I. Gr. Pagonakis

Systematic cavity design approach for a multi-frequency gyrotron for DEMO and study of its RF behavior

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.

A comparative study on the modeling of dynamic after-cavity interaction in gyrotrons

There are cases where gyrotron interaction simulations predict dynamic After-Cavity Interaction (ACI). In dynamic ACI, a mode is excited by the electron beam at a dominant frequency in the gyrotron cavity and, at the same time, this mode is also interacting with the beam at a different frequency in the non-linear uptaper after the cavity. In favor of dynamic ACI being a real physical effect, there are some experimental findings that could be attributed to it, as well as some physical rationale indicating the possibility of a mode being resonant with the beam at different frequencies in different regions. However, the interaction codes used in dynamic ACI prediction up to now are based on simplifications that put questions on their capability of correctly simulating this effect. In this work, the shortcomings of the usual simplifications with respect to dynamic ACI modeling, namely, the trajectory approach and the single-frequency boundary condition, are identified. Extensive simulations of dynamic ACI cases are presented, using several “in-house” as well as commercial codes. We report on the comparison and the assessment of different modeling approaches and their results and we discuss whether, in some cases, dynamic ACI can be a numerical artifact or not. Although the possibility of existence of dynamic ACI in gyrotrons is not disputed, it is concluded that the widely used trajectory approach for gyrotron interaction modeling is questionable for simulating dynamic ACI and can lead to misleading results.

Design of the EU-1MW gyrotron for ITER

EU is developing a 1 MW cylindrical cavity gyrotron. In the last year the design of the components of the new gyrotron has been finalized while the technological design of the new tube has been defined. In the present paper, the main characteristics of the new EU gyrotron for ITER are presented.

Interaction circuit design and RF behavior of a 236 GHz gyrotron for DEMO

The Demonstration Fusion Power Reactor (DEMO) to follow ITER by 2050 demands high frequency (>230 GHz), high power (in the range from 1 MW to 2 MW) gyrotrons as RF sources for electron cyclotron resonance heating and current drive (ECRH&CD). In the frame of the EUROfusion programme at KIT, the designs of conventional-cavity type and coaxial-cavity type DEMO-compatible gyrotrons are under investigation. In this presentation, the physical design of the interaction circuit of a 236 GHz conventional cavity gyrotron and its RF behavior are presented. The simulation results show a stable single mode RF output power without serious mode competition.

Magnetron injection gun for a 238 GHz 2 MW coaxial-cavity gyrotron

Karlsruhe Institute of Technology (KIT) has started the development of gyrotrons for the first demonstration fusion power plant DEMO. A coaxial-cavity 238 GHz 2 MW gyrotron design is under investigation. After having obtained an initial cavity design, one focus of current studies is the associated triode-type magnetron injection gun (MIG). Constraints, design approaches and an initial gun design are presented. An outlook on further investigations is given.

Study of the ITER Stray Magnetic Field Effect on the EU 170-GHz 2-MW Coaxial Cavity Gyrotron

In this paper, the effect of the ITER stray magnetic field (SMF) on the operation of the European gyrotron has been studied. The SMFs include the magnetic field generated by neighboring gyrotrons, the ITER tokamak, and also the field perturbations caused by the ferromagnetic structural materials used in the ECRH building.The 3-D self-consistent electrostatic code ARIADNE has been used for the evaluation of the SMF effect on the electron beam in the cavity and the collector region. It is shown that the beam parameters at the cavity of the gyrotron are not significantly affected by the SMF. On the other hand, the magnetic field lines in the collector region are significantly deformed in the presence of the SMF. A part of the electron beam is guided to the inappropriately cooled part of the collector surface, while the electron beam power is distributed in a significantly smaller area of the collector walls. In addition, the use of an advanced transverse sweeping system is proposed which would allow to compensate the SMF and to improve the collector operation.

