| In laser indirect-driven inertial confinement fusion(ICF),high power laser beams are converted to X-rays in a high-Z hohlraum,which ablate and compress a tiny capsule consisting of a plastic(Beryllium,or High Density Carbon etc.)shell and a Deuterium(D)Tritium(T)ice liner.The DT fuel is compressed to a state of high density and high temperature,and a huge number of DT fusion take place releasing an enormous amount of energy.Currently,there are three science and/or technology challenges lying on the road to ignition,?low mode x-ray drive asymmetry,?hydro-instabilities in the imploding shells,and ? laser plasma interaction instabilities.So,it is of great value and importance to study the effects of X-ray drive asymmetries and of hydrodynamics instabilities on the imploding performances of ignition capsules.In this thesis,two dimensional radiation-hydrodynamic codes are used to study the coupling rules of asymmetric X-ray drives with imploding capsules and the effects of drive asymmetry on capsule imploding performances.Moreover,the oscillation of the inner surface perturbations,which is responsible for the occurrence of "bubble against bubble" structure in imploding shells,are studied numerically.The oscillating mechanism is analyzed,which seems to be the result of coupling between gravity wavesand compression waves.First of all,the effects of P2 asymmetry of Au M-band flux in laser-driven gold hohlraums on capsule imploding performances are studied numerically,and the results show:(1)the M-band flux facilitates the ablation and the acceleration of the shell.Positive P2 M-band flux asymmetries(MFAs)can result in negative P2 asymmetries of hot spots and positive P2 asymmetries of shell'spR at the time of peak implosion velocity,both of which increase linearly with the P2 amplitudes of MFAs and are severely amplified by the growth of RT and RM instabilities during the deceleration phase.(2)An oblate or toroidal hot spot,depending on the P2 amplitude of MFA,forms at the end of deceleration,i.e.,stagnation.The surface area of such a hot spot is much or little larger than that of a spherical hot spot of the same volume,which certainly aggravates the energy loss of hot spot via electron thermal conduction across the enlarged hot-spot surface Meanwhile,vortices in hot spots of P2 shapes develop and facilitate the transferring of internal energy from the central to the marginal parts of hot spots,further intensifying the electron thermal conduction near the boundary of hot spots.Both mechanisms can increase the energy losing rate of hot spot,leading to the failure of ignition when the P2 amplitude of MFA is sufficiently large.(3)Mid-/high-Z dopants in ablators can smooth MFAs just before the ablation front,therefore mitigate the effects of MFA.By doping higher-Z materials,more dopants,or placing the doped layer to a larger radius,the smoothing effects of dopants on MFA can be enhanced,even though the implosion velocity may decrease inevitably.Mid-/high-Z dopants might not be the main method to mitigate the low-mode distortions of ICF implosion,but if combined with other approaches,it can be helpful in some situations where the performance is near the cliff and meanwhile the implosion velocity is not so critical.Secondly,we studied the effects on implosion performances of the P4 asymmetries of spectrally integrated flux irradiated onto the capsule,we find that:(4)the P4 asymmetry of spectrally integrated irradiation flux results in both P4 and P2 distortions of the imploding shell,and their phases are of the same sign but different to that of P4 asymmetrical flux;Reversing the phase of P4 asymmetry of irradiation flux reverses the phases both of the P4 and the P2 distortions of the shell.It is confirmed in our study to be of an actual phenomenon.(5)caretul study shows that the P2 distortion of the shell comes from the P2 asymmetric flux onto the ablation front of capsule,which is in turn resulted from the absorption and re-emission of X-rays by the hot plasma between the ablation front and the capsule-concentric spherical surface onto which the P4 asymmetric radiation flux is applied.First of all,the absorption of X-rays by plasma is nonlinear:the plasma locating in the trace of the intenser parts of the asymmetric radiation flux absorb more X-rays,making it get hotter and less opaque than other parts,therefore allows more radiation flux to reach.Therefore,the whole process forms a positive feedback,and is consequently nonlinear.The feature of nonlinear processes or systems is that a small difference of the input parameters makes a huge difference of the outputs,such as the so-called"butterfly effects".So,the non-linearity of the X-ray absorption by plasma amplifies the asymmetry of the radiation flux.Secondly,re-emission by hot plasma is isotropic and can help to smooth the asymmetry of radiation flux.Moreover,the smoothing effect depends on the modulation wavelength due to the finite X-ray mean free path in plasma:it is superior for short than for long modulation wavelength.Therefore,the cos 4? modulation,as one of the components of P4 asymmetric radiation flux,is more effectively smoothed than the cos 28 modulation.The synthetic result is that the amplitudes ratio of cos 20 modulation to cos 4? modulation exceeds the coefficients ratio of corresponding modulations in P4 polynomial,and a P2 modulation in radiation flux with a same phase sign as the P4 modulation occurs.(6)Different from the results of a model called "Thin Shell Model",the P2 distortion of the shell due to P4 asymmetric radiation flux occurs shortly after the application of the asymmetric flux onto the capsule,which means its occurrence begins at a small convergence ratio(CR),and is isochronous with the P4 distortion.Furthermore,it is resulted from the occurrence of a P2 driving flux that is of the same phase sign as the P4 flux,therefore the shell P2 distortion is also of the same phase sign as the P4 distortion.In the Thin Shell Model,the P2 distortion of the shell driven by P4 disturbed pressure occurs during the late stages of the implosion,the essence of which is that the growth of the cos 2? modulation in P4 distortion is slower than the cos 4? component at large CRs.As the result,a P2 distortion of the imploding shell occurs with an opposing phase as the P4 distortion.Finally,we studied the oscillating phenomena of the perturbations on the inner surface of the imploding shell.Our results show that the evolvement of the oscillating phase agrees well with that of a kind of standing wave that is from the coupling between surfacial gravity wave and compression wave;by contraries,a fast imploding shell without acceleration and compression cannot exhibit such oscillations.Both the positive and the negative arguments suggest to us that this kind of oscillation is from the coupling between the shell acceleration resulted gravity wave and compression wave from the ablation front. |
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