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A tapered-tube concentrator for Microwave Rocket was developed using a ray tracing method for sub-megawatt power transmission. An experiment was conducted using a 1 MW-class gyrotron as a beam source. Plasma ignition and ionization front propagation were achieved with the transmission system. The propagation velocity of an ionization front was measured at 555 m/s by a fast-framing camera.
The experimental study of the quasi-optical mode convertor for 0.42THz, TE26 mode terahertz gyrotron has been carried out. The results show that the power conversion efficiency is more than 80%, and the gaussian content of the field at the output window is about 80%.
A numerical procedure to implement non-reflecting radiation boundary conditions, based on the Laplace Transform, is described in details and tested successfully on a simple two frequencies test problem. The method is then implemented in the time-dependent gyrotron simulation code TWANG. In the case for which the code exhibits a multi-frequency spectrum with the same transverse mode, it is shown that...
An optical steerable terahertz zone plate is achieved utilizing the localized distribution of the transient electron plasma on a silicon wafer. Its focusing and imaging performances are experimentally demonstrated. The research prompts the development of steerable optical imaging elements.
Laser-induced plasmas with longitudinal size smaller than 40 μm are investigated as a source of broadband terahertz (THz) radiation. The angular dependent emission of the micro-plasma is measured with coherent detection, and the preferential directions of radiation are identified. The observations support electrons drift driven by the laser ponderomotive force as the principal generation mechanism...
Applications of THz photonics rely critically on efficient THz wave generation and detection. Here we report THz radiation from cesium and rubidium vapor plasma. By taking advantage of the very low ionization potential of cesium (3.89 eV) and rubidium (4.18 eV), we are able to produce a THz wave with an amplitude more than 8 times higher than nitrogen gas at the same pressure, showing the promise...
Millimeter-wave imaging diagnostics have been developed in the 50∼150 GHz region which provide 2D localized measurements of electron temperature and electron density fluctuations inside magnetic fusion plasma devices. New mixer and antenna geometries are being developed to extend these techniques to THz frequencies. Also under investigation are beam shaping techniques which offer the possibility of...
We have quantified the ultrafast birefringence induced in air plasma by measuring the THz-field-induced second harmonic (TFISH) along two orthogonal directions with THz air-biased-coherent-detection technique. The time domain TFISH waveforms and polarization state were measured under different polarizations of fundamental wave. The measured polarization state of the TFISH agree well with predictions...
The new gyrotron simulation code TWANG-PIC, a Particle-In-Cell (PIC)-version of the code will be presented, which solves the slow-timescale self-consistent wave-particle interaction equations for a single transverse mode. This code represents a considerable improvement with respect to the previous trajectory-code by relaxing the approximation of a time-frozen electromagnetic (EM)-field during the...
Using the simple unified theory, the beam-wave interactions are analyzed in high-power Cerenkov-type microwave generators composed of periodic metal slow-wave structure (SWS) with a magnetized plasma column.
We demonstrate ultrafast temporal slicing of phase-locked multi-THz pulses using a photoinduced electron-hole plasma in germanium. This approach enables us to extract single-cycle transients with a peak electric field of 10 MV/cm from a multi-cycle waveform at a carrier frequency of 24 THz.
Millimeter-wave radiation has been successfully generated by a ∼100GHz BWO driven by a high brightness electron beam derived from a 14-gap pseudospark discharge. Pseudospark electron beams have been studied recently with their application to millimeter-wave and terahertz radiation generation. A BWO utilizing the interaction between an axial electron beam and a slow-wave structure was designed and...
Cold test results of a G-band sheet beam backward wave oscillator (BWO) with a double grating waveguide, fabricated by wire cutting and a 3D printing technique, are presented in this paper. The dispersion curve and S-parameters of the BWO slow wave circuit and Bragg reflector were measured and the results compared to the numerical modelling and analytical theory.
During last years several new gyrotrons were developed at IAP/GYCOM. The main ITER requirements to a gyrotron have been demonstrated: 170 GHz frequency, 1MW power, 1000 seconds pulse duration, 53% efficiency. For a multi-frequency gyrotron a novel scheme for a tunable window was developed. Additionally some other new gyrotrons were developed for running plasma installations. New directions in the...
The upgraded Electron Cyclotron Resonance Heating (ECRH) system at ASDEX Upgrade (AUG) has been routinely used in the last experimental campaign. A further upgrade will replace an existing system of four short-pulse (140 GHz, 2s, 500 kW) gyrotrons. The final goal is to have around 6.5–7 MW at 140 GHz (or 5.5 MW at 105 GHz) from 8 units available in the plasma during the whole AUG discharge (10 s).
Beyond ITER, a future nuclear fusion demonstration power plant (DEMO) demands gyrotron frequencies above 200 GHz and gyrotron efficiencies of higher than 60 %. The unit power must well exceed 1 MW. Fast frequency tunability in steps of 2–3 GHz (in a few seconds) using broadband or tunable CVD-diamond windows is required. Beside these fundamental requirements, slow frequency tunability (in a few minutes)...
We have been investigating THz photon emission/absorption dynamics of high energy electrons channeled through crystalline nanostructures. Preliminary plasma wakefield simulations, employing particle-in-cell (PIC) computations, indicated that a short-pulse (σ ∼ 1 ps) electron beam, modulated with 28.4 THz, generates an intense plasmonic wave of ∼ 0.3 TeV/m in a plasma channel with a crystalline electronic...
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