Fig. 1. Perpendicular and parallel components of electron momentum (normalized to the mean electron momentum $p_{e0}$) for an evolved horseshoe distribution function, with the contours representing constant phase-space density.

Fig. 2. Imaginary part of the refractive index as a function of frequency for a mean beam energy of 100 keV and a thermal spread of 1 keV, and a mean beam energy of 500 keV and a thermal spread of 5 keV. The magnetic field ratio is taken to be 20.

Fig. 3. Composite overview of 3D VSim PIC simulation results in an $x$–$z$ plane ($y=0$) showing magnetic compression of an electron beam and subsequent cyclotron-maser emission in the X-mode at $t=1000t_{ce}$. The electron PIC particle trajectory is also overlaid (blue scatter plot) along with the corresponding velocity distribution over the simulation volume at $t=1000t_{ce}$.

Fig. 4. 3D VSim PIC simulation results showing (a) the spectrum of EM emission at $z=86\unicode[STIX]{x1D706}_{ce}$ and (b) the transverse Poynting flux in a $y$–$z$ plane displaced from the electron beam.

Fig. 5. Diagrammatic overview of the experimental setup showing the magnetic coil configuration, electron gun and the convergent axial magnetic field profile with peak-plateau region for cyclotron resonant energy transfer.

Fig. 6. Experimental measurements for the TE01 resonance, illustrating the spectrum of the output signal, displaying a strong resonance close to the electron-cyclotron frequency, 4.42 GHz.