Quasimonoenergetic Deuteron Bursts Produced by Ultraintense Laser Pulses

We report on the generation and laser acceleration of bunches of energetic deuterons with a small energy spread at about 2 MeV. This quasimonoenergetic peak within the ion energy spectrum was observed when heavy-water microdroplets were irradiated with ultrashort laser pulses of about 40 fs duration and high (${10}^{-8}$) temporal contrast, at an intensity of ${10}^{19}\mathrm{W}/{\mathrm{cm}}^2$. The results can be explained by a simple physical model related to spatial separation of two ion species within a finite-volume target. The production of quasimonoenergetic ions is a long-standing goal in laser-particle acceleration; it could have diverse applications such as in medicine or in the development of future compact ion accelerators.

Figure 1

Emitted ion spectra from laser-exposed heavy-water droplets: (a) Thomson-parabola traces of the ion emission (deuterons and oxygen ions) in the forward direction from a heavy-water droplet. The color picture shows the dominant deuteron emission in a narrow energy range at high energy. The bright circle in the upper left corner is formed by neutrals and energetic photons moving along the axis of the spectrometer. (b) Scan of the deuteron energy distribution in the forward direction. (c) Simultaneously emitted deuteron spectra in the backward direction. Here the monoenergetic part is left. Only low energy deuterons are visible. The total deuteron energy in the backward direction is less by a factor of 2 than in the forward direction, whereas their number in the low energy part is higher than in the forward direction.

Figure 2

Spectra of oxygen ions simultaneously emitted with the deuterons in the forward direction from the droplet [from Fig. 1a]. The reduced energy of the higher-charged oxygen ions is clearly visible.