Short pulse laser interactions with matter, P Gibbon

Tags: Wave Propagation, Metal Optics, Historical Background Technology & Physics Multiphoton Single, electron charge, electron dynamics, parabolic mirror, LOA LLC MBI IOQ RAL LLNL CEA Country USA UK, laser physics, TW, Pulse Laser, Historical Background, fs pulse, Femtosecond Lasers, Femtosecond TW laser system, Paul Gibbon, science Introduction
Content: Short Pulse Laser Interactions with Matter Paul Gibbon Forschungszentrum JuЁlich 1 / 115
Introduction: Historical background Technology & Physics Multiphoton Single electrons wave propagation Metal Optics ICF Lasers
Part I Introduction to Short Pulse Laser Physics
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Introduction: Historical Background Technology & Physics Multiphoton Single electrons Wave Propagation Metal Optics ICF Lasers
1 Introduction: Historical Background Progress in technology Multiphoton Physics Single-Electron Interaction with Intense electromagnetic fields Nonlinear Wave Propagation Metal Optics Long Pulse Laser-Plasma Interactions (ICF) Femtosecond Lasers
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Web site for lecture handouts www.fz-juelich.de/zam/splim Introduction: Historical Background Technology & Physics Multiphoton Single electrons Wave Propagation Metal Optics ICF Lasers 4 / 115
laser technology progress: chirped pulse amplification
Introduction: Historical Background Technology & Physics Multiphoton Single electrons Wave Propagation Metal Optics ICF Lasers
2
Intensity (W/cm )
1023 1020
Theoretical limit particle physics - ray sources Particle acceleration Fusion schemes Relativistic optics: vosc ~ c
Hard X-ray flash lamps Hot dense matter
1015
CPA Field ionisation of hydrogen Multiphoton physics Laser medicine
electron energy 1 GeV 1 MeV 1 keV 1 eV
1010 1960
1970
1980
1990
Year
2000
1 meV 2010
Figure: Progress in peak intensity since the invention of the laser in 1960. 5 / 115
What kind of physics does this field involve?
Introduction: Historical Background Technology & Physics Multiphoton Single electrons Wave Propagation Metal Optics ICF Lasers
Contributory fields are numerous and diverse: · laser physics · atomic physics · Plasma physics · astrophysics · nuclear & elementary particle physics Many Theoretical Models have roots in these more classical areas.
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Extreme conditions: violent science
Introduction: Historical Background Technology & Physics Multiphoton Single electrons Wave Propagation Metal Optics ICF Lasers
· Ordinary matter -- solid, liquid or gas -- rapidly ionized when subjected to high intensity irradiation. · Electrons released are then immediately caught in the laser field · Oscillate with a characteristic energy which then dictates the subsequent interaction physics. · Continual challenge to both theoreticians and experimentalists.
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Prehistory
Introduction: Historical Background Technology & Physics Multiphoton Single electrons Wave Propagation Metal Optics ICF Lasers
· Multiphoton physics · Single-electron interaction with intense EM fields · Nonlinear wave propagation · Metal optics (Drude model) · Long pulse laser-plasma interactions (ICF)
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Multiphoton physics
Introduction: Historical Background Technology & Physics Multiphoton Single electrons Wave Propagation Metal Optics ICF Lasers
· Standard lasers (0.25 µm ­ 13.4 µm ): cannot observe the Photoelectric effect on normal material because Ip. · Higher intensities in the 1960s and '70s (Fig. 1) led to possibility of multiphoton ionisation: n = Ip. · Electron absorbs n photons of moderate energy (eg laser photons with eV)
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Electrons in intense electromagnetic fields
Introduction: Historical Background Technology & Physics Multiphoton Single electrons Wave Propagation Metal Optics ICF Lasers
· Volkov (1935): electron `dressed' by field
· Schwinger (1949): radiated power
· Invention of laser (1960): theoretical works on electron dynamics
· Figure of merit q:
q = eEL ,
(1)
mc
e = electron charge, m = electron mass, c = speed of light; EL = laser electric field strength; = light frequency. · Ostriker & Gunn (1969) ­ electron dynamics in vicinity of pulsars: q 100
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Nonlinear wave propagation
Introduction: Historical Background Technology & Physics Multiphoton Single electrons Wave Propagation Metal Optics ICF Lasers
· Plasmas can support large-amplitude, nonlinear waves. · Early works by Akhiezer & Polovin (1956) and Dawson (1959) · Numerous studies on the behavior of: · large-amplitude Langmuir (electrostatic) waves · propagation of high-intensity electromagnetic radiation in plasmas · Tajima and Dawson (1979) proposed `laser electron accelerator' ­ fresh wave of interest in wave propagation, including from members of the particle accelerator community.
