W. Leonhardt

A 2 MW, 170 GHz coaxial cavity gyrotron

The feasibility of manufacturing a 2-MW CW coaxial cavity gyrotron at 170 GHz has been demonstrated and data required for fabrication of an industrial tube have been obtained. An engineering design of a prototype started recently with the goal to provide gyrotrons with 2-MW microwave output power for International Thermonuclear Experimental Reactor (ITER). The design of critical components of the prototype tube as electron gun, cavity and RF output system will be verified under realistic conditions at short pulses using the experimental coaxial gyrotron at Forschungszentrum Karlsruhe.

Development of a 140-GHz 1-MW continuous wave gyrotron for the W7-X stellarator

The development of high-power gyrotrons (118 GHz, 140 GHz) in continuous-wave (CW) operation for heating nuclear fusion plasmas has been in progress for several years in a joint collaboration between different European research institutes and industrial partners. The 140-GHz gyrotron being under development for the installation at the W7-X stellarator now under construction at the IPP Greifswald, Germany, operates in the TE/sub 28,8/ mode and is equipped with a diode type magnetron injection electron gun, an improved beam tunnel, a high mode-purity low-Ohmic loss cavity, an optimized nonlinear up-taper, a highly efficient internal quasi-optical mode converter, a single-stage depressed collector and an edge-cooled, single disk CVD-diamond window. RF measurements at pulse duration of a few milliseconds yielded an RF output power of 1.15 MW at a beam current of 40 A and a beam voltage of 84 kV. Depressed collector operation has been possible up to decelerating voltages of 33 kV without any reduction of the output power. Long pulse operation (10 s at 1 MW) was possible without any signs of a limitation caused by the tube. For this output power the efficiency of the tube could be increased from about 30% without to about 50% with depression voltage. The best performance reached so far has produced an energy per pulse as high as 90 MJ (power 0.64 MW, pulse length 140 s) which is the highest value achieved in gyrotrons operating at this frequency and power level. The pulse-length limitations so far are mainly due to the external system.

EU megawatt-class 140 GHZ CW gyrotron

The first series tube of the gyrotrons for the 10-MW electron cyclotron resonance heating system of the stellarator W7-X was tested at Forschungszentrum Karlsruhe (FZK) and yielded a total output power of 0.98 MW, with an efficiency of 31% (without a single-stage depressed collector) in short-pulse operation and of 0.92 MW in pulses of 180 s (efficiency of almost 45% at a depression voltage of 29 kV). The Gaussian mode output power was 0.91 MW. The pulselength at full power (1 MW) is limited at FZK by the available power supply. At a reduced electron beam current, it is possible to operate at longer pulselengths. At an output power of 0.57 MW (electron beam current of 29 A), the pulselength was increased to 1893 s. There was no physical reason for a limitation of this pulse: The pressure increase during the pulse was less than a factor of two and ended up at a very low value in the 10-9 mbar range. The tube was delivered to Max-Planck-Institut fuumlr Plasmaphysik Greifswald for tests at full power and up to 30-min pulselength. The Gaussian mode RF output power, measured in a calorimetric load after a 25-m-long quasi-optical transmission line (seven mirrors), was 0.87 MW at a total output power of 0.92 MW in 30-min pulses. Again, no indications for a limitation in pulselength were found. The second series tube was tested in short-pulse operation and showed a strange behavior concerning a mode hopping which has not yet been understood. The third series gyrotron delivers up to now 0.65 MW at a pulse duration of 180 s. Preliminary operation of the prototype tube as a two-frequency gyrotron delivered 0.41 MW in 10-s pulses at 103.8 GHz (TE21,6 mode)

High-power gyrotron development at Forschungszentrum Karlsruhe for fusion applications

