Fast electron transport and induced heating in solid targets from rear-side interferometry imaging

Fast adiabatic plasma heating of a thin solid target irradiated by a high intensity laser has been observed by an optical fast interferometry diagnostic. It is driven by the hot electron current induced by the laser plasma interaction at the front side of the target. Radial and longitudinal temperature profiles are calculated to reproduce the observed rear-side plasma expansion. The main parameters of the suprathermal electrons (number, temperature, and divergence) have been deduced from these observations.

Figure 1

Experimental setup. The plasma expansion is measured using a fast and time resolved interferometric diagnostic.

Figure 2

Experimental expansions at instant of (a) 240, (b) 480, and (c) 830 ps after the UHI laser pulse for the $9\mu \text{m}$ thickness Al target and at instant of 480 and 440 ps for the (d) 20 and (e) $50\mu \text{m}$ thickness targets. The laser beam arrives from left. In (e), there is no plastic foil. In (c), at later time, the two plasmas generated by particles collide with fast speed. The interfringe distance is $10\mu \text{m}$ and the full size of each image is $500\times 600\mu \text{m}$.

Figure 3

Experimental electron density profile along the longitudinal axis (perpendicular to the target surface) at 173 ps after the UHI laser pulse for the $9\mu \text{m}$ thick target.

Figure 4

Dependence of the plasma axial position vs the time for three target thicknesses for the electron density of ${10}^{20}{\text{cm}}^{-3}$.

Figure 5

Expansion velocity vs the target thickness.

Figure 6

Dependence of the diameter of expanding plasma on time for three target thicknesses.

Figure 7

On-axis initial temperature variation inside the target. Two analytical fits are shown: $T_e\left(z\right)=95e^{-z/30}$ (solid line) and $T_e\left(z\right)=-32.5z^{1/3}+135$ (dotted line).

Figure 8

(Color online) 2D plasma distributions 300 ps after the CPA pulse for two different initial temperature distributions. Panel (c) [(d)] shows the radial initial temperature distribution producing expansion (a) [(b)]. The target thickness is $9\mu \text{m}$ and the axial temperature is shown in Fig. 7.

Figure 9

(Color online) Initial 2D temperature distributions in the target. Panel (b) shows the radial initial temperature distribution (lineouts) at the front and rear sides.

Figure 10

(Color online) 2D temperature profiles of plasma expansion after 400 ps for (a) $9\mu \text{m}$, (b) $20\mu \text{m}$, and (c) $50\mu \text{m}$ target thickness.

Figure 11

Background electron temperature profile (solid line) with the model calculation for $r_0=23.5\mu \text{m}$, $\theta =20°$, $\eta =25\%$, and $T_h=300\text{keV}$ for a target initially cold. Heating has a resistive origin (circle) for the first $10\mu \text{m}$ and then, for deeper layers, collisions (square) become the dominant heating process. The on-axis initial temperature profiles from hydrodynamic simulations are in dotted lines (Fig. 7).

Figure 12

(Color online) (a) Density distributions in the target at the end of the prepulse $t=1.5\text{ns}$ for the $9\mu \text{m}$ target thickness. Panel (b) shows the axial $\left(R=0\right)$ density distribution (lineout) and panel (c) is a zoom-in on the dense zone of the target.

Figure 13

Background electron temperature profile (solid line) with the model calculation. The same parameters of the fast electron beam of Fig. 11 have been used but including the effect of the preplasma induced by the pedestal of the laser beam. This effect can be neglected for targets thicker than $1\mu \text{m}$.