Claes-Göran Wahlström Department of Physics

Lund University

Light Matter Interactions at Very High Intensities

Outline

• High-intensity lasers • Laser-produced plasmas • Electron motion in an intense pulse • Acceleration of protons • Acceleration of electrons

High-intensity lasers

Part of the old Nova Laser at LLNL

National Ignition Facility NIF - an Extreme Laser

180 m long

192 beams

1.8 MJ @351 nm

NIF Target Chamber

Laser Megajoule LMJ - another Extreme Laser

300 m long

240 beams

2 MJ @351 nm

NIF and LMJ : Indirect drive, MJ Energy

Lund Terawatt Laser

High-intensity lasers

Two approaches: ICF: τ ∼ 1 ns, E ∼ 100 kJ 100 TW T3: τ ∼ 10’s fs E ∼ 1 J 100 TW

API =

τEP =

ICF D = 1 mm I ~ 1 x 1016 W/cm2 T3: D = 5 µm I ~ 5 x 1020 W/cm2

Few shots/day 10 Hz

Strong-field ionization, Laser-produced plasmas

and Laser - electron interactions

High-intensity laser – matter interactions

• At high laser intensities: The Electro-Magnetic wave description is more

appropriate than the photon picture.

20

2EcI ε

= 271033.1 EI −⋅=

V/m W/cm2

Tunneling-ionization Multiphoton-ionization

IP

ground state

Eel

Over-the-barrier-ionization

Regimes of strong field ionization

Over-the-barrier-ionization

42

9104pth I

ZI ⋅

=

215 W/cm105.1

eV6.241

:

⋅=

==

→ +

th

p

I

IZ

HeHe

W/cm2

Charge state of the created ion

eV

219

1615

W/cm101.1

eV91816

:

⋅=

==

→ ++

th

p

I

IZ

ArAr

Plasma (Ionized matter)

Under dense

Over dense (no light propagation) < >

p

p

ω ω ω ω

Light propagation in plasma

Plasma frequency

e

e p m

n e

0

2

ε ω = At the critical density ωp=ω

nc~ 1021 cm-3 for near IR light

Refractive index <1 vp = c/n >c but vg<c vpvg=c2

c

e p

n

n n − = − = 1 1 2

ω

ω 2

High-intensity laser – matter interactions

• At ultra-high intensities: The magnetic field component becomes very important.

)( BvEeF

×+−=

Relativistic laser – matter interactions

2

2

2

)1(

)/(11

)(

mcEmcE

cv

vmp

BvEeFdtpd

k

tot

−=

=

−=

=

×+−==

γ

γ

γ

γ

Relativistic laser – matter interactions

)(10/101/10

10

2216

218

2

2182

laserCOmwithcmWmwithcmW

cvcm

mWI osc

µλ

µλ

µλ

=

=

≈⇒≥

Single electron in the laser field

Lorentz force:

Accelerates the electron in the laser forward direction.

)( BvEeF

×+−=

fsfsTm

87.2

,8.0

===

τ

µλ

fsfsTm

87.2

,8.0

===

τ

µλ

Single electron: Transverse acceleration vs time

Single electron: position

x

Figure-8 motion in drifting frame Figure-8 motion in lab frame

Lawson Woodward Criterion: no net acceleration in vacuum with an infinite plane wave

( )2pond IλF −∇∝

The ponderomotive force

• The light pressure pushes electrons away from regions with high intensity

Basic principles of Laser Particle Acceleration

The laser pulse pushes electrons away

Charge displacement

Quasi-static electric fields

Charged particle acceleration (e-, p+, Z+)

A high-intensity laser interacts with a plasma

Huge electrostatic fields possible in plasmas

• In RF-based accelerators: E-fields limited by electrical breakdown. E < 50 MV/m => Very long accelerators

• In a plasma: No such breakdown limit. Already a

plasma. => Very compact accelerators

e z n E ~ Ez = 300 MV/m for 1 % Density Perturbation at 1017 cm-3

Ez = 300 GV/m for 100 % Density Perturbation at 1019 cm-3

Ez = 30 TV/m for 100 % Density Perturbation at 1023 cm-3

Laser acceleration at the Lund High-Power Laser Facility

200 MeV Over 2 mm

10 MeV Over few µm

Electrons

Protons

Ion acceleration

few µm

Proton beam generation from overdense plasmas

Ion acceleration mechanisms

+ + +

Ponderomotive electron

acceleration

- - - - - -

+ + + + + +

Target Normal Sheath Acceleration

(TNSA)

Electron sheath

Protons (and other ions)

E~TV/m

Thin foil with H2O layers

Preplasma

µm

Typical characteristics

Applications of laser accelerated ions

Medical applications: Cancer therapy

PET isotope production.

