Gyrotron Operation Research Articles

Operation of a sub-terahertz CW gyrotron with an extremely low voltage

Decreasing the operating voltage for medium-power sub-terahertz gyrotrons aimed at industrial and scientific applications is highly attractive, since it allows size and cost reduction of the tubes and power supply units. In this paper, we examine such an opportunity both numerically and experimentally for the fundamental cyclotron resonance operation of an existing gyrotron initially designed for operation at the second cyclotron harmonic with a relatively high voltage. Simulations predict that output power higher than 10 W can be produced at the fundamental harmonic at voltages less than 2 kV. To form a low-voltage helical electron beam with a sufficiently large pitch-factor, a positive voltage was applied to the first anode of the gyrotron three-electrode magnetron-injection gun with a negative voltage at the cathode. CW gyrotron operation at voltages down to 1.5 kV has been demonstrated at a frequency about of 256 GHz.

RF Behavior of a 220/251.5-GHz, 2-MW, Triangular Corrugated Coaxial Cavity Gyrotron

In this paper, RF behavior studies of a dual regime coaxial cavity gyrotron designed for electron cyclotron resonance heating and current drive of magnetically confined plasmas in the future fusion reactors are presented. Considering all the design constraints, the mode pair is chosen as TE48,30 and TE55,34 for operation at 220 and 251.5 GHz, respectively. The interaction circuit is initially designed through the cold cavity design. A triangular corrugated insert offers good mode selection and also reduces the localized heating problem. Single mode computations are carried out to optimize the beam parameters for the maximum efficiency of the chosen mode pair. Time-dependent multimode simulations are carried out to conform the power in the desired mode pair and the possibility of power in the competing modes. Start-up analyses are performed before and after space-charge neutralization with nonuniform magnetic field using nominal electron beam parameters obtained from magnetron injection gun calculations. These studies ensure that the continuous wave operation of a coaxial cavity gyrotron is possible with the output power of ≈2 MW with the chosen mode pair.

CW Experiments With the EU 1-MW, 170-GHz Industrial Prototype Gyrotron for ITER at KIT

The European 1-MW, 170-GHz continuous wave industrial prototype gyrotron for electron cyclotron resonance heating and current drive on international thermonuclear experimental reactor was during 2016 under test at the Karlsruhe Institute of Technology (KIT) test facility. In order to optimize the gyrotron operation, the tube was at first thoroughly tested in the short-pulse regime, with pulses that did not exceed 10 ms, for a wide range of operational parameters. Then, and after proper conditioning of the tube, the operation was extended to longer pulses with duration up to 180 s, which is the maximum pulselength possible at the KIT test facility. In this paper, we present in detail the achievements of the long-pulse experimental campaign.

Terahertz broadband tunable pulse gyrotron

Based on the principle of a relativistic electron cyclotron maser, gyrotrons can generate high-power coherent radiation in the millimeter-terahertz (THz) waveband. A pulse magnet can generate an ultra-high field strength, and simultaneously reduces the volume by several times compared with a conventional superconducting magnet, which promotes a THz gyrotron to break the 1 THz barrier. However, only an extremely short duration around the peak field of the pulse magnet can be used for a conventional open-cavity gyrotron fixed-frequency operation. In this letter, a novel gyrotron interaction scheme is proposed to excite the broadband THz radiation by integrating a broadband pre-bunched interaction circuit with a pulse magnet, which is a promising way to expand the frequency tuning bandwidth, enlarge the magnetic field by utilizing the range of the pulse magnet, extend the operating pulse duration of a gyrotron, and realize the quasi-continuous operation of a pulse magnet gyrotron. After an investigation into the frequency and time domains, a broadband pulse gyrotron driven by a 20 kV low-voltage electron beam is predicted to generate radiation with a frequency of between 0.328 and 0.338 THz, with a peak power of 2.1 kW in a 6 ms pulse duration.

