John Jelonnek

First Operation of a Step-Frequency Tunable 1-MW Gyrotron With a Diamond Brewster Angle Output Window

Experimental results using a step-frequency tunable D-band gyrotron are reported. The short pulse (~3 ms) gyrotron is equipped with an elliptically brazed chemical vapor deposition (CVD) diamond Brewster angle output window. It is designed for the operation in the frequency range from 111.6 up to 165.7 GHz. Operating parameters for ten different frequencies corresponding to an equal number of different cavity operating modes has been measured. A minimum output power of 830 kW and a peak output power of 1.3 MW have been realized. For all frequencies, the parameters of the RF beam generated by the internal quasioptical converter, such as fundamental Gaussian contents and beam waist, are sufficiently good to allow an efficient coupling of the RF power out of the window. This is the first time a diamond Brewster angle window has been used in a high power gyrotron (~1 MW). Such a system offers the path to a simple and compact window solution for high power broadband applications using gyrotrons.

From Series Production of Gyrotrons for W7-X Toward EU-1 MW Gyrotrons for ITER

Europe is devoting significant joint efforts to develop and to manufacture MW-level gyrotrons for electron cyclotron heating and current drive of future plasma experiments. The two most important ones are the stellarator Wendelstein W7-X at Greifswald and the Tokamak ITER at Cadarache. While the series production of the 140 GHz, 1 MW, CW gyrotrons for the 10-MW electron cyclotron resonance heating system of stellarator W7-X is proceeding, the European GYrotron Consortium is presently developing the EU-1 MW, 170 GHz, CW gyrotron for ITER. The initial design had already been initiated in 2007, as a risk mitigation measure during the development of the advanced ITER EU-2-MW coaxial-cavity gyrotron. The target of the ITER EU-1-MW conventional-cavity design is to benefit as much as possible from the experiences made during the development and series production of the W7-X gyrotron and of the experiences gained from the earlier EU-2-MW coaxial-cavity gyrotron design. Hence, the similarity of the construction will be made visible in this paper. During 2012, the scientific design of the ITER EU-1-MW gyrotron components has been finalized. In collaboration with the industrial partner Thales electron devices, Vélizy, France, the industrial design of the technological parts of the gyrotron is being completed. A short-pulse prototype is under development to support the design of the CW prototype tube. The technological path toward the EU ITER-1 MW gyrotron and the final design will be presented.

Electron trapping mechanisms in magnetron injection guns

A key parameter for the gyrotron operation and efficiency is the presence of trapped electrons. Two electron trapping mechanisms can take place in gyrotrons: (i) the adiabatic trap and (ii) the magnetic potential well. Their influence on the gyrotron operation is analyzed. Two gun design criteria are then proposed to suppress both mechanisms in order to minimize the risk of possible problems. Experimental results of three high power gyrotrons are presented and their performance is correlated to the presence of populations of trapped electrons. Finally, some very general gun design principles are presented for the limitation of harmful electron trapping.

A generic mode selection strategy for high-order mode gyrotrons operating at multiple frequencies

High-power, high-frequency gyrotrons for electron cyclotron resonance heating and current drive, such as proposed for the demonstration thermonuclear fusion reactor DEMO, require operating modes of very high order. As it is shown, the selection of the operating modes for such gyrotrons can be based on multi-frequency operability. A general selection strategy is derived, suitable for multi-purpose multi-frequency gyrotrons with quasi-optical mode converter and single-disc output window. Two examples, one of them relevant for future DEMO gyrotron designs, are discussed.

Multistage depressed collector conceptual design for thin magnetically confined electron beams

The requirement of higher efficiency in high power microwave devices, such as traveling wave tubes and gyrotrons, guides scientific research to more advanced types of collector systems. First, a conceptual design approach of a multistage depressed collector for a sheet electron beam confined by a magnetic field is presented. The sorting of the electron trajectories, according to their initial kinetic energy, is based on the E × B drift concept. The optimization of the geometrical parameters is based on the analytical equations under several general assumptions. The analysis predicts very high levels of efficiency. Then, a design approach for the application of this type of collector to a gyrotron cylindrical hollow electron beam is also presented with very high levels of efficiency more than 80%.

