Experimental Classification and Enhanced Suppression of Parasitic Oscillations in Gyrotron Beam Tunnels

High-power gyrotrons may suffer from parasitic oscillations that are excited in the electron-beam compression zone. Different damping structures are proposed in the literature that reduce the possibility of parasitic excitation by increasing the starting currents of the modes. In this work, we focus on a dielectric-loaded (stacked) beam tunnel. Based on our previous theoretical studies, we make targeted modifications to the beam tunnel in order to classify the parasitic signals and localize the areas where they are excited. After two successive modifications, the beam tunnel exhibits improved behavior with higher starting currents of the parasitic modes. The experiments are performed by using a modular 170-GHz, 1-MW short-pulse gyrotron, which due to its flanged construction gives the possibility to modify the beam tunnel without affecting the rest of the tube

On the derivation of SPH schemes for shocks through inhomogeneous media

Smoothed Particle Hydrodynamics (SPH) is typically used for the simulation of shock propagation through solid media, commonly observed during hypervelocity impacts. Although schemes for impacts into monolithic structures have been studied using SPH, problems occur when multimaterial structures are considered. This study begins from a variational framework and builds schemes for multiphase compressible problems, coming from different density estimates. Algorithmic details are discussed and results are compared upon three one-dimensional Riemann problems of known behavior

Starting currents of modes in cylindrical cavities with mode-converting corrugations for second-harmonic gyrotrons

A self-consistent system of equations (known as single-mode gyrotron equations) is extended to describe the beam-wave interaction in a cylindrical gyrotron cavity with mode-converting longitudinal corrugations, which produce coupling of azimuthal basis modes. The system of equations is applied to investigate the effect of corrugations on starting currents of the cavity modes. For these modes, eigenvalues, ohmic losses, field structure, and beam-wave coupling coefficients are investigated with respect to the corrugation parameters. It is shown that properly sized mode-converting corrugations are capable of improving the selectivity properties of cylindrical cavities for second-harmonic gyrotrons

Identification of a response amplitude operator for ships

At the European Study Group Mathematics with Industry 2012 in Eindhoven, the Maritime Research Institute Netherlands (MARIN) presented the problem of identifying the response amplitude operator (RAO) for a ship, given input information on the amplitudes of the sea waves and output information on the movement of the ship. We approach the problem from a threefold perspective: a direct least-squares approach, an approach based on truncated Fourier series, and an approach using low-dimensional measures of the RAO. We give a few recommendations for possible further investigations

Multifaceted Simulations Reproducing Experimental Results from the 1.5-MW 140-GHz Preprototype Gyrotron for W7-X

A multifaceted simulation procedure, addressing the electron beam properties, the beam-wave interaction, and the internal losses, has been used for the simulation of the experimental operation of a 1.5-MW 140-GHz short-pulse preprototype gyrotron. The preprototype is related to the development of 1.5-MW gyrotrons for the upgrade of the electron cyclotron resonance heating system at the stellarator W7-X. A very good reproduction of experimental results has been achieved by simulation, without resorting to arbitrary speculations. This validated the numerical tools as well as the design and fabrication of the short-pulse preprototype, which fully reached the target of efficient 1.5-MW operation in millisecond pulses. Special attention has been given to simulating the possibility of parasitic after-cavity interaction in the gyrotron launcher. Also, parasitic backward-wave excitation in the gyrotron cavity has been demonstrated by simulation, at a frequency and voltage range in agreement with experimentally observed parasitic oscillations. This offers an additional possibility with respect to the origin of deleterious parasitic oscillations in high-power gyrotrons, which are usually attributed mainly to the gyrotron beam tunnel

Identification of a Response Amplitude Operator for Ships

At the European Study Group Mathematics with Industry 2012 in Eindhoven, the Maritime Research Institute Netherlands (MARIN) presented the problem of identifying the response amplitude operator (RAO) for a ship, given input information on the amplitudes of the sea waves and output information on the movement of the ship. We approach the problem from a threefold perspective: a direct least-squares approach, an approach based on truncated Fourier series, and an approach using low-dimensional measures of the RAO. We give a few recommendations for possible further investigations

Malignant triton tumour of the anterior mediastinum as incidental finding

A rare case of malignant peripheral nerve sheath tumour with rhabdomyoblastic differentiation (malignant triton tumour) of the anterior mediastinum in a 30-yearold male is reported. The tumour was an incidental finding during the diagnostic work-up following a motor vehicle accident. The patient underwent median sternotomy with a tumour resection performed. Local relapse was suspected one month later, as per the chest CT-scan, and post-operative chemoradiation was applied, which produced a response. Twelve months later the patient is doing well while radiological findings remain invariable. Localization of a triton tumour in the anterior mediastinum is extremely rare, adjuvant treatment is necessary, recurrence frequently occurs and the prognosis is dismal