Studies on monoenergetic proton beam generation and THz emission in relativistic laserplasma interac

1
Studies on monoenergetic proton beam generation
and THz emission in relativistic laser-plasma
interaction Z. M. Sheng Institute of Physics,
Chinese Academy of Sciences, BeijingDepartment
of physics, Shanghai Jiao Tong University,
Shanghai

1st international conference on ULtra-intense
Interaction Sciences (ULIS 07) October 1- 5,
2007, Bordeaux 1, France
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Collaborators
Institute of Physics, CAS, China H. C. Wu, M.
Chen, M. Q. He, Q. L. Dong, J. Zhang
MOE Key Laboratory of Heavy Ion Physics, Peking
University, China X.Q. Yan, C. Lin, Z.Y. Guo,
Y.R. Lu, J.X. Fang, J.E. Chen
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Outline

• High-power THz emission from laser wakefields
• Proton acceleration in laser solid interaction by
  shock waves
• Proton acceleration in the phase stability
  acceleration regime
• Summary

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Relativistic laser plasma is source for various
radiations including THz waves
Applications Material characterization by THz
spectroscopy Tomographic imaging Biomaterial
applications.
B. Ferguson and X.-C. Zhang, Nature Materials 1,
26 (2002)
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THz-emission power as a function of frequency
M. Tonouchi, Cutting-edge terahertz technology,
Nature Photonics, 1, 97 (2007)
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THz Free electron lasers
From Opportunities in THz Science, Report of a
DOE-NSF-NIH Workshop held February 12 14, 2004,
Arlington, VA, Ed. by M. S. Sherwin et al.
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An electron plasma wave is potentially a
high-power THz source

• Plasma waves that can be driven by ultrashort
  laser pulses oscillate typically at the THz range
  (e.g., ne1018cm-3, wp/2p9THz).
• The field strength before wave-breaking is as
  high as 100 GV/m for ne1018cm-3.

• How can an electrostatic wave be converted
• to an electromagnetic wave?

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THz radiations from a vacuum-plasma interface by
introducing an inhomogeneous plasma region
w 2p/tL
tL
ZM Sheng, HC WU, K Li, J Zhang, Phys. Rev. E 69,
025401(R) (2004). ZM Sheng, K Mima, J Zhang, H
Sanuki, PRL 94, 095003 (2005). ZM Sheng, K. Mima,
and J. Zhang, Phys. Plasmas.12, 123103 (2005).
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Dispersion of electromagnetic waves and electron
plasma waves
w
EM wave
Slope c
wpe
Langmuir waves
Slope 31/2vte
ES wave
klDe
1
They meet each other only at k0.
ZM Sheng, K Mima, J Zhang, H Sanuki, PRL 94,
095003 (2005).
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Mode conversion theory --- the direct problem
EM?ES
Conversion efficiency from electromagnetic waves
into electrostatic waves h
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1D simulation at oblique incidence
q15, L60l, dL10, a00.5
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Comparison with model
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Energy conversion efficiency scaling
C mainly depends upon the incident angle and the
pulse profile.
n0
dL
L
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Generation of single cycle THz pulse at the MW
level with few wavelength plasma oscillators
H. C. Wu, Z. M. Sheng et al., arXiv.org/physics/0
2/2007, submitted for publication
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The plasma layer must be thin enough
Mechanisms net current at the vacuum-plasma
boundaries.
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Scaling with the laser intensity and incident
angle
The THz field amplitude
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Outline

• High-power THz emission from laser wakefields
• Proton acceleration in laser solid interaction by
  shock waves
• Proton acceleration in the phase stability regime
  
• Summary

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Application of laser produced proton beams

• Proton therapy (200-300MeV, quasi-monoenergetic)
• PET Isotope Production (40MeV)
• Fast ignition with proton beams (of high flux)
• Isochoric heating of solid targets (in a short
  time)
• Diagnostic of dense matters (radiography)
• Injectors of ion accelerators (quasi-monoenergetic
  bunches)

