Experimental Study On The Effect Of Plasma Filling On Radiation Field In Hohlraum

In laser indirect-drive inertial confinement fusion(ICF),laser beams are injected into a high-Z hohlraum and the laser energy is converted into intense X-ray radiation,which ablates a capsule located at the center of the hohlraum and makes it implode.The convergence movement of the high-Z plasma from hohlraum inner wall is referred to as plasma filling,which will affect laser injection,hohlraum radiation uniformity and diagnosis of hohlraum radiation field.While complex hydrodynamic movements cannot be exactly calculated only by available simulation codes.Hence it is of great importance to study plasma filling in ICF experiments.In this thesis,the hydrodynamic movements of various plasmas are studied by laser entrance hole(LEIH)and diagnostic holes(DH),which are used to assess the effect of plasma filling on radiation field uniformity and radiation flux in hohlraum.Various suppression methods for plasma filling are studied and compared,such as lining low Z material on the inner wall of the hohlraum,prefilling low Z gas inside the hohlraum,foaming the high Z material of inner wall,imposing the strong magnetic field.In order to achieve the high density compression in laser indirect-drive inertial confinement fusion,the implosion symmetry and hohlraum radiation uniformity are strictly required.To study the variations of implosion asymmetry with hohlraum length and time,different lengths hohlraums are adopted in experiment.X-ray emission from capsule fuel is measured by an X-ray Framing Camera(XFC).Based on measured capsule compression process and ellipticity variation on SGIIIP laser facility,it is preliminarily judged that the hohlraum of ?1.0 mm × 1.7 mm is suitable for implosion symmetry demand on SGIIIP without beam smoothing,and the hohlraum of ?1.1 mm ×(1.8?1.9)mm is suitable with beam smoothing.Time-resolved implosion asymmetry is derived from an original simplified analytic model,in which used is the time-resolved hohlraum radiation nonuniformity derived from a view-factor code.The derived results of the time-resolved implosion asymmetry basically agree with experimental results.The physical mechanism for how hohlraum radiation nonuniformity evolution induces the variations of implosion asymmetry with hohlraum length and time is analyzed.Plasma filling is dominant for hohlraum radiation nonuniformity in the late stage.Therefore the study of suppression methods for plasma filling is important.Lining low Z material on the inner wall of the hohlraum,prefilling low Z gas inside the hohlraum,foaming the high Z material of inner wall,imposing the strong magnetic field and some other methods are used to suppress the plasma filling.Lining CH and other low Z materials on the inner wall of the hohlraum is easy to make,which can eliminate the high Z plasma jet and effectively suppress the high Z plasma filling.However,some low Z materials are still at a high density after ablation,and still probably form jets which are difficult to be diagnosed by X-ray and even hit the capsule.Moreover,there may be hydrodynamic instability at the materials interface.Therefore,lining CH and other low Z materials on the inner wall of the hohlraum is not selected as the main suppression method for the ignition hohlraum.Prefilling low Z gas,such as neopentane and helium gas,inside the hohlraum can also eliminate high Z plasma jet and effectively suppress high Z plasma filling.Moreover,density distribution of low Z gas is quite uniform,and it will not hit the capsule with high density.Therefore,it is selected as the main suppression method for the ignition hohlraum by National Ignition Campaign(NIC).However,hydrodynamic instability at the materials interface probably occurs.Moreover,the compressed high density gas will stimulate the laser plasma instability(LPI)and cause strong laser scattering and reduce the energy coupling efficiency,which is proved a main cause for the failure of NIC.One main direction of the current experimental study on National Ignition Facility(NIF)is to appropriately reduce the gas density and LPI.Foaming high Z material of the inner wall of hohlraum is a new important method after NIC,which mainly adopts tens to hundreds mg/cc Au,Ta2O5 or other foam materials.The foam materials can reduce the plasma velocity,and improve the X-ray conversion efficiency and the hohlraum radiation temperature.0.4 mg/cc Au foam plates were fabricated and verification experiment was carried out on SGIIIP.For emission region of Au foam,velocity and density are lower,which roughly agree with the simulation result.Reemission flux of Au foam increases 10-20%,which is diagnosed by Transmission Grating Spectrometer(TGS)and XFC.It is necessary to fabricate the whole hohlraum with lower density foam and carry out further experimental study.Using strong magnetic field to suppress plasma filling is a brand new method that we put forward in recent years.Megagauss magnetic field can restrain the radial motion of the high Z plasma in hohlraum,suppress the electron heat conduction,improve the electron temperature,reduce the backscatter and guild the superthermal electrons.These effects have been preliminarily confirmed by the simulation and experiment,and in-depth study and overall consideration are needed further.Generating methods of megagauss magnetic field in high power laser laboratory such as capacitor-coil target and pulsed power device are studied.Diagnostic hole used in indirect-drive inertial confinement fusion cannot be too large to cause severe radiation loss and affect the radiation uniformity in the hohlraum,or too small in case the plasma filling would block DHs and affect the diagnosis.An elongated small hole is chosen as an extreme case to study the plasma movement in DH in order to provide reference for the DH design.The elongated small DH on the Au hohlraum wall is 150 ?m in diameter and 100 ?m deep.The hydrodynamic movement in the elongated small hole is observed by XFC with a 2-5 keV backlighter on SGII laser facility.The plasma areal density distribution and evolution in the elongated small hole is quantitatively measured and can be used to assess the effect of hydrodynamic processes on the diagnosis from the DH.The result shows the DH of 300 ?m in diameter and 25 ?m deep is suitable for the hohlraum using 1 ns laser pulse on SGII.