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中国物理学会期刊

激光聚变黑腔中等离子体的热流研究

CSTR: 32037.14.aps.69.20191423

Heat flow of laser-ablated gold plasma in inertial confinement fusion hohlraum

CSTR: 32037.14.aps.69.20191423
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  • 辐射流体采用限流的局域Spitzer-Härm (S-H)电子热流近似, 在预估等离子体状态时可能与实验观察存在偏差. 利用一维(1D3V)含碰撞的粒子模拟程序, 研究了激光聚变黑腔中金等离子体的电子分布函数和电子热流. 分析表明, 在等离子体的冕区, \alpha=Z(v_\rmos/v_\rmte)^2 > 1, 电子分布函数表现为超高斯分布(m = 3.34), 克努森数\lambda_\rme/L_\rme=0.011大于局域S-H理论的临界值2\times10^-3. 这导致了局域S-H电子热流远大于实际热流. 这种实际热流受限现象将导致辐射流体模拟给出的冕区电子温度高于神光实验测量值. 而在等离子体的高密度区域, 电子分布函数仍表现为超高斯分布(m = 2.93), 克努森数\lambda_\rme/L_\rme=7.58\times10^-4小于局域S-H理论的临界值, 限流的局域S-H电子热流具有一定的适用性. 但电子热流严重依赖于限流因子f_\rme, 辐射流体模拟需要根据不同位置的光强和电子温度调整f_\rm e的大小.

     

    When evaluating the plasma parameters in inertial confinement fusion, the flux-limited local Spitzer-Härm (S-H) model in radiation hydrodynamics simulations may be invalid when electron temperature gradient is too large. In other publications, the electron distribution function (EDF) could be explained by comparing the energy equipartition rate R_\rm eq=\dfrac12m_\rm ev_\rm te ^2\nu_\rm ee with the heating rate R_\rm heat=\dfrac12m_\rm ev_\rm os ^2\nu_\rm ei. When the condition R_\rm heat\sim R_\rm eq is satisfied, the EDF deviates from Maxwell equilibrium distribution, and is well fitted to the super-Gaussian distribution f( v)=C_m\rm e^-(v/v_m)^m with the index m (2<m<5). The number of energetic electrons of the super-Gaussian distribution is less than that of the Maxwell distribution, which plays an important role in electron heat flux, especially for electrons of 3.7v_\rm te. So electron heat flux of the super-Gaussian distribution is smaller than that of the Maxwell distribution. In this paper, EDF and electron heat flux in laser-produced Au plasma are simulated by using 1D3V PIC code (Ascent). It is found that in the coronal region, the laser intensity is larger, and the electron temperature is lower than the high-density region. So \alpha=Z(v_\rm os/v_\rm te)^2>1, R_\rm heat>R_\rm eq, the EDF is well fitted to super-Gaussian distribution, where the index m is evaluated to be 3.34. In this region, the large electron temperature gradient leads to a small temperature scale length (L_\rm e=T_\rm e/(\partial T_\rm e/\partial x)), but the low e-e and e-i collision frequencies lead to a large electron mean-free-path (\lambda_\rm e). So the Knudsen number \lambda_\rm e/L_\rm e is evaluated to be 0.011, which is much larger than the critical value 2\times10^-3 of the S-H model, flux-limited local S-H electron heat flux is invalid. As a result, the limited-flux S-H predicts too large an electron heat flux, which results in much higher electron temperature of radiation hydrodynamics simulation than that of SG experiments. This heat flux inhibition phenomenon in coronal region cannot be explained by the flux-limited local S-H model, and non-local electron heat flux should be considered. In the high density region, the laser intensity is weaker, and the electron temperature is higher, so \alpha=Z(v_\rm os/v_\rm te)^2<1, R_\rm heat<R_\rm eq, but EDF is still well fitted to super-Gaussian distribution, where the index m is evaluated to be 2.93. In this region, L_\rm e is larger, \lambda_\rm e is smaller, so the Knudsen number is smaller, which is evaluated to be 7.58\times10^-4<2\times10^-3. As a result, The flux-limited local S-H electron heat flux is valid. However, the electron heat flux depends on the flux limiting factor (f_\rm e) that varies with laser intensity and electron temperature.

     

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