A First 2 MW-Class (136)/170/204 GHz Multi-Frequency Gyrotron Pre-Prototype for DEMO: Design, Construction and Key Components Verification

DEMO is planned to be the first DEMOnstration fusion power plant that will proof the production of electricity by facilitating the Tokamak concept. The fusion reaction takes place in a magnetically confined plasma that consists of Deuterium and Tritium. A temperature of up to 120 million Kelvin is required for the reaction. Electron Cyclotron Resonance Heating and Current Drive (ECRH&CD) is a possible method to heat and to control the plasma. The sources are high-power vacuum gyrotrons that produce microwave power at MW-levels in the microwave and sub-THz range. The European (EU) DEMO requirements associated with this work are adapted from those of ITER, from 1 MW to 2 MW output power per unit, from 170 GHz single-frequency operation to 136/170/204 GHz multi-frequency / multi-purpose operation and the ability to tune the frequency within a bandwidth of $\pm$ 10 GHz around the center frequency in steps of 2-3 GHz. In this work, the first 2 MW 170/204 GHz dual-frequency coaxial-cavity short-pulse pre-prototype has been designed and built on the basis of an existing 2 MW 170 GHz coaxial-cavity short-pulse pre-prototype. The Magnetron Injection Gun, the coaxial cavity and the quasi-optical output coupler are adapted to the magnetic field profile of a new 10.5 T SC magnet and to the operation at 204 GHz. Existing components, such as the cathode shall be used. The TE$_{40,23}$ mode is identified as the operating mode at 204 GHz. The mode fits to the parameters that are already determined by the TE$_{34,19}$ mode at 170 GHz. A new coaxial cavity is designed using an adapted systematic procedure. This design provides an output power of 2.1 MW at 204 GHz, which is an increase of about 20 %. The design allows an output power up to 2.6 MW at 170 GHz. In addition, a new 170/204 GHz dual-frequency quasi-optical output coupler has been developed providing a Gaussian mode content of > 95 %. Finally, simulations at 136 GHz show a principle operation satisfying the DEMO requirements. Frequency step-tunability around the given center frequencies has been investigated for the first time considering the insert loading constraint during the mode selection process. A novel mode series is presented that includes a jump in the azimuthal mode index from the nominal TE$_{m,p}$ mode to the TE$_{m+2,p-1}$ mode. The proposed mode series reduces the deviation of the relative caustic radii by a factor of two compared to classical approaches. Simulations are performed to tune the gyrotron in steps from high to low frequency and reverse. The deviation of the output power is in maximum 5 % at 170 GHz and 18 % at 204 GHz. An update of the frequency measurement system for gyrotron operation at KIT is performed to verify gyrotrons and their key components operating above 200 GHz. They are upgraded from the current frequency limitation of 175 GHz to at least 260 GHz. Particularly, the quasi-optical mode generator is optimized. It is used to validate the quasi-optical output coupler. The mechanically controlled linear drivers are replaced by high-precision computer-controlled linear drivers. A goniometer has been added for a full electronically adjustment. A novel automated measurement procedure with mode evaluation algorithms is implemented. The optimizations reduce the time needed to excite the desired gyrotron mode with higher quality from months to a few days. The excited cavity modes operating at 170 GHz and 204 GHz feed the 170/204 GHz dual-frequency quasi-optical output coupler for verification in cold measurement. A comparison shows an excellent agreement with simulations. The TE$_{40,23}$ mode at 204 GHz is the mode with the highest eigenvalue ever excited in cold measurements. Finally, a study on alternative mode series is performed considering an extended wall loading constraint of up to 2.5 kW/cm$^2$. These mode series benefit from low deviation of the relative caustic radius. One identified mode series has a beam radius and an insert radius, which is almost identical to the existing mode series. The result is that the most complicated and costly components to manufacture (insert and MIG) can be reused and do not need to be modified. A fast verification of the proposed mode series using several existing components is proposed

Design of a Quasi-Optical Mode Converter for a Dual-Frequency Coaxial-Cavity Gyrotron

A quasi-optical mode converter is under development for an 170/204 GHz coaxial-cavity gyrotron at KIT. It is operated in the TE 34,19mode at 170 GHz and the TE 40,23mode at 204 GHz. A mirror-line launcher should be used for such modes with the ratio of caustic to launcher radius of approximately 0.32. The optimum value of the launcher radius has been found to allow for a high Gaussian-mode content at both frequencies

Plasma Spraying of a Microwave Absorber Coating for an RF Dummy Load

The European fusion reactor research facility, called International Thermonuclear Experimental Reactor (ITER), is one of the most challenging projects that involves design and testing of hundreds of separately designed reactor elements and peripheric modules. One of the core elements involved in plasma heating are gyrotrons. They are used as a microwave source in electron–cyclotron resonance heating systems (ECRH) for variable injection of RF power into the plasma ring. In this work, the development and application of an alumina-titania 60/40 mixed oxide ceramic absorber coating on a copper cylinder is described. The cylinder is part of a dummy load used in gyrotron testing and its purpose is to absorb microwave radiation generated by gyrotrons during testing phase. The coating is applied by means of atmospheric plasma spraying (APS). The absorber coating is deposited on the inner diameter of a one-meter cylindrical tube. To ensure homogeneous radiation absorption when the incoming microwave beam is repeatedly scattered along the inner tube surface, the coating shows a varying thickness as a function of the tube length. By this it is ensured that the thermal power is distributed homogeneously on the entire inner tube surface. This paper describes a modeling approach of the coating thickness distribution, the manufacturing concept for the internal plasma spray coating and the coating characterization with regard to coating microstructure and microwave absorption characteristics

