M. Kuntze

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.

Coaxial cavity gyrotron- recent experimental results

The feasibility of fabrication of coaxial cavity gyrotrons with an output power up to 2 MW, continuous wave (CW) has been demonstrated and information necessary for a technical design has been obtained. Experiments with a gyrotron equipped with newly designed components-electron gun, cavity, RF output system-have been performed. In short pulses, a maximum radio frequency (RF) output power of 2.2 MW has been reached in stable operation. At the nominal output power of 1.5 MW an efficiency of 30% has been achieved. This has been enhanced to 48% in operation with a single-stage depressed collector. The stability of the coaxial insert has been measured to be within /spl plusmn/0.03 mm under operating conditions. The losses at the coaxial insert have been found to be about 0.1% of the RF output power. Investigations of the microwave stray radiation captured inside the tube have been performed with the following results: (1) the captured stray radiation due to diffraction losses is approximately uniformly distributed inside the mirror box; (2) about 8% of the captured microwave power is radiated through a relief window with 100-mm diameter in the used setup; and (3) the total amount of stray radiation has been found to be about 11% of the RF output power. Parasitic low-frequency oscillations have been successfully suppressed and stable operation has been achieved over a wide parameter range. Fast (/spl sim/0.1-ms) frequency tuning has been demonstrated by applying a rapid variable bias voltage at the coaxial insert. In particular, step frequency tuning by /spl plusmn/2.2 GHz due to switching from the nominal mode at 165 GHz to its azimuthal neighbors has been done and continuous tuning by up to 70 MHz within the bandwidth of the TE/sub 31,17/ mode has been performed.

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.

165 GHz, 1.5 MW-coaxial cavity gyrotron with depressed collector

A further step in the development of a coaxial-cavity gyrotron operated in the transverse electric TE/sub -31,17/ mode at 165 GHz is presented. The gyrotron has been equipped with a quasi-optical output system consisting of a Vlasov launcher with a single cut and two mirrors, one with a quasi-elliptic and the other with a nonquadratic phase correcting surface. The radio frequency (RF) power is transmitted through a single output window. A maximum output power of 1.7 MW has been achieved. At the nominal operational parameters an RF power of 1.3 MW with an efficiency of 27.3% has been measured. The efficiency increases to 41% in operation with a single-stage depressed collector.

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%.

Fast frequency-step-tunable high-power gyrotron with hybrid-magnet-system

The TE/sub 22,6/ 1 MW gyrotron operated at Forschungszentrum Karlsruhe (FZK) at frequencies between 114 and 166 GHz has been investigated with respect to the fast-frequency tunability in the frequency range from 132 to 147 GHz. For that purpose, the gyrotron has been equipped with a special hybrid-magnet system consisting of superconducting (sc) magnets in the cryostat and additional normal conducting (nc) copper magnets with a fast time constant. Some special problems due to the magnetic coupling between the different magnets were investigated by calculation and experiment. Making use of these investigations some different current regulation systems for the nc magnets were implemented and tested experimentally. Finally a step-tuning operation between the modes from TE/sub 20,6/ to TE/sub 24,6/ in time steps of 1 s has been achieved.

Experimental results and numerical simulations of a high power 140 GHz gyrotron

The design of a 0.5 MW 140 GHz gyrotron with axial RF output operating in the TE/sub 10,4/ mode is presented. Experimental results and numerical simulations are compared. In particular the effects of RF reflections at the output window, velocity spread of the helical electron beam and startup of oscillations by simultaneous increase of pitch factor /spl alpha/ and relativistic factor /spl gamma/ are discussed. In short pulse operation (/spl les/5 ms) an output power of 690 kW has been obtained with an electronic efficiency of 31% and a mode purity of about 99%. The experiments have shown that it is possible to use for step frequency tuning the azimuthal neighbours of the TE/sub 10,4/ mode, TE/sub 9.4/ at 132.6 GHz (420 kW) and TE/sub 11,4/ at 147.3 GHz (300 kW) as working modes by decreasing and increasing the magnetic field, respectively. >