CW Gyrotron Research Articles

Progress Toward Steady-State Operation in LHD Using Electron Cyclotron Waves

Trials of steady-state operation (SSO) in the Large Helical Device (LHD) were started when a continuous wave (cw) gyrotron with the output power up to 0.2 MW was introduced to the electron cyclotron heating (ECH) system on LHD in 2003. During the first trial of SSO in the seventh LHD experimental campaign in 2004, severe temperature increase on the waveguide transmission line and, as a result, intense pressure increase in the evacuated waveguide occurred, which terminated the operation at 756 s. Additional pumping sections and cooling structures on the transmission line worked well, allowing a 3900-s sustainment of plasma with ne = 0.15 × 1019 m−3 and Te0 = 1.7 keV by 0.1 MW injection power in 2005. The improvement of the ECH system by introducing cw gyrotrons with higher power for further improvement of plasma performance in SSO is in progress.Investigations on electron cyclotron current drive (ECCD) physics have been advanced a few years after the proof of ECCD in LHD. By obliquely injecting second-harmonic X-mode EC waves in toroidal direction, stable current up to 5.5 kA is driven, which was evaluated as a difference in plasma currents of the co- and counter-ECCD discharges with 0.1-MW EC wave power. It takes a few seconds for the driven current to saturate. Change in profile of rotational transform by ECCD and profile of driven current density are investigated by use of motional Stark effect measurement. Peaked and localized driven current profile at the plasma core region was confirmed for on-axis second-harmonic ECCD discharges.

DESIGN OF A 60 GHz, 100 kW CW GYROTRON FOR PLASMA DIAGNOSTICS: GDS-V.01 SIMULATIONS

In this work, the design studies of a 60GHz, 100kW CW gyrotron have been presented. Mode selection is carefully studied with the aim of minimizing mode competition and to yield a perfect solid beam output through an RF window with a suitable dimpled-wall quasi-optical launcher. Cavity design and interaction computations are then carried out. In addition, preliminary design of the magnetron injection gun, magnetic guidance system, launcher, and RF window are presented. Thus, we present a feasibility study, which indicates that the operation of such a gyrotron is possible and can give a power in excess of 100kW at an e-ciency > 35%. As a part of this work, a complete Graphical User Interface package \GDS V.01 (Gyrotron Design Suit Ver.01) has been developed for the design and conceptualization of speciflc gyrotrons.

Highly Efficient Quasi-Optical Mode Converter for a Multifrequency High-Power Gyrotron

A highly efficient quasi-optical mode converter with a bandwidth of 38 GHz has been designed and tested. The mode converter combines low-diffraction losses and a Gaussian mode content up to 97% for a set of nine modes in the range of 105 to 143 GHz for a 1-MW CW gyrotron. This was achieved using a dimpled-wall waveguide antenna (launcher), one quasi-elliptical mirror, and two toroidal mirrors. The optimization of the launcher was done using coupled-mode theory. The simulation results show a well-focused Gaussian beam for all nine operating modes. The curvature radii of the toroidal mirrors were determined by Gaussian mode transformation (ABCD-law) and subsequently optimized for a multimode operation. The simulations of the quasi-optical mode converter are based on the electric field integral equation and, thus, are 3-D. Experimental low-power measurements show close agreement with predictions.

Self-Consistent Electromagnetic–Thermal Model for Calculating the Temperature of a Ceramic Cylinder Irradiated by a High-Power Millimeter-Wave Beam

The high-power millimeter-wave beams that can be generated by CW gyrotrons represent a promising energy source for rapid high-temperature processing of materials. A program is under way at the Naval Research Laboratory to investigate the heating of ceramic tubes and cylinders using an 83-GHz beam for joining and sintering applications. In this paper, we discuss the scattering and absorption of microwave power by a ceramic cylinder and calculate the resulting temperature profile. The analysis accounts for the temperature dependence of the dielectric properties, an effect that can dramatically alter the microwave coupling during the heating process. In the process under investigation, the cylinder is rotated to promote azimuthally uniform heating so that only the radial dependence of the dielectric properties needs to be included.

Design and Optimization of Nonlinear Tapers using Particle Swarm Optimization

In this paper the design and optimization of a nonlinear diameter taper, connecting the output section of a gyrotron cavity to the uniform output waveguide section, is presented. The design of a nonlinear taper of a 42 GHz, 200 kW CW gyrotron operating in the TE0,3 cavity mode with axial output collection has been taken as a case study. The taper synthesis has been carried out considering a raised cosine type of nonlinear taper and the analysis is done using a dedicated scattering matrix code. In addition, an improved particle swarm optimization - an evolutionary optimization - algorithm is used for the design optimization of this nonlinear taper. The optimum design of the taper shows the effectiveness of the presented method.