Status and First Operation of Gyrotron Teststand FULGOR at KIT

FULGOR, the new KIT gyrotron teststand for megawatt-class gyrotrons, will be presented. Results of initial experiments using a 1.5 MW 140 GHz short pulse pre-prototype gyrotron will be discussed

170/204 GHz Dual-Frequency Mode Generator for Verification of the Quasi-Optical Output Coupler of a 2 MW Coaxial-Cavity Gyrotron

The 2 MW 170 GHz single-frequency coaxial-cavity short-pulse pre-prototype is upgraded to operate also at 204 GHz. Therefore, the quasi-optical output coupler, which is a gyrotron key component, has been modified. Before the newly manufactured quasi-optical output coupler is installed into the gyrotron, a low-power cold measurement for the verification is performed. Therefore, a mode generator is designed and adjusted to excite the relevant operating gyrotron modes, namely the TE 34,19 mode at 170 GHz and TE 40,23 mode at 204 GHz, with excellent purity and a low counter-rotating amount of < 0.5 % for both modes. The TE 40,23 mode is the mode with the highest eigenvalue ever excited in cold tests. After the successful mode excitation, first the fabricated launcher and then the entire quasi-optical output coupler are verified, showing excellent agreement with the simulation

Theoretical Study on the Operation of the EU/KIT TE34,19-Mode Coaxial-Cavity Gyrotron at 170/204/238 GHz

The 170 GHz 2 MW TE34,19-mode coaxial-cavity modular short-pulse pre-prototype gyrotron at KIT was recently modified in order to verify the multi-megawatt coaxial-cavity technology at longer pulses. In parallel, theoretical investigations on a possibility to operate the 170 GHz TE34,19-mode coaxial-cavity prototype at multiple frequencies up to 238 GHz have been started, with a goal to find a configuration at which the tube could operate in the KIT FULGOR gyrotron test facility using the new 10.5 T SC magnet. This paper indicates which adjustments have to be made and show the feasibility of the multi-frequency operation. Small modifications at the gyrotron cavity will support an RF output power of more than 2 MW at 170/204 GHz. Furthermore, a new gyrotron launcher has been designed capable of producing a Gaussian microwave beam with a Gaussian mode content of more than 96% at these frequencies

Computer-Controlled Test System for the Excitation of Very High-Order Modes in Highly Oversized Waveguides

The generation of a specific high-order mode with excellent mode purity in a highly oversized cylindrical waveguide is mandatorily required for the verification of high-power components at sub-THz frequencies. An example is the verification of quasi-optical mode conversion and output systems for fusion gyrotrons. A rotating high-order mode can be excited by taking a low-power RF source (e.g. RF network analyser) and by injecting the RF power via a horn antenna into a specific adjustable quasi-optical setup, the so-called mode generator. The manual adjustment of the mode generator is typically very time-consuming. An automatized adjustment using intelligent algorithms can solve this problem. In the present work, the intelligent algorithms consist of five different mode evaluation techniques to determine the azimuthal and radial mode indices, the quality factor, the scalar mode content and the amount of the counter-rotating mode. Here, the implemented algorithms, the design of the computer-controlled mechanical adjustment and test results are presented. The new system is benchmarked using an existing TE28,8 mode cavity operating at 140 GHz. In addition, the repeatability of the algorithms has been proven by measuring a newly designed TE28,10 mode generator cavity. Using the described advanced mode generator system, the quality of the excited modes has been significantly improved and the time for the proper adjustment has been reduced by at least a factor of 10

Triode magnetron injection gun for the KIT 2 MW 170 GHz coaxial cavity gyrotron

Considering the recent understanding of the physics of electron trapping mechanisms taking place in the magnetron injection gun (MIG) region of gyrotrons and the sensitivity of the emitter ring manufacturing tolerances on the electron beam quality, a MIG has been designed and manufactured for the 2 MW, 170 GHz coaxial cavity gyrotron developed at Karlsruhe Institute of Technology. The new MIG has the following novelties: (i) the design satisfies the criteria for the suppression of the electron trapping mechanisms, (ii) a new type of emitter ring is used for the suppression of the influence of the manufacturing tolerances and misalignments on the quality of the generated electron beam, and (iii) the design was optimized to generate a good beam quality in a wide variety of magnetic field profiles to increase the flexibility. An additional important feature of the new triode MIG design is the possibility to operate with only two power supplies by using a special start-up scenario. The first experimental results of the coaxial cavity gyrotron with the new MIG are presented