Charge-state resolved laser acceleration of gold ions to beyond 7 MeV/u

Abstract

In the past years, the interest in the laser-driven acceleration of heavy ions in the mass range of $$\text {A}\approx 200$$ A ≈ 200 has been increasing due to promising application ideas like the fission-fusion nuclear reaction mechanism, aiming at the production of neutron-rich isotopes relevant for the astrophysical r-process nucleosynthesis. In this paper, we report on the laser acceleration of gold ions to beyond 7 MeV/u, exceeding for the first time an important prerequisite for this nuclear reaction scheme. Moreover, the gold ion charge states have been detected with an unprecedented resolution, which enables the separation of individual charge states up to 4 MeV/u. The recorded charge-state distributions show a remarkable dependency on the target foil thickness and differ from simulations, lacking a straight-forward explanation by the established ionization models.

The role of collisional ionization in heavy ion acceleration by high intensity laser pulses

“…9 of Ref. 6 ). We can see a good agreement between simulations and experiments for thin targets, 25–100 nm.…”

“…The experiment 6 shows that the 100 nm gold foil produces the highest dominant charge state, Z = 69. Seemingly, it is on the border between the FI-dominant and CI-dominant regimes : first, the target becomes semi-transparent and neutral atoms are ionized fast; then, CI increases the ion charge states: at t = 1 ps the minimum-to-maximum range of Au charge states is Z = 57–69, while at t = 2 ps it becomes Z = 66–77.…”

“…4 of Ref. 6 ), carbon and oxygen ions were detected as contaminants, whose recombination can reduce their charge especially that it is important also for light ions 36 and for this reason, lower charge states are seen for these species. For protons, charge transfer with other atoms and electron-ion recombination in the high-density plasma near the target was unable to explain the neutralization, and instead, the recombination of copropagating electrons and ions far away from the target accounted for neutralization observed in the experiment 37 .…”

“…Recently, an experiment 6 performed at the PHELIX laser facility has reported the acceleration of gold (Au) ions up to 7 MeV/nucleon showing remarkable dependency of their charge-state distribution on target thickness and the observation of dominant charge states up to Z = 69 (Ne-like), well above the field-ionization (FI) prediction at such intensities. Previous simulations failed to account for these observations and predicted a sharp, dominant charge state of Z = 51 for 0.5 m Au foils (sub-ps laser pulse; W/cm 2 ) 7 , which is not seen experimentally, along with negligible populations from other charge states (600 fs; W/cm 2 ) 8 , 9 .…”

“…In the following, we present 2 dimensional (2D) simulations incorporating a new collisional ionization (CI) module and show for the first time that we are able to reproduce numerically high charge states up to Z = 77, in particular for thicker targets 6 . We believe that the CI process is the most important reason for increasing charge states well beyond Z = 51 up to Z = 77.…”