High-Efficiency Quasi-Optical Mode Converter for a 1-MW

A 1-MW, continuous wave, 170-GHz, TE32,9-mode gyrotron for use in International Thermonuclear Experimental Reactor (ITER) is under development within the European Gyrotron Consortium. A quasi-optical mode converter is employed in the gyrotron to transform the high-order cavity mode into a fundamental Gaussian wave beam. The quasi-optical mode converter contains a launcher and a mirror system. The launcher is numerically optimized to provide Gaussian mode content of 98.43% at the launcher aperture. The mirror system consists of three mirrors. The first mirror is a quasi-elliptical mirror, the second and third mirrors are beam-shaping mirrors, which are used to change the beam parameters, such as the beam waist and the position of the focusing plane. The field distribution in the mode converter has been analyzed. The simulation results show that the fundamental Gaussian mode content of the wave beam is 98.6% at the window plane. A first numerical estimation of the stray radiation generated by the mode converter is 1.75%, to be verified in future measurements. The proper synthesis of the quasi-optical mode converter has been verified by comparison of the simulation results from TWLDO with results obtained using the commercial 3-D full-wave vector analysis SURF3D code.

{\rm TE}_{32,9}

The KIT two-dimensional beam optics code ESRAY is an efficient tool for the design and optimization of magnetron injection guns and collectors of gyrotrons or comparable vacuum tubes. The applied numerical methods and latest modifications and enhancements will be presented. To achieve more realistic results, a new emitter surface roughness model and the treatment of secondary electrons has been implemented recently. An extension to three dimensions (without taking space-charge effects into account) permits the simulation of various collector sweeping concepts, including the very efficient transverse sweeping.

-Mode Gyrotron

In this paper, a measurement system for the detection of time-dependent effects in broadband spectra of high-power millimeter-wave sources is demonstrated. The heterodyne approach with sub-harmonic mixers enables high dynamic range and configuration flexibility, but typically also imposes severe problems on the analysis of transient or instationary phenomena through frequency ambiguity. A key feature of the presented system is an unambiguous reconstruction of the RF spectrum from the detected IF spectra. This is done by evaluating the upper and lower mixer sidebands of two parallel heterodyne receiver channels simultaneously, resulting in a comparatively high instantaneous measurement bandwidth of 7 GHz per channel pair and a high dynamic range of 50-60 dB in the frequency range 110-170 GHz. Sample measurements obtained during the occurrence of an arc at the dielectric output window of a 140-GHz megawatt-class gyrotron demonstrate the unique capabilities of the system for detecting highly transient and broadband effects in the tubes output spectrum.

Gyrotron electron gun and collector simulation with the ESRAY beam optics code

High frequency (>230 GHz) megawatt-class gyrotrons are planned as RF sources for electron cyclotron resonance heating and current drive in DEMOnstration fusion power plants (DEMOs). In this paper, for the first time, a feasibility study of a 236 GHz DEMO gyrotron is presented by considering all relevant design goals and the possible technical limitations. A mode-selection procedure is proposed in order to satisfy the multi-frequency and frequency-step tunability requirements. An effective systematic design approach for the optimal design of a gradually tapered cavity is presented. The RF-behavior of the proposed cavity is verified rigorously, supporting 920 kW of stable output power with an interaction efficiency of 36% including the considerations of realistic beam parameters.

Transient Millimeter-Wave Signal Analysis With Unambiguous RF Spectrum Reconstruction

An inverse magnetron injection gun (MIG) has been designed for the 2-MW, 170-GHz, coaxial-cavity gyrotron built at the Karlsruhe Institute of Technology. The inverse gun design could offer the possibility for the implementation of a larger emitter ring without the need for a bigger bore hole in the magnet compared with the conventional type of MIGs. Considering the fundamental beam parameters, an excellent beam quality has been achieved in numerical simulation. Electron-trapping suppression criteria were considered during the design phase of the MIG.