Study of the Influence of Stray Magnetic Fields on the Operation of the European Gyrotron for ITER

In this paper, the influence of stray magnetic fields (SMFs) in the ITER electron cyclotron resonance heating and current drive (ECRH-CD) building on the operation of the European (EU) 170 GHz, 1 MW, continuous wave gyrotrons is investigated. There are mainly two sources of SMF in the ITER ECRH-CD building: 1) the tokamak and 2) the neighboring gyrotrons which are placed in close proximity. The possible electron beam degradation caused by the influence of the SMF is numerically investigated. Furthermore, the collector operation under the influence of an SMF is also studied. A proposal to compensate the influence of the SMF is discussed.

Generation of Rogue Waves in Gyrotrons Operating in the Regime of Developed Turbulence.

Within the framework of the average approach and direct 3D PIC (particle-in-cell) simulations, we demonstrate that the gyrotrons operating in the regime of developed turbulence can sporadically emit "giant" spikes with intensities a factor of 100-150 greater than the average radiation power and a factor of 6-9 exceeding the power of the driving electron beams. Together with the statistical features such as a long-tail probability distribution, this allows the interpretation of generated spikes as microwave rogue waves. The mechanism of spikes formation is related to the simultaneous cyclotron interaction of a gyrating electron beam with forward and backward waves near the waveguide cutoff frequency as well as with the longitudinal deceleration of electrons.

A Trial Experiment on Water-Cooled 1.8-T 50% Duty Solenoid

A copper solenoid for continuous operation of 95-GHz gyrotron has been developed and operated. The solenoid was designed to provide magnetic field of 1.8 T for the interaction of 95-GHz gyrotron at the second harmonic. The solenoid is comprised of concentric cylindrical segments of copper foil windings. Cooled water at 4 °C is pumped into the system to sink the heat, as well as to reduce the copper temperature, the electric resistance, and the dissipated power inside the system. In this paper, we present experimental results of the solenoid operated with a 50% duty cycle for a long period of time until steady state is obtained.

Multi-physics analysis of a 1 MW gyrotron cavity cooled by mini-channels

The interaction cavity of the European 170GHz, 1MW Continuous Wave (CW) gyrotron for ITER, which could also be water-cooled using mini-channels as recently proposed, experiences during operation a very large heat load (>15MW/m2) localized on a very short (<1cm) axial length. Such heat loads are typical for high power gyrotrons.As the thermal deformation of the cavity influences the electromagnetic field structure and consequently the gyrotron operation, the analysis of the cavity performance requires the simultaneous solution of the coupled thermal-hydraulic, thermo-mechanic and electro-magnetic fields. In this paper, the thermal behaviour of the cavity under nominal heat load is computed first by CFD. Then a 3D thermo-mechanical model of the cavity is developed, based on the temperature maps computed by CFD, to evaluate the resulting deformation of the inner cavity surface. Finally the deformation is used to compute the updated heat load coming from the electromagnetic field generated by the electron beam in the deformed cavity, which becomes the input for a new iteration of the thermal-hydraulic, thermal-mechanical and electromagnetic analyses. It is shown that this iterative procedure converges to a self-consistent heat-load/temperature-field/deformation-field picture in nominal operating conditions, without exceeding a temperature of ∼230°C on the inner surface of the cavity.

Demonstration of synchronous control of EC TL switch and gyrotron for ITER EC system

