Analysis of an actively-cooled coaxial cavity in a 170 GHz 2 MW gyrotron using the multi-physics computational tool MUCCA

Continuous Wave gyrotrons are the key elements for Electron Cyclotron Resonance Heating and Current Drive(ECRH&CD) in present fusion experiments and future fusion reactors. In the frame of the EUROfusion activities,a 170 GHz, 2 MW short-pulse (∼1 ms), water-cooled coaxial gyrotron, already tested at Karlsruhe Institute ofTechnology (KIT), is being upgraded for operation at longer pulses (∼100–1000 ms).Here we use the MUlti-physiCs tool for the integrated simulation of the CAvity (MUCCA), recently developedin collaboration between Politecnico di Torino and KIT, to analyze the evolution of the operating condition ofthe coaxial gyrotron cavity, self-consistently coupling thermal-hydraulic, thermo-mechanical and electro-dy-namic models. The main results are presented in terms of evolution of temperature, heat load and deformation ofthe heated surface of the resonator and of the coaxial insert during thefirst few seconds of operation. We showthat the system evolves towards stable operating conditions (no beam loss), with a peak temperature stronglydependent on the cooling configuration, where a large room for the improvement of the current cavity coolingdesign is found.1. IntroductionIn the EU-DEMO perspective [1], high power coaxial-cavity gyro-trons for heating the plasma and for driving a non-inductive currentinto it are under development at the Karlsruhe Institute of Technology(KIT) within the framework of EUROfusion activities [2]. The aim is toextend the∼1 ms pulse length of the existing 170 GHz, 2 MW coaxial-cavity tube up to 1 s [3,4]. Fig. 1shows the cross section of the bottompart of the 170 GHz, 2 MW gyrotron which is characterized by thepresence, inside the∼150 mm long cavity, of the coaxial insert (∼1200 mm long, in total). The high-frequency wave is generated insidethe cavity by resonant interaction between the electron beam, producedby a Magnetron Injection Gun (MIG) at the lower part of the gyrotron,and the high-frequency electromagneticfield in the cavity [5].A very high ohmic heat-load results on the cavity and insert sur-faces; therefore, both are subject to a deformation that modifies theresonating volume and changes the working condition of the gyrotron.A forcedflow of pressurized subcooled water, passing around the re-sonator as well as through the insert by means of two independentcooling circuits, is used to maintain the cavity region as cold as possibleto avoid damages (Tmax< 250 °C) and to reduce the thermal de-formation of the surfaces.2. Description of the coaxial cavity geometryThe cross section of the cavity and coaxial insert are shown inFig. 2a, where the internal structure of the two components is alsodisplayed.The cavity is made of two different regions: the resonator and theexternal structure. The coolantflows in the annular region between theresonator and the external structure (seeFig. 2a, blue arrows) and exitsfrom the outlet pipe on the top–inlet and outlet pipe are located onopposite parts of the cavity, inducing a non-fully uniform massflowaround the cavity. The part of the coaxial insert, which is simulated inthis work is shown in purple inFig.2a. The main coolant pathway in thetop region of the coaxial insert is described by blue arrows inFig. 2b.The waterflows toward the upper region of the insert in the innerchannel, then moves to the external region by means of four rectangular.