B. Piosczyk

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)

European high-power CW gyrotron development for ECRH systems

The development of high power CW gyrotrons for ECRH heating of fusion relevant plasmas has been in progress for several years in a joint collaboration between different European research institutes and an industrial partner. Two development are on going, aiming, respectively, towards a 0.51-MW-210-s gyrotron at 118 GHz for the tokamaks TCV of CRPP (2 s pulse length) and Tore Supra of CEA (210 s pulse length), and towards a 1 MW-CW gyrotron at 140 GHz for the stellarator W7-X under construction in Greifswald. Series 118 GHz gyrotrons have been delivered to CRPP and CEA. Long pulse results (15.5 s at 400 kW) as well as considerations on power modulation capabilities of the tube and on long pulse effects are discussed. In a second development program, a 1-MW/CW 140 GHz gyrotron with a CVD diamond window and a single-stage depressed collector has been designed and constructed as a first prototype for the 10-MW ECRH (Elecron Cyclotion Resonance Heating) system of the new stellarator experiment Wendelstein 7-X of IPP Greifswald/Germany. The gyrotron operates in the TE28.8 cavity mode and provides a linearly polarized, TEM0.0 Gaussian RF beam. It is composed of a diode MIG 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 employing an improved launcher together with one quasi-elliptical and two beam shaping reflectors, a large single stage depressed collector at ground potential with a beam sweeping magnet, and a horizontal RF output

First experimental results from the European Union 2 MW coaxial cavity ITER gyrotron prototype

Abstract The European Union is working toward providing 2-MW, coaxial-cavity, continuous-wave (cw) 170-GHz gyrotrons for ITER. Their design is based on results from an experimental preprototype tube having a pulse length of several milliseconds, in operation at Forschungszentrum Karlsruhe (FZK) for several years now. The first industrial prototype tube was designed for cw operation but, in a first phase, aimed at a pulse length of 1 s at the European Gyrotron Test Facility in Lausanne, Switzerland, as part of a phased testing/development program (1 s, 60 s, cw). The first experimental results of the operation of this prototype gyrotron are reported here. The microwave generation was characterized at very short pulse length (<0.01 s) using a load on loan from FZK, and the highest measured output power was 1.4 MW, at a beam energy significantly lower than the design value (83 kV instead of 90 kV), limited by arcing in the tube. The radio-frequency (rf) beam profile was measured to allow reconstruction of the phase and amplitude profile at the window and to provide the necessary information permitting proper alignment of the compact rf loads prior to pulse extension. Arcs in the tube limited the pulse length extension to a few tens of milliseconds. According to present planning, the tube is going to be opened, inspected, and refurbished, depending on the results of the inspection, to allow testing of an improved version of the mode launcher and replacement of some subassemblies.

Recent results of the 1-MW, 140-GHz, TE/sub 22,6/-mode gyrotron

The TE/sub 22,6/-mode gyrotron operated at Forschungszentrum Karlsruhe at a frequency of 140 GHz has been investigated with respect to the behavior of different emitter materials, step tunability and reflections of the output beam. Two different materials of an emitter ring, LaB/sub 6/ and a coated dispenser cathode, were used to test the features of the gyrotron. The output power was found to be independent from the cathode material, as long as a new emitter ring was used. Aging of the emitter led to a slightly decreased output power. The gyrotron also was operated with a Brewster window. The broad-band characteristics of this window made it possible to measure the neighboring frequencies in a frequency range extending from 114 to 166 GHz. Only a slight dependence of the output power has been found over the whole frequency range. The Brewster window also allows us to investigate the influence of reflections on the output power. A strong decrease of the output power was found even for very small reflections. Tilting the power calorimeter (the reflections were measured to be less than 1%) increased the output power by about 20% to 1.6 MW at an efficiency of 36.2%. With a collector depression voltage of 35 kV for energy recovery, efficiencies of 60% at the above-mentioned output power were obtained.

