A. Macor

Experimental study from linear to chaotic regimes on a terahertz-frequency gyrotron oscillator

Basic wave-particle interaction dynamics from linear to chaotic regimes is experimentally studied on a frequency tunable gyrotron generating THz radiation in continuous mode (200W) at 263GHz which will be used for dynamic nuclear polarization nuclear magnetic resonance spectroscopy applications. In the studied system, the nonlinear dynamics associated to the waveparticle interaction is dominated by longitudinal mode competition of a given transverse TEm;p cavity-mode. This study covers a wide range of control parameter from gyro-traveling wave tube (gyro-TWT) to gyro-backward wave oscillator (gyro-BWO) like interactions for which extensive theoretical studies have been performed in the past on a simplified system. Besides the common route to chaos characterized by period doubling, other routes have been identified among which some are characterized by line-width frequency-broadening on the side-bands. The complex nonlinear dynamics is in good agreement with the theory and the experimental results are discussed on the basis of the prediction obtained with the nonlinear time-dependent selfconsistent codes TWANG and EURIDICE both based on a slow-time scale formulation of the self-consistent equations governing the wave-particle dynamics. VC

Note: stacked rings for terahertz wave-guiding.

We demonstrate the construction of corrugated waveguides using stacked rings to propagate terahertz frequencies. The waveguide allows propagation of the same fundamental mode as an optical-fiber, namely, the HE(11) mode. This simple concept opens the way for corrugated wave-guides up to several terahertz, maintaining beam characteristics as for terahertz applications.

Note: Three-dimensional stereolithography for millimeter wave and terahertz applications

Metal-coated polymers shaped by 3D stereolithography are introduced as a new manufacturing method for passive components for millimeter to terahertz electromagnetic waves. This concept offers increased design capabilities and flexibilities while shortening the manufacturing process of complex shapes, e.g., corrugated horns, mirrors, etc. Tests at 92.5, 140, and 170 GHz are reported.

THz signal transmission in a compact modular waveguide system

The transmission properties of a compact circular corrugated waveguide system are investigated experimentally in the WM-380 band from 500 to 750 GHz. Up to 700 GHz, the average transmission losses range from undetectable to 3.4 dB/m, which is more than four orders of magnitude lower than in rectangular waveguides. The propagation of the lowest-loss HE11 mode is found to be dominant, and long-term measurements of phase and amplitude demonstrate high transmission stability.

THz-waves channeling in a monolithic saddle-coil for Dynamic Nuclear Polarization enhanced NMR

A saddle coil manufactured by electric discharge machining (EDM) from a solid piece of copper has recently been realized at EPFL for Dynamic Nuclear Polarization enhanced Nuclear Magnetic Resonance experiments (DNP-NMR) at 9.4 T. The corresponding electromagnetic behavior of radio-frequency (400 MHz) and THz (263 GHz) waves were studied by numerical simulation in various measurement configurations. Moreover, we present an experimental method by which the results of the THz-wave numerical modeling are validated. On the basis of the good agreement between numerical and experimental results, we conducted by numerical simulation a systematic analysis on the influence of the coil geometry and of the sample properties on the THz-wave field, which is crucial in view of the optimization of DNP-NMR in solids.

Manufacturing of a 263 GHz continuously tunable gyrotron

A 263 GHz gyrotron dedicated to Dynamic Nuclear Polarization (DNP) enhanced Nuclear Magnetic Resonance (NMR) spectroscopy has been designed by EPFL and manufactured by Thales. With this device, we have demonstrated an output power up to 160 W in continuous wave. Design and main experimental results are discussed in this paper.