F. Noke

Diplexers for Power Combination and Switching in High Power ECRH Systems

Electron Cyclotron Resonance Heating (ECRH) systems for next step large fusion-devices operate at a Continuous Wave (CW) power well beyond 10 MW generated by a large number of gyrotrons with typically 1 MW power per unit. The combination of the power of two (or more) gyrotrons and switching of the power between different launchers for different physics applications is an attractive feature for such systems. The combination of beams from different gyrotrons would reduce the number of transmission lines and the requirements on port space. Fast switching between two antennas synchronously with the Magneto-Hydro Dynamic (MHD) modes frequency would increase the efficiency of mode stabilization. Both combination and switching as well as power sharing between different ports can be performed with high-power four-port diplexers using small frequency differences or small frequency-shift keying of the gyrotrons, respectively. Fast directional switches (FADIS) and beam combiners (BC) can be designed on the basis of different physical mechanisms: some selected design variants were investigated and the results are presented. Considerations on the integration of FADIS/BCs into large ECRH systems and their use in test arrangements are presented.

High-power performance of a resonant diplexer for advanced ECRH

Abstract Electron cyclotron resonance heating (ECRH) systems for next-step large fusion devices operate in continuous wave power in the multimegawatt range. The unique feature of narrow and well-localized power deposition assigns a key role to ECRH for different tasks, such as plasma start-up, electron heating, current drive, magnetohydrodynamic (MHD) control and profile shaping. The integration of high-power microwave diplexers in the transmission lines will improve the flexibility and efficiency while simultaneously reducing the complexity of large ECRH systems. They can serve as power or beam combiners, as slow and fast directional switches to toggle the power from continuously operating gyrotrons between two launchers, and as discriminators of low-power electron cyclotron emission (ECE) signals from high-power ECRH using a common transmission line and antenna. Among various design options a resonant diplexer with a narrow resonance was selected for application at ASDEX Upgrade. The design is driven by the specific physics requirements for MHD control experiments and possible use for line-of-sight ECE. The compact, waveguide-compatible design features a feedback-controlled mirror drive for tracking of the resonator to the gyrotron frequency. High-power, long-pulse tests were performed with the 140-GHz ECRH system for the stellarator W7-X. Results on the transmission characteristics, power combination, and stationary and controlled distribution of the input power to two outputs are presented. The qualification for in-line ECE was investigated.

High-power tests of a remote steering launcher mock-up at 140 GHz

This paper reports the results of the high-power test of a remote steering launcher mock-up at 140 GHz, which were performed at the ECRH installation for the future stellarator W7-X at IPP Greifswald. The mock-up test system consists of a 6.62 m long corrugated square waveguide with a steerable optic at the entrance and various diagnostics at the exit of the waveguide. A straight and a dog-leg version of the launcher were investigated. The high-power tests of the straight setup have been performed with powers up to P0 = 700 kW (typically 500 kW) and pulse lengths of up to 10 seconds. For both polarizations (parallel and perpendicular to the steering plane), no arcing was observed in spite of the fact, that the experiments were performed under ambient atmospheric conditions. After the integration of 2 mitre bends in the setup, arcing limited the usable parameter range. The ohmic loss PΩ of the waveguide was measured via the temperature increase of the waveguide wall, and was used to calibrate the calculated angular dependence of the total ohmic losses of the waveguide. Short-pulse radiation pattern measurements with thermographic recording show high beam quality and confirm the steering range of −12° < < 12°.

Enhanced transversal collector sweeping for high power CW gyrotrons

Amplitude modulation of the 50 Hz AC sweeping current of transversal sweeping systems reduces the maximum averaged power density on the collector wall as compared to an unmodulated system. This makes transversal sweeping a very attractive alternative to conventional longitudinal sweeping systems for high power CW Gyrotrons.

