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

Design of MW-Class Coaxial Gyrotron Cavities With Mode-Converting Corrugation Operating at the Second Cyclotron Harmonic

This article presents investigations on the design of coaxial gyrotron cavities with mode-converting corrugations, operating at the second harmonic of the electron cyclotron frequency with output power of the order of megawatts. The suppression of the competing modes interacting at the fundamental cyclotron frequency is achieved by the combination of a corrugated coaxial insert and mode-converting corrugation on the outer wall. The outer corrugation couples the key competing modes to lower order modes with reduced quality factor. The design steps, which form a generally applicable design procedure, are described in detail. As an illustrative example, the proposed procedure is used for the design of a cavity for a fusion-relevant, second-harmonic MW-class gyrotron, operating at 170 GHz with the TE 37,1837,18 mode. From the simulations, it is found that for the proposed design, this mode is excited with an output power of around/ ∼ 1.5 MW. Two additional paths for cavity optimization toward even higher output power are also presented

Generation of 1.5MW-140GHz pulses with the modular pre-prototype gyrotron for W7-X

In anticipation of an Electron Cyclotron Resonance Heating system upgrade for the stellarator Wendelstein 7-X, a 1.5 MW – 140 GHz continuous-wave gyrotron is under development. In order to provide a first experimental verification of the scientific RF and electron beam optics design of the gyrotron with ms pulses, the Karlsruhe Intitule of Technology has developed a short-pulse pre-prototype gyrotron. In this work, we present details regarding the construction of the pre-prototype as well as measurements from the first experimental campaign delivering up to 1.6 MW in short pulses

European 1 MW, 170 GHz CW Gyrotron Prototype for ITER - long-pulse operation at KIT -

The upgraded EU 1 MW, 170 GHz continuous wave (CW) industrial prototype gyrotron (TH1509U) for Electron Cyclotron Resonance Heating and Current Drive (ECRH&CD) in ITER was tested at the Karlsruhe Institute of Technology (KIT). The gyrotron surpassed the performance of the previous TH1509 tube. In particular, TH1509U delivered (i) 0.9 MW in 180 s pulses (max. pulse length of the KIT test stand) and (ii) more than 1 MW at a pulse length limited to 40 s, due to a problem with the test stand cooling circuit at that time. In addition, it was possible to demonstrate gyrotron operation at (iii) 0.5 MW in 1600 s pulses

Towards a 1.5 MW, 140 GHz gyrotron for the upgraded ECRH system at W7-X

For the required upgrades of the Electron Cyclotron Resonance Heating system at the stellarator Wendelstein 7-X, the development of a 1.5 MW 140 GHz Continuous Wave (CW) prototype gyrotron has started. KIT has been responsible to deliver the scientific design of the tube (i.e. the electron optics design and the RF design), with contributions from NKUA and IPP. The prototype gyrotron has been ordered at the industrial partner, Thales, France, and is expected to be delivered in 2021. In parallel, a short-pulse pre-prototype gyrotron has been developed at KIT, to provide the means for a first experimental validation of the scientific design in ms pulses, prior to the construction of the CW prototype. This paper reports on the status of the 1.5 MW CW gyrotron development, focusing on the scientific design and its numerical and experimental validation

A new 3MW ECRH system at 105 GHz for WEST

The aim of the WEST experiments is to master long plasma pulses (1000s) and expose ITER-like tungsten wall to deposited heat fluxes up to 10 MW/m$^2$. To increase the margin to reach the H-Mode and to control W-impurities in the plasma, the installation of an upgraded ECRH heating system, with a gyrotron performance of 1MW/1000s per unit, is planned in 2023. With the modifications of Tore Supra to WEST, simulations at a magnetic field B$_0$∼3.7T and a central density n$_{e0}$∼6 × 10$^{19}$ m$^{−3}$ show that the optimal frequency for central absorption is 105 GHz. For this purpose, a 105 GHz/1MW gyrotron (TH1511) has been designed at KIT in 2021, based on the technological design of the 140 GHz/1.5 MW (TH1507U) gyrotron for W7-X. Currently, three units are under fabrication at THALES. In the first phase of the project, some of the previous Tore Supra Electron Cyclotron (EC) system components will be re-installed and re-used whenever possible. This paper describes the studies performed to adapt the new ECRH system to 105 GHz and the status of the modifications necessary to re-start the system with a challenging schedule

Recent experiments with the European 1MW, 170GHz industrial CW and short-pulse gyrotrons for ITER

The European Gyrotron Consortium (EGYC) is developing the European 1 MW, 170 GHz Continuous Wave (CW) industrial prototype gyrotron for ITER in cooperation with Thales Electron Devices (TED) and Fusion for Energy (F4E). This conventional, hollow-cavity gyrotron, is based on the 1 MW, 170 GHz Short-Pulse (SP) modular gyrotron that has been designed and manufactured by the Karlsruhe Institute of Technology (KIT) in collaboration with TED. Both gyrotrons have been tested successfully in multiple experiments. In this work we briefly report on the results with the CW gyrotron at KIT and we focus at the experiments at the Swiss Plasma Center (SPC). In addition, we present preliminary results from various upgrades of the SP tube that are currently tested at KIT

Simulation of electron cyclotron interaction in gyrotron beam tunnels for the study of parasitic oscillations