In the first part of this paper, the status of the 140-GHz continuously operated gyrotrons with an output power of 1 MW for the stellarator Wendelstein 7-X will be described. With the first series tube, an output power of 1000 kW has been achieved in short pulse operation (milliseconds) with an electron beam current of 40 A, and of 1150 kW at 50 A. With a pulse length of 3 min limited by the available high-voltage (HV) power supply, an output power of 920 kW at an electron beam current of about 40 A with an efficiency of 45% and a mode purity of 97.5% has been obtained. At a reduced beam current of 29 A, an output power of 570 kW was measured with a pulse length of 1893 s without significant increase in tube pressure. The energy content of this pulse is almost 1.1 GJ. For the next fusion plasma device, International Thermonuclear Experimental Reactor (ITER), gyrotrons with a higher output power of about 2 MW are desirable. In short-pulse experiments, the feasibility of the fabrication of coaxial cavity gyrotrons with an output power up to 2-MW, continuous wave (CW), has been demonstrated, and the information necessary for a technical design has been obtained. The development of a long-pulse 2-MW coaxial cavity gyrotron started within a European cooperation. In parallel to the design and fabrication of an industrial prototype gyrotron, a short-pulse preprototype gyrotron has been operated to verify the design of critical components. An output power of 1.2 MW with an efficiency of 20% has been achieved. The development of frequency tunable gyrotrons operating in the range from 105 to 140 GHz for stabilization of current driven plasma instabilities in fusion plasma devices (neoclassical tearing modes) is another task in the development of gyrotrons at the Forschungszentrum Karlsruhe.

A 2 MW, 170 GHz coaxial cavity gyrotron - experimental verification of the design of main components

The feasibility of manufacturing a 2-MW CW coaxial cavity gyrotron at 170 GHz has been demonstrated and data required for fabrication of an industrial tube have been obtained. An engineering design of a prototype started recently with the goal to provide gyrotrons with 2-MW microwave output power for International Thermonuclear Experimental Reactor (ITER). The design of critical components of the prototype tube as electron gun, cavity and RF output system will be verified under realistic conditions at short pulses using the experimental coaxial gyrotron at Forschungszentrum Karlsruhe.

Development of multimegawatt gyrotrons for fusion plasma heating and current drive

High frequency gyrotrons with high output power are mainly used for microwave heating and current drive in plasmas for thermonuclear fusion experiments. Electron cyclotron resonance heating (ECRH) has proven to be an important tool for plasma devices, especially for stellarators, as it provides both net current free plasma start up from the neutral gas and efficient plasma heating. The development of high power gyrotrons (118 GHz, 140 GHz and 170 GHz) in continuous wave operation (CW) has been in progress for several years in a joint collaboration between different European research institutes and industrial partners. This paper describes the work of the Forschungszentrum Karlsruhe for the development of conventional-cavity 1-MW CW gyrotrons, coaxial cavity 2-MW short-pulse gyrotrons and a frequency step-tunable gyrotron in the frequency range between 105-140 GHz.

Development of a 140 GHz, 1 MW, continuous wave gyrotron for the W7-X stellarator

The development of high power gyrotrons in continuous wave (CW) operation for heating of plasmas used in nuclear fusion research has been in progress for several years in a joint collaboration between different European research institutes and industrial partners. A recent RD program aims at the development of 140 GHz gyrotrons with an output power of 1 MW in CW operation for the 10 MW ECRH system of the new stellarator plasma physics experiment Wendelstein 7-X at IPP Greifswald, Germany. The work is performed under responsibility of FZK Karlsruhe in collaboration with CRPP Lausanne, IPF Stuttgart, IPP Garching and Greifswald, CEA Cadarache and TED Velizy. The gyrotron operates in the TE28.8 mode and is equipped with a diode type magnetron injection electron gun, an improved beam tunnel, a high-mode purity low-ohmic loss cavity, an optimized non-linear up-taper, a highly efficient internal quasi-optical mode converter, a single-stage depressed collector and an edge-cooled, single disk CVD-diamond window. RF measurements at pulse duration of a few milliseconds yielded an RF output power of 1.15 MW at a beam current of 40 A and a beam voltage of 84 kV. Depressed collector operation has been possible up to decelerating voltages of 33 kV without any reduction of the output power, and an efficiency of 49 % has been achieved. Long pulse operation of the gyrotron was possible with an output power of 1 MW at a pulse length of 10 s without any signs of a limitation caused by the tube. For this output power the efficiency of the tube could be increased from about 30 % without depression voltage to about 50% with depression voltage. At an output power of 640 kW, a pulse length of 140 s could be achieved.

