Specific features of radiation emitted upon tunnel ionisation of atoms in extremely intense laser fields

Radiation emitted by fast electrons in the process of multiple tunnel ionisation of heavy atoms in the focus of a laser pulse with extreme intensity exceeding 10²² W cm^-2 is studied. It is shown that the spectral-angular distribution of emitted photons in a wide range of angles is qualitatively determined by relations of the synchrotron radiation theory. The dependences of the number and the characteristic frequency of emitted photons on the laser pulse parameters are estimated. The obtained results can be used to determine the maximum laser intensity in the focus.