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Numerical Simulations of Laser Wakefield�in inhomogeneous Plasmas�

Hayashi Yoshiaki(1), Habara Hideaki(1), Yao-Li Liu(2), Kuramitsu Yasuhiro(1)

(1)Graduate school of engineering, Osaka University

(2)Institute of Atomic and Molecular Sciences, Academia Sinica Taipei

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Flying mirror concept

Incident

light

Reflected

light

Pump laser

Laser wakes as a flying mirror

Direction of

wakes propagation

  • A mirror flying at relativistic speed have many interesting proerties, such as frequecy upshifting and reflected light intesification.
  • Nonlinear wake waves excited by a short laser pulse in under dense plasmas form thin dense electron layers moving at a relativistic speed which can be viewed as a relativistic flying mirror.
  • Accelerating flying mirror have another interesting property, which is a radiation analogous to Hawking’s black hole radiation called Unruh effect[1].
  • It is considered that the speed of plasama wakes is controlled with�inhomogeneous density profile.

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Simulation setup

Similation parameters

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Simulation code

epoch2d

(Nx , Ny)

(3072,3072)

(Δx , Δx)

(9.77 nm, 9.77nm)

Plasma

Hydrogen plasma

Particle count

16

Boundary condition

outflow

Frame

Moveing frame moving with light speed

Laser parameters

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Wavelength

810 nm

Pulse Width

23.55 fs

Spot Size

7.1 μm

Intensity

1019 W/cm2

Normalized vector potential

1.53

Gamma

1.83

nc

1.7 × 1020 cm-3

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Density profile

L = 1 μm

n0 = 10-2 nc

(1) g = 0.01 μm-1

(2) g = 0.1 μm-1

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  • L = 1 μm
  • n0 = 10-2 nc

(1) g = 0.01 μm-1

(2) g = 0.1 μm-1

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Method

  • The line profiles along the laser axis are obtained for all time series. Time-space plot is produced by stacking them vertically.
  • The positions of peaks of the wakefield is tracked and fitted with 10th order polynomial. Velocity and acceleration of the peaks are obtained by differentiation.

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30 um

c*τ : 240 μm

g = 0.01 μm-1

800 fs

Space

Time

300 fs

c*τ : 90 μm

g = 0.1 μm-1

Space

Time

30 um

c*τ : 240 μm

g = 0

800 fs

Space

Time

300 fs

Space

Time

g = 0.1 μm-1 with focusing laser

c*τ : 90 μm

Laser focal point

Results

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g = 0.1 μm-1 with focusing laser

g = 0.1 μm-1

g = 0.01 μm-1

30 um

30 um

30 um

300 fs

  • The position of the first peak is shifted to the right edge so that change of the wavelength is easy to.
  • White dashed line represent a local plasma wavelength
  • The wavelength of wakes is well characterized by local plasma wavelength

Change of wavelength

300 fs

300 fs

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The velocity of plasma wakes decreases from superluminal velocity.

It eventually reaches to light speed before the wakes collapse.

g = 0.01 μm-1

g = 0.1 μm-1

Change of velocity

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  • The speed of plasma wakes in inhomogeneous density profile changes in time by two factors. The first is the change of the laser group velocity and the second is the change of the wavelength of plasma wakes.
  • The laser group velocity and the wavelength of plasma wakes is well characterized�by the dispersion relation of plasma wave and light in a plasma.
  • Acceleration of plasma wakes is not obtained from increasing density profiles with parameters we searched.

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  • This work is supported by JSPS Grant-in-Aid for Scientific Research(S) #15H05751 and (C)#18K03577

Summary