Power modulation capabilities of the 140 GHz/1 MW gyrotron for the stellarator Wendelstein 7-X

In current tokamaks and, in particular, in future larger devices such as ITER, the control of neo-classical tearing modes (NTM) is essential for achieving high performance in terms of the beta limit. A commonly used scheme for NTM stabilization consists in driving a helical current at the resonance surface of interest with electron-cyclotron-current-drive. Depending on the ratio between the magnetic island size and the RF beam width, complete stabilization of the NTM will only be achieved with deep RF power modulation in phase with the mode. In the frame of the European development program of high power sources for ECRH applications between Forschungszentrum Karlsruhe, IPP Garching/Greifswald, EPFL Lausanne, IPF Stuttgart, CEA Cadarache and Thales Electron Devices, the modulation capabilities of the 140 GHz/1 MW gyrotron have been experimentally investigated. RF-power modulation depths higher than 80% at a frequency of 50 kHz with cathode modulation and 1.5 kHz with depression voltage modulation have been obtained. The limitations in frequency were given by the corresponding power supplies and not by the gyrotron itself. Detailed analysis of the collector loading with respect to the modulation scheme will be presented and the intrinsic gyrotron limitations for long-pulse operation with deep modulation will be discussed

Advanced high-power gyrotrons

Gyrotrons at high frequency with high-output power are mainly developed for microwave heating and current drive in plasmas for thermonuclear fusion. For the stellarator Wendelstein 7-X, now under construction at IPP Greifswald, Germany, a 10-MW electron-cyclotron-resonance-heating (ECRH) system is foreseen. A 1-MW 140-GHz gyrotron with synthetic diamond window for continuous wave operation and with a single stage depressed collector for energy recovery and improvement of efficiency has been designed, constructed, and tested in collaboration with CRPP Lausanne and TED Ve/spl acute/lizy. It operates in the TE/sub 28,8/-cavity mode and provides a linearly polarized TEM/sub 0,0/ Gaussian RF beam. In short pulse operation at the design current of 40 A, an output power of 1-MW could be achieved for an accelerating voltage of 82 kV without depression voltage, an output power of 1.15 MW at an accelerating voltage of 84 kV with a depression voltage of 25 kV. These values correspond to an efficiency of 49%. After some problems with the RF-load, long-pulse operation was performed. The power measurements were done by the calibrated signal of the diode detector placed at the second mirror. Output powers of 1 MW could be achieved for 10 s, and an energy as high as 90 MJ per pulse has been produced with an output power of 0.64 MW. The pulselengths were mainly determined by the preset values. Only for the 100-s pulse at 0.74 MW, a limitation was found due to a pressure increase beyond about 10/sup -7/ hPa. The ITER task (task for the future international thermonuclear experimental reactor) on development of coaxial cavity gyrotrons ended in 2001. In accordance with the goal of the task, the potential of coaxial gyrotrons has been investigated and, as a result, data necessary for an industrial realization of a 2-MW CW 170-GHz tube have been obtained. In addition, first work on tube integration has been done. The results will be presented and discussed. By biasing the coaxial insert a fast (within 0.1 ms) frequency tuning has been demonstrated. In particular, a fast step tuning between the 165-GHz nominal mode and the azimuthal neighbors at 162.5 and 167.2 GHz have been performed. In addition, at the nominal mode a continuous frequency variation within the bandwidth of up to 70 MHz has been done.

Progress in the 10 MW ECRH system for the stellarator W7-X

Summary form only given, as follows. Electron cyclotron resonance heating (ECRH) has proven during the last few years to be one of the most attractive heating schemes for stellarators, as it provides net current free plasma start tip and heating. Extensive measurements on stellarators at IPP Garching provide a solid physical and technological basis for ECRH systems. Therefore, ECRH will be the main heating method for the Wendelstein 7-X stellarator (W7-X) now under construction at Greifswald / Germany. A 10 MW ECRH system with continuous wave (CW) possibilities operating at 140 GHz will be built to meet the scientific goals of the stellarator at Greifswald with inherent steady-state capability at reactor relevant plasma parameters. A prototype gyrotron with an output power of 1 MW was developed in collaboration between European research laboratories, and European industries. The gyrotron is equipped with a single stage depressed collector for increasing the efficiency, an optimized quasi-optical mode converter and a CVD-diamond window. The prototype gyrotron has been successfully tested with an output power of 0.89 MW at a pulse duration of three minutes and all output power of 0.54 MW for about 15 minutes. The specified value of Gaussian beam output power exceeding 0.9 MW has been obtained for about 1 minute. Further results will be given.