W7-X Gyrotron Research Articles

Fabrication and assembly of the gyrotron multi-stage depressed collector prototype at KIT

In this paper details of the fabrication and assembly of the first Multi-stage Depressed Collector (MDC) prototype developed at KIT for megawatt-class gyrotrons are presented. Utilizing the E×B drift concept for electron trajectory separation, the cylindrical Short-Pulse (SP) MDC prototype features a design compatible with applications in different fusion gyrotrons at KIT, for W7-X, ITER, and DEMO. The fabrication process includes inner electrodes out of copper-chromium-zirconium (CuCr1Zr) with a triple helix isolation design, modular vacuum housing consisting of four parts, and additional external coils for electron beam confinement. Detailed assembly procedures are provided for two configurations: the 170 GHz 2 MW coaxial cavity gyrotron and the W7-X upgrade SP gyrotron. The successful manufacturing of the modular collector and a robust design for vacuum tightness are demonstrated. The prototype is now primed for verification in the KIT FULGOR test stand with the W7-X gyrotron for validation of the E×B drift concept and improvement of the gyrotron efficiency. This comprehensive work bridges theoretical concepts with practical implementation, offering insights crucial for refining and advancing MDCs for megawattclass gyrotrons for fusion applications.

Frequency-Based Investigation of Charge Neutralization Processes and Thermal Cavity Expansion in Gyrotrons

During the first hundred milliseconds, the frequency and RF output power of long pulse operating gyrotrons undergo deterministic variation. This well-known behavior is caused by the thermal expansion of the cavity and internal electrostatic processes related to the ionization of residual gas. A macroscopic analytical investigation of the gas conditions in modern gyrotrons indicates that ionization processes are unlikely to influence the overall internal gas pressure. In combination with electrostatic potential considerations, it was found that the beam depression voltage is not fully neutralized; in the case of W7-X gyrotrons, a maximum value of about 60 % neutralization is expected, in conflict with the common assumption of full neutralization in steady state. Using experimentally measured frequency shifts and the Evridiki gyrotron interaction simulation code, a fitting process was employed to further investigate these effects. The results are in very good agreement with the theoretical predictions and allow a separation of the time constants of the two processes causing the frequency tuning.

Gyrotron development at KIT: FULGOR test facility and gyrotron concepts for DEMO

At the Karlsruhe Institute of Technology (KIT), theoretical and experimental foundations for the development of future gyrotrons for fusion applications are being laid down. This includes the construction of the new Fusion Long Pulse Gyrotron Laboratory (FULGOR) test facility as well as physical design studies towards DEMO-compatible gyrotrons. Initially FULGOR will comprise of a 10MW CW power supply, a 5MW water cooling system (upgradeable to 10MW), a superconducting 10T magnet, one or two 2MW ECRH test loads and a new control and data acquisition system for all these elements. The test facility will then be equipped to test the conventional 1MW or coaxial 2MW gyrotrons for DEMO, currently under design, as well as possible upgraded gyrotrons for W7-X and ITER. The design of the new high voltage DC power supply (HVDCPS) is flexible enough to handle gyrotrons with 4MW CW output power (conceivably up to 170GHz), but also test gyrotrons with higher frequencies (>250GHz) which, due to physical limitations in the gyrotron design, will require less power but have more stringent demands on voltage stability.

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.

Recent achievements on tests of series gyrotrons for W7-X and planned extension at the KIT gyrotron test facility

Parasitic beam tunnel oscillations have been hampering the series production of gyrotrons for W7-X. This problem has now been overcome thanks to the introduction of a specially corrugated beam tunnel. Two gyrotrons equipped with the new beam tunnel have fully passed the acceptance tests. Despite excellent power capability, the expected efficiency has not yet been achieved, possibly due to the presence of parasitic oscillations suspected to be dynamic after-cavity-oscillations (ACI's) or due to insufficient electron beam quality. Both theoretical and experimental investigations on these topics are ongoing. On previous W7-X gyrotrons collector fatigue has been observed, not (yet) leading to any failures so far. The plastic deformation occurring on the collector has now been eliminated due to the strict use (on all gyrotrons) of a sweeping method which combines the conventional 7Hz solenoid sweeping technique with a 50Hz transverse-field sweep system.Starting in 2013, the gyrotron test facility at KIT will be enhanced, chiefly with a new 10MW DC modulator, capable of testing gyrotrons up to 4 MW CW output power with multi-stage-depressed collectors.

Technical developments at the KIT gyrotron test facility

Parasitic beam tunnel oscillations have been discovered on some of the series production gyrotrons for W7-X and also on the coaxial pre-prototype gyrotron for ITER. Solutions to remedy these problems have resulted in a modified beam tunnel design, technologically close to the existing beam tunnel. The new design has successfully been tested on both the coaxial and also the f-step-tunable gyrotrons and has subsequently been implemented on one of the W7-X series-production-tubes presently undergoing factory acceptance tests in Karlsruhe. The ECRH test loads at KIT are operated under normal atmospheric conditions. Several loads have eventually failed in 1 MW long pulse experiments and KIT has therefore started to design its own loads. The first KIT-load is based on a fixed conical mirror and an aluminum cylinder coated with a lossy material for increased absorption. The new load has so far successfully been used during the acceptance tests of two 1-MW CW gyrotrons. Nevertheless a new load based on pure (uncoated) stainless steel absorbers is being developed as a backup solution for the ongoing high priority gyrotron testing. A superconducting magnet capable of rapid field changes between 4.15 and 5.67 T for frequency step-tunable gyrotrons has been procured, has demonstrated a (static) field of 7.2 T and its capability of rapid field-changes.

04/00212 Next step spherical torus design studies: Neumeyer, C. et al. Fusion Engineering and Design, 2003, 66–68, 139–145

Parasitic beam tunnel oscillations have been discovered on some of the series production gyrotrons for W7-X and also on the coaxial pre-prototype gyrotron for ITER. Solutions to remedy these problems have resulted in a modified beam tunnel design, technologically close to the existing beam tunnel. The new design has successfully been tested on both the coaxial and also the f-step-tunable gyrotrons and has subsequently been implemented on one of the W7-X series-production-tubes presently undergoing factory acceptance tests in Karlsruhe.The ECRH test loads at KIT are operated under normal atmospheric conditions. Several loads have eventually failed in 1 MW long pulse experiments and KIT has therefore started to design its own loads. The first KIT-load is based on a fixed conical mirror and an aluminum cylinder coated with a lossy material for increased absorption. The new load has so far successfully been used during the acceptance tests of two 1-MW CW gyrotrons. Nevertheless a new load based on pure (uncoated) stainless steel absorbers is being developed as a backup solution for the ongoing high priority gyrotron testing.A superconducting magnet capable of rapid field changes between 4.15 and 5.67 T for frequency step-tunable gyrotrons has been procured, has demonstrated a (static) field of 7.2 T and its capability of rapid field-changes.