Influence of background plasma on operation efficiency of high-power continuous wave (cw) gyrotrons

A slow wave non-linear single mode stationary theory describing influence of background plasma which occurs in the gyrotron cavity on gyrotron operation has been developed. It is shown that the main effects of a low density background plasma on efficiency are concerned with decreasing the mismatch between operational and cyclotron frequencies and decreasing the beam-wave coupling coefficient. Numerical calculations are performed for the 140 GHz 1 MW CW gyrotron developed in Karlsruhe for fusion applications. Interpretation of some experimental effects which are not yet clearly understood is addressed.

Performance Test of CW 300 GHz Gyrotron FU CW I

A 300 GHz CW gyrotron FU CW I has been developed and installed in the Research Center for Development of Far-Infrared Region, University of Fukui as a power source of a high frequency material processing system. Its performance was tested and the maximum power of 1.75 kW / CW has been attained at the beam voltage of 15 kV and the beam current of 1A. The maximum window power efficiency of 15.5% has been obtained at the cathode voltage slightly lower than 15 kV. This gyrotron is designed to deliver a Gaussian beam after mode conversion from the oscillation mode TE22,8 in the cavity with a complex of an internal radiator and beam shaping mirrors. The detailed measurement with an infrared camera has confirmed that a Gaussian beam is radiated when the magnetic field strength B c at the cavity is adjusted at a proper value. However, within a range of B c , the output power is emerged into multiple directions, which suggests simultaneous oscillation of competing cavity modes.

Increasing Power and Efficiency of Gyrotrons

Until recently, the development of new gyrotrons was directed mainly at the increase of their operating frequency, power, and efficiency. The output power of modern continuous-wave (cw) gyrotrons has reached 1 MW, and there is a clear tendency to increase this power further to at least up to 1.5 to 2 MW. The efficiency of the best gyrotron tubes reaches 40% without recovering the residual energy of the spent electron beam [collector potential depression (CPD)] in the continuous regimes and 50% in the pulsed one and achieves 50% with one-step CPD in the cw regimes and near 70% in the pulsed regimes. We analyze limitations of the gyrotron output power and efficiency imposed by systems forming helical electron beams, the cavity interaction processes, the transmitting capability of the output window, and the losses of stray radiation in the built-in converter and power dissipation on the collector (including CPD). Some specific examples in applying the different limits to real cases of gyrotrons are discussed. Ways to enhance the power and efficiency of gyrotrons based on the results of this analysis are shown.

Transverse field collector sweep system for high power CW gyrotrons

Abstract The spent electron beam of megawatt-class CW gyrotrons requires magnetic sweeping to keep the specific heat dissipation on the collector surface within technically acceptable limits of about 500 W per cm2. Conventionally this is realised with a solenoid coil placed over the collector, which sweeps the strike point of the electron beam up and down the collector-wall at low frequency (

Advances in high power calorimetric matched loads for short pulses and CW gyrotrons

The development of high power gyrotrons for plasma physics research needs properly matched calorimetric loads able to absorb and measure the power, which nowadays is foreseen to be as high as 2 MW during CW operations. To this end IFP/CNR has developed a family of matched loads useful in the mm-wave frequency band for applications ranging from a few ms to CW in pulse length. The different loads in the family, made of an integrating sphere with a partially reflecting coating on the inner wall, are characterized by having the same absorbing geometry for the incoming beam and a different heat removal system for the specific application. Some important advances have been recently achieved from the point of view of the uniformity of power distribution on the absorbing wall and of the load construction. With the high precision achieved in the coating thickness a better control of the heating power distribution is possible by proper shaping of the local reflectivity, in addition to the shaping of the mirror dispersing the input beam. A more sophisticated model describing the power distribution has been developed, taking into account a variable thickness of the absorbing coating, the proper shape of the spreading mirror, the frequency of the incoming radiation and the shape of the input beam. Lower coating thickness is shown to be preferable, for a given local reflectivity, from the point of view of a lower peak temperature and thermal stress. The paper describes a load with variable coating thickness along the meridian of the sphere, showing a uniform power deposition on the inner walls. The cooling pipe is completely electroformed on the spherical copper shell, ensuring the maintenance of the correct curvature of the inner surface and fast heat conduction from the absorbing coating to the water through the thin copper body. For CW use all heated parts of the load must be cooled and this is achieved with 16 electroformed spiral channels. Both short pulse loads (0.1–1 s) and the CW version at 2 MW, 170 GHz, are described in the paper. High power tests on short-pulse loads have been done using a double frequency gyrotron, 105 GHz/600 kW for 0.5 s and 140 GHz/800 kW for 1 s. Also a method for emulating 2 MW conditions while using 1 MW gyrotron has been applied to test the load to be used for the European 2 MW coaxial cavity gyrotron development programme.

