European research activities towards a future DEMO gyrotron

J. Jelonnek, G. Aiello, S. Alberti, K. Avramidis, A. Bertinetti, A. Bruschi, J. Chelis, T. Franke, G. Gantenbein, S. Garavaglia, G. Granucci, G. Grossetti, S. Illy, Z.C. Ioannidis, J. Jin, P. Kalaria, G.P. Latsas, H. Laqua, A. Leggieri, F. Legrand, A. Marek, I.Gr. Pagonakis, D. Peponis, L. Savoldi, T. Rzesnicki, S. Ruess, T. Ruess, T. Scherer, M. Schmid, D. Strauss, I. Tigelis, M. Thumm, M.Q. Tran, F. Wilde, C. Wu, R. Zanino, A. Zein IHM, IHE, IAM-AWP, Karlsruhe Institute of Technology (KIT), D-76131 Karlsruhe, Germany Swiss Plasma Center (SPC), EPFL, CH-1015 Lausanne, Switzerland Institute of Plasma Physics “P.Caldirola”, National Research Council of Italy, Milan, Italy NCSRD/National and Kapodistrian University of Athens, Faculty of Physics, Athens, Greece EUROfusion Consortium, Boltzmannstr. 2, D-85748 Garching, Germany Max-Planck-Institut für Plasmaphysik, Boltzmannstr. 2, D-85748 Garching, Germany Max Planck Institut für Plasmaphysik, Teilinstitut Greifswald, Wendelsteinstrasse 1, D-17491 Greifswald, Germany Thales Electron Devices, 2 rue Marcel Dassault, Vélizy-Villacoublay, F-78141, France 11 NEMO group, Dipartimento Energia, Politecnico di Torino, 24, c. Duca degli Abruzzi, I-10129 Torino, Italy


Coordinated by the EUROfusion Consortium, several
European research institutes are working on fusion technologies towards options for a European DEMOnstration Fusion Power Plant (FPP), as a single step between ITER and a commercial FPP, to deliver net electricity by mid of this century.One of the focus areas is the research on a proper Electron Cyclotron Resonance Heating (ECRH) and Current Drive (ECCD) system for which the fusion gyrotron is one of its major key components [1].
A future FPP will probably require an ECCD operating frequency ranging from 170 GHz up to 240 GHz depending on the DEMO baseline.An RF output power of significantly higher than 1 MW (target: 2 MW) and a total gyrotron efficiency better than 60% are required.Multi-purpose operation at multiples of the λ/2-resonance frequency of the vacuum window of the gyrotron, hence in leaps of about 30 GHz (e. g. 170 / 204 / 238 GHz) needs to be considered for plasma start-up, heating and current drive.Optionally for possible steering of the absorption layer the gyrotron shall allow a fast frequency tuning in steps of around 2-3 GHz.The R&D work within the EUROfusion work package "WP HCD EC Gyrotron R&D and Advanced Developments (AD)" is focusing on all of the named targets.

Verification of the coaxial-cavity technology
The coaxial-cavity gyrotron is a promising technology for future multi-MW fusion gyrotrons [2].In [3] a world record RF output power of 2.2 MW at short pulses (ms-range) was demonstrated.Nevertheless, the 2 MW coaxial-cavity technology, already considered for the first installation in ITER earlier [4], is still lacking its proofof-concept regarding long-pulse operation.Major concerns are the proper alignment and thermal loading of the cavity wall and its inner conductor as well as the thermal loading of the collector.Its feasibility shall be finally demonstrated by upgrading the existing KIT 2 MW 170 GHz short-pulse pre-prototype to pulse lengths up to 1 s [5].In parallel, work is ongoing in the field of advanced cooling concepts [6,7].Additionally, two new coaxial-cavity Magnetron Injection Guns (MIGs) are under manufacturing.The first is employing an advanced emitter technology whose major element is a new non-emissive coating.That will significantly reduce the velocity spread of the electrons at the emitter [8].Secondly, a newly designed Inverse Magnetron Injection Gun (IMIG) will allow for a significant larger emitter radius and therefore increased output power at operating frequencies significantly above 200 GHz by keeping the same or even smaller size of the bore hole of the gyrotron SC magnet [9].

Studies towards a 240 GHz gyrotron
A frequency up to 240 GHz was selected for the theoretical research work towards a future FPP, considering the requirements for "multi-purpose" and "fast frequency step-tunable" operation at high-field tokamaks and for a wide range of RF beam steering.The coaxialcavity gyrotron technology, and, as a possible fallback solution, the conventional hollow-cavity gyrotron are under investigation.Both technologies were studied regarding to the maximum achievable output power versus efficiency and stability in operation due to tolerances.A generic design strategy was developed to find the optimum operating mode for the two different cavity topologies [10].Operating scenarios close to 2 MW for the coaxial-cavity technology and around 1.5 MW for the conventional cavity technology have been found in the theoretical analyses (ref.[11][12][13][14]).

Advances in Window Technologies
Fundamental for frequency-step tunable operation is the utilization of a proper broadband window technology.At KIT the CVD-Diamond Brewster-angle window is the favorite.The successful operation of a step-frequency tunable 1 MW short-pulse (ms) gyrotron with a synthetically manufactured diamond Brewster angle output window was demonstrated earlier [15].Nevertheless, for future DEMO gyrotrons the development of diamond discs of larger size for higher power capabilities, advanced cooling and brazing technologies are mandatorily required and pushed forward at KIT [16,17].

Towards a total efficiency of higher than 60%
A DEMO gyrotron will require a total efficiency of above 60% to minimize the electrical power requirements, and, ultimately the recirculating power in the bal-ance of plant.Considering an interaction efficiency of typical 35% between the electron beam and the microwave, a large fraction of the input energy remains in the spent electron beam.Up to 60% of the spent beam energy might be recovered by a single-stage depressed collector (SDC) which will lead to an overall gyrotron efficiency of about 50% in theoretically best case.An overall efficiency of higher than 60% requires the use of advanced multi-stage depressed collectors (MDCs).Two concepts are under investigation [18] at KIT.In the first one, the gyrotron magnetic field is unwound utilizing wellcontrolled non-adiabatic transitions, whereas in the second one, the electrons are sorted by an E × B drift [19][20][21][22].

Intelligent control systems for gyrotrons
In future, intelligent control systems shall allow higher output powers of gyrotrons by reducing the necessary safety margins, hence allowing for an operation close to the stability limits [23].Firstly applied to the W7-X gyrotrons, a new control concept is under investigation.The idea is to detect the stray radiation caused by the excitation of parasitic modes at the stability limits and to use that as an indicator for the operational stability of the W7-X gyrotrons.A feedback control system is under development.

Advanced gyrotron tests environments
In 2015, the final design review and procurement of the High-Voltage Power Supply (HV PS) for the new KIT FULGOR gyrotron teststand started [24].FULGOR will allow CW operation of gyrotrons with a required input power of 5 (2 nd phase: 10) MW DC.A 10.5 T SC magnet will allow the operation of gyrotrons up to 240 GHz.That includes the upgrade of all the measurements systems, which have been developed for verification of the frequency spectrum, calorimetry and quasioptical transmission [25,26].