- E < 10 MeV are produced

- Lasers in hep

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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 = 2****e·[****N****e**** = 10****10 ****]·[****E****e**** = 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 ~ 10****4**** Hz****  **
- which is ***10******7****** 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 = 2.7×1017 /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 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

-                                      **P****s**** = P****0**** (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 m

- Filled beam buckets

- The cavity has excited modes spaced at the “free spectral range”
-                                      **f = c/2L = 10 MHz**

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## Metric perturbation induced at a distance b from the beam,

-                **< h > ~ (4G/c****2****)** **γ****m (N/2****π****R) ln(2****γ****)**
- Bunch length cτB >> b, γ = 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    MS > 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)

- Lasers in hep

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