W environment in steady-state tokamak (WEST) experiments aim to achieve long plasma pulses (1000 s) and to expose ITER-like tungsten divertor to power fluxes up to 10 MW/m2. To increase the margin to reach H-Mode regimes and control W-impurities in the plasma, the WEST ECRH system is upgraded to a power capability of 3 MW/1000 s at the frequency of 105 GHz. The Tore Supra ECRH antenna has first been improved and is now installed in the equatorial plane of WEST to start ECRH operation on plasma. On the power generation side, a new gyrotron (1 MW/1000 s) designed jointly by KIT and NKUA and manufactured by THALES has been tested in the FULGOR test bench at KIT. The validation of the gyrotron electron optic and Radio Frequency (RF) design with performances of 1 MW/5 s triggered the assembly of the second and third series gyrotrons. In parallel to the gyrotron validation and the installation of the upgraded antenna in the tokamak, the ECRH transmitter has undergone profound changes to adapt the auxiliaries to the new components (gyrotron, superconducting magnet, RF load) such as the cooling system, the high voltage power supply (HVPS) system and the plant environment itself. To control, monitor and ensure the safety of staff and components, the control system, software and communication protocols with the auxiliaries of the gyrotron have been completely refurbished. The commissioning of the complete system now equipped with the first gyrotron started in September 2024 in WEST with the goal of achieving the first ECRH shots during plasma experiments in April 2025. This article reports the main results obtained with the first gyrotron, focuses on the upgrades of the ECRH plant and its commissioning and finally gives a summary of the first experiments with ECRH in WEST.

The starting pitch factor of a transverse electric (TE) mode in a gyrotron cavity is defined as the minimum pitch factor (electron velocity ratio) that an electron beam of given current must have, in order for the mode to be excited by the beam. In high-power gyrotrons with dense spectra of competing modes, the knowledge of the starting pitch factor of several modes at different operating parameters would provide a valuable insight into the expected mode competition. Therefore, a numerical code is developed for the calculation of the starting pitch factor, based on the mathematical model of the beam-wave interaction in the gyrotron cavity. Self-consistency is achieved by solving simultaneously the equations for the electron motion and for the axial profile of the high-frequency field. Numerical results for the calculation of the starting pitch factor as well as the starting current of TE modes in various cavities are presented. The code validation, involving comparisons with the time-dependent large-signal code EURIDICE as well as with the small-signal spectral code TWANGlinspec, is also presented. Using an example of a challenging coaxial cavity design for second harmonic MW-class operation, it is shown that, in high-power gyrotrons, the calculation of the starting pitch factors of the competing modes is much more advantageous than that of their starting currents because it can immediately identify practical gyrotron start-up scenarios with a triode-type electron gun that mitigate mode competition.

In the need of development of gyrotrons capable of fast switching between two widely separated operating frequencies, allowing the same device to deliver power for both Electron Cyclotron Resonance Heating (ECRH) and rapid Radiative Instability (RI) suppression in fusion reactors, the existing 2nd harmonic coaxial cavity at KIT, designed to provide an output power of 1.55 MW at 170 GHz, is studied to assess whether it allows room also for efficient MW-class operation at the fundamental cyclotron frequency. The feasibility of dual-harmonic operation at 170/85 GHz and at 170/88 GHz is theoretically demonstrated, together with a novel concept for fast switching (in the range of milliseconds) between the two harmonics, using a triode-type electron gun. The results build a foundation for future coaxial-cavity designs for MW - class continuous-wave dual-harmonic gyrotron operation.

The experimental verification of a quasi-optical TE28,12-mode launcher antenna for the Wendelstein 7-X (W7-X) gyrotron is presented. A step-type mode converter was used to convert the TE28,10 mode, excited in an existing mode generator to the targeted TE28,12 mode. The TE28,12 field pattern was measured with high spatial resolution, achieving a scalar mode purity of 84.6 % and a counter-rotating component below 0.2 %. The measured mode purity fits well to the theoretical design values. The launcher radiation pattern is in excellent agreement with simulation and has a scalar correlation factor of 96.4 %. These results demonstrate the reliability of the step-type mode converter. It provides a reliable and efficient method for mode conversion and preliminary launcher verification with already at KIT existing mode generators.

