ИСТИНА

Bright coherent attosecond x-ray pulses have a large number of applications both in science and technology. One of the ways to generate such x-ray pulses is to use a reflection of a probe laser pulse off a relativistic electron mirror [1]. In this case, a reflected pulse can belong to the x-ray band and its parameters, such as amplitude, frequency, envelope, phase, etc., can be controlled easily. An idea for synchronous acceleration of electrons from a nanofilm with a superintense nonadiabatic laser pulse was considered in [2] and characteristics of generated relativistic electron bunches were studied in [3]. For a nonadiabatic laser pulse of relativistic amplitude incident normally at the nanofilm, all electrons can be expelled simultaneously out of the nanofilm in the longitudinal direction (along the laser beam axis) due to the action of the longitudinal component of the Lorentz force. This force accelerates electrons to relativistic velocities. As a result, a relativistic electron mirror can be formed - an electron bunch with diameter of several micrometers and thickness of several nanometers or less. For relativistic electron mirror formation, the laser pulse amplitude should be relativistic and exceed some threshold, which is determined by material and thickness of the nanofilm, besides the front of the laser pulse should be sharp enough. In this presentation, we first consider laser pulse front shaping using plasma layers with thickness of about laser wavelength or more. Their electron density should be considerably smaller than the solid-state density (about 2-10 times larger than the critical density). Such layers allow fully suppressing the part of the laser pulse with small amplitude, and the transmitted pulse maximal amplitude can be about that of the incoming laser pulse. As a result, petawatt class nonadiabatic electromagnetic pulses required for effective generation of relativistic electron mirrors can be produced. Next, we studied numerically characteristics for relativistic electron mirrors generated with shaped pulses. It was shown that, for a nanofilm solid-state target with a thickness of several nanometers, a lifetime of relativistic electron mirror can be tens of femtoseconds with good homogeneity and with momenta spread of no more than 1-2 %. The parameters of such a mirror are well suited for reflection of the counter-streaming probe pulse to generate coherent X-rays. At last, the reflections of the counter-propagating probe laser pulses off generated relativistic electron mirrors were investigated also. It was shown that, as a result of reflection, the single coherent X-ray pulse with duration of less than 100 as can be generated having the wavelength of ten nanometers or less and power of several hundreds of gigawatts. Possible characteristics for the generated bright coherent attosecond x-ray pulses were investigated such as dependence of the reflected pulse parameters on amplitude and duration of the probe pulse, nanofilm thickness and electron density, etc. It was shown that if the relativistic electron mirror stays in the field of accelerating pulse during interaction with the probe pulse, a reflection coefficient of the mirror and a frequency up-shift coefficient for the probe pulse do not depend on the amplitude of the probe pulse up to that of the accelerating pulse. Optimization of the system will allow increasing the frequency and power of the coherent reflected pulse by several times.