1 GENERALIDADES
2.4 MEJORAMIENTO DE PROCESOS 4
As already mentioned in section 5.3.2, the ion emission characteristics depend strongly on the preplasma gradient. Therefore, a comparison of neutron spectra
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Figure 5.13: Neutron spectra with (top) and without (bottom) artificial prepulse. The neutron spectra derived from the 10-µm gradient PIC ion distribution of Fig. 5.10 are also plotted as dashed lines for the two detector positions.
with and without an artificially added prepulse was performed. Before this exper- iment was made, the contrast ratio of the ATLAS laser was carefully measured and prepulses caused by a misaligned regenerative amplifier were removed (see section 5.2). This resulted in a contrast ratio of > 10−7 on the ns-scale, but at a peak intensity of 1019W/cm2 the intensity of the ASE background is still in the 1011−1012W/cm2-range, sufficient for noticeable plasma creation. An artificial prepulse with an energy and intensity content of 1% of the main pulse can be added 400 ps before the main pulse. Neutron spectra were recorded with and without this prepulse. The result of this experiment is shown in Fig. 5.13. In this figure, the sum of all runs with and without artificial prepulse from the measure- ment campaign. Also plotted is the modeled neutron spectrum, which is based on the PIC calculation for a 10-µm plasma gradient, as shown in Fig. 5.10. The resulting experimental spectra exhibit small, but distinct differences.
These can be realized most clearly by comparing them to the PIC results. While the model curve agrees quite well with the experiment in the case with prepulse (the large discrepancy at neutron energies below 2 MeV is due to scattered neutrons), no close match can be achieved for the high contrast case. In the following, we will discuss the differences for both detector locations separately.
1. Forward (45◦) emitted neutrons: For high contrast, the low-energy slope
of the forward detected neutron peak is far more red-shifted than in the runs with prepulse. This is quite surprising, since redshifted neutrons can only be caused by ions streaming away from the target surface (and hence from the detector) into the coronal plasma, where they undergo fusion. Fig. 5.5 sug- gests a higher preplasma density for the case with prepulse, and consequently one would expect more red-shifted neutrons in this case. Since the exper- imental findings are the exact opposite of that, something must be wrong with this simple model; however, a conclusive solution to this problem is not yet found. A possible reason for the observed result may be a snow-plow effect of the prepulse, in the sense that the prepulse creates a void in the preplasma in front of the critical surface. This effect cannot be observed in 1-D hydrodynamic or PIC calculations, since here the plasma cannot move laterally out of the focal region. In 3-D, the prepulse could act in a similar way as the main pulse to create a density depression in the focus, since the time for the plasma to move is quite long. However, at prepulse intensities of 1017W/cm2, the relativistic effects are rather small, so this explanation is just a hypothesis. It cannot be tested in 3-D PIC simulations, since a 400 ps time interval between the pre- and main pulses are a factor of 1000 too long to treat in reasonable computing time, and a 3-D hydrodynamics code that can resolve the fs-prepulse does not exist to my knowledge.
2. Sideways (135◦) emitted neutrons: The predominantly radial emission
of ions with respect to the laser axis in the 10-µm gradient case (see Fig.
5.10) causes a relatively broad neutron spectrum in the sideways direction. This agrees well with the experiment for the low contrast case with prepulse. In the high-contrast case, as expected from the steeper gradient, the ion emission is more forward directed, leading to a narrower sideways peak. These result of that last experiment shows that the dynamics of the laser-plasma interaction is still far from being fully understood, leaving enough room for further interesting research. Especially the effects of fs-prepulses acting on a long-pulse generated preplasma seem to be not yet completely described.
All in all, in this chapter, the first steps toward neutron spectroscopy taken in this thesis were described. We could show that by a moderate increase in pulse energy by a factor of 4 and simultaneously rising the laser intensity by a factor of 20, the neutron yield could be pushed by nearly two orders of magnitude, which can be mainly attributed to the steep increase of the d-d reaction cross-section with ion energy. Now enough neutrons can be generated to perform neutron spectroscopy. First results in this field show that, contrary to Pretzler’s suggestion [12], the ion emission characteristics is either normal to the target or isotropic, which together with PIC simulations hints at an ion origin close to the critical density. The spectra can be modeled to determine approximate ion temperatures and numbers. If the laser energy is varied while the focusability is maintained, the variation of the neutron yield can be explained by the ponderomotive scaling law for the ion temperature and a fixed conversion efficiency of laser light into ions. The neutron spectra also show an increase in ion temperature. The preplasma scalelength does have an influence on the neutron spectra, but it cannot yet be fully modeled and understood.