Experimental Study Of Thin-layer Plasma And Surface Plasmons

With the study of plasma physics going deeper and deeper,plasma science and technology has been developed and applied more and more widely.Not only in the area of industry,agriculture,biology and space technology,but also in the region of military applications of several foreign countries in recent years.Plasma stealth,the developing trend of the stealth of flight vehicles,has attracted great attention from the world.And the investigation into plasma radome is one of the directions among them. The antenna is a necessarily important component in Radar system,whose stealth is a problem that puzzled the technology researchers all the while.The flight vehicle needs antenna to find its aim when reentering the aerosphere,and it also needs the protection by radome at the same time.The RCS of Radar Antennas is the region of strong reflelection,and how to reduce RCS is a difficult problem of the country's security.Based on this subject,the dissertation carried out the work on plasma layer radome.Besides the plasma stealth,the excitation of surface plasmons(SPs) can also realize the total absorption of the incident electromagnetic waves(EMW) in special frequency band.Surface plasmon is a kind of surface waves that propagate along the interface but decay exponentially from the interface into both mediums.One characteristic of surface plasmons is that it can be excited by the EMW in free space under special coupling conditions,which renders the energy of incident EMW converting into that of SPs.Reversely,SPs could also couple out into free-space EMW under suitable coupling conditions.Therefore,SPs can be applied in a great deal of research on EMW phenomenon,such as via exciting SPs to control the propagation of EMW,via the excitation of SPs to achieve total absorption of incident EMW.Moreover,SPs also has important applications in the area of Plasmonics,SP etching on nanometer scale,SP novel light source and so on.In the microwave region,the dispersion models of EMW in metal aluminium and plasma have the same format,and metal can be regarded as an overdense plasma whose electron density is much greater than the critical density of the incident microwaves.Therefore,SPs at A1 surface and the boundary of overdense plasma have properties in common.So in this dissertation,besides the research on radome plasma, we also studied the excitation and applied technology of SPs.In the broad sense of plasma definition,the work on radome plasma and the coupling between EMW and SPs are both the interaction between plasma and EMW.And the surface-plasmon-induced total absorption of EMW energy provides a new physical mechanism for the stealth of Radar antennas.The main contents and highlights in this dissertation are as following.1.Based on the principle of low pressure capacitively coupled discharge,we designed a plasma layer radome,built a radome plasma generation system,and carried out a series of experiments to highten the electron density of the radome plasma layer.We studied the discharge of Argon,Xenon,and the discharge of Ar+Hg at different mixed ratio in detail.2.By controlling the density of the radome plasma,we studied the influence of the plasma radome on the microwave reflection in the frequency region 5.09-6.8GHz. The experiment results show that the radome plasma can greatly attenuate energy of the incident EMW in special frequencie band when the plasma density is proper. These results will be of use to the plasma stealth of Radar antennas.3.By employing the diffraction grating coupling technology and the basic theory of surface plasmons,we successfully excited the SPs along Al/air interface,and first observed the extraordinary enhancement and beaming effect of EMW transmission through a single subwavelength metallic slit in X-band microwaves. Our experiments demonstrated that the extraordinary enhancement and beaming effect results from the excitation of surface plasmons,and the period corrugations in the output surface results in the beaming effect.The SPs we excited in the X-band microwaves have the longest wavelength that has been experimentally achieved so far.It is of fundamental importance to the applications of SPs in the X-band microwaves.4.In studying the extraordinary enhancement of microwave transmission through single subwavelength slit,we also examined the influences of the groove number and the periodical parameters on the enhancement factor in detail.These works provide an experimental premise to the design of metallic lens and SP coupler in the microwave region.At the same time,our results are also of reference value to the SP study in the optical regime and other EMW frequency regions.5.We experimentally investigated the influences of SPs at boundary of overdense plasma and that at Al surface on microwave reflection.By coupling the incident microwaves into SPs at metal surface or the boundary of overdense plasma,we can convert the incident EMW energy into that of SPs.SPs at Al surface can attenuate the microwave of 10.6GHz to about 30dB(0.1%) at the most,and the band-width at the attenuated value of 15dB is about 0.4GHz,with that of 10dB being about 0.8GHz.At boundary of overdense plasma,SPs can attenuate the microwave of 5GHz to about 23dB(0.5%) at the most,and the band-width at R=0.5 is about 0.1GHz(R:reflection coefficient).The mechanism of surface-plasmon-assisted total absorption of incident EMW energy provides a new effective mean to the absorption of EMW energy.