Electromagnetic waves with helical phase surfaces arise in different fields of physics such as space plasmas, laboratory plasmas, solid-state physics, atomic, molecular and optical sciences. Their common features are the wave orbital angular momentum associated with the circular wave propagation around the axis of wave propagation. In plasmas these waves are called helicons. When particles or waves change the field momentum they experience a pressure and a torque which can lead to useful applications. In plasmas electrons can damp or excite rotating whistlers, depending on the electron distribution function in velocity space. A magnetized plasma is an anisotropic medium in which electromagnetic waves propagate differently than in space. Phase and group velocities are different such that wave focusing and wave reflections are different from those in free space. Electrons experience Doppler shifts and cyclotron resonance which creates wave damping and growth. All media exhibit nonlinear effects whic...

https://www.tandfonline.com/doi/pdf/10.1080/23746149.2016.1240017

Whistler waves with angular momentum in space and laboratory plasmas and their counterparts in free space

Measurements have been performed in a laboratory experiment modeling the interaction of a modulated electron beam with a magnetized plasma under conditions relevant to space experiments involving beam injection. Both whistler emission through Cherenkov resonance and a nonresonant mechanism of transition radiation from the point of beam injection into the plasma have been observed. Electrons injected from the gun into the plasma pass from one medium (gun chamber) into another (plasma volume) and electromagnetic fields change as charges cross the metallic interface between both media, giving rise to transition radiation. This type of beam radiation, observed separately from the resonant Cherenkov emission owing to adequate choices of the physical conditions, has been characterized as a function of the beam and plasma parameters. Moreover, in the case of beams injected from satellites in the ionospheric and magnetospheric plasmas, this nonresonant emission, mainly located in the near gun region, can be governed by an adequate control of the radiator parameters and separated from resonant emissions.

Whistler wave emission by a modulated electron beam through transition radiation

Helicons are whistler modes with azimuthal wave numbers. They arise in bounded gaseous and solid state plasmas, but the present work shows that very similar modes also exist in unbounded uniform plasmas. The antenna properties determine the mode structure. A simple antenna is a magnetic loop with dipole moment aligned either along or across the ambient background magnetic field B0. For such configurations, the wave magnetic field has been measured in space and time in a large and uniform laboratory plasma. The observed wave topology for a dipole along B0 is similar to that of an m = 0 helicon mode. It consists of a sequence of alternating whistler vortices. For a dipole across B0, an m = 1 mode is excited which can be considered as a transverse vortex which rotates around B0. In m = 0 modes, the field lines are confined to each half-wavelength vortex while for m = 1 modes they pass through the entire wave train. A subset of m = 1 field lines forms two nested helices which rotate in space and time like cor...

Helicon modes in uniform plasmas. I. Low m modes

The first observations of whistlers excited spontaneously by a modulated electron beam through normal Doppler shifted resonance have been reported in a laboratory experiment. The excited waves are propagating opposite to the beam direction and their phase and group velocities are characteristic of beam-whistler resonant cyclotron coupling. These results should shed light on mechanisms of whistler waves excitation in space plasmas, either by artificial beams injected from spacecraft in the ionosphere and the magnetosphere or by fluxes of energetic particles present in many astrophysical and space phenomena.

Resonant Cyclotron Emission of Whistler Waves by a Modulated Electron Beam

Emission of whistler waves through the Cherenkov resonance mechanism has been studied in a laboratory experiment modeling the interaction of a modulated electron beam with a magnetized plasma under conditions relevant to space experiments involving electron beam injection. The characteristics of the emission and the propagation of the radiated whistlers, and namely the features of their directive radiation patterns, have been controlled as a function of the beam and plasma parameters. This beam radiation process can be distinguished from another mechanism of wave emission, the nonresonant transition radiation from the beam injection point, owing to adequate choices of the physical conditions.

Resonant Cherenkov emission of whistlers by a modulated electron beam

Emission of whistler waves by a density modulated electron beam is studied in a laboratory plasma and compared to excitation by loop antenna. Single and double pole excitation are predicted by a simple linear model which agrees with the experiments. The fraction of the power of the modulated beam radiated by the whistler wave has been measured and is of the order of ${10}^{\mathrm{\ensuremath{-}}5}$.

Whistler wave emission by a modulated electron beam.

When injecting an electron beam with a significant pitch-angle in a magnetoplasma, one has to take care that the parallel velocity of the electrons measured in the plasma does not only depend on the acceleration potential between the anode and the cathode of the electron gun, but also on the amplitude of the ambient magnetic field. The latter can play an important role when the actual acceleration distance of electrons in the gun vicinity, determined partly by collective processes that lead to potential modification in the gun environment, is large enough. Consequently, the actual beam pitch-angle can be noticeably different from the orientation of the electron gun.

Oblique electron-beam injection into plasma: effect of external magnetic field upon gun environment

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Whistler wave emission by a modulated electron beam.

Whistler wave emission by a modulated electron beam through transition radiation

Resonant Cherenkov emission of whistlers by a modulated electron beam

Oblique electron-beam injection into plasma: effect of external magnetic field upon gun environment