The ITER EC system includes both an equatorial port launcher and upper port launchers as RF power injection devices, The waveguide switch in the transmission line (TL) is used to select the operating launcher. Its operation is required even during plasma operation, primarily for the mid-pulse switch operation. Changing the waveguide switch requires that the gyrotron stop RF power during the switch operation since the direction is changed by mechanical movement of the mirror position which takes a few seconds. Since the ITER EC system is based on a multi-subsystem concept, each subsystem has its own subsystem control unit (SCU). The EC main controller supervises all subsystem controllers. Hence, cooperative operation requires the sharing of the information of both RF power status and switch status between the gyrotron SCU and TL SCU via the main controller. Since the design of the inter-subsystem control scheme is a key issue for ITER EC system control, its evaluation is required. At QST, the gyrotron and ITER-relevant TL test stand were utilized for demonstration of mid-pulse switch operation. For this purpose, SCUs for each subsystem and the main controller were developed using the ITER-relevant control system. The operation of the mechanical switch during gyrotron pulse was demonstrated. During the 150s operation of the high power gyrotron at 400kW level, the waveguide switch in the TL was operated to change the direction of RF power. The time duration for the switch operation with inter-subsystem control scheme took 1.5s in total. The synchronizing of RF power suspend and resume with switch motion has therefore been realized, and RF power direction control during the gyrotron operation was successfully demonstrated.

Magnetic field profile analysis for gyrotron experimental investigation

The external applied magnetic field plays a main role on the gyrotron operation. Even if the gyrotron design is optimized for the nominal magnetic profile, it is possible the performance to be better for an alternative one. This unexpected result can happen for several reasons, such as the manufacturing imperfections, the misalignment issues, and several unknown factors in gyrotron theory and design. The systematic experimental investigation of the gyrotron in different magnetic profiles is very important for the optimization of the gyrotron operation and for the better understanding of the gyrotron behavior. In this sense, an analytical approach for the definition of the appropriate magnetic profiles based on the beam characteristics instead of the coil currents definition is proposed for a systematic experimental study. Using this technique, operational maps in the space of the important magnetic profile parameters are developed, which are very useful for the characterization of the gyrotron performance. For the demonstration of this idea, the operational maps of the short-pulse prototype of the 170 GHz, 1 MW European ITER gyrotron project are presented.

Experimental verification of the European 1 MW, 170 GHz industrial CW prototype gyrotron for ITER

The EU 1MW, 170GHz gyrotron with hollow cylindrical cavity have been designed within the European GYrotron Consortium (EGYC) in collaboration with the industrial partner Thales Electron Devices (TED) and under the coordination of Fusion for Energy (F4E). In the frame of the EU program, the short-pulse (SP) version of this tube was designed and manufactured by KIT in collaboration with TED. The experimental verification of the SP gyrotron prototype was successfully completed in 2015. The achieved experimental results show a very stable gyrotron operation with RF output power above 1MW at good cavity interaction efficiency around 35%. The gyrotron was operated up to 10ms pulse length; the nominal cavity mode TE32,9 has been excited at the frequency 170.1GHz in agreement with the corresponding ITER specification. The Gaussian mode content of the output RF beam was about 98% and the total level of internal stray radiation was in the range of 2–3%. The manufacturing of the first industrial continuous-wave (CW) prototype gyrotron, based on the SP gyrotron design, was completed in November 2015 at TED. The tube was delivered to KIT and the experimental verification started in February 2016. Operation of the gyrotron with ms pulse duration resulted in stable excitation of the nominal mode at 170.22GHz for a wide range of operating parameters and with a level of stray radiation comparable with the SP version. The maximum RF power achieved up to now in short-pulse operation at a level of 1MW. The measured Gaussian mode content of the RF beam is 97%. Currently, further preparation for the CW operation of the gyrotron is in progress.

Physics-based modelling of the ECRH system for MHD control applications

The state of the art in NTM control involves electromagnetic wave beams as primary actuator. These are generated by gyrotron sources and injected into the plasma via the steerable mirrors of controlled electromechanical launchers. In connection to the control design for the overall process, a theoretical model of the ECRH system should include, among others, the gyrotron operation, the launcher mirror dynamics, the motion transmission to the mirror by the associated servomotors, and the beam propagation in the transmission lines and the plasma. In this work, we present fragments of the physics-based modelling which is involved in the control system design.

