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Abstract

The motion of an electron in an external field of arbitrary intensity is one
of the most fundamental outstanding problems in electrodynamics. Remarkably,
there is no unanimously accepted theoretical solution for high intensities and
little or no experimental data to date. The basic challenge is the inclusion of
the self-interaction of the electron with the fields emitted by the electron
itself -- the so-called radiation reaction (RR). As is well known, solutions
exist for specific parameter ranges. At low field-strength, where RR is
negligible, the equation of motion is the well-known Lorentz equation. At
higher field-strength in the classical limit the Landau-Lifshitz equation is
the accepted solution. For the strongest fields a full quantum description is
required and this is currently the subject of active research. We report on the
first experimental evidence of strong radiation reaction during the interaction
of a relativistic electron beam with an intense laser field.