Detailed Consideration of Experimental Results of Gyrotron FU CW II Developed as a Radiation Source for DNP-NMR Spectroscopy

A CW gyrotron for the sensitivity enhancement of NMR spectroscopy through dynamic nuclear polarization has been designed. The gyrotron operates at the second harmonic and frequency of 394.6 GHz with the main operating mode TE0,6. Operating conditions of other neighboring cavity modes such as TE2,6 at frequency of 392.6 GHz and TE2,3 at frequency of 200.7 GHz were also considered. The experimental conditions of the gyrotron at low and high voltages are simulated. The output power of 56 watts corresponds to the efficiency of 2 percent at low voltage operation and frequency of 394.6 GHz is expected.

CONCEPTUAL DESIGN STUDIES OF AN 84 GHz, 500 kW, CW GYROTRON

The feasibility of an 84 GHz, 500 kW, CW gyrotron for ECRH on an experimental tokamak will be presented in this paper. Mode competition and mode selection procedures are carefully investigated by considering various candidate modes and the TE10,4 mode is chosen as the operating mode. A conventional cylindrical cavity resonator with weak input and output tapers and parabolic roundings is considered for interaction studies. Self-consistent, both single mode and time-dependent, calculations are carried out and power and efficiencies are computed for a typical set of beam parameters. The results show that an output power of well over 500 kW, CW and efficiency around 40% can be reached without a depressed collector.

Efficient Low-Voltage Operation of a CW Gyrotron Oscillator at 233 GHz.

The gyrotron oscillator is a source of high average power millimeter-wave through terahertz radiation. In this paper, we report low beam power and high-efficiency operation of a tunable gyrotron oscillator at 233 GHz. The low-voltage operating mode provides a path to further miniaturization of the gyrotron through reduction in the size of the electron gun, power supply, collector, and cooling system, which will benefit industrial and scientific applications requiring portability. Detailed studies of low-voltage operation in the TE(2) (,) (3) (,) (1) mode reveal that the mode can be excited with less than 7 W of beam power at 3.5 kV. During CW operation with 3.5-kV beam voltage and 50-mA beam current, the gyrotron generates 12 W of RF power at 233.2 GHz. The EGUN electron optics code describes the low-voltage operation of the electron gun. Using gun-operating parameters derived from EGUN simulations, we show that a linear theory adequately predicts the low experimental starting currents.

Observation of Simultaneous Oscillation of Multiple Modes in a CW 300 GHz Gyrotron

Multi-mode oscillation was observed in a 300 GHz fully CW gyrotron. It has been developed and installed in the Research Center for Development of Far-Infrared Region, University of Fukui as a power source of a submillimeter-wave material processing system. This gyrotron delivers 1.75 kW/CW at maximum. The radiation pattern is a Gaussian beam when the magnetic field strength Bc at the cavity is properly adjusted. However, within a range of Bc values, simultaneous oscillation of competing modes is observed, manifesting in radiation of the output power in multiple directions.

Development of material processing system by using a 300 GHz CW gyrotron

High power THz-frequency material processing has great potential for the applications. The gyrotron is a microwave tube capable of delivering very high microwave power in the pulse and CW operation at the THz frequencies. The frequency of gyrotron is proportional to the intensity of magnetic field, because the operation results from the mechanism of "cyclotron resonance maser". Therefore, in order to achieve high frequency operation, we need a high magnetic field which superconducting magnets or pulsed-magnets produce. We developed the material heating system by using a 300 GHz gyrotron. This system consists of a 300 GHz, 3.5 kW, CW gyrotron (Gyrotron FU CW I) with a cryogen-free 12 T superconducting magnet, a corrugated circular waveguide and an applicator.

Joining of ceramic tubes using a high-power 83-GHz Millimeter-wave beam

Summary form only given. The high power beams that can be generated by CW gyrotrons represent a promising energy source for high-temperature processing of materials. An experimental facility based on an 83-GHz, 15 kW CW industrial gyrotron has been set up at the Naval Research Laboratory to investigate novel mm-wave-beam-based approaches to processing ceramic materials including sintering, coating, and joining applications. Current experimental and theoretical studies are investigating the formation of strong bonds between ceramics and between ceramics and metals using controlled, selective heating of the joint area. In this paper, the results of current joining experiments were described. In particular, the results from the joining of ceramic tubes for a dielectric-loaded accelerator was presented. Additionally, novel methods for temperature sensing as well as monitoring of the ongoing process was given.

