Enhanced Isochoric Heating from Fast Electrons Produced by High-Contrast, Relativistic-Intensity Laser Pulses

Thin, mass-limited targets composed of $\mathrm{V}/\mathrm{Cu}/\mathrm{Al}$ layers with diameters ranging from 50 to $300\mu \mathrm{m}$ have been isochorically heated by a 300 fs laser pulse delivering up to 10 J at $2\times {10}^{19}\mathrm{W}/{\mathrm{cm}}^2$ irradiance. Detailed spectral analysis of the Cu x-ray emission indicates that the highest temperatures, of the order of 100 eV, have been reached when irradiating the smallest targets with a high-contrast, frequency-doubled pulse despite a reduced laser energy. Collisional particle-in-cell simulations confirm the detrimental influence of the preformed plasma on the bulk target heating.

Figure 1

$\mathrm{Cu}{K}_{\alpha }$ spectra for different target diameters irradiated at $2{\omega }_0$ (green, top-three curves) and $1{\omega }_0$ (red, bottom curve). The dashed lines correspond to the FLYCHK profiles for varying temperatures.

Figure 2

Rear-view images at 8 keV of $300\mu \mathrm{m}$ diameter targets irradiated at $1{\omega }_0$ (a) and $2{\omega }_0$ (b).

Figure 3

Temperature measurements for varying target sizes at $1{\omega }_0$ and $2{\omega }_0$. EOS-predicted simple estimates are shown as hatched areas for a 15%-25% laser-to-hot-electron conversion efficiency.

Figure 4

Side-view images at 8 keV of $50\mu \mathrm{m}$ diameter targets irradiated at $1{\omega }_0$ (a) and $2{\omega }_0$ (b) and corresponding horizontal profiles (c). The laser comes from the left-hand side.

Figure 5

Mean energy of bulk ($<10\mathrm{keV}$) electrons at $t=400\mathrm{fs}$ at $1{\omega }_0$ (a) and $2{\omega }_0$ (b). The dashed contour at $1{\omega }_0$ delimits the maximum density region.

Figure 6

Kinetic energy density (in ${log}_{10}$ scale and normalized to $m_e{c}^2{n}_c$) of the fast ($>10\mathrm{keV}$) electrons at $1{\omega }_0$ (a) and $2{\omega }_0$ (b). The dashed contour at $1{\omega }_0$ delimits the maximum density region.

Figure 7

Quasistatic magnetic field $B_Z$ (normalized to $m_e{\omega }_0/e\sim {10}^4\mathrm{T}$) at $t=100\mathrm{fs}$ after the pulse maximum at $1{\omega }_0$ (a) and $2{\omega }_0$ (b).