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abstract:
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Focusing of a 15 MeV, 19 nC electron bunch by an un-
derdense plasma lens operated just beyond the threshold
of the underdense condition has been demonstrated in ex-
periments at the Fermilab NICADD Photoinjector Labora-
tory (FNPL). The strong 1.9 cm focal-length plasma-lens
focused both transverse directions simultaneously and re-
duced the minimum area of the beam spot by a factor of 23.
Analysis of the beam-envelope evolution observed near the
beam waist shows that the spherical aberrations of this un-
derdense lens are lower than those of an overdense plasma
lens, as predicted by theory. Correlations between the beam
charge and the properties of the beam focus corroborate this
conclusion.
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