Powerful Ultrashort Pulse Laser And Cluster Interaction Experimental Research

As a main branch of the high field physics,Ultra-short ultra-intense laser(UUL) interaction with atom cluster has become very active in recent years.Fementosecond laser with near PW power has been built.And the laser intenstiy 10²⁰W/cm² was achieved,which makes it possible to generate extreme regime such as intense electromagnetic field,high temperature medium,ultra-high pressure.The UUL offers well conditions for high field physics and high energy density physics.Atom clusters are aggregates of atoms(molecules)containing between two and a few thousand atoms that have properties intermedate between those of the isolated monomer and solid material.The properties of the cluster vary with its size from several angstrom to a few hundred of angstrom.Clusters can be placed in the following categories depending on the bonding between the mononers in the clusters such as ionic cluster,metallic cluster,covalent cluster,hydrogen-bonded cluster and Van der Waals cluster.The simplist way to get Van der Waals cluste is based on the expansion of a high pressure gas in a vcuum through a nozzle.As a result,the temperature of the expanding gas drops dramatically,and the gas pressure becomes higher than the saturated vapor pressure at this low temperature,which leads to the formation of the clusters.Forming clusters grow in time by three-body collisions and suitable conditions.Cluster size and density profile are the two main parameters,which are vital to the UUL interaction with clusters.We determined the average size of the clusters by Rayleigh scattering experiment.The relative factors such as backing pressure,delay time,and nozzle geometry were discussedThe laser interferometer is a kind of precision technology to diagnose density field profile.A Mach-Zehnder interferometer was built to measure the phase shift after the probe beam propagating the gas target.This phase is always proportional to the product of the index of refraction with the optical path length.It is necessary to know the index of refraction everywhere in the medium.Since the gas medium is cylindrically symmetrical,we can deduce the radial distribution of the index of refraction by a transformation Abel inversion.The relations between the gas density and backing pressure and delay time were obtained.These results are important to the ultra-shor ultra-intense laser interaction with atom clusters and helpful to understand the process of cluster generation.Unlike solid target and gas target,clusters interaction with ultra-shor ultra-intense laser is distinctive.Firstly,a part of electrons was stripped by above threshold ionization once the laser intensty is high enough.More and more free electrons generate in the clusters.Then the nanometer dense plasma forms,which shields the laser field and reduces the ionization rate.The cluster heating and expansion become slowly.However,another ionization mechanism dominates after the plasma shielding.With the laser radiation,cluster plasma continue getting energy through inverse bremsstrahlung and collisional absorption.With the ionization process,two forces,hot electron pressure and Coulomb repulsion act on the cluster,causing it to expand during and after the laser pulse.The expanding behave varies with different kind of clusters or clusters with different size. Experiments were performed on the 20 TW laser facility to explore the evolution of the cluster in the intense laser field.Deuterium ions with high energy were observed. Using the Coulomb explosion model,we calculate the ion energy spectra considering the different cluster size profile.The calculation indicate that the Coulomb explosion model could describe the evolution of deuterium cluster very well.For deuterium clusters,the laser field is usually strong enough to remove ionized electrons directly from clusters.In this case,the clusters explode by Coulomb repulsion and high energy ions eject in all directions.These ions with substantial kinetic energy can be harnessed to drive nuclear fusion if the ions have a sufficient average density to permit collision between ions from different clusters.A model was developed to calculate the UUL interaction with large size deuterium clusters.We found the fusion neutrons main come from the hot plasma region radiated by the intense laser when the cluster size is less than 5 nm.The neutrons from the close vicinity are more and more as the cluster size becomes larger.Interestingly,it is found that the clusters with a narrow size profile are benefit for the fusion neutron production.Deuterium cluster fusion was achieved using the Ti;sapphire laser system based on chirped-pulse amplification at Korea Atomic Energy Research Institute(KAERI), which deliverd pulses of 300 mJ in a duration of 30 fs.The average neutron yield of 10³ per shot was observed at backing pressure 50 atm.Since the cluster size is less than 5 nm,DD fusion events mainly take palce at hot plasma filament.To gain a greater insight into the mechanism we also studied the yield as a function of the laser spot.Surprisingly,the neutron yields enhanced with the increasing of the laser spot size.No neutron was observed when the focal spot was larger than 550 Mm.So the laser intensity of 4.3×10¹⁵W/cm²(laser spot size 550μm)could be the threshold for neutron generation,which is important to future application research.Accelerator is an efficient way to get high energy particles.Laser wake field accekeration is a scientific breakthrough.The acceleration of electrons using laser-plasma interaction has attracted more attention due to the greater acceleration gradient than radio frequency accelerators.The photon pressure or ponderomotive force pushes plasma electrons out of the pulse while ions stay the original position due to their large mass.Thus a plasma wave forms and propagates behind the laser pulse.The plasma wave can trap the background electrons and grow until nonlinear process wavebreaking takes place in the laser wake field.Energy is transferred from plasma wave to electrons by wavebreaking.High energy electron acceleration in a wake field generated in the intense fs laser pulse cluster-gas mixture interaction is experimentally demonstrated.Relativistic electrons with 60 MeV energy were obtained and these high energy electrons split into two beams.Two monoenergetic electron peaks at 36.2MeV and 16.7MeV were observed.It is noted that the electrons quiver with velocity closed to c for the plasma generated above the laser intensity 10¹⁸W/cm².The electron mass exceeds its rest mass greatly,which cause the plasma frequency decrease.Accordingly,refractive index of the plasma will increase,which is like a lens for the propagating laser pulse. This is a positive feedback mechanism resulted in ununiformity profile of the laser intensity.Two main self-channels form and the electrons split into two groups when they escape the acceleration region.That is why two electron beams were observed after transmitting a dipole magnet.Except for the laser wake field,many other mechanisms can generate and accelerate hot electrons in the laser cluster plasma interaction.Vacuum heating and resonance absorption can also generate hot electrons depending on the plasma density scale length.We experimentally investigated the electron emission along the side and backward directions.A 50 micron Aluminium foil was used to block the low energy electrons.The electrons jet along the side direction are due to the resonance absorption.And the those jet along the backward are due to the reflecting laser acceleration.