Measurements of satellite modes in 140 GHz wendelstein 7-X gyrotrons: An approach to an electronic stability control

The stellarator Wendelstein 7-X (W7-X) has been designed to show that optimized stellarators can achieve and sustain fusion relevant plasma conditions. One of the main optimization criteria was the reduction of the neoclassical transport, which is considered as one of the most critical issues of the stellarator concept. While the demonstration of the neoclassical optimization will definitely be one of the most important aspects of W7-X, several additional topics are equally important due to the boundary condition that fusion relevant conditions should be demonstrated and sustained. Hence, low neoclassical transport needs to be achieved in a scenario which is compatible with divertor operation, high densities, and an acceptable impurity concentration without further accumulation during stable operation. So far, these issues have mostly not been tackled experimentally in the first experimental campaign. W7-X had its first plasma in December 2015 and the first operational campaign (OP1.1) was mainly intended for testing and commissioning purposes. In OP1.1, W7-X was operated in a limiter configuration without divertor and at low densities. Nevertheless, valuable insights could be gained which help to prepare and understand the first experiments with a test divertor planned to start in the second half of 2017. As will be shown in this contribution, one of the interesting observations in OP1.1 was the presence of an operational limit, where above a critical density the power balance seems to be dominated by radiative losses. Such a behavior is well-known from other stellarator experiments and while it can be expected that this critical density is particularly low for OP1.1 (high impurity concentration connected to the limiter operation and conservative wall conditioning), this observation also shows that impurity related radiation losses will be an important issue to keep track of as the density is progressively increased in the next experimental campaigns of W7-X.

Experimental results and outlook of the 2 MW 170 GHz coaxial-cavity gyrotron towards long pulse operation

The development of the modular short-pulse pre-prototype as base for the 2 MW, 170 GHz, CW coaxial-cavity gyrotron for international fusion project ITER and beyond is in progress at Karlsruhe Institute of Technology (KIT). The current modular pre-prototype configuration allows the generation of > 2 MW RF output power in short-pulses with a reasonable electronic efficiency of approximately 30 % without depressed collector operation. Recently, experiments with a new glidcop resonator has been performed. In depressed operation the overall efficiency of the gyrotron has been increased to ~48 %. In addition, an advanced water cooling system for a long pulse 2 MW gyrotron was developed. In this paper the experimental results, the suppression of the beam-halo and future plans for the 2 MW gyrotron will be presented.

A fast frequency step-tunable 236 GHz gyrotron design for DEMO

As part of the EUROfusion WP HCD EC project, the conceptual design of a 1 MW, 236 GHz hollow-cavity gyrotron is ongoing at IHM, KIT for a Demonstration Power Plant (DEMO), along with the 2 MW coaxial-cavity design concept. Fast frequency-tunable gyrotrons (tuning within a few seconds) are recommended for plasma stabilization using a non-movable antenna. In this work, the mode-selection approach for such a frequency-tunable gyrotron is presented and suitable operating modes for fast frequency tunability are suggested. Magnetic field tuning has been confirmed as an effective technique to tune the gyrotron operating frequency. The step-tunability of the 236 GHz gyrotron within the frequency range of ±10 GHz in steps of 2-3 GHz is demonstrated in numerical simulations.

Considerations on the selection of operating modes for future coaxial-cavity gyrotrons for DEMO

At KIT, a modular 170 GHz, 2 MW TE34, i9-mode coaxial-cavity gyrotron with advanced water cooling is ready for tests. The successful operation of this tube will be a first important step towards a possible future DEMO gyrotron. Nevertheless, looking forward, there are two questions to be answered: (i) what potential does the existing coaxial cavity offer with regards to MW-class multi-frequency operation also at higher frequencies, and (ii) what could be a different mode selection to achieve an even higher output power in a more compact gyrotron design. To provide an answer to (i), based on the 170 GHz, 2 MW pre-prototype the multi-frequency operation at multiples of the resonance frequency of the diamond disc RF output window was carried out. Additionally, a slightly modified cavity design was introduced. To answer the question (ii), the TE25,22-mode was chosen and compared with the results got for the TE34, i9-mode. The extreme volume TE25,22-mode allows to reduce the beam radius by around 25 % and to increase the RF output power of the gyrotron by up to 30 %.