<title>Hot electrons and hard x-ray emission from relativistically strong laser pulses in underdense plasma</title>

Abstract

Significant success in the field of creation of ultra high power (I(lambda) <SUP>2</SUP> greater than or equal to 10<SUP>18</SUP> W(DOT)micrometer<SUP>2</SUP>/cm<SUP>2</SUP>) short-pulse (less than or equal to 1 ps) lasers results in strong interest in such a laser radiation interaction with matter. In the case of such intensity dense (n<SUB>e</SUB> approximately equals 10<SUP>23</SUP> cm<SUP>-3</SUP>) and hot (T<SUB>e</SUB> approximately equals 10 keV) plasma is produced practically immediately (just several periods of light oscillation) and further interaction of the pulse occurs with this plasma. One of the results of such an interaction is generation of fast particles and of rather intense hard x-ray emission. X-ray pulse duration is determined by a free path of the produced fast electrons (approximately equals 0.1 ps). This pulse intensity is sufficient for registration by the common methods. Laser pulse repetition rate can be very high -- up to approximately equals 100 MHz. Hence, it is possible to obtain the instantaneous x-ray photographs of the fast mechanical, chemical and biological processes; the stage of these processes is fixed on the temporal level of approximately equals 1 ps. High energy of the x-ray quanta produced (up to 100 keV) makes it possible to study the objects of small size -- even the separate large molecules. Ultrashort x-ray pulse can be also applied for fast decoupling of molecular bonds with monitoring of the processes by the optical laser. Laser spot diameter on the target is small (approximately equals 10 micrometers); depth of thermal wave penetration into the target is also small (approximately equals 10<SUP>-3</SUP> cm). That is why the very object or deposed onto it thin layer of heavy metal can be used as the target. In the latter case the object is irradiated only by x-ray pulse. Noteworthy, that the high power pulse can be produced by the comparatively cheap and small, table-top laser. Numerous papers, both experimental and theoretical were devoted to the study of short laser pulse interaction with plasma. The function of electrons distribution in the laser field was calculated, absorption coefficient was determined as well as structure of electromagnetic field inside the plasma. In this paper we determine the distribution function and parameters of the fast particles flux in plasma. In the nonrelativistic approximation our results are in agreement with those previously published. A novel feature is calculation of the energy and spectrum parameters of the hard x-ray pulse, produced by the intense laser pulse interaction with the solid state target.