MeV Ion Jets from Short-Pulse-Laser Interaction with Thin Foils

Collimated jets of carbon and fluorine ions up to $5\mathrm{M}\mathrm{e}\mathrm{V}/\mathrm{\text{nucleon}}$ ($\sim 100\mathrm{M}\mathrm{e}\mathrm{V}$) are observed from the rear surface of thin foils irradiated with laser intensities of up to $5\times {10}^{19}\mathrm{W}/{\mathrm{c}\mathrm{m}}^2$. The normally dominant proton acceleration could be surpressed by removing the hydrocarbon contaminants by resistive heating. This inhibits screening effects and permits effective energy transfer and acceleration of other ion species. The acceleration dynamics and the spatiotemporal distributions of the accelerating $E$ fields at the rear surface of the target are inferred from the detailed spectra.

Figure 1

(a) Ion traces (on CR39) from an unheated $\mathrm{A}\mathrm{l}\mid \mathrm{C}$ target and (b) corresponding spectra. The gap in the proton signal is due to the CR-39 detector which is optimized for heavier particles. The dotted line illustrates the ${\mathrm{H}}^{+}$ spectra as obtained with the proton spectrometer. (c) Ion traces from a heated $\mathrm{A}\mathrm{l}\mid \mathrm{C}$ target and (d) corresponding spectra. The ion signals are strongly enhanced. The spot in the upper right corner of (a),(c) is the pinhole image formed by neutral atoms.

Figure 2

(a) Overlayed signals of heated (orange) and unheated (blue) $\mathrm{W}\mid {\mathrm{C}\mathrm{a}\mathrm{F}}_2$ targets: The proton signal vanishes for heated targets; the fluorine signals (especially ${\mathrm{F}}^{\mathrm{7}\mathrm{+}}$) go up to much higher energies. (b) Corresponding ${\mathrm{F}}^{\mathrm{7}\mathrm{+}}$ spectra: ${\mathrm{F}}^{\mathrm{7}\mathrm{+}}$ ions achieve more than 100 MeV ($5\mathrm{M}\mathrm{e}\mathrm{V}/\mathrm{\text{nucleon}}$).

Figure 3

Space-time history of the accelerating electric fields (red curves) and the ${\mathrm{C}}^{\mathrm{4}\mathrm{+}}$-dominated ion distributions (white curves) as calculated by our model: The field maximum moves out into the vacuum. Already during the pulse there exists an ion distribution on a $\mu \mathrm{m}$ scale.