extreme relativistic optics: turning matter transparent
TRANSCRIPT
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Aaron Bernstein
Associate Director
Texas Center for High
Intensity Laser Science
Mike Donovan
Associate Director
Texas Petawatt Laser
Alan Wootton
Associate Director IHEDS
John Keto
Research Scientists:
Gilliss Dyer Hernan Quevedo
Post Docs:
Rashida Jafer Ishay Pomerantz
Erhard Gaul
Chief Scientist &
Engineer, TPW
Mikael Martinez
Chief of Operations, TPW
TPW Professional Staff 15 Ph.D. Students, + 7 Undergrads
+ 9 part-time Undergraduate Research Assistants
Admin Staff: Maria Aguirre Sharee Aery
+ 1 undergrad
Todd Ditmire Mike Downer Roger Bengtson B. Manuel Hegelich
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Parameter space and motivation
Status at the Texas PW laser
How to reach ultrahigh intensities Conventional optics
Plasma optics
Science at I > 1022 W/cm2 and what can be done now Single electron dynamics
Radiation reaction in plasmas
Pair cascades
Outlook
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Elementary processes
in strong external field
have been extensively
studied.
Classical
relativistic
plasma
QED
plasma?
Strongly damped
regime?
How will these
processes affect
collective plasma
phenomena?
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Hybrid OPCPA – Glass system
160 J
150 fs
Two target areas TA-1: short focal
length ○ F/3, r ~ 5mm
○ I ~ 1021 W/cm2
TA-2: long focal length ○ F/40, r ~ 100mm
○ I ~ 1018 W/cm2
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Pulse energy ~ J - kJ
Focal spot size ~ 1 - 100 mm
Pulse duration ~ 10 – 1000 fs
Pulse shape deviation from ideal
Gaussian
Spatial Pulse Shape
Temporal Pulse Shape
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Conventional optics
well understood
material damage threshold
Examples
high bandwidth amplifiers
CPA
(non)linear pulse
cleaning/shaping
focusing mirrors
adaptive optics
Plasma optics
no damage threshold
hard to control
Examples
Raman amplifiers
self-focusing
self phase modulation
plasma mirrors
wavelength conversion
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Current status
E = 160 J
r = 5 mm (30%)
t = 150 fs FWHM
I ~ 1021 W/cm2
Energy increase is not feasible.
Use 2nd adaptive mirror after the
compressor to improve the wave
front.
Use F/1 focusing optics to reduce
spot size to less than 2 mm.
Use Pockels cell, USP booster
amplifier, and OPAPE pulse cleaner
to improve the temporal pulse profile.
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Plasma Mirrors $ 0.1
( Antireflective coated 100$)
Target Prepulse passed!
Throughput: 50-60%
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Elec
tric
Fi
eld
Dl(t)
t
Initial Pulse
Fiel
d E
nve
lop
e Dl(t)
SPM with dispersion and delayed nonlinearities
Pulse Modulation
Pulse Energy
0.3 mJ
3.5 mJ
450 fs 0 ps -450 fs
4 m 10 m 23 m
450 fs 0 ps -450 fs
450 fs 0 ps -450 fs
Pulse self-compression Pulse modulation
Bernstein, A.C. (2004). Measurements of Ultrashort Pulses Self-focusing in Air. Ph.D. Thesis, University of New Mexico, USA.
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Pulse compressed from 120 fs to 29 fs
(~4x).
53% of energy is contained in the first peak.
Long gas cell could enable > 2x power
compression on TPW.
<2 mm thick window supported by
a thick aluminum mesh
Mesh reduces internal glass
stress to safe levels (<650 PSI).
Max displacement: 30 μm
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The energy accumulated by a
nanobunch during a fraction of the
laser period is radiated out as a
short burst of XUV and x-ray.
Optimization of this process
(optimal incidence angle and target
density) allows a x10 increase of
intensity.
A groove-shaped target can focus
the emitted radiation.
>1 keV forward directed, <1° div.
A laser beam produces
nanobunches of electrons
irradiating a solid density target.
current
conditions
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Above threshold ionization
(ATI) at ultrarelativistic
intensities (I >> 1018 W/cm2) is
expected to change.
Regime of interest: electrons
ionized at peak intensity.
These electrons are
accelerated forward by a plane
wave.
Electrons are ejected from the
focal spot with high energies in
the forward direction at a small
angle.
This effect has not been
measured.
Monte Carlo simulation for Ar ionization
by a 100 fs 5x1021 W/cm2 laser beam
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Radiation reaction (still an active area of research) becomes
important with the increase of laser intensity.
The classical description breaks down, but the problem remains
tractable because
only photon emission has to be treated quantum mechanically for ultra-
relativistic electrons
the motion between two photon emissions is classical
Laser field can be treated classically (relatively small number of
laser photons absorbed in each interaction).
EPOCH code developed in UK (Univ. of Warwick) implements this
approach to account for photon emission (radiation reaction) and
pair production.
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Laser with peak intensity 5x1022 W/cm2 irradiates an opaque target, ne = 1022 cm-3.
Electron interaction with the laser beam in the sheath field leads to g-ray production.
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The energy absorption by
electrons scales roughly
linearly with density.
The energy conversion by
electrons into g-rays
depends on the
transparency of the target.
Electrons move along with
the laser beam in a
transparent target,
experiencing a low field.
Using a dense opaque
target is critical for
enhancement of the g-ray
yield.
We expect for 10% of the laser energy to be converted into photons for the projected TPW intensity of 5x1022 W/cm2.
Radiation reaction is mainly classical in this regime.
Onset of QED effects is possible in this setup [Ridgers et al. PRL 2012].
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c0 – ratio of the electric field in the
rest frame to the Schwinger field
It controls the magnitude of pure
quantum effects.
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Pair cascades require for the pair creation time to be much
shorter than the time of ejection from the laser focus.
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Use two counter-propagating beams I = 8x1022 W/cm2
t = 10 fs
A 10 nm diamond foil provides energetic seed electrons.
The beams are temporally offset by t/2 to allow the foil electrons accelerate.
g spectrum
g phase-space
number of positrons
pair
cascade
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Reaching QED relevant fields requires new techniques.
TPW intensity can be enhanced to 5x1022 W/cm2, allowing
observation of the onset of QED effects
access to radiation reaction dominated plasmas
We have designs for c0 > 1 experiments using pulse splitting.
Higher fields will allow cascading and QED-dominated plasmas.
Higher intensities require new laser systems and/or plasma optics.
Increase in repetition rate will allow for increase in statistics enabling new experiments on high rep rate systems.