Coaxial Cavity Gyrotron Research Articles

MW gyrotron development for fusion plasma applications

High power gyrotron oscillators are mainly used as millimetre wave sources for electron cyclotron resonance heating (ECRH), electron cyclotron current drive (ECCD), stability control and diagnostics of magnetically confined plasmas for energy generation by controlled thermonuclear fusion. The maximum pulse length of commercially available 1.0 MW gyrotrons, employing open-ended cylindrical resonators and chemical vapour deposition diamond output windows is 12 s at 140 GHz and 9 s at 170 GHz, with efficiencies slightly above 30%. The energy world record of 160 MJ at a power level around 0.9 MW is held by the European FZK-CRPP-CEA-TED collaboration. Total efficiencies of up to 50% have been obtained using a single-stage depressed collector (SDC). To achieve output powers of around 2 MW in continous wave operation at the ITER reference frequency 170 GHz, it is necessary to use a coaxial cavity geometry. A maximum output power of 2.2 MW at 165 GHz was obtained at FZK with an efficiency of 28% (48% in SDC operation). The availability of gyrotrons with fast frequency tunability would permit the use of a simple, fixed, non-steerable mirror antenna for local current drive experiments on ITER. This work reports on the progress in gyrotron development and the status of advanced coaxial cavity gyrotrons and step-wise frequency tunable gyrotrons.

Influence of reflections on the operation of the 2 MW, CW 170 GHz coaxial cavity gyrotron for ITER

The influence of reflections on the operation of the 2 MW, CW 170 GHz coaxial cavity gyrotron oscillating in the mode is studied. In particular, frequency-independent power reflections, e.g. from plasma or some elements of the transmission line, larger than 1% and occurring in a poor phase may lead to oscillation breakdown and should be avoided. Frequency-dependent reflections from a specially designed diamond window influence mode competition, but contract the oscillation region of the operating mode noticeably only under the unrealistic assumption that all reflected power returns to the cavity. The conclusion is that the gyrotron is sufficiently robust against possible negative effects related to reflections.

Design study of a test stand for ITER gyrotron

In the frame of development of the ITER electron cyclotron wave (ECW) system, a two MW CW coaxial cavity gyrotron will be developed during the Sixth Framework Program (2003–2006). Such development relies on the availability of a test stand capable of providing the electrical energy and cooling capacity. This test stand will possibly be used, in a later stage, for the component test of the ITER ECW system. This paper will first present the main parameters of this new coaxial gyrotron. Then we describe the test stand itself, including the general requirements for testing and evaluating the behaviour of the RF source and then a description of the electrical system design. Compared to the ITER reference design, the test stand emphasises the requirement of flexibility, which is necessary during the development of the gyrotron. The additional electrical equipment is included in the overview of the electrical system. The cooling system will be an important part of the design study. Indeed, the design efficiency of such a depressed collector gyrotron is ≈50%, implying >4 MW of continuous heat dissipation and evacuation by the cooling equipment. The specifications of the cooling system must also comply with ITER reference design values.

Progress in gyrotron development

Gyrotrons are microwave oscillators based on the Electron Cyclotron Maser (ECM) instability. The free energy is the rotational energy of weakly relativistic electrons in a longitudinal magnetic cavity field. In contrast, to klystrons the interaction circuit is a high-order-mode cavity allowing higher power at higher frequencies. At present gyrotrons are mainly used as high power millimeter wave sources for electron cyclotron resonance heating (ECRH), electron cyclotron current drive (ECCD), stability control and diagnostics of magnetically confined plasmas for generation of energy by controlled thermonuclear fusion. The maximum pulse length of commercially available 1.0 MW gyrotrons employing open-ended cylindrical resonators and synthetic diamond output windows is 5 s at 110 GHz, 12 s at 140 GHz and 9 s at 170 GHz, with efficiencies slightly above 30%. The energy world record of 160 MJ (0.89 MW at 180 s pulse length and 140 GHz) at power levels higher than 0.8 MW has been achieved by the European FZK-CRPP-CEA-TED collaboration. Total efficiencies around 50% have been obtained using a single-stage depressed collector (SDC). To reduce the costs of the ECRH system on ITER and to make its poloidal launcher for neoclassical-tearing-mode stabilization more compact, a further increase of the output power per gyrotron is desirable. To achieve output powers in excess of around 2 MW in continuous wave (CW) operation at the ITER reference frequency 170 GHz it is necessary to use a coaxial cavity geometry. A maximum output power of 2.2 MW at 165 GHz (1 ms pulse length) was obtained at FZK with an efficiency of 28%. At the nominal output power of 1.5 MW the efficiency increases from 30 to 48% in operation with a SDC. The availability of sources with fast frequency tunability would permit the use of a simple fixed, non-steerable mirror antenna for local current drive experiments. This work reports on the progress in gyrotron development and the status of advanced coaxial cavity gyrotrons and step-wise frequency tunable gyrotrons.

