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Theory of ionization-induced trapping in laser-plasma accelerators
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10.1063/1.3689922
/content/aip/journal/pop/19/3/10.1063/1.3689922
http://aip.metastore.ingenta.com/content/aip/journal/pop/19/3/10.1063/1.3689922
View: Figures

Figures

Image of FIG. 1.
FIG. 1.

(Color online) Typical laser plasma accelerator structure and injection scheme. The light black line shows a laser electric field distribution along its propagation direction. The thick red/gray line shows the electric field of the wake. The thin blue/dark gray line shows the momentum distribution of the background electron fluid. The thin green/gray line shows the trapping momentum threshold of the electrons in the wakefield. The thick black line shows the momentum gap between the trapping threshold and the background electron’s momentum. Here, (where is the plasma wavelength), and .

Image of FIG. 2.
FIG. 2.

(Color online) Schematic profile of the gas density distribution. The density of the pre-ionized electrons () and the neutral nitrogen atomic density () satisfy . So the total density of the pre-ionized electrons and the electrons from ionization up to the 5th level of the nitrogen is uniform ().

Image of FIG. 3.
FIG. 3.

(a) Threshold laser intensity required to trap an electron that was ionized at a phase for a Gaussian laser centered at with and . (b) Threshold laser intensity required to trap at peak of laser envelope versus laser pulse length .

Image of FIG. 4.
FIG. 4.

(Color online) Normalized laser vector potential (light blue/gray curve), wake potential (blue/up dark gray curve), wake electric field (red/lower gray curve), (green/middle gray curve), and degree of ionization versus . The dashed black curve shows the degree of ionization for , and the solid black curve shows . Here, (where is the laser wavelength), and .

Image of FIG. 5.
FIG. 5.

(Color online) (a) Trapped electron number versus the laser propagation distance (position of the center of the laser pulse) in the mixed gas. Inset shows the electron orbits in longitudinal phase space: (1) orbits of an electron trapped near the separatrix (solid line) and (2) a deeply trapped electron. The laser pulse is represented by a colorful ellipse in the inset. (b) Trapped number evolution versus laser propagation distance. Here, the mixed gas length is fixed at . Laser-plasma parameters are , and a uniform plasma density .

Image of FIG. 6.
FIG. 6.

(Color online) (a) Distribution of electrons in longitudinal phase space (x-) and normalized electric field of the wake (arbitrary units). The color bar represents the relative density in the phase space. (b) Electron energy spectrum (number versus electron energy ) beyond dephasing length. Laser pulse: and . The uniform plasma density is with the full length being mixed gas (1% nitrogen).

Image of FIG. 7.
FIG. 7.

(Color online) Electron beam energy spread (black circles) and injected electron number (blue squares) versus the mixed gas length. The laser-plasma parameters are , and a uniform plasma density with .

Image of FIG. 8.
FIG. 8.

(Color online) Dependence of energy spread and final electron injection number on the concentration of nitrogen. Here, the mixed gas length is . Laser pulse: 8 and . The uniform plasma density is .

Image of FIG. 9.
FIG. 9.

(Color online) (a) Dependence of energy spread and final electron injection number on the mixed gas length. Here, the product of mixed gas length (unit in ) and concentration (percentage) is fixed to be 2. (b) Electron energy spectrum when the mixed gas length is 1000 long (black line) or 11 long (red/gray line). Laser pulse: , and the uniform plasma density .

Image of FIG. 10.
FIG. 10.

(Color online) Electron momentum distribution versus initial ionization phase with respect to laser peak for (a) and (b) .

Image of FIG. 11.
FIG. 11.

(Color online) (a) Skewed (b = −0.8) laser pulse transverse field (thin black) and excited wakefield (thick blue/dark gray) normalized by . (b) Electron beam energy spectra using a laser with a positive (red/gray) and negative skew (black).

Image of FIG. 12.
FIG. 12.

(Color online) Transverse momentum distribution of trapped electrons using a laser with (a) linear polarization or (b) circular polarization. The laser-plasma parameters are , and the plasma density . (c) Number of trapped electrons and (d) transverse momentum (root mean square) versus using a linearly polarized laser pulse.

Image of FIG. 13.
FIG. 13.

(Color online) Typical trajectories of trapped electrons via ionization. Laser-plasma parameters are , focus spot size , uniform plasma density with , and mixed gas length . The nitrogen concentration is 1%. Top half shows and bottom half shows .

Image of FIG. 14.
FIG. 14.

(Color online) Transverse spatial structure of the accelerated electron beam using (a) S-polarized laser pulse and (b) P-polarized laser pulse when . Typical trajectories of trapped electrons for the (c) S-polarized and (d) P-polarized cases, respectively. Except polarization, other laser and plasma parameters are the same as in Fig. 13.

Image of FIG. 15.
FIG. 15.

(Color online) Distribution of ionization rate in the space of the laser vector potential and the position along the laser pulse. Here, the ionization and normalized laser intensity and length of are considered.

Image of FIG. 16.
FIG. 16.

(Color online) Ionization effect on wakefield generation. The light blue/gray line shows the normalized laser electric field. The other lines show the wakefield intensity (, see text). The normalized laser intensity and length of are considered. The total electron density is .

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/content/aip/journal/pop/19/3/10.1063/1.3689922
2012-03-05
2014-04-20
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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Theory of ionization-induced trapping in laser-plasma accelerators
http://aip.metastore.ingenta.com/content/aip/journal/pop/19/3/10.1063/1.3689922
10.1063/1.3689922
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