| The interaction of an intense laser pulse with a metallic target provides a wealth of phenomena appealing as electromagnetic emissions, which has always been a research area attracting considerable interest. Studies of these emis-sions are of great importance for enhancing our understanding of the underlying physics, as well as leading to a variety of important promising applications. Ex-tensive works have been carried out focusing on generating higher frequency electromagnetic radiation, such as extreme-ultraviolet (XUV), X-rays and γ-rays, and lower frequency radiation mainly at terahertz (THz) region. Study of electromagnetic pulse (EMP) at the low frequency end of the electromagnetic spectrum, i.e., Microwave to radio-frequency (RF), however, has been rare to our knowledge. Some related physics are still not clear, and experimental evidence is sparse. With the increase of laser intensity, EMP generated in the interaction of intense laser pulses with metallic targets has been showing more and more special impacts that can not be ignored. It is necessary to identify the origin and properties of these EMP.In this thesis, EMP from intense pulsed laser irradiated metallic targets has been studied experimentally and theoretically, including the characteristic, strength, radiation mechanism of and target size effect on the EMP, with the hope of enhancing our understanding of the fundamental process and providing reference for research in related areas. The main content and results are as following:1. EMP emission in the microwave range of about0.5-4GHz from a～102ps laser of intensity about1012W/cm2normally incident on a metallic disk has been investigated experimentally. The corresponding laser energy to microwave energy conversion efficiency is found to be10-7～10-8. The radiation process might be reasonably interpreted as being magnetic dipole or electric quadrupole radiation from the laser-induced symmetric poloidal current distribution. As can be predicted, EMP from ultraintense laser-metal interactions could be so intense that shielding should be considered to mitigate the EMP effect.2. Effect of target size on EMP emission in pulsed laser interaction with metallic target has been investigated experimentally. EMP signal was observed to increase with increasing target size. which suggests the benefit oi using smaller electrically isolated target to mitigate EMP effects in ultraintense laser-metal interactions. The temporal behavior of electrostatic electric field on the target surface is believed to be responsible for the effect. In addition, visible light emission was measured simultaneously as a function of target size, which may be helpful in understanding the energy partition in the process.3. Directional radio-frequency radiation by a line focused laser pulse obliquely incident on a copper wire target has been studied experimentally. The angular radiation pattern was measured and found to be directed in the direction cor-responding to the specular reflection of the incident pulse, which demonstrated the radiation mechanism is due to Cherenkov effect by a superluminal electron emission current, source propagating along the target surface. The results also show an easy way to generate EMP with good directional property.4. A simple one-dimensional analytical model for electromagnetic emission from an unmagnetized wakefield excited by an ultrashort ultraintense laser pulse in the nonlinear regime has been developed. The expressions for the radiated fields and for the spectral and angular distribution of the radiated energy have been derived. The model suggest that the origin of the radiation can be at-tributed to the violent sudden acceleration of plasma electrons experiencing the accelerating potential of the laser wakefield. The results could help to qualita-tively interpret some existing experimental results. |
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