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Proton/Ion acceleration in laser interaction with
ultra-thin films
Laser
Thin proton layer
1.) Acceleration via Coulomb explosion 2.)
Target normal sheath acceleration
S. C. Wilks et al, Phys. Plasmas 8, 542 (2001)
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Acceleration at extremely high laser intensity
laser piston acceleration
Laser 8x25x25l3 a0316 I
1.37x1023W/cm2 Sharp front from a0100
EL10kJ N05.5x1022cm-3
T. Esirkepov et al., PRL 92, 175003 (2004).
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We look into the mechanisms of ion
acceleration for the laser parameters between the
sheath field acceleration and laser piston
acceleration in laser solid interactions.
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Field structure of collisionless shock at the
shock front
a05.0, T20T0, n30nc
The field inside the target becomes dipolar, and
a collisionless electrostatic shock wave (CESW)
is formed. The condition for the CESW formation
1ltM lt 1.6 (see F. F. Chen, Plasma Phys.
Contr. Fusion)
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Ion acceleration by shock waves with mixed ion
components
Heavy ion components Light ion component
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Shock wave propagation through the interface of
two materials
Same charge but different masses
b) m1m241 c) m1m2101 d) m1m2110
For unequivocal experimental demonstration of
shock wave acceleration?
Shock remaining but with a higher speed
Forming a solitary wave
Shock wave-breaking
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Can the proton energy be increased further more?
The light ion speed has been increased to 3 times
of the shock speed after the second reflection.
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Formation of Colliding Electrostatic Shock Waves
m11836, m2100, q1q21.0e? The light ions in
the middle layer of the target can be reflected
many times between the two shocks until their
kinetic energy is larger than its potential
energy in the shock fields.
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Successive Ion Acceleration by Colliding
Electrostatic Shock Waves
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Outline

• High-power THz emission from laser wakefields
• Proton acceleration in laser solid interaction by
  shock waves
• Proton acceleration in the phase stability
  acceleration regime
• Summary

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The characteristics of Surface Acceleration

• Normally linear polarized laser irradiates the
  high density target, the ponderomotive force
  heats plasma and plasma expands into vacuum.
• Resulted electrostatic field can rapidly
  accelerate proton/ions.

• No phase oscillation!
• Hot electrons go away quickly acceleration
  length is short (1µm).

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Phase Stability Acceleration Regime (PSA Regime)

• .

Phase Stability
Phase Oscillation

• Helps to decrease the energy spread obviously !

Beam curled round the reference particle!
The protons can be accelerated and bunched in
the same time like in the conventional
accelerators
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Plasma Heating (JB)

• Linear polarized laser

Circular polarized laser
1D simulation a5, n0/nc10, L0.2l
No oscillation component, it pushes electrons
forward!
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Equilibrium condition

• Electrons are pushed forward and pile up in the
  front of laser pulse .
• Theyll be kept in an equilibrium !
• Condition

Skin depth ls, target thickness d ls
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The electric field distribution in 1D model
Protons are Bunched by Ex2 Debunched by
Ex1 . Phase Oscillations!
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Dynamics equations in PSA
Reference particle

Test charge
Protons are oscillated around the reference
particle, which is called as Phase motion (?,t)!
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Dynamics in PSA - Phase motion
Motion equations Phase motion equation
The phase motion of proton is harmonic with
frequency ?.
The energy spread is derived
X. Q. Yan et al., submitted (2007).
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Phase oscillations in 1D Simulations
a5, n0/nc10, L0.2l, t100 TL
The periods are 8, 8, 10 and 14 TL ,
respectively, which well agrees with theoretical
results!
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a5, n0/nc10, L0.2l, t100 TL
1D simulation results at t200 TL
(a) Phase space of electrons.
(b) Phase space distribution of protons.
(c) Electrons and protons density profiles (d)
Energy spectrum of protons
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Scaling law and GeV proton beam
Mono-energetic GeV proton t1ps , a5

• L 0.2 µm, n0/nc10 , Spot size 10µm, 100TW,

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Multi-dimensional effects
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Summary

• Single-cycle THz emission can be produced from
  laser wakefields excited in a thin plasma layer.
• It is found that CES can be formed with the high
  intensity laser pulses currently available.
  Protons can even be reflected by colliding shock
  waves and be accelerated for several times.
• Monoenergetic (energy spreadlt4 ) proton beam (gt
  300MeV) can be obtained by a laser with focused
  intensity of 1019W/cm2 by circularly polarized
  laser pulses in the so-called phase stability
  acceleration regime.

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Plasma oscillations in inhomogeneous plasmas
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Wave vector of a plasma wave in inhomogeneous
plasmas