Generation of a GeV, nC monoenergetic beam using laser plasma acceleration

1
Generation of a GeV, nC monoenergetic beam using
laser plasma acceleration
W.Lu, M.Tzoufras, F.S.Tsung, C.Joshi,
W.B.Mori UCLA, USA L.O.Silva, R.A.Fonseca IST,
Portugal
2
Outline

• Motivation.
• Simulation parameters Show how to choose the
  simulation parameters given the laser power
  (matched profile, guiding). Find the expected
  results based on the theory.
• Physical picture Description of the physical
  picture of the simulation. Additional effects
  that could lead to results different than the
  theoretical predictions.
• Simulation results Phasespace of the self
  generated electron beam. Evolution of the beam
  with time and its characteristics at the end of
  the simulation.
• Conclusions.

3
Motivation Recent results
Phys. Rev. Lett. by Tsung et al. (September
2004), where energy up to 0.8 GeV and
monoenergetic beam with energy 260 MeV were
observed.
3 Nature papers (September 2004), where
monoenergetic electron beams with energy
exceeding 100 MeV were measured.
The laser induced ultrarelativistic blowout
regime is very effective in accelerating
particles.
4
Explicit Particle-In-Cell code OSIRIS
Typical simulation parameters 109
particles 105 time steps
c
5
Simulations Nature papers, agreement with
experiment
3D Simulations for Nature 431, 541 (S.P.D.
Mangles et al)

• In experiments, the of electrons in the spike
  is 1.4 108.
• In our 3D simulations, we estimate of 0.9 108
  electrons in the bunch.

6
Simulations Laser parameters and plasma density
What can we do with a 200 TW laser?
Matched laser profile
Require self focused propagation
Matched laser profile
7
Simulations Expected results

• For a 30fs pulse the depletion length is shorter
  than the dephasing length (we could have chosen a
  longer pulse).
• Energy about 1.5GeV, however what is the energy
  spread since there is no dephasing?

8
Simulations Parameters

• We need to resolve (30 cells/wavelength)
• The laser wavelength in the direction of the
  laser propagation.
• The plasma wavelength in the plane perpendicular
  to it.

After 5 Zr / 7.5 mm
(Took about 1 month on 200 G5s)
Total charge 1.1 nC
9
Physical picture Geometry - fields

• An ion channel is generated due to the
  ponderomotive blowout of the electrons.
• Its shape is almost a sphere.
• This structure moves with the speed of (laser)
  light, supporting huge accelerating fields.
• Particles at rear of the channel are injected in
  the blowout region.
• The force on these particles is both
  accelerating and focusing.

10
Physical picture Evolution of the nonlinear
structure

• The front of the laser pulse looses energy to
  the particles and etches back.
• Beam loading eventually shuts down the self
  injection.
• The pulse forms its own channel and remains
  self-focused until its power falls below a
  certain value.
• The laser can be chosen long enough so that the
  pump depletion length is longer than the
  dephasing length.

11
Laser Guiding 3D movie - not exactly matched but
stable
If the laser profile is not exactly matched,
the laser size of the envelope oscillates and as
a result so does the blowout spheroid. However
the propagation is still stable!
Electron density isosurfaces from 3D simulation
for a 200 TW 30 fsec pulse
12
Simulations The 200 TW run

• At early times the accelerating fields are
  higher.
• A beam has not been formed yet.

Beam loading
13
Simulations The 200 TW run

• A beam starts to form as beam loading becomes
  significant.
• Some particles feel the spike of the wakefield
  again!

Beam loading
14
Simulations The 200 TW run

• Even though dephasing wasnt reached a beam with
  low longitudinal spread does emerge.
• The divergence is also very low.

Beam loading
15
Conclusions

• We have shown how the theory allows us to design
  laser plasma accelerators operating in the
  ultrarelativistic blowout regime.
• Given the power of a laser we can
• Pick the density for self-focused propagation.
• Choose the rest of the laser parameters by
  assuming a matched profile.
• Predict the energy and the charge of the
  monoenergetic beam.
• Our theoretical estimates are very robust,
  despite of the very complicated interplay of
  phenomena that occur in this regime.
• For these accelerators, since the energy is
  proportional to the laser power
• we can expect beams, with energy above 10 GeV and
  5nC charge using 2 PW lasers.