Positron Bunch Radiation in the System of Tightly Packed Nanotubes
Radiation emitted by a bunch of positrons channeled in nanotubes at zero emission angle is studied taking into account medium polarization. The formation of radiation is characterized by an energy threshold that depends on the oscillation amplitude of each positron. When the bunch energy reaches the maximum value of the threshold energy, radiation is produced by all positrons in the bunch. The nanotube potential barrier is described using a harmonic model. The spectral line shape of the radiation from the positron bunch, the fundamental radiation frequency, and the number of emitted photons are determined. It is shown that a system of tightly packed carbon nanotubes can generate an intense, quasi-monochromatic, and directed beam of circularly polarized soft X-ray photons with an energy of about $3$~\SI{}{\kilo\electronvolt} (wavelength $4.1$~\SI{}{\angstrom}).