Towards a 2 MW, CW, 170 GHz coaxial cavity gyrotron for ITER

The development work on coaxial cavity gyrotrons has demonstrated the feasibility of manufacturing of a 2 MW, CW 170 GHz tube that could be used for ITER. The problems specific to the coaxial arrangement have been investigated and all relevant information needed for an industrial realization of a coaxial gyrotron have been obtained in short pulse experiments (up to 17 ms). The suitability of critical components for a 2 MW, CW coaxial gyrotron has been proved and a first integrated design has been done.

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.

Numerical Study of Mode Competition in Coaxial Cavity Gyrotrons with Corrugated Insert

A time-dependent self-consistent multimode code which allows to model the interaction between the modes of a coaxial cavity gyrotron operating at the fundamental and/or at higher harmonics of the cyclotron frequency is presented. The Method of Moments (MoM) is used to obtain a rigorous solution of the eigenvalue problem for a coaxial waveguide with corrugated insert. It is shown that the use of the Surface Impedance Model for the calculation of modes operating at higher harmonics leads to incorrect results in high-power coaxial cavity gyrotrons. However, the numerical simulations based on the MoM agree well with the available experimental results.

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.

Novel method of improving performance of coaxial gyrotron resonators

A new method of improving performance of coaxial gyrotron resonators is presented. It is based on the use of a corrugated insert with variable depth of longitudinal corrugations. Such an insert mitigates the mode competition problem in oversized cavities and redistributes heat losses along the axial direction. The method is illustrated in the case of the resonator used in the coaxial cavity gyrotron operating at Forschungszentrum Karlsruhe.

Development of gyrotrons with 2 MW output power

A TE 22,6 gyrotron with conventional hollow cylindrical resonator has attained a maximum output power of 2.1 MW at 140 GHz and an efficiency of 34% (53% with depressed collector). Fast stepwise frequency-tunability with 1-s time steps has been demonstrated with this TE 22,6 gyrotron at a power level of 1 MW. Fast frequency-tunable gyrotrons are of interest for controlling instabilities in magnetically confined plasmas. Two small auxiliary normal conducting magnets in the cryostat bore hole outside the cryostat have been added to the superconducting magnet system. A fast change of the magnetic field by ±326 mT results in a fast frequency change of about ±7.3 GHz with the center frequency at 140 GHz. A TE 31,17 coaxial cavity gyrotron operated at 165 GHz has been developed. A maximum output power of 2.2 MW with an efficiency of 28% at a beam current of 84 A has been achieved with a pulse length of 1 ms. The maximum output efficiency of 30% (48% with depressed collector) has been measured at an output power of 1.5 MW at 56 A.

Gyrokinetic Analysis of Influence of Electron Beam Eccentricity on Operation of Higher-Order Mode in a Coaxial Cavity Gyrotron

A gyrokinetic analysis is presented to the influence of the electron beam eccentricity on the power and starting current of a coaxial cavity gyrotron, which operates in a higher-order mode TE31, 17, 1 with a frequency of 165 GHz. It is found that the starting current becomes larger because of the existence of the electron-beam eccentricity. Especially, the power will be decreased substantially by the electron beam eccentricity, for example, down to 93% of the power without any eccentricity even if the eccentricity is 1% of the outer conductor radius. The acceptable range of the electron beam voltage and operating magnetic field for the establishment of the electromagnetic oscillation is narrowed by the electron-beam eccentricity.

A novel 4.5-MW electron gun for a coaxial cavity gyrotron

A novel 4.5-MW (90 kV, 50 A) electron gun for a 165-GHz coaxial cavity gyrotron operated in the transverse electric mode TE/sub 31, 3.7/ has been designed, fabricated, and operated in a gyrotron. The electrons are extracted toward the anode as in a conventional magnetron injection gun (MIG) and not toward the coaxial insert as in the previously used so-called inverse gun. The main advantage of this arrangement is the reduced overall radial size which becomes comparable to the size of a conventional gun with the same diameter of the emitter. The design and the technology of the electron gun fulfil the requirements for a high-power gyrotron operated at long pulses up to continuous wave. The coaxial insert is fully cooled and can be adjusted when the tube is completely assembled. The amplitude of mechanical vibrations has been measured to be less than 0.03 mm under operating conditions. This is sufficiently small for stable long-pulse operation. In operation at short pulses (ms) a microwave RF output power of 2.2 MW at 165 GHz has been obtained with a beam current of 84 A. The best operating conditions have been observed with an intermediate type of electron flow.