Terahertz Broadband-Tunable Minigyrotron With a Pulse Magnet

A minigyrotron scheme controlled by a compact pulse magnet to excite broadband terahertz (THz) radiation is presented here. In comparison to an open-cavity circuit, the adopted prebunched backward-wave interaction circuit can expand tuning bandwidth tenfold under the control of time-varying magnetic field strength, which also significantly extends the available duration time of the pulse magnet for gyrotron operation. A quasi-optical mode convertor and a Brewster window constitute the output system to transfer the broadband radiation from the circuit into free space. A systematic gyrotron design is also presented. Driven by a low-voltage electron beam, the minigyrotron is predicted to generate radiation with 10-GHz tuning bandwidth around 0.33 THz and a maximum peak power of 2.1 kW with 6-ms pulse duration, using a TE6,2 mode interaction. Such a THz gyrotron with broad tunable bandwidth, kilowatt level power, and with the unique advantage of a compact configuration is the key to high-power THz scientific and industrial applications.

Embedded control systems for gyrotrons applications based on NI solutions

The Swiss Plasma Center is involved in the development and the operation of gyrotrons for fusion application (TCV tokamak, W7-X, ITER, DEMO) as well as for basic science applications such as Nuclear Maggnetic Resonance (NMR) spectroscopy. In this framework, embedded control systems based on National Instrument (NI) compact Reconfigurable Input Output (cRIO) and compact Data AcQuisition (cDAQ) offer versatile solutions for dedicated applications. Three specific developments are briefly presented and discussed here. First, a complete control system based on cRIO material has been implemented in a Modulator Power Supply. Second, the development of a compact system, based on a cDAQ solution used for the characterization of the RF losses on the various gyrotron components. Third, the installation of a whole control system for a Dynamic Nuclear Polarization (DNP/NMR) gyrotron dedicated to non-specialist gyrotron users.The flexibility given by these compact control systems and acquisition could offer multiple solutions in many applications for fusion research.

Instrumentation and control system architecture of ECRH SST1

The Electron Cyclotron Resonance Heating (ECRH) system is an important heating system for the reliable start-up of tokamak. The 42GHz and 82.6GHz Gyrotron based ECRH systems are used in tokomaks SST-1 and Aditya to carry out ECRH related experiments. The Gyrotrons are high power microwave tubes used as a source for ECRH systems. The Gyrotrons need to be handled with optimum care right from the installation to its Full parameter control operation. The Gyrotrons are associated with the subsystems like: High voltage power supplies (Beam voltage and anode voltage), dedicated crowbar system, magnet, filament and ion pump power supplies and cooling system. The other subsystems are transmission line, launcher and dummy load. A dedicated VME based data acquisition & control (DAC) system is developed to operate and control the Gyrotron and its associated sub system. For the safe operation of Gyrotron, two level interlocks with fail-safe logic are developed. Slow signals that are operated in scale of millisecond range are programmed through software and hardware interlock in scale of microsecond range are designed and developed indigenously. Water-cooling and the associated interlock are monitored and control by data logger with independent human machine interface.

Experimental Results of the EU ITER Prototype Gyrotrons

The European 1 MW, 170 GHz CW industrial prototype gyrotron for ECRH&CD on ITER was under test at the KIT test facility during 2016. In order to optimize the gyrotron operation, the tube was thoroughly tested in the short-pulse regime, with pulse lengths below 10 ms, for a wide range of operational parameters. The operation was extended to longer pulses with a duration of up to 180 s. In this work we present in detail the achievements and the challenges that were faced during the long-pulse experimental campaign.

