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Abstract

Dense cryogenic hydrogen is heated by intense femtosecond infrared laser pulses at intensities of ${10}^{15}-{10}^{16}\;$ W cm$^{−2}$. Three-dimensional particle-in-cell (PIC) simulations predict that this heating is limited to the skin depth, causing an inhomogeneously heated outer shell with a cold core and two prominent temperatures of about $25$ and $40\;\mathrm{eV}$ for simulated delay times up to $+70\;\mathrm{fs}$ after the laser pulse maximum. Experimentally, the time-integrated emitted bremsstrahlung in the spectral range of 8–18 nm was corrected for the wavelength-dependent instrument efficiency. The resulting spectrum cannot be fit with a single temperature bremsstrahlung model, and the best fit is obtained using two temperatures of about 13 and $30\;$eV. The lower temperatures in the experiment can be explained by missing energy-loss channels in the simulations, as well as the inclusion of hot, non-Maxwellian electrons in the temperature calculation. We resolved the time-scale for laser-heating of hydrogen, and PIC results for laser–matter interaction were successfully tested against the experiment data.

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