A. Schlaich

2.2-MW Record Power of the 170-GHz European Preprototype Coaxial-Cavity Gyrotron for ITER

A 2-MW continuous-wave (CW) 170-GHz coaxial-cavity gyrotron for electron cyclotron heating and current drive in the International Thermonuclear Experimental Reactor (ITER) is under development within the European Gyrotron Consortium (EGYC1), a cooperation between European research institutions. To support the development of the industrial prototype of a CW gyrotron, a short-pulse tube (preprototype) is used at KIT Karlsruhe (former FZK) for experimental verification of the design of critical components, like the electron gun, beam tunnel, cavity, and quasi-optical RF output coupler. Significant progress has been achieved recently. In particular, RF output power of up to 2.2 MW with 30% output efficiency has been obtained in single-mode operation at 170 GHz. Furthermore, a new RF output system has been designed, with an efficient conversion of the generated RF power into a Gaussian RF output beam. The results have been successful, yielding a Gaussian mode content ~96%.

Experimental Investigations and Analysis of Parasitic RF Oscillations in High-Power Gyrotrons

Megawatt gyrotrons are found to suffer from various parasitic oscillations, in particular, RF oscillations in the beam tunnel prior to the desired interaction zone (the cavity). This paper describes the experimental results from a gyrotron experiment which was dedicated to investigate parasitic oscillations in the beam tunnel and to verify improved beam-tunnel structures. A system for improved spectral measurements and a new analysis method are presented. The results verify theoretical predictions on the parasitic oscillations, and in effect validate the corresponding improved beam-tunnel structure. In addition, other types of parasitic oscillations were observed and explained.

First Operation of a Step-Frequency Tunable 1-MW Gyrotron With a Diamond Brewster Angle Output Window

Experimental results using a step-frequency tunable D-band gyrotron are reported. The short pulse (~3 ms) gyrotron is equipped with an elliptically brazed chemical vapor deposition (CVD) diamond Brewster angle output window. It is designed for the operation in the frequency range from 111.6 up to 165.7 GHz. Operating parameters for ten different frequencies corresponding to an equal number of different cavity operating modes has been measured. A minimum output power of 830 kW and a peak output power of 1.3 MW have been realized. For all frequencies, the parameters of the RF beam generated by the internal quasioptical converter, such as fundamental Gaussian contents and beam waist, are sufficiently good to allow an efficient coupling of the RF power out of the window. This is the first time a diamond Brewster angle window has been used in a high power gyrotron (~1 MW). Such a system offers the path to a simple and compact window solution for high power broadband applications using gyrotrons.

Transient Millimeter-Wave Signal Analysis With Unambiguous RF Spectrum Reconstruction

In this paper, a measurement system for the detection of time-dependent effects in broadband spectra of high-power millimeter-wave sources is demonstrated. The heterodyne approach with sub-harmonic mixers enables high dynamic range and configuration flexibility, but typically also imposes severe problems on the analysis of transient or instationary phenomena through frequency ambiguity. A key feature of the presented system is an unambiguous reconstruction of the RF spectrum from the detected IF spectra. This is done by evaluating the upper and lower mixer sidebands of two parallel heterodyne receiver channels simultaneously, resulting in a comparatively high instantaneous measurement bandwidth of 7 GHz per channel pair and a high dynamic range of 50-60 dB in the frequency range 110-170 GHz. Sample measurements obtained during the occurrence of an arc at the dielectric output window of a 140-GHz megawatt-class gyrotron demonstrate the unique capabilities of the system for detecting highly transient and broadband effects in the tubes output spectrum.

Simulation and experimental investigations on dynamic after cavity interaction (ACI)

In recent years, the so called after cavity interaction (ACI) in high power gyrotrons operating in the 100–200 GHz range gained attention as an influence factor on overall efficiency. While investigations concentrated on ACI as a stationary effect until now, recent simulations show that an undesired interaction in the uptaper region can also result in additional parasitic oscillations. In this paper, such non-stationary, dynamic processes are investigated in first simulations and experiments.

Investigations on parasitic oscillations in megawatt gyrotrons

Recently megawatt gyrotrons are found to suffer from parasitic oscillations induced by the electron beam prior to the desired interaction zone (the cavity). This paper describes experimental and theoretical investigations on both high and low frequency parasitic oscillations. A new measurement method is introduced to determine and characterize these undesired oscillations, and a dedicated experimental study is presented.

140 GHz, 1 MW, CW gyrotron development for the ECRH system of the stellarator Wendelstein 7-X

A 10 MW, 140 GHz ECRH system with a pulse duration of 30 minutes is currently under construction for the stellarator W7-X at Greifswald. The RF power will be provided by 10 gyrotrons. A European collaboration has been established to develop and build 9 (out of 10) tubes each with an output power of 1 MW for continuous wave (CW) operation. This contribution reports on recent results with the series gyrotrons.

Observation of Discrete Frequency Hopping in MW-Class Gyrotrons During Long-Pulse Operation

A detailed investigation of the RF oscillation frequency produced by megawatt-class continuous wave 140 GHz gyrotrons for the stellarator W7-X found discrete jumps of a few megahertz during the initial frequency downtuning phase, as well as in quasi-steady long-pulse operation. The common explanations for such behavior, such as voltage ripple and the classical long-line effect, are reviewed, and are found not to match the experimental observations properly. As alternative hypotheses, we suggest a long-line effect involving higher-order reflections inside the gyrotron, or discrete changes in the cavity radius, caused by discontinuous thermomechanical expansion.

Separation of thermal expansion and beam charge neutralization effects in high power 140 GHz CW gyrotrons

Summary form only given. During the first few seconds of longer pulses in high-power gyrotrons, the main mode frequency decreases over a few hundred MHz, until a stable operation is reached. This is due to the combination of two effects, namely the thermo-mechanical expansion of the cavity and a shift in the effective electron energy through ionization of residual gas in the tube. Most investigations focus on the quasi-stationary “short-pulse” range of the first few milliseconds, and the long-pulse stationary range after both tuning effects have settled. As the fusion gyrotron development is clearly headed towards higher output power and higher operating frequency, the corridors for operation parameters narrows with the desired cavity mode [1]. Furthermore, modern fusion reactors tend to demand gyrotron power modulation, which will cause operation in the mentioned non-stationary transition regime. In this work, the characterization and time constant separation of the named effects is attempted for the 1 MW, 140 GHz W7-X gyrotron. From the experimental side, this is done with the help of a transient spectrum measurement system [2], which with some modifications is capable of recording continuously in the transient pulse range. The measured time-dependent frequencies are used for the modeling of the two effects, which is supported through parameter studies using the self-consistent gyrotron simulation code EURIDICE. In addition, previous theoretical descriptions of the ionization processes in the device [3] were modified for the application to modern high power gyrotrons, and brought into context with the experimental and modeling data.

Operation of a step-frequency tunable gyrotron with a diamond Brewster angle output window

This paper reports on a diamond Brewster angle window which has been used as a vacuum barrier in a 140 GHz step-frequency tunable high power gyrotron. This configuration ensures operation of the gyrotron at different frequencies without reflection of the RF power and provides excellent material properties of CVD diamond in the mm wave range.