Experimental Studies On Hot Electron Generation And Transition Radiation In The Physics Of Intense Field

Hot electron generation and transportation and optical transition radiation in the Interaction of ultrashort ultra-intence(US-UI) laser pulse with plasma were studied in this paper. As the relativistic laser intensity reached 100TW, we obtained some new phenomena that have been reported for the first time. These results are greatly helpful for us to understand the physical mechanisms of the laser-plasma interaction, and very important for the research of Fast ignition(FI) for inertial confinement fusion(ICF).Since the fast ignition scheme was first suggested by M.Tabak in 1994,it now becomes an increasing research field for its merits that traditional inertial confine fusion (ICF) did not possess. But for its short time research, there are lots of physical mechanisms need to be studied carefully, in which the transportation of relativistic electrons in dense plasmas is an interesting and important subject.Firstly, some aspects were introduced :the development of the laser technology, the process of the interaction of the US-UI laser pulse with plasma(Physics of Intense Field),the research of the potential applications of physics of intense field, and hot electron generation and transportation, transition radiation(TR);Secondly, some basic concepts and theories for the laser-plasma interaction, the research of the physics of intense field ,fast ignition and its correlative phenomenon were introduced;The basic theories of hot electronic TR in the interaction of the US-UI laser pulse with plasma were introduced;The acceleration mechanism of hot electrons were studied. The experimental setup at the Laser Fusion Research Center in China Academy of Engineering Physics(CAEP) were also introduced. Finally, the emphases were focused on the analyses of the experimental results of the hot electron generation ,transportation and optical radiation of the hot electrons during the interaction of ultrashort ultra-intence(US-UI) laser pulse with plasma.In this paper, such four points were studied as following: a) experimental studies on the characteristic of hot electron bunches by transition radiation, the experimental results of the transition radiation compared with the results of theoretical simulations of the transition radiation;the compared to the experimental studies of the transition radiation(TR) from the rear side of different kind and different target thickness. b) the generation and transportation of the hotelectrons emission in the solid targets, the energy spectrum of hot electron in the different direction, the relation of injection influence of the hot electrons to the target thickness, the relevancy of the total energy of the hot electrons with the target thickness, the relevancy of the numbers of the hot electrons with the laser intensity, the laser pulse width influence on the range of energy deposition of the hot electrons, c) the study on the instabilities of the hot electrons transport, The special distributions of the time-integrated image pattern of transition radiation(TR) from the rear side of different target thickness, the relevancy of the intensity of optical radiation with the target thickness;and d) the spectrum of transition radiation and the time-differentiate image resulting from the interaction of ultrashort ultra-intence(US-UI) laser pulse with solid targets.In the experiment of on the characteristic of hot electron bunches by transition radiation, the transition radiation from the rear side of the target thickness 20 u m Ta was presented smoothness round, center brightness higher that around, contained incoherent transition radiation(ITR) and coherent transition radiation(CTR), was close to the results of theoretical simulations;The transition radiation from the rear side of the target thickness 100 u m Ta was presented a star-structure, a facula of the transition radiation become smaller, it resembles a star-structure of the energetic proton emission;A facula of the transition radiation from the rear side of the target thickness 10 u m Ta + 10 u mCu+5 u mTa was presented a ring structure, and the facula become biggest, was presented about two apart facula. These facula of the transition radiation can reflected some trait of the hot electron bunches.In the experiment of hot electrons generation and transport, an anisotropy energy distribution of hot electrons emission has been observed. The energy spectrum of hot electrons in the direction of target normal and in the laser reverberation direction of the target surface is a Maxwellian-like distribution;The energy spectrum of hot electrons along laser propagation direction at target rear surface was presented energetic tail;The yield of hot electrons in the direction of target normal was higher than that at the laser reverberation direction of the target surface and along laser propagation direction at target rear surface. In the study of the relation of the injection fluence and the total energy of hot electrons with target thickness, the injection fluence of the hot electrons rapid decreases with the increase of the target thickness, when the target thickness increased to a certain value, the injection fluence and the total energy of the hot electrons decreases slowly. This Indicates that the deposition of total energy of hot electrons mainly occured in the front region.and the region was influenced by the laser pulse width, and the thickness increased with the increase of laser pulse width.In the study of the instability of hot electrons transportation, the integrated image of transition radiation(TR) of hot electrons passing through the solid targets was observed from the rear surface. The integrated image taken by CCD camera shows that the optical emission forms a ring, similar to the ring-structure of the energetic proton emission, on the edge of the ring there was a bright localized signal, which was the optical transition radiation(OTR). It was filament effect during Weibel instability of the hot electron transport . The OTR intensity decreases withthe increase of the target thickness.The time resolution pattern and the spectrum of optical transition radiation (OTR) were measured in the interaction of ultrashort ultra-intence(US-UI) laser pulse with plasma employing optical streak camera and OMA optical multi-channel spectrometer. The time resolution pattern and spectrum of transition radiation (TR) were measured in the normal direction from the rear side of targets that have not been reported before in China. The spectrum of transition radiation (TR) presented sharp peaks, which were attributed to the coherent transition radiation (CTR) of hot electrons, which passing through solid targets produced from the rear surface due to peculiar microbunches of hot electron beams generated in different acceleration mechanism.These peculiar microbunches passing through solid targets were the signals of coherent transition radiation (CTR) of different spectrum produced from the rear surface.. The time resolution pattern of the optical radiation more proves that the transition radiation was intense, which had short duration (ps);and other optical radiation was weak, which had very long duration (ns).