Ioannis Gr. Pagonakis

First experimental results from the European Union 2 MW coaxial cavity ITER gyrotron prototype

Abstract The European Union is working toward providing 2-MW, coaxial-cavity, continuous-wave (cw) 170-GHz gyrotrons for ITER. Their design is based on results from an experimental preprototype tube having a pulse length of several milliseconds, in operation at Forschungszentrum Karlsruhe (FZK) for several years now. The first industrial prototype tube was designed for cw operation but, in a first phase, aimed at a pulse length of 1 s at the European Gyrotron Test Facility in Lausanne, Switzerland, as part of a phased testing/development program (1 s, 60 s, cw). The first experimental results of the operation of this prototype gyrotron are reported here. The microwave generation was characterized at very short pulse length (<0.01 s) using a load on loan from FZK, and the highest measured output power was 1.4 MW, at a beam energy significantly lower than the design value (83 kV instead of 90 kV), limited by arcing in the tube. The radio-frequency (rf) beam profile was measured to allow reconstruction of the phase and amplitude profile at the window and to provide the necessary information permitting proper alignment of the compact rf loads prior to pulse extension. Arcs in the tube limited the pulse length extension to a few tens of milliseconds. According to present planning, the tube is going to be opened, inspected, and refurbished, depending on the results of the inspection, to allow testing of an improved version of the mode launcher and replacement of some subassemblies.

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.

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}

A sensitivity analysis of manufacturing imperfections and possible misalignments of the emitter ring in the gyrotron cathode structure on the electron beam quality has been performed. It has been shown that a possible radial displacement of the emitter ring of the order of few tens of microns can cause dramatic effects on the beam quality and therefore the gyrotron operation. Two different design approaches are proposed in order to achieve an electron beam which is less sensitive to manufacturing imperfections.

-Mode Gyrotron

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.

Influence of emitter ring manufacturing tolerances on electron beam quality of high power gyrotrons

Multistage depressed collectors are challenges for high-power, high-frequency fusion gyrotrons. Two concepts exist in the literature: (1) unwinding the spent electron beam cyclotron motion utilizing non-adiabatic transitions of magnetic fields and (2) sorting and collecting the electrons using the E × B drift. To facilitate the collection by the drift, the hollow electron beam can be transformed to one or more thin beams before applying the sorting. There are many approaches, which can transform the hollow electron beam to thin beams; among them, two approaches similar to the tilted electric field collectors of traveling wave tubes are conceptually studied in this paper: the first one transforms the hollow circular electron beam to an elongated elliptic beam, and then the thin elliptic beam is collected by the E × B drift; the second one splits an elliptic or a circular electron beam into two arc-shaped sheet beams; these two parts are collected individually. The functionality of these concepts is proven ...

An Inverse Magnetron Injection Gun for the KIT 2-MW Coaxial-Cavity Gyrotron

An important issue in gyrotron operation is the halo of the electron beam of the magnetron injection gun (MIG). It is formed by magnetically trapped electrons between the cathode and the cavity. These magnetically trapped electrons are generated by several sources, such as the roughness of the emitter ring, the secondary emission from other parts of the cathode surface, etc. The effects of the beam halo can be dramatic in gyrotron operation. As an example of such a dramatic influence of the electron beam halo on the operation of the gyrotron, the instabilities observed in the operation of the first industrial prototype EU coaxial cavity gyrotron will be presented. The positions of the damages found during the inspection of the tube indicate that the observed instabilities were absolutely correlated with the electron beam halo. A simple criterion for MIG design is proposed in order to limit the generation of the beam halo. This criterion has been already taken into account for the gun design of the refurbished 1st prototype coaxial cavity gyrotron. The experiments on this tube showed no electron beam halo effects and instabilities. This important criterion has also been applied for the gun design of the new EU 1MW conventional cavity gyrotron for ITER.

Conceptual designs of E × B multistage depressed collectors for gyrotrons

In this paper, the design of a 10-kW/28-GHz gyrotron is presented. The main characteristic of the new gyrotron is the emitter ring, which is assembled from twelve individually supplied emitters based on controlled-porosity reservoir (CPR) technology. The main goal is to evaluate the use of CPR emitters for gyrotron applications. In addition, the azimuthally segmented emitter ring could be used for the generation of controlled nonuniform electron beams. In this way, it is planned to experimentally study the effect of nonuniform emission on the gyrotron operation. In this context, the effect of nonuniform emission on the beam quality is numerically investigated using the 3-D, electrostatic, parallel-code Ariadne, while the effect of the degraded beam on mode coupling and stability of the wave-particle interaction in the cavity is numerically studied using the nonstationary, self-consistent multimode cavity code Euridice.