Gyrotrons are essential sources for electron cyclotron resonance heating in magnetic confinement fusion devices. Achieving high overall efficiency and the nominal design power in these devices is a critical requirement. However, discrepancies between theoretical models and experimental results often arise because real systems exhibit additional effects that are not fully captured in idealized simulations. One such effect is the static After-Cavity Interaction (ACI) of the same transverse electric (TE) mode in the cavity uptaper and quasi-optical launcher, which can significantly reduce performance. A thorough understanding of ACI, supported by advanced simulation tools, is essential to identify effective mitigation strategies. Practical measures include fine adjustments of the magnetic field, operation at alternative working points, or modifications to the launcher geometry. Such approaches offer promising potential to enhance overall efficiency in gyrotrons for present and future fusion applications.

This paper presents the design, fabrication and preliminary assessment of the THALES TH1512 1 MW 117.5 GHz CW gyrotron, dedicated to the DIII-D fusion facility. Short pulse tests at KIT demonstrated 1.3 MW output and $48\%$ efficiency with readiness for long-pulse operation.

This paper reports the design, manufacturing, and testing of the THALES TH1511 $105 \mathrm{GHz}, 1 \mathrm{MW} \mathrm{CW}$ gyrotron for WEST. The prototype was tested in short pulses at KIT, demonstrating 1.2 MW power and $46 \%$ efficiency capabilities with readiness for long pulse operation.

Following the recent upgrade of the Electron Cyclotron Resonance Heating (ECRH) system at the stellarator Wendelstein 7-X (W7-X) with a 1.5-MW, 140-GHz gyrotron (TH1507U), efforts are now focused on further advancing gyrotron technology by developing a 2-MW, 140-GHz Continuous Wave (CW) prototype. This paper proposes two RF and electron beam optics designs of a conventional-cavity 2-MW, 140-GHz gyrotron, operating with the TE28,12 mode. Each design corresponds to different operating conditions, namely Low-Voltage HighCurrent (LVHC) and High-Voltage Low-Current (HVLC). The primary objective of the work is to leverage the existing infrastructure at W7-X while minimizing design modifications to the existing 1.5-MW gyrotron, therefore ensuring cost efficiency and increasing the possibility for a rapid implementation of the proposed designs. Given the significant (and challenging) space-charge depression associated with the TE28,12 mode, several gyrotron startup scenarios are investigated thoroughly using the existing TH1507U diode Magnetron Injection Gun (MIG) as well as a new triode-type MIG design, which is based on the diode design. The findings of this study offer key insights into the design and operational challenges of future 2 MW-class gyrotrons operating at 140 GHz and beyond.

Gyrotrons are essential sources for electron cyclotron resonance heating in magnetic confinement fusion devices. Achieving high overall efficiency and the nominal design power in these devices is a critical requirement. However, discrepancies between theoretical models and experimental results often arise because real systems exhibit additional effects that are not fully captured in idealized simulations. One such effect is the static after-cavity interaction (ACI) of the same transverse electric (TE) mode in the cavity uptaper and quasi-optical launcher, which can significantly reduce performance. A thorough understanding of ACI, supported by advanced simulation tools, is essential to identify effective mitigation strategies. Practical measures include fine adjustments of the magnetic field, operation at alternative working points, or modifications to the launcher geometry. Such approaches offer promising potential to enhance overall efficiency in gyrotrons for present and future fusion applications.

The international race toward realizing the first economically viable fusion power plant is in full progress. Start-ups, in particular, are proposing compact high-field tokamak experiments that require electron cyclotron heating at frequencies well above 200 GHz. This demand drives the intensive pursuit of high-power gyrotrons operating beyond 200 GHz. Second harmonic operation offers a promising route to reach such frequencies without necessitating stronger magnetic fields. However, the intrinsically lower interaction efficiency and strong competition from first harmonic modes pose significant challenges. To enhance mode selectivity, we propose a cavity design with a novel scheme of profiled impedance corrugations, changing the surface impedance on the inner conductor along the axis of a coaxial gyrotron cavity. This design achieves unprecedented suppression of competing first harmonic modes while maintaining ohmic wall loading levels compatible with continuous-wave (CW) operation. As a result, this novel corrugation scheme enables higher output power, broadens the range of stable gyrotron operation, and substantially reduces sensitivity to electron beam quality. These advances establish tapered impedance corrugations as a powerful tool for realizing robust, efficient, and scalable harmonic gyrotrons at sub-terahertz frequencies.