A 1.5-MW, 140-GHz, TE/sub 28,16/-coaxial cavity gyrotron

The design of a 1.5-MW, 140-GHz, TE-/sub 28,16/-coaxial cavity gyrotron is presented and results of experimental operation are given. A cavity with a cylindrical outer wall and a radially tapered inner rod with longitudinal corrugations was used. A maximum output power of 1.17 MW has been measured in the design mode with an efficiency of 27.2%. Single-mode operation has been found over a wide range of operating parameters. The experimental values agree well with the results of multimode calculations. Frequency-step tuning has been performed between 115.6 and 164.2 GHz. In particular, an output power of 0.9 MW has ben measured in the TE/sub 25,14/ mode at 123.0 GHz and 1.16 MW in the TE/sub 32,18/ mode at 158.9 GHz. At frequencies its with strong window reflections the parameter range for which stable operation is possible is reduced significantly. In order to obtain results relevant for a technical realization of a continuously operated gyrotron, a tube with a radial radio frequency (RF)-beam output through two output windows and a single-stage depressed collector has been designed and is under fabrication. A two-step mode conversion scheme-TE-/sub 28,16/ to Te/sub +76.2/ to TEM/sub 00/-which generates two narrowly directed (60/spl deg/ at the launcher) output wavebeams has been chosen for a quasioptical (q,o) mode converter system. A conversion efficiency of 94% is expected.

Design and experimental operation of a 165-GHz, 1.5-MW, coaxial-cavity gyrotron with axial RF output

The development of a coaxial-cavity gyrotron operating in TE/sub 31,17/ mode at 165 GHz is presented. The selection of the operating frequency and mode are based on the limitations imposed by the maximum held of the superconducting (sc) magnet at Forschungzentrum Karlsruhe, Institut fur Technische Physik (FZK), the use of the inverse-magnetron injection gun (IMIG) of the 140-GHz, TE/sub 28,16/ coaxial gyrotron and the possibility of transforming the cavity mode to a whispering gallery mode (WGM) appropriate for the dual-beam quasioptical (q.o.) output coupler and the two output windows, which are foreseen for the next lateral output version of the tube. The tube with axial output has been tested at FZK to deliver maximum output power of 1.17 MW in the designed TE/sub 31,17/ mode with 26.7% efficiency at 164.98 GHz. Maximum efficiency of 28.2% was achieved at 0.9-MW output power. The design operating point with output power 1.36 MW and 36.7% efficiency was net accessible because of beam instabilities at high electron-velocity ratio /spl alpha/, presumably caused due to high electron-velocity spread. Power at higher frequencies was also detected: 1.02 MW at 167.16 GHz in TE/sub 32,17/ mode with 26.88 efficiency, 0.63 MW at 169.46 GHz in TE/sub 33,17/ mode with 18% efficiency, and 0.35 MW at 171.80 GHz in TE/sub 31,17/ mode with 13.3% efficiency.

Long-pulse operation of a 0.5 MW TE/sub 10.4/ gyrotron at 140 GHz

The operation features of a TE/sub 10.4/-mode gyrotron oscillator with a quasi-optical mode converter and a single-stage depressed collector at 140 GHz with an output power of 500 kW in long pulses of 0.2 s are presented. Measurements on long-pulse operation of the tube are described in detail, and the significant differences between short- and long-pulse operation concerning efficiency and output power are pointed out. The variation of frequency during a pulse and an irreversible frequency shift during long-pulse operation were measured and are discussed with respect to gyrotron design.

Coaxial cavity gyrotron with dual RF beam output

A 140-GHz, 1.5-MW, TE/sub 28,16/-coaxial cavity gyrotron with a dual RF beam output has been designed, built, and tested. For the first time, the generated RF power has been split into two parts and coupled out through two RF output windows in order to reduce the power loading in the windows. The quasioptical output system is based on a two-step mode conversion scheme. First, the cavity mode TE/sub -28,16/ is converted into its degenerate whispering gallery mode TE/sub +76,2/ using a rippled-wall mode converter. Then, this mode is transformed into two TEM/sub 00/ output wave beams. A maximum rf output power of about 950 kW with an output efficiency of 20% has been measured. According to numerical calculations, an rf power above 1.5 MW is expected to be generated in the cavity. Even if all losses are taken into account, a discrepancy between experiment and calculations remains. The power deficit seems to be partly caused by the influence of the stray radiation captured inside the tube. However, the two main reasons are probably an incomplete mode conversion from TE/sub -28,16/ to TE/sub +76,2/ and a large energy spread of the electron beam due to trapped electrons. An increased amount of captured stray radiation resulted in a reduced stability of operation. A single-stage depressed collector was used successfully, increasing the RF output efficiency from 20% to 29%.

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.