Diagnostics Of Laser Ablation Plasma Based On Electromotive Force

Laser ablation plasma presents very different nature in its nascent phase and mature phase.The nascent phase refers to the duty period of laser pulse and the successive short duration for the plasma to grow during and after the laser pulse.In the nascent phase,plasma is much dense and has not made diffusion,therefore should be a hot plasma with very high pressure,density and temperature but very small volume,forming steep spatiotemporal distribution.The mature phase plasma is generated after the laser pulse by diffusion from the nascent plasma.Especially at the later stage of diffusion,it comes a low-pressure,non-equilibrium low-temperature plasma.This dissertation focuses on the nascent plasma phase in which electron emission,plasma formation occurs successively in the ablation zone during laser on period.A diagnostic principle targeting at the plasma phase based on electromotive force effect is proposed and two different realization circuits are designed based on the principle.For electromotive force characterization,shorted current and opened voltage measurements are basically used according to Ohm's Law.The measured current and voltage can be used to evaluate the electron yield during the nascent phase.In order to verify the method,the relationship between electron yield and ablation parameters was investigated and proved a new possibility for diagnosing laser ablation.On the onset of laser ablation,the surface is heated quickly and temperature of the ablation zone rise firstly to high enough for electron emission before melting and ionization.An instantaneous electromotive force can form between the ablated surface and the other surrounding conductors by the emitted electrons.After that,with the occurrence of melting and ionization,hot and dense nascent plasmas are formed and inertially confined in the ablation zone.Therefore the nascent phase generates extremely large gradient and drives originally the so-called non-bipolar diffusion.Principally,non-bipolar diffusion means electrons separation from ions by diffusion which forms another electromotive force between the ablated surface and the surrounding conductors.In order to measure the above-mentioned electromotive forces,an electrode structure is designed based on the ablation position to facilitate the collection of electrons emitted from the ablation zone and forms a battery configuration.Short-circuit measurement and open-circuit measurement were carried out for the battery electromotive forces.Based on the shorted current of the battery,an analysis scheme for estimating the amount of electron production by one laser pulse.Using the scheme,the tendencies of the electron generation on ablation parameters such as material type,laser energy and gas pressure is studied to verify the applicability of the battery configuration.For comparison,the opened voltage of the battery configuration was further measured and the relationship between the voltage waveform and the electron generation mechanism is clarified to estimate the electron yield again.By investigating the laser energy,pressure and battery electrode gap,the calculation results from the opened voltage are consistent with those from shorted current.In order to further illustrate the dynamic characteristics of the laser ablation process,the luminescence morphology of ablation plasmas was recorded by color video recorder.The diffusion behavior of laser ablation plasmas under different pressures were analyzed and correlated with the waveform of the opened voltage,it is preliminarily shown that the electromotive force is induced in the nascent phase of laser ablation.