A coaxial metal rod with partial dielectric coating is considered as a means for efficient suppression of all volume competing modes in cavities for secondharmonic gyrotrons operated in whispering gallery modes. The rod radius is selected small enough to have only a slight effect on operating mode, which therefore remains insensitive to fabrication tolerances and a misalignment of the coaxial insert. By contrast, for the competing modes such a rod is shown to reduce the effective cavity length and thereby greatly increases the starting currents. Such a method of mode selection is demonstrated to be more versatile, as compared to that provided by a tapered coaxial conductor. The advantageous use of a dielectriccoated coaxial insert is illustrated by the example of a cavity for a 100-kW 300-GHz pulsed gyrotron operated in the second-harmonic mode.

Scattering from a dielectric nanowire coated by helical graphene strips (nanoribbons) is investigated for dual-polarized wave at oblique incidence. In the long-wavelength approximation, the helical strips are treated as a homogeneous layer with averaged tensor conductivity. It is shown that performance of well-known surface cloaks in the form of graphene monolayer, axial and azimuthal graphene strips can be deteriorated in a wide range of incidence angles. To overcome this problem, helical graphene strips are proposed as an advanced metasurface for dual-polarized all-angle cloaking of dielectric nanowire in the terahertz band. It is found that such metasurface suppresses scattering from nanowire more effectively as compared to graphene monolayer, regardless of the angle of wave incidence. Moreover, dual-polarized all-angle cloaking of dielectric nanowire can be broadly tuned in frequency with parameters of helical graphene strips. arXiv:1904.08556v1 [physics.optics]

Possible fusion reactivity enhancement due to 3 He minority ICRF heating in D-3 He toroidal plasma is demonstrated in present numerical simulations. On this purpose the particle code based on test-particle approach is developed. This code solves guiding center equations for 3 He ions in toroidal magnetic field including Coulomb collisions of these ions with the background deuterons and electrons. A simple Monte Carlo model for ICRF heating is implemented in this code as well. The transformation of 3 He distribution function from Maxwellian to non-Maxwellian due to heating plays the key role for reactivity enhancement. The formation of significant energetic tail gives rise to the reactivity enhancement. This is an important issue for the performance of fusion reactors with minority heating of ICRF.