European research activities towards a future DEMO gyrotron

Abstract

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 technologyThe 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 nonemissive 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 gyrotronA 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. ...

Systems engineering perspective to the integration of the heating and current drive system in the EU DEMO: Analysis of requirements and functions

“…These are: Electron Cyclotron (EC) [5], Ion Cyclotron (IC) [6] and Neutral Beam Injection (NBI) [7] systems. In addition, within the scope of WPHCD, technical development in the fields of EC radiofrequency (Gyrotrons) [8] and NBI sources that could be used both in a pulsed tokamak (DEMO1) and in a steady state tokamak (DEMO2) are also assessed. The HCD system must be fully integrated in the machine design and a number of outstanding technologies and physics issues need to be solved consistently to comply with the system requirements.…”