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Drude model of metal optics
Introduction: Historical Background Technology & Physics Multiphoton Single electrons Wave Propagation Metal Optics ICF Lasers
· Atoms in a metal share a limited number of `valence' electrons, forming a conduction band ­ Drude (1906) · These carry current and heat through the material. · For an element with mass density and atomic weight A, free electron density is given by: ne = NAZ /A where NA is Avogadro's constant and Z is the number of valence electrons per atom.
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Drude model: conductivity
Introduction: Historical Background Technology & Physics Multiphoton Single electrons Wave Propagation Metal Optics ICF Lasers
· Electrical Conductivity of a metal:
e = ne e2 /me
where is the collision or relaxation time.
· Ohm's Law:
j = e E
· Resulting AC conductivity:
() = e . 1 - i
(2) 13 / 115
Drude model: dielectric constant
Introduction: Historical Background Technology & Physics Multiphoton Single electrons Wave Propagation Metal Optics ICF Lasers
· Combine () from Eq. (2) with Maxwell's equations to get complex dielectric constant:
= 1 - p2 , ( + i)
where
p2 = 4ne e2/me
is the plasma frequency of the valence electrons, and -1 is their collision frequency.
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Long pulse interactions: Inertial Confinement Fusion (ICF)
Introduction: Historical Background Technology & Physics Multiphoton Single electrons Wave Propagation Metal Optics ICF Lasers
· Principle: micrometer-sized pellet filled with DT fuel compressed to enormous densities by many laser beams focused symmetrically onto its surface · Pellet shell material ablates radially outwards, pushing the fuel inwards via rocket effect · Fuel implodes, reaching densities 500 - 1000 gcm-3 and temperatures T of 10 keV (107 degrees Kelvin) · Laser fusion became official in 1972 (previously classified): paper in Nature by Nuckolls et al.
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Requirements for ICF
Introduction: Historical Background Technology & Physics Multiphoton Single electrons Wave Propagation Metal Optics ICF Lasers
· Aim to satisfy Lawson criterion for thermonuclear confinement:
nT > 1015 keV s cm-3.
(3)
· Target must release fusion energy before it blows apart · ­ leads to requirement for the areal fuel density R 0.3, where R is the final capsule radius · `Hot Spot' scenario: hot, low density core surrounded by cold, high density fuel · Laser driver energy 1MJ · Facilities currently being built: NIF, Livermore, USA; LMJ, Bordeaux, France
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ICF issues relevant to short pulse interactions
Introduction: Historical Background Technology & Physics Multiphoton Single electrons Wave Propagation Metal Optics ICF Lasers
· Hydrodynamics ­ ion motion, target expansion (prepulse physics)
· Coronal processes ­ Fig. 2:
· parametric instabilities ­ Raman and Brillouin scattering
· resonance absorption - kinetic wave-particle interactions
· fast electron generation & heating: `suprathermal' temperature
TH given by:
TH 14 (I 2)1/3 keV,
where I is the laser intensity in units of 1016 Wcm-2 and the laser wavelength in microns.
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Coronal physics in ICF
Introduction: Historical Background Technology & Physics Multiphoton Single electrons Wave Propagation Metal Optics ICF Lasers
uneven laser irradiation
Raman: em em + l
2 p : em l + l fast e-
Brillouin em + em ia
1/4 critical
Inverse bremsstrahlung
ne= nc /4
absorption
Resonance absorption
Filamentation
Nonlinear heat current
em l fast e-
large plasma waves critical density
heat flow instability
x-radiation
ne= nc = p
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Pa (max)
Figure: Laser-plasma interactions in the corona of an imploding micro-balloon.
