O. Dumbrajs

Theory of gyro-backward wave oscillators with tapered magnetic field and waveguide cross section

A theory is developed which describes gyro-backward wave oscillators (gyro-BWOs) with arbitrarily relativistic electron beams and tapered external magnetic field and waveguide wall radius. For the case of linear tapering the optimal parameters are found that correspond to highly efficient operation. The frequency tunability as the function of magnetic field and voltage deviations has been analyzed. It is shown how to use the results given in terms of normalized parameters for designing concrete gyro-BWOs. For one concrete design the effect of electron velocity spread on the efficiency has been studied.

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.

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

Note: IAEA-TM on ECRH Physics and Technology for ITER, Kloster Seeon, July 14-17, 2003 Reference CRPP-CONF-2003-084 Record created on 2008-05-13, modified on 2017-05-12

Nonstationary oscillations in gyrotrons

The onset of stochastic oscillations in gyrotrons is studied by means of the self-consistent theory describing nonstationary processes. Complicated alternating sequences of regions of stationary, automodulation, and chaotic oscillations are found in the plane of the generalized gyrotron variables: cyclotron resonance mismatch and dimensionless current. The results of the investigations are important in connection with attempts to increase the output power of gyrotrons by raising the current.

Eigenvalues and Ohmic Losses in Coaxial Gyrotron Cavity

The authors present the mathematical analysis for the calculation of the dispersion relation, the field distributions, and the ohmic losses for TEm,p modes in an infinite coaxial waveguide with a longitudinally corrugated insert. The method employed is based on an appropriate eigenfunction expansion, and its main advantage is the very fast convergence with a few spatial harmonics. The analysis is properly extended to include tapered cavities with varying, in respect to the z-coordinate, outer and/or inner radius. Numerical results are presented for several tapered cavity geometries and compared with already published methods

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.

Generalized gyrotron theory with inclusion of electron velocity and energy spreads

The influence of the electron velocity and energy spreads on gyrotron operation is studied by generalizing the well-known η⊥(μ,F) plots by means of the Monte Carlo techniques. Such contour plots may be useful for a quick quantitative estimation of the detrimental effect of the spreads on the efficiency of a gyrotron.

Stochastic sawtooth reconnection in ASDEX Upgrade

In this paper we investigate non-complete sawtooth reconnection in the ASDEX Upgrade tokamak. Such reconnection phenomena are associated with internal m/n = 1/1 kink mode which does not vanish after the crash phase (as would be the case for complete reconnection). It is shown that this sawtooth cannot be fully described by pure m/n = 1/1 mode and that higher harmonics play an important role during the sawtooth crash phase. We employ the Hamiltonian formalism and reconstructed perturbations to model incomplete sawtooth reconnection. It is demonstrated that stochastization appears due to the excitation of low-order resonances which are present in the corresponding q-profiles inside the q = 1 surface which reflects the key role of the q0 value. Depending on this value two completely different situations are possible for one and the same mode perturbations: (i) the resonant surfaces are present in the q-profile leading to stochasticity and sawtooth crash (q0 ≈ 0.7 ± 0.1); (ii) the resonant surfaces are not present, which means no stochasticity in the system and no crash event (q0 ≈ 0.9 ± 0.05). Accordingly the central safety factor value is always less than unity in the case of a non-complete sawtooth reconnection. Our investigations show that the stochastic model agrees well with the experimental observations and can be proposed as a promising candidate for an explanation of the sawtooth reconnection.

Development of a 2-MW, CW Coaxial Gyrotron at 70 GHz and Test Facility for ITER

In ITER, EC heating and current drive (H&CD) is foreseen not only as a principal auxiliary system for plasma heating and as assist for plasma start-up, but is considered essential in meeting the key requirement of neoclassical tearing mode (NTM) stabilisation, by localized current drive. In the reference ECH design, ITER requires a total of 20 MW/CW power at 170 GHz using gyrotrons with a unit power of 1 MW. A higher power per unit (2 MW/gyrotron) would result in a strong reduction of the cost of the whole ECRH system, and would also relax the room constraints on the launcher antenna design. In view of the capability of coaxial cavity gyrotrons demonstrated with short pulse experiments at FZK, the European Fusion Development Agreement (EFDA) has started in 2003 the development of an industrial 170 GHz 2 MW/CW coaxial cavity gyrotron, in a collaborative effort between European research associations CRPP/EPFL, FZK, TEKES and Thals Electron Devices (TED). The development plan includes three steps to reach successively 2 MW/1s, 2 MW/60s and finally 2 MW/CW operation. The procurement of the first prototype is in progress and it scheduled to be delivered during the first quarter of 2006. The experimental tests of the prototypes will be carried out at CRPP/EPFL, where an ITER relevant test facility is presently under construction and will be achieved during the second half of 2005. The test facility is designed to be flexible enough, allowing the possible commissioning of tubes with different characteristics, as well the tests of the launcher antenna at full performances.

Design of a high order volume mode cavity for a l MW/140GHz gyrotron

Abstract A high-power cylindrical cavity gyrotron operating at 140 GHz is studied. The possible working modes TE22,4, TE22,5, and TE22,6,are compared. The output power envisaged is 1 MW. The influence of the space charge effect of the electron beam and the RF losses in the resonator are discussed. Results of numerical calculations of the interaction of the electron beam and the RF wave are presented. The problem of single-mode excitation is addressed in mode competition calculations. Stability regions of the working modes are shown for some interesting parameter ranges. From these plots an optimized variation of the parameters during start-up of the gyrotron can be deduced in order to ensure single-mode operation. On the basis of kinetic theory the influence of misalignment of the electron beam and resonator axis (eccentricity) on the starting currents and frequency shifts of the operating and parasitic modes is studies for several values of eccentricity.