High Quality Electron Bunches up to 1 GeV from Laser WakefieldAcceleration at LBNL Page: 4 of 7
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Electron energy (MeV)
FIGURE 2. Electron bunch. The electron energy spectrum of the channeled accelerator shows the
appearance of monoenergetic features, here with 2 x 10' electrons in a bunch with energy spread of 4%
FWHM at 86 MeV. Divergence was near 3 mrad FWHM.
Transmission at 4 TW was 35%, a reduction of one third from the low power case,
indicating substantial power was deposited in plasma waves. This is consistent with
particle in cell simulations (below), which indicate that a plasma wave averaging 2 -
300 GVfm in the last 0.5 mm of guide length. No electrons are trapped at 4 TW,
indicating a structure for controlled injection experiments [23,24], and colliding pulse
injection  experiments are now under way which may increase beam stability.
Channeled Wakefield Acceleration
At guided drive pulse powers above 4 TW, electrons were trapped and accelerated,
verifying that an intense plasma wake was driven in the channel. At 9 TW, the
channel guided accelerator produced high charge electron beams at high energy with
low energy spread and low divergence . Figure 2 shows a bunch of 2 x 109
electrons within an energy spread of 2% centered at 86 MeV. Optimal performance
was found in a channel with an axial density of 1.9 x 1019 cm3 and with a parabolic
profile with 40% less rise in density over a spot diameter than the low power matched
case. The normalized geometric emittance obtained from assuming the bunch comes
from a source on order of the laser spot size is 1-2 mm-mrad, competitive with state of
the art RF facilities.
The accelerator was operated in the same gas jet without the guiding channel.
Optimum unchanneled performance was at ne ~ 4 x 1019 cm-a, producing an
exponential energy distribution with a 2.6 MeV temperature below 10 MeV and an 8
MeV temperature above 10 MeV, and no detectable electrons above 40 MeV. No
difference was observed between operation in a neutral gas jet and a pre-ionized (but
not channeled) plasma, confirming that channeling greatly enhanced accelerator
Simulations and Dephasing
Two-dimensional particle-in-cell (PIC) simulations using the code VORPAL
(developed at U. of Colorado Tech X ) performed with parameters close to the
experiment indicated that the high quality electron bunches are formed by wake
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Esarey, E.; Nagler, B.; Gonsalves, A.J.; Toth, Cs.; Nakamura, K.; Geddes, C.G.R. et al. High Quality Electron Bunches up to 1 GeV from Laser WakefieldAcceleration at LBNL, article, July 1, 2006; (digital.library.unt.edu/ark:/67531/metadc898223/m1/4/: accessed December 10, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.