Following the pre-validation of the design of the short-pulse gyrotron for ITER which is being developed by Europe, work is moving ahead. The European Continuous-Wave (CW) gyrotron prototype has successfully passed the final Factory Acceptance Tests – an important sign of progress.
The gyrotron is part of ITER’s Electron Cyclotron Heating (ECH) system, one of the three systems that will heat the plasma in the ITER machine to the sweltering temperature of 150 million degrees C necessary for the fusion reaction to occur. The ECH system provides the heating by transferring the energy from electromagnetic waves into the plasma electrons. The other two heating systems are the Neutral Beam and the Ion Cyclotron Heating. The ECH system will be used to start-up every plasma and, in addition, to sustain a longer plasma duration by driving additional current, and improve the plasma confinement thanks to its unique capability to heat very localised parts of the plasma.
These radiofrequency (RF) microwaves are generated by gyrotrons located some 100 metres away from the tokamak and will be guided to the launchers attached to port plugs of the vacuum vessel for injection into the plasma. Responsible for providing six of the ITER gyrotrons (the remaining 18 gyrotrons will be delivered by the Russian, Japanese and Indian ITER Domestic Agencies), F4E is working on the development of the final gyrotrons for ITER which will exude radio frequency microwaves of 170 GHz in collaboration with the European Gyrotron Consortium (EGYC) (which consists of several European Fusion Laboratories (EFLs), namely KIT – Germany, SPC (formerly CRPP) – Switzerland, HELLAS – Greece, CNR – Italy, and USTUTT- Germany, and ISSP – Latvia as third parties) and an industrial partner, Thales Electron Devices (TED), a French company.
The Factory Acceptance Tests for the long-pulse CW gyrotron prototype, which will produce radiofrequency microwaves of 1MW of output power for a duration of several minutes, took place at the TED facility near Paris and comprised the checking of ultra-high vacuum level in order to guarantee long-pulse stable operation; the cooling circuits which are important to dissipate the high heat fluxes of some internal components (up to state-of-the-art maximum values of ~20MW/m2 ); and the high voltage withstand-off of the different gyrotron parts which are needed to accelerate the electrons and have a good efficiency.
“It’s the first time in Europe that a gyrotron is fabricated using this design. The work which has been and which is currently being done in relation to this component is a wonderful example of strong and productive collaboration during many years between European Fusion Laboratories, European industry and F4E, in close collaboration with ITER IO Central Team”, enthuses Ferran Albajar, F4E Technical Officer dealing with the development of Europe’s gyrotron contribution to ITER in the EC and NB Power Supplies and Source Team. This gyrotron incorporates novel concepts developed by the EFLs to improve further the quality of the electron beam for an efficient RF generation and of the output RF beam, which is important for an efficient transmission and launching of the RF waves into the plasma, while making an optimal use of the technical heritage in Europe of the successful series production of gyrotron tubes for the W7-X Stellarator.
With the Factory Acceptance Tests now completed, the next step entails the gyrotron being shipped to KIT for the start of the RF tests in order to check that it fully complies with ITER requirements. The KIT facility in Germany is one of the only two facilities in Europe that has the unique infrastructure where this type of testing can be done. The testing in KIT will start in January and is expected to last six months.
F4E is progressing towards a critical phase of the validation of the European gyrotron for ITER. The design needs to give solutions that meet the high ITER standards. Design and technology of high power and long-pulse gyrotrons is still rare. Requirements on the manufacturing techniques, tolerances, and materials used, such as the synthetic diamond for the output window, are outside the current industrial practices. The experience that both European industry and European Fusion Laboratories gain in working within this particular project will advantageously serve them in developing spin-off applications for other projects.