  In many high-profile applications, such as nuclear fusion and pumping of
active media of short-wavelength lasers, it is necessary to achieve high
specific input of power of an electromagnetic beam in a target. Diffraction
sets the lower limit to the transverse dimensions of electromagnetic beams and
represents a fundamental obstacle for electromagnetic heating of small or
inaccessible regions. It was found, however, that it is possible to achieve
efficient heating of cylindrical targets by electromagnetic beams with
transverse dimensions that are several orders of magnitude greater than those
of the cylinder. These counter-intuitive results have the following physical
mechanism: the absorption in the cylinder causes a deep fall in the field
distribution, and this fall causes diffractive diffusion of the field towards
the axis from a large volume of the beam. The heating efficiency was rigorously
calculated using the exact solution of the problem of diffraction on an
infinite homogeneous cylinder (J.R. Wait, 1955). Several non-resonant domains
of parameters were found that provide efficient absorption of the energy of a
broad beam in a thin conducting cylinder. The typical asymptotic efficiency for
very thin cylinders is (2-5)/L, where L=ln(w/d), where d is the diameter of the
cylinder and w is the width of the beam waist in the longitudinal geometry or
the wavelength in the transverse geometry. This efficiency is high even if the
beam is several orders of magnitude broader than the cylinder. The relevant
conditions are not too rigid and may be used to heat cylindrical targets to
high temperatures in a number of important applications. External magnetic
field may further relax the conditions of efficient heating.
