The creation of well-thermalized, hot and dense plasmas is attractive for warm dense matter studies. We investigate collisionally induced energy absorption of an ultraintense and ultrashort laser pulse in a solid copper target using particle-in-cell simulations. We find that, upon irradiation by a $2\times 10^{20}~\text{W}\,\text{cm}^{-2}$ intensity, 60 fs duration, circularly polarized laser pulse, the electrons in the collisional simulation rapidly reach a well-thermalized distribution with ${\sim}3.5~\text{keV}$ temperature, while in the collisionless simulation the absorption is several orders of magnitude weaker. Circular polarization inhibits the generation of suprathermal electrons, while ensuring efficient bulk heating through inverse bremsstrahlung, a mechanism usually overlooked at relativistic laser intensity. An additional simulation, taking account of both collisional and field ionization, yields similar results: the bulk electrons are heated to ${\sim}2.5~\text{keV}$ , but with a somewhat lower degree of thermalization than in the pre-set, fixed-ionization case. The collisional absorption mechanism is found to be robust against variations in the laser parameters. At fixed laser pulse energy, increasing the pulse duration rather than the intensity leads to a higher electron temperature.

中文翻译：

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圆偏振超强短脉冲激光驱动的薄箔中的快速碰撞电子加热和弛豫

产生良好热化的热致密等离子体对温致密物质研究具有吸引力。我们使用细胞内粒子模拟研究了固体铜靶中超强和超短激光脉冲的碰撞诱导能量吸收。我们发现，在被 $2\times 10^{20}~\text{W}\,\text{cm}^{-2}$ 强度，60 fs 持续时间，圆偏振激光脉冲，碰撞模拟中的电子迅速达到良好的热化分布 ${\sim}3.5~\text{keV}$ 温度，而在无碰撞模拟中，吸收要弱几个数量级。圆极化抑制超热电子的产生，同时通过逆轫致辐射确保有效的体加热，这种机制通常在相对论激光强度中被忽视。考虑到碰撞电离和场电离的附加模拟产生了类似的结果：体电子被加热到 ${\sim}2.5~\text{keV}$ ，但热化程度略低于预设的固定电离情况。发现碰撞吸收机制对激光参数的变化具有鲁棒性。在固定激光脉冲能量下，增加脉冲持续时间而不是强度会导致更高的电子温度。