DIAGRAMA DE FLUJO

Neutron time-of-flight spectroscopy was used to determine the neutron energy spectrum. The setup shown in Fig. 5.1 was slightly modified in a way that the detectors were moved to various directions, and the laser incidence angle on the target was switched from 45◦ _{to 0}◦ _{to change the polarization conditions (see Fig.}

5.9). The laser was polarized in the plane of incidence (p-polarized) in the case of oblique incidence. A typical neutron time-of-flight spectrum and a scattering- corrected neutron energy spectrum is shown in Fig. 5.7, where the laser was inci-

dent under 45◦ _{to the target normal.}

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Figure 5.7: Neutron time-of-flight spectrum from a single detector (left). The separa- tion between the prompt γ-peak and the delayed neutron signals is a measure for their energy. Right: Energy for two angles of the neutron detector arrays with respect to the target normal. The low energy cutoff is determined by the time window for the neutron detection.

The two energy spectra on the right side of Fig. 5.7 clearly exhibit a peak, which occurs at different energies for different detector directions. This kinematic shift away from the 2.45 MeV center-of-mass energy reflects the ion kinematics, as discussed in Chapter 4. The low energy tail is typical for these spectra and is caused by incomplete treatment of neutron scattering or by excess neutrons from the 12_C(d,n)13_{N reaction. A comparison with model calculations is shown in} Fig. 5.8, where an isotropic ion emission from the laser focus with an exponential energy spectrum with a slope of 75 keV was assumed.

The model curves represent the experimental data quite well. For other shots the temperature lay a bit higher, and a typical value for these laser conditions is 75-100 keV. The total number of accelerated ions based on that temperature estimate is approximately 3.5_×1011_{, which corresponds to a coupling efficiency of} laser light to fast ions of _∼1.75%.

For a larger number of experimental runs, in Fig. 5.9 the peak position in the neutron energy spectrum is plotted versus the laser energy for different detection

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Figure 5.8: The neutron spectrum from Fig. 5.7 compared to model calculations with MCNEUT. The model parameters and results are explained in the text.

directions and laser incidence angles.

Figure 5.9: Position of the kinematically shifted neutron energy peak plotted against the laser energy for 0◦ _{and 45}◦ _{laser incidence direction under various detection direc-} tions.

1. Oblique (45◦_{) incidence (p-polarized):}

(a) The two detectors placed symmetrically behind the target (0◦ _{and 90}◦₎ measure the same blue shift (i.e. a peak at energies higher than 2.45 MeV).

(b) The detector at 225◦ _{measures redshifted neutrons.}

2. Normal (0◦_{) incidence (s-polarized):}

(a) The detector at 0◦ _{sees a strong blue shift.}

(b) The detector at 90◦ _{measures unshifted neutrons.} (c) The detector at 225◦ _{measures redshifted neutrons.}

From 1(a) it follows that the ion motion is symmetrical to the target nor- mal. Points 1(a) and 1(b) implicate either isotropic emission, where the red-and blueshift are caused by the difference in plasma density inside and in front of the target, or by a overall velocity component directed symmetrically into the target. The same general picture is valid for case 2, so the ion emission is not related to the laser direction, but either to the target orientation or isotropic. In figure 5.10

the ion momentum space from 3-D PIC calculations is plotted for a number of preplasma scalelengths which are within the range determined by the estimate in section 5.2. These simulations represent four different cases:

• Acceleration into and out of the target even for an obliquely incident laser occurs at short preplasma scalelengths (Fig.5.10(c)). A steep density gradi- ent belonging to a small scalelength corresponds to quasi 1-D situation with target normal acceleration.

• When the plasma scalelength is large (Fig. 5.10 (a)), hole boring becomes possible favoring ion acceleration in the radial direction normal to the laser axis.

• At intermediate scalelengths, a transition between the two extreme cases is observed (Fig. 5.10 (b)).

• A more isotropic acceleration occurs at even longer scalelengths when the focus is large (Fig.5.10 (d)). Here the divergence of the ion beam is broader because of the ”softer” plasma boundary and goes roughly into 2π. This situation would also explain the results from Fig. 5.9, and is in good agree- ment with the neutron spectrum in Fig.5.8. Since this scenario calls for a less

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Figure 5.10: Projection of the ion momentum space on the px-py-plane for varying pre- plasma scalelengths and focal spot sizes. Spot size 4µm, 2_×1019_W/cm2_{: (a) Scalelength} l=10µm, (b) l=2.5µm and (c) l=1.5µm. (d) Spot size 9µm, 5_×1018W/cm2, scalelength l=7.5µm. The pulse duration was 158fs, and the snapshots were taken 40 fs after the pulse maximum. For orientation, the laser direction and target surface position in space coordinates is marked. Although the plots are momentum space, this gives an impression where the ions move. The simulation box was x=19.2µm_×y=64µm_×z=16µm in size and the laser was obliquely incident onto the target. The neutron detector directions are given for the following experiments (sections 5.3.3 and 5.3.4)

steep density gradient, it is more likely that the real emission characteristics is broad.

A comparison of p- and s-polarized laser pulses allows to rule out that the Brunel mechanism (see chapter 2, section 2.1.2) dominates the electron acceleration in our case. Since the Brunel mechanism works only for p-polarized light, one would expect a difference in the number and energy of the fast electrons for the two cases, which in turn would result in different ion and neutron energies and numbers. This difference is not seen in the experiment.

Taking into account the PIC results, this interpretation of the experimental data looks quite conclusive.

1. Due to the large focus the ions are accelerated either in a quasi 1-D geometry in a direction into and out of the target surface or are emitted completely isotropic. The neutron diagnostic is not able to clearly distinguish between these two cases.

2. The interaction takes place in a plasma with a gently density gradient, be- cause obviously the Brunel mechanism plays no role for fast-electron gener- ation.