Konstantinos A. Avramidis

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.

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.

Efficient Frequency Step-Tunable Megawatt-Class

Results of latest experimental studies on the frequency step-tunable (D-band) megawatt class gyrotron which is under development at IHM (KIT) are presented. The goal of the short pulse (~ 1 ms) experiments was to study the performance of an upgraded cavity with longer cylindrical section. Target was to achieve significantly better efficiencies by introducing a cavity with a higher quality factor. The new design of the cavity was numerically optimized using the EURIDICE code.

D

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.

-Band Gyrotron

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.

A gun design criterion to limit the effects of the electron beam halo in gyrotrons

Along with high power and high efficiency operation around 240 GHz, the requirements of DEMO gyrotrons include the possibility of multi-frequency operation and fast frequency tunability in 2-3 GHz steps. In this paper, multi-frequency operation of a proposed design for a conventional cavity DEMO gyrotron is presented, including operating parameters at different frequencies. The performance of the gyrotron is simulated considering realistic electron beam parameters.

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

Coaxial-cavity gyrotrons for electron cyclotron heating in plasma experiments for nuclear fusion can operate with very high-order modes, having reduced mode competition and decreased voltage depression compared with hollow-cavity tubes. However, since exact alignment of coaxial insert and cavity wall can only be ensured up to a certain precision, the effects of misalignment must be properly understood. In this paper, an efficient method is presented to determine the voltage depression on beam electrons for arbitrary misalignment between cavity wall and insert, and for a beam with arbitrary shape and density distribution. The method has been verified using a 3-D code, and it can be generalized to some other geometries.

Multi-frequency operation of DEMO gyrotron with realistic electron beam parameters

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.