Frequency step-tunable (114–170 GHz) megawatt gyrotrons for plasma physics applications

Successful gyrotron experiments at FZK employing a conventional hollow cylindrical waveguide cavity, a quasi-optical mode converter with dimple-type launcher, a broadband silicon nitride Brewster window and a single-stage depressed collector (SDC), gave up to 1.6 MW output power at efficiencies between 48 and 60% for all operating mode series in the frequency range from 114 to 166 GHz. Frequency tuning in 3.7 GHz steps has been achieved by slow variation (minutes) of the magnetic field strength in the cavity. A specific hybrid magnet system for fast frequency tuning (1 s) is being manufactured. These experiments confirm the preliminary results achieved with a prototype fused quartz glass Brewster window. Up to now, the pulse duration has been 1–5 ms. However a water-edge-cooled silicon nitride composite Brewster window with its thermal conductivity of k=60 W/mK, permittivity ε r=7.85 ( θ Brewster=70.35°), tan δ=3.5×10 −4, excellent mechanical properties and clear window aperture of 100 mm diameter, will allow long-pulse operation (1 s) in the 1 MW power range. By increasing the beam current to 70 A (70% of the limiting current!), a maximum power of 2.14 MW, the highest value ever generated by a weakly relativistic gyrotron, has been obtained at 140 GHz with an efficiency of 34% (53% with SDC). With a TE 31,17 coaxial cavity gyrotron, also equipped with a quasi-optical mode converter and a SDC, a maximum RF-output power of 1.7 MW with an efficiency of 26.2% has been achieved at a beam current I b=69.7 A and a cathode voltage U c=93.2 kV. Around the nominal beam parameters ( I b=52.2 A, U c=91.8 kV, B cav=6.65 T) an RF-output power of 1.3 MW, with an efficiency of 27.2% (41% with SDC), has been measured. In experiments on frequency step tuning, also using the silicon nitride composite Brewster angle window, 19 different modes with ≈2.2 GHz frequency spacing have been excited with ≥1 MW power at frequencies in the range between 134 and 169.5 GHz.

Experimental investigation of a 140-GHz coaxial gyrotron oscillator

We report experimental results on a megawatt power level, 140-GHz coaxial gyrotron oscillator. The gyrotron has an inverted magnetron injection gun (IMIG) designed for operation at up to 95 kV and 88 A. The IMIG has an inner grounded anode which extends from the center of the gun down through the entire length of the tube including the cavity and collector. The IMIG was tested at up to 105 kV and 93 A in 3 /spl mu/s pulses, achieving an electron beam power of 10 MW. The output power from the coaxial gyrotron cavity was transported to an internal mode converter and a single mirror that coupled the power out transversely from the tube axis. A maximum output power of up to 1 MW was obtained in the TE/sub 27,11/ mode at 142 GHz at an efficiency of 16%, about one half of the design efficiency. The reduced efficiency was attributed to nonuniformity of the cathode emission and the sensitivity to the relative alignment of the electron gun, coaxial insert, and cavity. The cathode emission over the azimuthal angle was measured for two cathodes and was shown to be nonuniform due to both temperature and emitter work function nonuniformity. The gyrotron was also tested in two alternate configurations: 1) with the internal mode converter removed (axial output), and 2) with both the internal converter and the coaxial insert removed (empty cavity). In operation in the empty cavity configuration, which is equivalent to a conventional gyrotron oscillator, output power of up to 0.9 MW was observed.

Step-frequency operation of a coaxial cavity gyrotron from 134 to 169.5 GHz

Experiments on frequency step tuning have been performed in a coaxial cavity gyrotron equipped with a Brewster window. In single-mode operation, 19 different modes have been excited with high power In a frequency range between 134-169.5 GHz. In all modes, the radio frequency (RF) power was efficiently transmitted through the output window with only slightly varying RF power distribution. The step-frequency tuning is restricted by the coaxial insert. In particular, the strong increase of the quality factor for small values of the parameter w, which describes the corrugation impedance, limits the maximum frequency range. Furthermore, the increase of ohmic losses at the insert for modes with reduced caustic radius may restrict the selection of usable modes.

