Frequency and mode measurement techniques for megawatt-class gyrotrons

State-of-the-art vacuum electron tubes such as gyrotrons, deliver RF output powers up to more than 2 MW at frequencies up to 170 GHz. In terms of the very high power levels, a proper verification of the gyrotron components itself and measurements during gyrotron operation are vital to prevent possible fatal errors. Several basic RF measurement setups, which are used at IHM/KIT, are discussed. Currently, their upper frequency limit is 175 GHz. In terms of future gyrotron operation above 200 GHz, upgrades of the measurement setups for operation up to 260–330 GHz are prepared. The experimental devices discussed herein are a quasi-optical mode generator for the verification of the quasi-optical gyrotron output system, the window measurement test stand to verify the ceramic gyrotron output window and the frequency diagnostic system to measure the operating frequency and thereby the excited mode.

Demonstration of k-vector selective microscopy for nanoscale mapping of higher order spin wave modes

Time resolved STXM provides deep insights into efficient excitation of symmetric and antisysmmetric higher order spin wave modes in magnonic waveguides. k-selective imaging reveals real space information of simultaneously excited mode patterns.

SIW resonator fed horn mounted compact DRA with enhanced gain for multiband applications

A novel design of compact and light-weight horn mounted cylindrical dielectric resonator antenna (CDRA) fed by substrate integrated waveguide (SIW) resonator has been investigated for high gain and multiband applications. SIW resonator contains two closely spaced longitudinal slots of equal length and unequal width to excite the CDRA. These slots are responsible for introducing triple resonating bands. The excited mode in dielectric resonator by longitudinal slots is EH11δ mode. The achieved impedance bandwidths are 65, 180, and 240 MHz at resonant frequencies 9.78, 10.58, and 11.84 GHz, respectively, for |S11| <−10 dB. Copper-taped horn enhances the gain of antenna more than 2 dB for all resonating bands. The measured peak value of gain is 9.3 dBi at ~11.84 GHz.

An adaptive higher order finite element model and modal energy for the vibration of a traveling string

A nonlinear equation describing the transverse vibration of an axially traveling string with constant and time-varying length is obtained by developing a new finite element model described by quadratic shape functions. A novel nonlinear coordinate transform is introduced with regard to its nonlinear terms. Subsequently, a new hybrid Newmark-beta/time varying degree of freedom method, which can adjust the element number automatically according to the change of string length, is proposed to improve accuracy. The proposed method as well as normal numerical methods are compared with an analytical solution. Results show that the proposed method is in good agreement with the Newmark-beta method for the case of small variations in string length, whilst it is superior in accuracy to the latter in the case of large length variations. Complex mode theory is adopted firstly to obtain the modal components as well as subsequently the modal energy for a traveling string. A phenomenon is observed where the free vibration energy leaks from one mode to the others in a traveling string. The higher the speed of translation and the modal order, the more energy that is leaked into the modes close to the initially excited mode.

Experimental Investigation on the Suppression Device of VIV of a Flexible Riser

Experimental investigations were conducted on a flexible riser with and without helical strakes. A uniform current was obtained by towing a riser model in a tank, and the vortex-induced vibration (VIV) suppression of strakes with different heights and pitches was studied. The results of the bare riser show that the characteristics of the synchronization of the VIV for a flexible riser have many orders, and the excited mode jumps from one to another abruptly. During the high order synchronization regime, the VIV response decreases with the increased order of the synchronization. The experimental results also indicate that the response characteristics of a bare riser can be quite distinct from those of a riser with helical strakes, and the suppression performance depends on the geometry of the helical strakes. The fatigue damage in the CF direction is of the same order as that in the IL direction for the bare riser. However, for the riser fitted with helical strakes, the fatigue damage in the CF direction is much smaller than that in the IL direction. The experimental results also confirmed that strake height has a greater influence on the VIV response than the strake pitch, and the drag exerted on the riser increases with strake pitch and height.

Active Control of a Reduced Scale Riser Undergoing Vortex-Induced Vibrations

This article concerns the use of an active control applied to a reduced scale riser undergoing vortex-induced vibrations (VIV). The control system relies upon the fact that a flexible structure undergoing VIV oscillates at frequencies corresponding to the structure resonant modes. These experiments were carried out in a recirculating water channel at IFP. For control design, the structure dynamic behavior is approximated by the dynamic behavior of its most excited mode. This is achieved through modal analysis and leads to a simple linear second order system. Its input is a displacement at the structure top end; its output is the structure displacement of a point away from the top end. The input is computed to attenuate the vibrations associated the most excited mode. This control strategy has been tested on a reduced scale experiment. These results are shown to agree with numerical results obtained on a phenomenological model. Both suggest a VIV reduction about 30%.