Gyrotron oscillators are microwave sources covering the millimeter bands of the electromagnetic spectrum with main application being the generation of high-power microwave radiation for heating of plasmas in experimental fusion devices. One serious problem of high-power high-frequency gyrotron experiments is the excitation of parasitic oscillations inside the beam tunnel prior to the cavity. This kind of parasitic oscillations degrade the quality of the electron beam, reduce the efficiency of the main interaction and may cause overheating and arcs possibly leading to the failure of the overall device.The topic of this dissertation is the theoretical study of parasitic oscillations in the beam tunnel and its aim is to offer a deeper insight of the corresponding physics and suggest possible methods for their suppression. More specifically, for the simulation of the beam tunnel its mathematical model and the pertinent numerical code are developed. The beam tunnel is modeled as a finite-length cavity by applying outgoing-wave boundary conditions at its boundaries. By using such an approach, the characteristics of the resonant modes, which exhibit dielectric and diffraction losses, can be calculated.For the simulation of the beam-wave interaction in the beam tunnel, a simulation scheme is developed by combining our code (NESTOR) with EURIDICE that is a simulation and design code for gyrotron cavities. With this combination of codes specific beam-tunnel configurations are studied and the starting currents of the resonant modes are calculated. In addition, parametric studies are performed by varying the geometrical and physical properties of the beam tunnel, like the real and imaginary part of the permittivity and the thickness of the dielectric rings. Next, extensions of the model for coaxial geometries and for a structure with a total-reflection boundary condition at the right boundary are developed. Finally, specific directions for future research are presented and discussed, which can simulate realistically and precisely actual beam-tunnel structures.Τα γυροτρόνια είναι διατάξεις παραγωγής μικροκυμάτων που καλύπτουν τη χιλιοστομετρική περιοχή του ηλεκτρομαγνητικού φάσματος με κύρια εφαρμογή την παραγωγή μικροκυματικής ακτινοβολίας υψηλής ισχύος για τη θέρμανση του πλάσματος στα πειράματα ελεγχόμενης θερμοπυρηνικής σύντηξης. Ένα από τα σοβαρά προβλήματα, το οποίο αντιμετωπίζουν τα γυροτρόνια υψηλής ισχύος και συχνότητας, είναι η διέγερση ανεπιθύμητων παρασιτικών ταλαντώσεων στον δίαυλο της δέσμης πριν την κοιλότητα. Τέτοιου είδους παρασιτικές ταλαντώσεις υποβαθμίζουν την ποιότητα της δέσμης, μειώνουν την απόδοση της κύριας αλληλεπίδρασης και είναι πιθανό να προκαλέσουν τοπική υπερθέρμανση, ηλεκτρικά τόξα έως και πλήρη διακοπή της λειτουργίας όλης της διάταξης.Η παρούσα διατριβή ασχολείται με τη θεωρητική μελέτη των παρασιτικών ταλαντώσεων στον δίαυλο της δέσμης και φιλοδοξεί να προσφέρει μια βαθύτερη κατανόηση της φυσικής του φαινομένου και να προτείνει πιθανούς τρόπους αντιμετώπισης του προβλήματος. Συγκεκριμένα, για τη μελέτη του διαύλου δέσμης αναπτύσσεται το μαθηματικό μοντέλο και ο σχετικός κώδικας για την προσομοίωση της διάταξης. Ο δίαυλος της δέσμης αντιμετωπίζεται σαν πεπερασμένου μήκους κοιλότητα εφαρμόζοντας οριακές συνθήκες εξερχόμενων κυμάτων στα άκρα. Έτσι, μπορούν να υπολογισθούν τα χαρακτηριστικά των συντονιζόμενων ρυθμών της διάταξης, οι οποίοι παρουσιάζουν διηλεκτρικές απώλειες και απώλειες περίθλασης. Για την προσομοίωση της κυκλοτρονικής αλληλεπίδρασης κύματος-δέσμης στον δίαυλο αναπτύσσεται ένα σχήμα προσομοίωσης συνδυάζοντας τον κώδικα μας (NESTOR) με τον κώδικα προσομοίωσης της κοιλότητας EURIDICE. Με τον συνδυασμό αυτό μελετώνται συγκεκριμένες διαμορφώσεις διαύλων δέσμης και υπολογίζονται τα ρεύματα εκκίνησης των ρυθμών. Επίσης, πραγματοποιούνται παραμετρικές μελέτες της εξάρτησης των απωλειών και της αλληλεπίδρασης από γεωμετρικές και φυσικές παραμέτρους του διαύλου, όπως το πραγματικό και φανταστικό μέρος της επιτρεπτότητας και το πάχος των δαχτυλιδιών. Κατόπιν, αναπτύσσονται επεκτάσεις για ομοαξονικές γεωμετρίες διαύλου και εφαρμόζονται οριακές συνθήκης ολικής ανάκλασης στο ένα άκρο. Τέλος, συζητούνται και προτείνονται συγκεκριμένες κατευθύνσεις μελλοντικής έρευνας που υπόσχονται την πλήρη και ακριβή προσομοίωση του διαύλου δέσμης στον υπολογιστ

1.5 MW, 140 GHz Gyrotron for W7-X - development status and experimental results -

For the upgrade of the Electron Cyclotron Resonance Heating (ECRH) system of the stellarator Wendelstein 7-X (W7-X), a 1.5 MW, 140 GHz continuous-wave (CW) gyrotron is under development. In order to provide a first experimental verification of the scientific microwave and electron beam optics design of the gyrotron at pulse lengths of a few milliseconds (ms), Karlsruhe Intitule of Technology (KIT) has developed and tested successfully a short-pulse pre-prototype gyrotron. The tube was stably operated at up to 1.6 MW output power. The status of the project as well as the results achieved during the first experimental campaign, are presented in this paper