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First step towards the understanding of the interaction of ions with dense plasmas

The understanding of the physical mechanisms involved in the interaction of heavy ions with “dense and hot matter” is fundamental within the framework of the researches on thermonuclear fusion induced by ion beams. It is even true more generally when dealing with physics of high energy densities, a scientific field in full expansion. Compared with “cold matter”, the elementary processes involved in ion-plasma interaction are drastically disturbed when matter is hot and ionized. Nevertheless, these modifications remain so far barely characterized and poorly known. Recent experiments, led by a team of the LULI , and supported by a data analysis based on a model of the ion charge state evolution developed by the INSP team “Clusters and Surfaces under Intense Excitation”, allowed to obtained the very first results in this domain.

In the experimental setup, (see figure 1), a laser pulse (B1) of 320 fs and 1J produces two ultrashort ion bunches that are identical on both sides of the irradiated Mylar target. A Thomson parabola spectrometer (TP1) measures the ion beam energy spectrum as well as the charge state distribution of the ion bunch produced by laser interaction. The other bunch going through a thin aluminum foil is analyzed by a second Thomson parabola (TP2). The aluminum target can be warmed up to temperatures ≥ 1eV by isochoric heating induced by a proton beam. The picosecond fast proton beam is synchronously generated by a second short laser pulse, B2 (with the same pulse duration of 320 fs and 4J in energy). These experiments showed that for the relatively modest temperatures reached here, we do not observe significant difference in the spectrum recorded for either a cold or a hot target.

GIF

Figure 1
Experimental setup.

The mean charge of carbon ion beam after passing through cold (300 K) solid aluminum is shown in figure 2 as function of beam energy. We can see that the Shima’s code, which is generally used, clearly disagrees with the measurements. Let us remind that this empirical model established from a compilation of experimental data, gives access only to the average charge state at the exit of the target. On the other hand, simulation results from a quite recent version of our ETACHA code are found to be in very good agreement. Our ETACHA (French acronym for charge states) code can predict the evolution of population of projectile electronic states traversing targets of different thicknesses, by solving a set of coupled differential rate equations. Elementary ion-atom cross-sections for capture, ionization and excitation processes are used and radiative and Auger deexcitations are included as well. The validity domain of the ab-initio calculations made in this code has been extended recently towards the intermediate velocity regime, relevant for the present case of carbon ion in aluminum.

GIF Figure 2
Comparison between experimental data with Shima’s empirical model (dotted line) and ab-initio calculations from the ETACHA model (black stars).

From this work, two important conclusions can be drawn :

  • For moderated temperatures, as those reached here (11.6 103 K), the degree of ionization and excitation of the target remaining quite small we do not observe significant difference in the charge state distribution and in the energy loss of the incident ion beam compared to cold matter ;
  • The ETACHA model, in its extended new version, is able to describe the physical processes occurring in cold matter, and should thus allow to quantify the modifications induced on the ion-atom elementary processes when ions interact with hot and dense plasmas.

Next steps will be to perform such kind of experiments for other highly charged ions and certainly at higher temperatures, like those that can be reached (up to 100 eV, i.e. more than 106 K) by using x-ray free-electron laser beams (XFEL).

Reference
“Charge Equilibrium of a Laser-Generated Carbon-Ion Beam in Warm Dense Matter”
M. Gauthier, S. N. Chen, A. Levy, P. Audebert, C. Blancard, T. Ceccotti, M. Cerchez, D. Doria, V. Floquet, E. Lamour, C. Peth, L. Romagnani, J. –P. Rozet, M. Scheinder, R. Shepherd, T. Toncian, D. Vernhet, O. Willi, M. Borghesi, G. Faussurier, J. Fuchs
Physical Review Letters, Vol. 110, N°135003, 2013

Contact
Dominique Vernhet