Laboratory Exploration Of Astrophysical Outflow Regulated By Magnetized Ambient Gas

Interaction between plasma and magnetic field are ubiquitous in interstellar envi-ronment,low strength magnetic field can alter the transport coefficient of energy and matter,while high magnetic field strength affect plasma evolution dynamics.Limited by current observation capabilities,quantitative studies on the dynamic behavior of in-terstellar magnetized plasma are proved to be difficult.Progresses made in pulsed power technology enable the production of magnetized high energy density plasma in a con-trolled laboratory environment.Based on the scaling invariance property of magneto-hydrodynamics(MHD)formulae,it is now possible to quantitatively study the evolution dynamics of astrophysical magnetized plasma by laboratory scale experiments.This thesis employ magnetohydrodynamics computer simulation,experiment and theoretical analysis,performing an integrated study on the interaction between laser-produced plasma and its magnetized gas background,aiming at explaining the formation mechanism of a series of collimated astrophysical outflows which resides in magnetized background.In both simulation and experiment setup,the original isotropic outflow are pro-duced by irradiating planar Aluminum or Silicon target using nanosecond laser,while the astrophysical magnetized background is mimicked by Helium gas,in which hosts externally applied magnetic field parallel to the outflow axis.A full 3-dimensional radiative magnetohydrodynamics code is selected for simulation works,physics ingre-dients like laser energy deposition,realistic equation of state,radiation transport and non-ideal diffusion are all included.The long pedestal laser temporal profile ensures a stable outflow,laser intensity in the simulation is I=1012-1013W/cm2,magnetic field strength is B?0-60Tesla,background gas density ranging from vacuum up to?=10-6g/cm3.Actual experiments consisting of a 7ns width,620mJ energy Nd:YAG laser,8Tesla externally applied magnetic field from pulsed generator,and background gas filling the chamber with variable density;Interferometry,plasma self-emission and other optical methods are used for diagnosing plasma evolution in magnetized back-ground.From simulation results we find that:hydrodynamics behavior of the originally high-speed,large angle ablated plasma can be greatly altered by changing external mag-netic field strength or background gas density,though background stay magnetically dominated all the way,meaning field pressure always greater than thermal pressure.In scenario of low background density and fixed magnetic strength,field-traverse mo-tion of ablated plasma is suppressed,plasma forming a prolonged ellipsoid cavity along the field lines,outflow that follows get refracted by the cavity wall and converged,transforming into a well-collimated jet through a conical shock on top of the ellipsoid.Fixing magnetic field strength and slightly elevating the background density results in a blocked jet,plasma gets stagnated on the jet tip and form a knotty jet with density bump inside.If the background density exceeding a certain value,jet can be fully suppressed,conical shock also found to be replaced by a bow shock,outflow transform into a less collimated lobe.Similar changing patterns also emerge when fixing background den-sity and changing magnetic field,that the ellipsoid cavity gradually transform to knotty jet to a highly collimated jet as field strength increases.Detailed analysis on various fluid behaviors show outflow morphology is deter-mined by its external Alfven Mach number and corresponding shock types excited in the ambient.Sub-Alfvenic expansion with M?<1 excites magneto-sound mode in the background,and the anisotropic Lorentz force in the background can effectively guide the large angle outflow into a collimated jet;Low Mach number super-Alfvenic expansion within range 1<Ma<2 excites multiple layer”switch-on" MHD shock,shock front suppress the jet tip,degenerate it into knotty jet;Super-Alfvenic expansion with Mach number M?>2 excites single layer classical bow shock,whose speed and structure are identical to shocks without any magnetic field,Lorentz force in the back-ground becoming isotropic,jets are fully suppressed,only ellipsoid cavity seen in this case.It shows that transition boundaries between above three situations can be exclu-sively categorized by outflow external Alfven-Mach number,making its generalization to astrophysical objects straightforward.We have summarized all the possible outflow morphology into a two-dimensional phase map,its two axes consist of external Alfven Mach number in strong magnetization and acoustic Mach number in weak magnetization,map covered all five kind of outflow morphology in strong,weak magnetic fields or fast,slow outflow driving speed:low field weak driving results in sound mode perturbation and spherical expansion,low field strong driving results in blast wave and spherical outflow lobe,strong field weak driving results in collimated jet,strong field and intermediate driving strength results in switch-on shock and knotty jet,strong field strong driving results in MHD bow-shock and less-collimated lobe.Actual experiments results verify the simulation patterns,transformation from jet to knotty jet to ellipsoid cavity when elevating magnetic field strength are observed both in self-emission and interferometry imaging,field strength and background density transforming thresholds are coincide with that predicted by the phase map.At the end we have established connection between experimental outflow morphology and three series of related astrophysical objects:jets in young stellar objects(YSO),bi-polar lobes of planetary nebula(PN)and the heliosphere(HS)bow shock,all possess central ex-panding wind and high density magnetized background wind.Validity of dimensionless comparison with these objects are guaranteed by making sure parameters representing heat conduction,magnetic diffusion and cooling etc.are all negligible,along we present temporal and spatial scaling coefficients from laboratory to astrophysical systems.As a specially interesting case,the density knot in HH212 jet is reproduced in experiments with a scaled high density background,this can potentially provide new understanding of the generation mechanism of knotty jet in astrophysical outflow systems.Summarizing above simulation and experiment works,we have reach two impor-tant conclusions:one,our research results strongly support the poloidal field jet collima-tion theoretical model in YSO and other objects,showing that few tens of milli-Gauss embedded field is sufficient for collimating the nested central wind;two,we demon-strate a common physics model that can unify morphology including collimated jet,knotty jet and ellipsoid cavity:sub-Alfvenic expansion corresponding to well-collimated jet,intermediate 1<Ma<2 super-Alfvenic expansion corresponding to knotty jet,Ma>2 super-Alfvenic expansion corresponding to ellipsoid cavity.These results can help explaining some of the formation mechanism of YSO and PN,and may also be useful in understanding the bow shock structure of HS boundary that separating inner solar wind and interstellar medium.