Study Of Directed Electromagnetic Pulse Emission From Magnetic Reconnection

Magnetized high-energy-density plasmas can radiate nonthermal electro-magnetic pulse with high energy conversion efficiency, that is confirmed by as-trophysical observations. In this thesis we study directed electromagnetic pulse emission generated by magnetic reconnection and discuss its limitation under magnetized high-energy-density conditions. It is the electric field, as the original form of energy existence in traditional antenna radiation, which causes the peak power and energy conversion efficiency of electromagnetic radiation of traditional antenna mechanism are approaching to its physical limitation. For example, when the strength of electric field approaches to the value of that in atoms, the electric field energy will be dramatically released on the level of energy density1011J/m3, generating plasmas, which is harmful to low frequency radiation. The requirement of neutral condition indicates it is impossible for existence of strong electric fields in plasmas, specially for large scale plasmas. Besides, space charge effect further limit the possible radiation power and energy conversion efficiency of traditional antenna mechanism. On the contrary, existence of strong magnetic fields does not violate neutral condition. It is not sure about whether magnetic field strength has physical limitation or what is its limitation. If the limitation of magnetic field do exist, the limitation of it should be larger than1015G ac-cording to astrophysical observations, and the corresponding energy density is1027J/m3, eleven orders of magnitude higher than that of traditional antenna radiation. The complicated relativistic motions of plasmas due to self-generated magnetic fields reconstruct topology of magnetic field lines, which will generate ultra-strong electromagnetic pulse. Relativistic effects may dramatically enhance efficiency of nonthermal radiation of magnetic reconnection, as well as directed and coherent radiation. Although relativistic and magnetized conditions required by radiation of magnetic reconnection is more common in astrophysical systems, the still increasing capability of creating high-energy-density conditions makes our study meaningful. Based on the above-mentioned facts, in this thesis we present our consid-eration on nonthermal radiation mechanism of magnetized high-energy-density plasmas, and investigate the generation of current filamentation structure and electromagnetic pulse emission process of magnetic reconnection due to current filamentation in order to identify differences relative to traditional antenna radi-ation processes. The main conclusions of our research include:1. Possible mechanism of formation of current filamentation or current loop structure in magnetized high-energy-density plasmas has been discussed. A temperature perturbation parallel to magnetic field lines will grow due to positive feedback arising from the fact that electrical resistivity of high-energy-density plasmas decreases with temperature. Current filamentation structure with wavevector parallel to magnetic fields will formed consequently. Colliding of current filaments or current loops with relativistic velocity accelerated by self-generated magnetic fields will generate ultra-intense electromagnetic pulse. The dispersion relation of current filament array implies the existence of eigenmode of current filament structure with wavelength close to the space period of ar-ray. The deexcitation of eigenmode could radiate electromagnetic pulse with eigen wavelength, which provides a secondary nonthermal radiation mechanism. The excitation of above-mentioned instability with wavevector parallel to mag-netic fields requires plenty of input energy beyond the capability of our device. Based on the pulsed power facilities available to us, experiments of instability with wavevector perpendicular to magnetic field have been carried out. The experimental results imply wavelength of eigenmode increases with time, and electromagnetic waves in plasmas can provide initial temperature perturbation as well as fluctuations. Anyway, perturbation and positive feedback confirm the generation of current filament structure.2. Discussion on electromagnetic pulse produced by moving current loops and current filaments has been presented. Theoretical analysis indicates the mo-tions perpendicular to current would benefit to electromagnetic pulse emission, and the Lorentz force by self-generated or external magnetic field satisfied this requirement. Relativistic effects have dramatical influence on radiation power and angular distribution, including:peak power of radiation of current loop or current filament satisfies scaling law Prad∝β2γ2-3, which will be enhanced by three orders of magnitude when velocity is increased from0.1c to0.9c. Relativis-tic effects also have advantages on directed radiation. The preferred direction of electromagnetic pulse of rotating current loop with non-relativistic speed locates in the plane of the loop. On the contrary, the preferred direction will turn to the direction of magnetic dipole (perpendicular to the plane of the loop) in relativistic case. The electromagnetic pulse maximizes in the direction of parallel to velocity and anti-velocity for current filament with non-relativistic speed, and there is no radiation in direction of perpendicular to motion. In relativistic case, radiation of current filament locates in a narrow cone close to the direction of velocity. Two current filaments with tilt angle or with mass modulation in axial direction can produce superluminal perturbations and radiate electromagnetic pulse with the same angular distribution as that of Cerenkov radiation. Symmetrical motions of current filaments would not mitigate electromagnetic radiation in relativistic case, that is different in non-relativistic case where electromagnetic radiation will be mitigated by symmetrical motions. Peak power and radiation energy of elec-tromagnetic pulse from magnetic reconnection is proportional to β3γ2-3and of no physical limitation in principle, that shows great advantages over traditional antenna radiation.3. Experimental results from magnetic reconnection under weak driven con-ditions have been discussed. Microwave, optical and X-ray radiation from double metallic wires driven by pulsed currents have been measured. Thermal radiation as expected can explain X-ray radiation. Optical emission results from thermal radiation and characteristic line radiation of elements. Microwave radiation under weak driven conditions probably results from wires undergoing phase transitions from solid to liquid, gas and plasma.