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Femtosecond TW laser system: front end Introduction: Historical Background Technology & Physics Multiphoton Single electrons Wave Propagation Metal Optics ICF Lasers 19 / 115
Femtosecond TW laser system: components
Introduction: Historical Background Technology & Physics Multiphoton Single electrons Wave Propagation Metal Optics ICF Lasers
1 Oscillator : produces short, low-energy pulse 2 Stretcher : converts fs pulse to 50­200 ps 3 Amplifier : increase the pulse energy by a factor of 107­109 4 Compressor : performs optical inverse of the stretcher to deliver an amplified fs pulse
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Femtosecond TW laser system: schematic
Introduction: Historical Background Technology & Physics Multiphoton Single electrons Wave Propagation Metal Optics ICF Lasers
s tre tc h e r 150 ps
5W cw
N d :Y V O 4
fs - o s c illa to r
10 nJ 4 5 fs 80 M H z
is o la to r
p u ls e p ic k e r 1 0 H z
N d :Y A G 40 m J,10 H z
r e g e n e r a tiv e a m p lifie r PZ T iS a
2mJ
p r e p u ls e
l /2
E = 0 ,8 J , t = 8 0 fs , 1 0 H z l = 800 nm , D l = 16 nm O = 70 m m f/2
N d :Y A G 500 m J,10 H z N d :Y A G 500 m J,10 H z
l /2 4 - p a s s - a m p lifie r T iS a
p o la r iz o r 300 m J
1 0 2 0 W /c m 2 a u f 5 m m 2 v a k u u m c o m p re s s o r
N d :Y A G 5 J,10 H z
T iS a
1 ,2 J
3 - p a s s - a m p lifie r
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Chirped pulse amplification
Introduction: Historical Background Technology & Physics Multiphoton Single electrons Wave Propagation Metal Optics ICF Lasers
· Invented by Gerard Mourou and co-workers in 1985 · Way of increasing intensities beyond damage thresholds for `long' pulses (100-200 ps) · Fluence 0.16 Jcm-2p1s/2 · ­ way below saturation levels of amplifying medium 1 Jcm-2 for Ti:sapphire. · Stretcher-compressor separates pulse generation and amplification stages · ­ permits standard techniques & components in amplifier chain
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Oscillator
Introduction: Historical Background Technology & Physics Multiphoton Single electrons Wave Propagation Metal Optics ICF Lasers
· Femtosecond laser sources are · mode-locked: output pulse is superposition of many electromagnetic waves (or laser modes) · transform or bandwidth limited:
p 1/
· Large bandwidth is essential to generate a short pulse.
· Example: 10 fs Gaussian pulse = 0.44, giving
= 4.4 Ч 1013 Hz, which for a central wavelength of 800 nm,
translates to:
2 = = 94 nm. c
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Amplification & recompression
Introduction: Historical Background Technology & Physics Multiphoton Single electrons Wave Propagation Metal Optics ICF Lasers
· Regenerative preamplifier ­ gain 107 · power Amplifiers ­ gain 10 - 1000 · Amplified pulse recompressed using grating pair or quadruple
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Final focus
Introduction: Historical Background Technology & Physics Multiphoton Single electrons Wave Propagation Metal Optics ICF Lasers
· Off-axis, parabolic mirror (f /4 - f /2)
· Focal spot of Ti:sapphire laser with 3 µm diameter containing more than 50% of the pulse energy. · Peak intensity here: 4 Ч 1019 Wcm-2. 25 / 115
Multi-Terawatt laser systems and laboratories worldwide
Introduction: Historical Background Technology & Physics Multiphoton Single electrons Wave Propagation Metal Optics ICF Lasers
Name Petawatta VULCANb PW Mod.c PHELIXd LULI PW APR PW ­ ALFA 2 S. Jaune Lund TW MBI Ti:Sa Jena TW ASTRA USP UHI 10
Lab LLNL RAL ILE GSI LULI APR FOCUS FOCUS LOA LLC MBI IOQ RAL LLNL CEA
Country USA UK JP D F JP USA USA F SW D D UK USA F
Type Nd:glass Nd:glass Nd:glass Nd:glass Ti:Sa Ti:Sa Ti:Sa Ti:Sa Ti:Sa Ti:Sa Ti:Sa Ti:Sa Ti:Sa Ti:Sa Ti:Sa
(nm) 1053 1053 1054 1064 800 800 800 800 800 800 800 800 800 800 800
Energy (J) 700 423 420 500 30 2 1.2 4.5 0.8 1.0 0.7 1.0 0.5 1 (10) 0.7
L (fs) 500 410 470 500 300 20 27 30 25 30 35 80 40 100 (30) 65
P (TW) 1300 1030 1000 1000 100 100 45 150 35 30 20 12 12 10 (100) 10
L ( µm) ­ 10 30 ­ 11 (1) (1) 10 3
IL ( Wcm-2 ) > 1020 1.06 Ч 1021 1020 ­ ­ 2 Ч 1019 (8 Ч 1021 ) (1022 ) 1019 > 1019 > 1019 5 Ч 1019 1019 5 Ч 1019 5 Ч 1019
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P Gibbon

File: short-pulse-laser-interactions-with-matter.pdf
Author: P Gibbon
Published: Mon Nov 6 13:00:48 2006
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