A new approach for a multistage depressed collector for gyrotrons

A feasibility study for a two-stage depressed gyrotron collector has been performed. A new approach for an adiabatic magnetic decompression of the hollow electron beam has been used. It permits control of the radius of the constant magnetic flux surface, which determines the radial extension of the electron beam. Independent of the value of the magnetic field around the beam. For this purpose, either solenoidal coils or a ferromagnetic insert can be placed inside the hollow electron beam. Thus, the radial dimensions of a multistage depressed collector of a high-power high-frequency gyrotron can be kept within limits given by technological constraints. The energy sorting of the electron beam is improved by using electrodes inside the hollow electron beam for controlling the potential distribution. The additional control electrodes make it possible to eliminate almost all of the effect of secondary electrons on the operation of the collector. In order to demonstrate the proposed approach, a compact two-stage depressed collector has been designed for a 1.5-MW coaxial cavity gyrotron operating at 165 GHz in the transverse electric (TE)/sub 31,17/ mode, which is under development at FZK, Karlsruhe, Germany. Including the effect due to secondary electrons, a collector efficiency of 73% has been calculated with an average and peak heat dissipation density of about 240 W/cm/sup 2/ and 500 W/cm/sup 2/, respectively. This results in an increase of the output gyrotron efficiency from 36.5% to 62.6% when internal radio frequency (RF)-losses inside the gyrotron tube of 15% are taken into account.

Design of rapid-frequency step-tunable powerful coaxial-cavity harmonic gyrotrons

A design of a new powerful high-frequency short-pulse coaxial-cavity harmonic gyrotron operating at first, second, and third harmonic is presented. The design includes the analysis of mode competition between the fundamental and higher harmonic modes, as well as the investigation of the possibility of the corresponding fast frequency tuning of the gyrotron by changing the accelerating voltage. The possibilities of modification of the existing coaxial cavity gyrotron operating at present at the Forschungszentrum Karlsruhe (FZK) for operation at the second harmonic are discussed. Potentialities of such gyrotrons for plasma physics applications are noted.

Comparison of Electron-Beam-Misalignment Effect on Starting Current and Output Power in Coaxial-Cavity and Cylindrical-Cavity Gyrotron Oscillators

Comparative study is presented to the effect of the electron-beam misalignment on the starting current and output power of the coaxial-cavity and cylindrical-cavity gyrotron oscillators operating in the millimeter wave ranges. The numerical analysis is based on the gyrokinetic formulas for a TE28,16,1 mode at a frequency of 140 GHz. Results show that the coaxial-cavity gyrotron oscillator has lower starting current and less power loss than the cylindrical-cavity gyrotron oscillator when the electron-beam axis has a misalignment to the cavity axis.

Eigenvalue equations and numerical analysis of a coaxial cavity with misaligned inner rod

Based on the Helmholtz equation, the superposition of cylindrical wave functions, and coordinates transformation, the eigenvalue equation is derived rigorously for a coaxial gyrotron cavity with a misaligned inner rod. It is shown that, due to the existence of the structural misalignment, any single normal mode of a perfect coaxial structure (i.e., without misalignment) no longer simultaneously satisfies both the outer and inner boundary conditions; consequently, the superposition of cylindrical wave functions must be taken into account. A numerical approach of solving the eigenvalue equation is proposed in this paper. As a practical application, analysis is given to the higher mode coaxial cavity employed in a 140-GHz/1.5-MW gyrotron device at the Forschungszentrum Karlsruhe, Karlsruhe, Germany. Result shows that the eigenvalue of the operating mode in a misaligned coaxial cavity is affected noticeably by the structural misalignment.

Gyrokinetic description of the structural eccentricity influence on the starting current of a coaxial-cavity gyrotron

Based on the gyrokinetics of free-electron masers, the structural eccentricity influence on the starting current of a 1.5 MW/140 GHz, TE28,16,1 coaxial-cavity gyrotron oscillator is studied. Results show that the structural eccentricity substantially increases the starting current. The stable term of the instability, which results from the contribution of the guiding-center distribution, is of significance in a coaxial-cavity gyrotron, although it has usually been neglected in a cylindrical-cavity gyrotron. It is found that the influence of the structural eccentricity on the performance of the coaxial cavity gyrotron is more serious than that of the beam misalignment. The influences of the operating magnetic field and the beam energy on the starting current are also taken into account.