Angular distribution of neutrons from deuterated cluster explosions driven by femtosecond laser pulses

We have studied experimentally the angular distributions of fusion neutrons from plasmas of multi-keV ion temperature, created by $40\mathrm{fs}$, multi-TW laser pulses in dense plumes of ${\mathrm{D}}_2$ and ${\mathrm{CD}}_4$ clusters. A slight anisotropy in the neutron emission is observed. We attribute this anisotropy to the fact that the differential cross section for DD fusion is anisotropic even at low collision energies, and this, coupled with the geometry of the gas jet target, leads to beam-target neutrons that are slightly directed. The qualitative features of this anisotropy are confirmed by Monte Carlo simulations.

Figure 1

Time of flight spectrum from a neutron detector placed $2.2\mathrm{m}$ from the cluster fusion plasma.

Figure 2

Schematic of the setup used for the angular measurement of the neutron yield.

Figure 3

Azimuthal neutron scan in ${\mathrm{D}}_2$ and comparison with data from a particle simulation. The error bars indicate the standard deviation obtained from the standard deviation of the signal areas.

Figure 4

Measured neutron emission from a ${\mathrm{CD}}_4$ plasma in the plane perpendicular to the laser propagation as a function of angle with respect to the laser polarization.

Figure 5

Azimuthal neutron scan in ${\mathrm{CD}}_4$ and comparison with data from a particle simulation.

Figure 6

Ratio of the average cross section for ion-ion interaction in the filament (subscript F) and for interactions in the beam target (subscript BT).