Transverse field collector sweeping for the W7-X gyrotrons — Modulation techniques

High power cw gyrotrons operate with an RF-power of up to 1 MW and efficiencies of 35–60 %. The waste power remains in the spent electron-beam and must be dissipated in the gyrotron-collector. Such collectors have strong sophisticated water-cooling systems for continuous operation. The electron-beam strike area on the collector surface is moved between an upper and a lower turning point by sweeping magnet fields in order to achieve a smooth power deposition.

The electron cyclotron heating system for the stellarator W7-X: Status and recent achievements

The status and recent achievements of the electron cyclotron heating system at the stellarator W7-X in Greifswald, Germany are discussed. Three prototype gyrotrons and one from a series of 7 are operable now, one additional is under test. The transmission system is ready up to the stellarator hall; part of it has passed high-power tests (900 kW, 30 min). The test of the multi-beam waveguide is in preparation. At present, optics in front of the stellarator, the antennas, as well as in-vessel components are constructed.

Extended operation of the 1 MW, CW gyrotrons for W7-X

Electron Cyclotron Resonance Heating (ECRH) is the main heating method for the Wendelstein 7-X Stellarator (W7-X), which is under construction at IPP- Greifswald. A 10 MW ECRH plant with CW-capability at 140 GHz is under construction to meet the scientific objectives. The microwave power is generated by 10 gyrotrons with 1 MW each. Three gyrotrons are already operational at IPP in Greifswald. The W7-X gyrotrons are designed for single frequency operation at 140 GHz. The operation regime of the W7-X stellarator experiments is restricted to magnetic fields matched to the ECRH-frequency and it would be interesting to extend the operation regime to a second resonant magnetic field. Therefore gyrotron operation at a second frequency was investigated. The standard vertical collector sweep systems of gyrotrons inherently display a pronounced peaking of the collector heat loading. An improved collector sweep system was successfully tested on the TED gyrotrons, which creates an almost homogenous heat loading. This system is of particular interest for next generation gyrotrons with an output power up to 2 MW.

High power gyrotron development at KIT for ECH&CD of fusion plasmas

ECH & CD is widely used in fusion plasmas. Gyrotrons are applied as power sources as they offer continuous wave operation at MW power level. This contribution reports on the status and plans of major projects of KIT towards high power gyrotrons, i.e. results from series production of the W7-X gyrotron, the development of the EU pre-prototype coaxial-cavity gyrotron, the 2 MW, 170 GHz CW coaxial cavity gyrotron for ITER and a frequency step-tunable gyrotron.

Collector loading during high frequency power modulation

The high frequency power modulation of cw gyrotrons with an rf-output power up to 1 MW and efficiencies of 35–55 % is limited by collector loading. Two alternative power modulation schemes are commonly applied which show a significant difference in the resulting collector loading. In any case waste power remains in the spent electron-beam and must be dissipated in the gyrotron-collector. Such collectors have strong sophisticated water-cooling systems and the electron-beam strike area is moved on the collector surface by sweeping the magnetic field. The technically less demanding power modulation method unfortunately generates the highest level of collector loading. Different sweep technologies have been developed in order to avoid overheating.

High-Power Experiments with 140 GHz Series Gyrotrons for W7-X

Electron-cyclotron-resonance heating (ECRH) is an important heating system for the stellarator W7-X. For this system a total power of 10 MW at 140 GHz is needed in 30 min operation. In a collaboration with European research laboratories and European tube industry (Thales Electron Devices (TED) in France), cylindrical cavity gyrotrons designed for an output power of 1 MW, CW at 140 GHz have been developed. They are equipped with a single-stage depressed collector for increasing the efficiency and reducing the power loading at the collector wall, an advanced quasi-optical (q.o.) mode converter system with minimized stray radiation, and a single-disk diamond window made by chemical vapor deposition (CVD diamond). Details of the design can be found in Ref. (Dammertz et al, 2002). The first series gyrotron (SN1) has been tested successfully in long pulse operation with full power (30 min, 920 kW, efficiency close to 45 %) and meets the specifications (Dammertz et al., 2006). In this contribution we discuss experimental results of the second and third series gyrotron (SN2, SN3).