Ion injection to heavy ion accelerators

Proton imaging of electric fields in plasma

Proton Therapy

Prostate tumor

The rectal portion of the bowel

Depth in tissue

Bragg peak

γ

p+ Abso

rbed

dos

e

10 -3

10 -2

10 -1

10 0

10 1

10 2

10 3

10 100 1000 10 4

Ran

ge (c

m)

Ion energy

Proton

Carbon

H20 absorber

Radiation therapy region

/ MeV

• Protons are deposited over a shorter range than x-rays (Bragg peak)

• Insignificant sideways spread (straggling range is ~ 1% of the penetration range)

• A short burst of high-energy protons can maybe breaks more DNA strands (non-linear effects)

Electron acceleration

Laser ⇒ Relativistic electrons ⇒ Non-relativistic protons

Electrons (mec2=0.5 MeV) Protons (mpc2=0.9 GeV)

v~c v~5% c

Wkin=1 MeV

Intense Laser

Gas-Jet

Electron Beam

Nozzle

Underdense Plasma

Electron beam generation in underdense plasmas

Electron acceleration in plasma wake wave

Plasma wave generation

• The ponderomotive force pushes electrons out of regions of high intensity • Ions are stationary on the fs timescale. • Induced charge separation pull back electrons • Wave generation most efficient when w0 = cτ = λp

• Plasma wave propagates with a velocity close to c • Strong accelerating and focusing electric fields

( )wavephase

lasergroup vv =

Gas medium

~10 um c v ≈

Wakefield acceleration 3D PIC simulation of a plasma wave (UCLA)

100 um

1 m radio frequency cavity

100 GV/m

20 MV/m

Gas targets - Wake-field acceleration

Electron acceleration:

Underdense plasmas: ~ 100 GV/m Over few mm giving few 100 MeV

Proton acceleration:

Thin solid targets: ~TV/m Over few µm giving ~few 10 MeV

Solid targets - Sheet acceleration

Wave breaking

ne = 5 x 1018 to 5 x 1019 cm-3

High intensity laser pulse 1 J, 35 fs, 800 nm

Supersonic gas jet target

Collimator + Electromagnet

LANEX screen

Electron energy spectrum

Thomson scattering plasma imaging

f/10 focusing

100 μm

Experimental arrangement

0 100 2000

0.2

0.4

0.6

0.8

1

E / MeV

Num

ber o

f ele

ctro

ns /

a.u.

0 100 2000

0.2

0.4

0.6

0.8

1

E / MeV

Num

ber o

f ele

ctro

ns /

a.u.

e- e- e- e- e- e-

Quasi-monoenergetic wakefield acceleration

Increasing the maximum energy

5.12

3 1

eL

pd n

L ∝=λλThe electrons reach their maximum

energy after the dephasing length

ep n

emc

E ∝=ω

maxMaximum electric field is

ed n

LeEW 1maxmax ∝=The maximum energy is

To increase Wmax a factor 10: Decrease ne a factor 10 Increase interaction length a factor 30

(from a few mm to several cm)

Diffraction limits the interaction to the order of the Rayleigh length

LR

wzλ

π 20=

m 400nm 800m 10

L

0

µλ

µ

=⇒==

Rz

w

A Relativistic Channel in Helium Plasma

Helium gas

Gas nozzle

Relativistic Self-Focusing

I(r)

r

n(r)

r

Focusing Lens ! n(ωp)

ωp(me)

me(v)

v(I)

Light intensity in the focus:

Refractive index in the focus:

Relativistic channelling

Helium gas

Gas nozzle

ne = 5 x 1018 to 5 x 1019 cm-3

High intensity laser pulse 1 J, 35 fs, 800 nm

Supersonic gas jet target

Collimator + Electromagnet

LANEX screen

Electron energy spectrum

Thomson scattering plasma imaging

f/10 focusing

100 μm

Experimental arrangement

Waveguide capillaries

Hollow dielectric capillaries • Preliminary results show excellent guiding • Guiding over several cm possible • Sensitive to laser pointing variation and

spot quality (damage)

L

Material: Glass Length: 3 to 10 cm Inner diam: 100 µm

Electron beam properties

+ Small source size (~10 µm) + Low divergence (~5 mrad) + Quasi-monoenergetic (∆E/E~10%) + Short duration (<25 fs) + High charge (~100 pC)

- Repetition rate (~1 – 10 Hz) - Stability (today)

100 pC/25 fs= 4 kA

Progress of accelerator technology

LLC

LBNL

electrons

Thank you !