Micrometer-sized negative-ion accelerator based on ultrashort laser pulse interaction with transparent solids

We report here energetic $(>100\mathrm{keV})$ negative hydrogen ions $\left({\mathrm{H}}^{-}\right)$ generated in the interaction of moderately intense $\left({10}^{18}\mathrm{W}\mathrm{c}{\mathrm{m}}^{-2}\right)$ ultrashort laser pulses (45 fs) with transparent hydrogen containing solid targets. An unambiguous and consistent detection of negative hydrogen ions, with a flux of $8\times {10}^{11}{\mathrm{H}}^{-}$ ions/sr, has been observed in every single laser shot, using a Thomson parabola ion spectrograph. Simple estimates based on charge transfer cross sections match well with experimental observations. Our method offers the implementation of an intense, ultrashort laser based negative-ion source at a higher repetition rate, which can be important for various applications.

Figure 1

A schematic diagram of the experimental setup for producing energetic negative hydrogen ions using ultrashort laser pulse irradiation of solids.

Figure 2

Typical parabolic traces obtained using the TPIS, showing the protons and ${\mathrm{H}}^{-}$ ions.

Figure 3

(a) Energy distribution of the ${\mathrm{H}}^{-}$ ions at different focal positions of the laser beam. The positive direction implies the sample has been moved towards the laser incident direction. The inset shows the self-focusing channels formed inside PMMA, perhaps by the ASE prepulse. (b) The effect of the laser prepulse on ${\mathrm{H}}^{-}$ production. The laser prepulse levels at different settings are shown in the inset.

Figure 4

The ion energy spectra corresponding to parabolic traces shown in Fig. 2: ${\mathrm{H}}^{+}$ (dashed curve, black) and ${\mathrm{H}}^{-}$ (thick solid curve, red). The thin solid curve (blue) shows the estimated spectrum of the ${\mathrm{H}}^{-}$ ions, derived from the observed ${\mathrm{H}}^{+}$ spectrum.