Sub-THz and THz Cherenkov radiation source with two-dimensional periodic surface lattice and multistage depressed collector

Abstract

We present the theory, concept and design of an efficient, megawatt coherent Cherenkov radiation source based on a two-dimensional periodic surface lattice (2D-PSL) cavity combined with a novel energy recovery system for the generation of highly efficient (> 50%) single-frequency radiation. We demonstrate the scalability of the transverse dimension of the 2D-PSL cavity of the Cherenkov source and thus the potential for efficient, continuous-wave, high-power (> 1 MW) operation; fundamental to the eventual realization of clean, fusion energy. These new sources, with the capacity to operate in the 0.1-10THz range, hold strong promise to address the long-standing "Terahertz gap". By combining a Cherenkov oscillator driven by a non-gyrating beam with an innovative four-stage depressed collector energy recovery system, the overall device efficiency can be increased to be competitive with gyrotrons in the requirements for heating and current drive in fusion plasma. In these Cherenkov devices, the frequency independence of the magnetic guide field enables advantageous frequency scaling without deployment constraints, making them especially attractive for high-impact applications in fusion science, turbulence diagnostics, non-destructive testing and biochemical spectroscopy. The novel energy recovery techniques presented in this paper have broad applicability to many electron-beam driven devices, bringing revolutionary potential to future THz source technologies.

Fig. 1

Conceptual diagram of oversized 2D-PSL oscillator illustrating the key parameters of the novel interaction structure.

Fig. 2

Illustration of the synchronism conditions showing crossing of the analytical dispersion and electron beam line (non-gyrating electrons, solid line) and electron cyclotron beam line (gyrating electrons, dashed line). The axial wavenumber is normalized by the electron cyclotron wavenumber,. The points of intersection illustrate (i) the backward wave interaction with the 120 keV electron beam at the operating frequency f ≅ 87 GHz and (ii) the frequency where the strongest cyclotron absorption (f_ce = 87 GHz) can be expected for B = 4.6T.

Fig. 3

(a) Output radiation spectrum (logarithmic scale where the numbers on the axis represent 10^N) showing output at 83 GHz and second harmonic 166 GHz and (b) Time dependence of the output power and interaction efficiency for the surface wave source based on the 2D-PSL cavity with . The inset shows the contour plots of the E_z (top) and H_z (bottom) eigenmode field components.

Fig. 4

(a) A 2D contour plot showing the surface field of the 2D PSL extending beyond the 2D corrugation (colored gray) where it is overlapped by the electron beam (orange/yellow annulus). The inset shows the close-up view of the edge of the 2D-PSL cavity. The orange and yellow coloring of the beam (see inset colorbar) shows the relative variation in the electron energies. The contour plot of the E_z field is shown by the colorbar on the right. (b) The dependences of the normalized electronic efficiency (left axis) and transient/saturation period  (right axis) on the electron beam-grating separation measured from the peak of the corrugation to the outer radius of the electron beam. The smallest value of is 0.001 mm .

Fig. 5

The current normalized to the peak current as a function of the electron energy. The vertical lines (orange, yellow, purple, green) illustrate the optimal potentials of the 4-stage depressed collector.

Fig. 6

Configuration of superconducting magnet and the magnetic field profile used for the simulations.

Fig. 7

Plot of particle trajectories showing (a) the beam (primary) electron trajectories and (b) the trajectories of the secondary electrons. The secondary electrons are emitted from the collisions of the beam electrons with the electrodes of the 4-stage MDC.