The radiation from a charge moving along a helical trajectory inside a cylindrical hole in homogeneous dielectric medium is investigated. Prompted by availability of materials with large dielectric permittivity F and small absorption, we discuss the features of this type of radiation for media with epsilon >> 1. It is shown that there are high peaks in the angular distribution of radiation intensity at well-defined harmonics. The conditions are specified for the cavity-to-helix radii ratio, rho(1)/rho(0), under which the angle-integrated radiation intensity on some harmonics exceeds that in the empty space. Though the amplification of radiation intensity increases with increasing E, the corresponding "resonant" values of rho(1)/rho(0), ratio are practically independent of the dielectric permittivity of surrounding medium. It is shown that an analogous amplification of radiation takes place essentially for the same values of rho(1)/rho(0) also for the radiation in a cylindrical waveguide with conducting walls. An explanation of this phenomenon is given. (C) 2008 Elsevier B.V. All rights reserved.
