| Profited by the improvement of intense laser technology,frontiers of high energy density physics have been developed into two categories: one is the physical researches based on high-energy long-pulse lasers;the other is based on high peak power ultrashort-pulse lasers.Laboratory astrophysics using long pulse laser-plasma interactions to model celestial events has attracted much interest.An important topic therein is the generation of magnetically collimated jets.It is generally believed that magnetic fields play an important role.Employing strong magnetic fields obtained in laboratory may uncover the physics of magnetic fields in astronomical jets.The first work of this dissertation is studying the strong magnetic fields in laboratory and the jets driven by them in laboratory astrophysics,detailed as below.To understand the Omega-shaped coil target experiments performed in Shenguang-II laser facility previously by our group,a rotation slip model is proposed to explain the observed collimated jets driven by poloidal magnetic field.In this model,the rotational slip between plasma disk and magnetic field with Hall effect construct a configuration of θ pinch which would generate bipolar jets.Semiquantitative numerical simulations of hydrodynamics and magnetohydrodynamics verify this physical model.To carry out quantitative simulations with high precision,we developed a compressible magnetohydrodynamic solver to simulate transonic flows,based on an open-source computational fluid dynamics platform Open FOAM.The solver is achieved by modifying the density-based Riemann solver rho Central Foam with central differencing scheme,which is available in Open FOAM.To improve simulation accuracy and avoid non-physical oscillations,a specialized pressureimplicit algorithm with splitting of operators is implemented to guarantee the incompressibility of magnetic fields.The solver has been benchmarked and the convergence rate is between the first and the second order.We apply this solver in magnetohydrodynamic simulations of intense-laser-produced plasmas.The impact of uniform axial magnetic fields and nonuniform coil-current-induced magnetic fields on laser-produced plasma jets are investigated.We obtain the spatial distributions of nozzle and knot structures in jets.This magnetohydrodynamic solver is also suitable for calculations of laser plasma problems with relatively sophisticated configurations.High-power ultrashort-pulse laser can produce secondary high-brightness radiation sources in multiple bands.Therein strong-field terahertz attract great interest due to its features of unique frequency range(0.1-10 THz),high peak field strength(～MV/cm)and etc.To satisfy the application requirements of intense terahertz in condensed matter physics,materials physics and bioscience,the corresponding detection techniques must be advanced.The second work in this dissertation is studying the imaging camera and diagnosis in intense terahertz detection,detailed as below.A strong-pulsed-field terahertz camera based on light-emitting diode(LED)is developed in this dissertation.It is based on the principle that LED can generate nanosecond pulse-duration photovoltaic signals with a reproducible response to picosecond pulse-duration intense terahertz irradiation,due to impact ionization when terahertz electric field strength is larger than 50 k V/cm.By employing this effect,we fabricate scanning and array LED-THz cameras.These devices have successfully captured images of focused terahertz beam profile generated in lithium niobate via the tilted pulse front technique.The proposed camera has characteristics of low cost,strong photovoltaic signal,rapid response and large imaging area.It would be a useful way in developing terahertz imaging technology based on strong-field nonlinear effect.On the terahertz imaging diagnosis,we propose a spatial diffraction diagnostic method via inserting a millimeter-gap double slit into the collimated terahertz beam.It can monitor the minute variation of the terahertz beam in strong-field terahertz sources,which is difficult to be resolved in conventional terahertz imaging systems.To verify the method,we intentionally fabricate tiny variations of the terahertz beam through tuning the iris for the infrared pumping beam before the tilted-pulse-front pumping setups.The phenomena can be well explained by the theories based on tilted-pulse-front sources and terahertz diffraction. |
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