Recent ITER-Relevant Gyrotron Tests

Recent tests of two different megawatt-class gyrotrons at CPI provide a design baseline for the 120 GHz, 1 MW CW gyrotrons required by ITER for plasma start-up. The two gyrotron designs include a 140 GHz, 900 kW CW device that was delivered to the Max Planck Institute for Plasma Physics in Greifswald, Germany, and a 110 GHz, 1.3 MW CW gyrotron that was recently tested at CPI. Both gyrotrons utilize many of the same features that are foreseen for the ITER 120 GHz gyrotrons. These features include: single-anode magnetron injection electron guns, high-efficiency internal converters that produce a pure TEM00 output mode, 88-mm-diameter diamond output windows and single-stage depressed collectors. In tests at CPI, the 140 GHz gyrotron achieved 500 kW power levels for 700-s pulses and 930 kW for short pulses. Long-pulse tests of the gyrotron, at beam currents above the 25-A test-set limit at CPI, were performed in Greifswald and resulted in output power levels of up to 900 kW for 30-minute pulses. In short-pulse operation at CPI, the 110 GHz gyrotron has achieved power levels of 1.28 MW. In long-pulse tests of the gyrotron, power levels of 500 kW were obtained for 10-s pulses.

04/00212 Next step spherical torus design studies: Neumeyer, C. et al. Fusion Engineering and Design, 2003, 66–68, 139–145

Parasitic beam tunnel oscillations have been discovered on some of the series production gyrotrons for W7-X and also on the coaxial pre-prototype gyrotron for ITER. Solutions to remedy these problems have resulted in a modified beam tunnel design, technologically close to the existing beam tunnel. The new design has successfully been tested on both the coaxial and also the f-step-tunable gyrotrons and has subsequently been implemented on one of the W7-X series-production-tubes presently undergoing factory acceptance tests in Karlsruhe.The ECRH test loads at KIT are operated under normal atmospheric conditions. Several loads have eventually failed in 1 MW long pulse experiments and KIT has therefore started to design its own loads. The first KIT-load is based on a fixed conical mirror and an aluminum cylinder coated with a lossy material for increased absorption. The new load has so far successfully been used during the acceptance tests of two 1-MW CW gyrotrons. Nevertheless a new load based on pure (uncoated) stainless steel absorbers is being developed as a backup solution for the ongoing high priority gyrotron testing.A superconducting magnet capable of rapid field changes between 4.15 and 5.67 T for frequency step-tunable gyrotrons has been procured, has demonstrated a (static) field of 7.2 T and its capability of rapid field-changes.

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 TE 28,8 cavity mode and provides a linearly polarized, TEM 0,0 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.

Cold test measurements on components of the 1 MW, 140 GHz, CW gyrotron for the stellarator Wendelstein 7-X

For the development of a 1 MW, 140 GHz gyrotron for CW operation which will be installed at the stellarator facility Wendelstein 7-X at IPP Greifswald, a collaboration between different European research institutes and an industrial company has been established. In order to prove the proper functioning of the millimeter wave components installed in the gyrotron — such as the cavity, the waveguide taper and the quasioptical mode converter — these components should be cold tested, preferably before installation. However, due to lack of time as well as long delivery times, this was not possible. Therefore, two units of the quasioptical mode converter and the cavity were fabricated with identical geometry, one of those being used for measurements on the low power test device. To perform these cold tests for tapers and mode converters, the gyrotron cavity output mode has to be simulated. This means that a high order rotating mode (TE 28,8 mode) must be generated at low power. This can be achieved by means of a mode generator consisting of two mirrors and a coaxial cavity with a perforated outer wall. Before applying the mode generator to the components, its proper behavior and the accurate alignment of the system must be verified either by radiation pattern measurements or k-spectrometer measurements. As the coupling through the holes of the k-spectrometer is extremely low, a special vector network analyzer with a dynamic range of at least 100 dB had to be developed. This has been achieved by integration of a phase locked backward-wave oscillator with a line width of 100 Hz and an output power of 10 mW. A non-destructive measurement of the resonance frequency and the quality factor of the cavity does not seem possible. The second cavity will be prepared for the cold measurement by drilling a small radial hole into its wall in the plane of the field maximum. This hole is then used for the input coupling. The accuracy required for this hole is rather critical. The coupling coefficient must be high for sufficient excitation of the rf field, but on the other hand it must neither change the frequency nor the quality factor strongly. The transmission is measured by a probe at the output of the uptaper.