Update on the DIII-D ECH system: experiments, gyrotrons, advanced diagnostics, and controls

The ECH system on DIII-D is continuing to be upgraded, while simultaneously being operated nearly daily for plasma experiments. The latest major hardware addition is a new 117.5 GHz gyrotron, which generated 1.7 MW for short pulses during factory testing. A new gyrotron control system based on Field Programmable Gate Array (FPGA) technology with very high speed system data acquisition has significantly increased the flexibility and reliability of individual gyrotron operation. We have improved the performance of the fast mirror scanning, both by increasing the scan speeds and by adding new algorithms for controlling the aiming using commands generated by the Plasma Control System (PCS). The system is used for transport studies, ELM control, current profile control, non-inductive current generation, suppression of MHD modes, startup assist, plasma density control, and other applications. A program of protective measures, which has been in place for more than two years, has eliminated damage to hardware and diagnostics caused by overdense operation. Other activities not directly related to fusion research have used the ECH system to test components, study methods for improving production of semiconductor junctions and materials, and test the feasibility of using ground based microwave systems to power satellites into orbit.

RF Behavior and Launcher Design for a Fast Frequency Step-tunable 236 GHz Gyrotron for DEMO

AbstractAs part of the EUROfusion project, the conceptual design of a 1 MW 236 GHz hollow-cavity gyrotron is ongoing at IHM, KIT for a DEMOnstration Power Plant (DEMO), along with a 2 MW coaxial-cavity design concept. Fast frequency-tunable gyrotrons (tuning within a few seconds) are recommended for plasma stabilization using a non-steerable antenna. In this work, the mode-selection approach for such a frequency-tunable gyrotron is presented and suitable operating modes for fast frequency tunability are suggested. Magnetic field tuning has been studied as an effective technique to tune the gyrotron operating frequency. The step-tunability of the 236 GHz gyrotron within the frequency range of ±10 GHz in steps of 2–3 GHz is demonstrated in numerical simulations. A hybrid-type Quasi-Optical Launcher (QOL) has been designed for a step-frequency tunable gyrotron with sufficiently high Fundamental Gaussian Mode Content (FGMC).

Development of 50-kV 100-kW Three-Phase Resonant Converter for 95-GHz Gyrotron

This paper describes the development of a 50-kV 100-kW cathode power supply (CPS) for the operation of a 30-kW 95-GHz gyrotron. For stable operation of the gyrotron, the requirements of CPS include low output voltage ripple and low arc energy less than 1% and 10 J, respectively. Depending on required specifications, a three-phase series-parallel resonant converter (SPRC) is proposed for designing CPS. In addition to high-efficiency performance of SPRC, three-phase operation provides the reduction of the output voltage ripple through a minimized output filter component that is closely related to the arc energy. For allowing symmetrical resonant current from three-phase resonant inverter, the high-voltage transformers are configured as star connection with floated neutral node. This facilitates balanced voltage on each secondary winding. In addition, distinctive design of the high-voltage rectifier is introduced, taking into consideration the effective series stacking of diodes by means of the parallel resonant capacitor. In particular, the implementation of the high-voltage part including transformer and rectifier is presented in detail. For providing high power density and high reliability, effective methods for winding the high-voltage transformer and stacking rectifier diodes are discussed. Finally, the developed CPS achieves 95.5% of maximum efficiency, 0.92 of maximum power factor, 500 W/liter of power density, 0.6% of output voltage ripple, with 8.3-J arc energy.

I/O System for A 77/154-GHz, 0.5-MW Dual Regime Gyrotron

This paper contributes to the design of a common input/output (I/O) system to support a dual regime gyrotron operation. I/O system comprises of different subassemblies such as magnetron injection gun (MIG), quasi-optical launcher, and RF window. The operating frequencies selected are 77/154 GHz with the desired mode-pair TE15,4/TE28,8, respectively. The necessary selection criteria of such unified design for these subassemblies has been briefly discussed. The initial design parameters have been selected using our in-house code Gyrotron Design Suite version 3.0 2014 (GDSv3.0 2014). Final optimization of MIG to generate electron beam with proper parameters to support the maximum power transfer has been carried out using a particle trajectory code ESRAY. A dimpled wall, helical cut launcher has been designed to convert the complex higher order modes to a simple Gaussian-like beam and has been optimized with a commercial software launcher optimization tool. The design and optimization of RF window has been done using GDSv3.0, which will allow the chosen frequency beam to be extracted out from the gyrotron.