The scheduled upgrade of the Electron Cyclotron Resonance Heating (ECRH) system at the stellarator Wendelstein 7-X (W7-X) has now focused studies from the available 1.5-MW, 140-GHz gyrotron (TH1507U), towards a future 2-MW, 140-GHz gyrotron. To advance in 2-MW operation a higher-order operating mode than the TE28,10 mode used in the 1.5-MW gyrotron is required and its selection depends on various criteria. This paper explores operation with three TE mode candidates by taking into account the compatibility with the existing infrastructure at W7-X as well as the possibilities of similarity to existing successful tubes. Using either the conventional or the coaxial-cavity concept was also investigated.

Relativistic oscillator calculation kit (ROCK) is a self-consistent time-dependent multimode multi-harmonic code recently developed at KIT to simulate the interaction between electrons and microwaves in gyrotron cavities. Its key features include a flexible software design, the use of the finite element method, the capability for precisely resolving the electron motion, and an enhanced formulation of field excitation. The last two features are essential model differences and are discussed in detail. Preliminary simulation comparisons demonstrate the validity and highlight the potential advantages of the improved model implemented in ROCK.

Gyrotrons are high-power microwave sources that play an important role in the heating of plasmas for magnetically confined thermonuclear fusion applications. This paper presents a comprehensive study of two potential strategies for operating high-power megawatt-class fusion gyrotrons at the second harmonic of the electron cyclotron frequency which requires only half of the gyrotron cavity magnetic field. The first approach focuses on a coaxial cavity design that effectively suppresses fundamental competing modes, making it a robust solution for second harmonic operation. The second strategy discusses the injection of an external locking signal. Therefore, a quasi-optical mode converter was designed and tested capable of handling both, co- and counter-rotating modes.

Gyro-TWTs with helically corrugated interaction region (HCIR) have shown to be effective for broadband and high-power amplification of signals up to the W-band. In this publication, versions for 263 GHz (G-band) are proposed, which are able to generate above 1 kW of output power with a gain of 30 dB and 50 dB respectively. To avoid self-oscillations, a dielectric lined drift section is introduced and its effect on the interaction investigated. Furthermore, the effect of a non-ideal electron beam on the amplifier gain is examined. It is found that it has a great influence on the bandwidth; its influence on the maximum gain, however, is limited.

Karlsruhe Institute of Technology (KIT) is advancing the research and development of a short-pulse pre-prototype as the base for a continuous-wave (CW) industrial gyrotron with an output power exceeding 1 Megawatt (MW) at an operating frequency of around 240 GHz. It shall become the key component of an Electron Cyclotron Resonance Heating (ECRH) system for a future fusion power plant operating at a toroidal magnetic field of around 9 Tesla, specifically designed for Proxima Alpha. The pre-prototype shall be built-up and tested at the KIT FULGOR gyrotron test-facility, that will receive a 10.5 Tesla superconducting gyrotron magnet latest by 2025. This paper outlines the development for advancing the 240 GHz gyrotron. Possible designs are evaluated for their potential to manage the thermal loading, to minimize mode competition, and, to advance operating stability. The fundamental design choices and key challenges, including thermal management, cavity mode selection, and precision alignment, as well as simulation results for electron beam propagation and cavity performance are discussed.

The Short-Pulse (SP) ExB Multistage Depressed Collector (MDC) prototype developed and built at Karlsruhe Institute of Technology (KIT) is theoretically investigated for its robustness to manufacturing tolerances and non-optimized gyrotron operating scenarios. The tolerance analysis is done based on a parametric definition of the collector electrodes for the optimized gyrotron operation with the KIT 2 MW 170 GHz coaxial-cavity gyrotron. The six most dominant geometrical parameters are considered. The prototype design is then adapted to 5 different gyrotron configurations and operation modes, which include the Wendelstein 7-X (W7-X) Upgrade SP gyrotron for 140 GHz and 105 GHz operation, as well as the coaxial-cavity gyrotron in multi-frequency operation at 136 GHz and 204 GHz (in addition to the nominal operation at 170 GHz) and operation at the second harmonic at 170 GHz. The necessary modifications to the mechanical adaption, the magnetic configuration and electrode potentials are highlighted for promising MDC operation in all cases.

Gyro-traveling-wave amplifiers (TWAs) with a helically corrugated interaction region (HCIR) are used for broadband, high-power amplification of microwave signals. In this publication, an HCIR for 265 GHz (G-band) and the influence of manufacturing tolerances on the HCIR performance are presented. This is done by using three different approaches. First, the influence of uniform deviations along the HCIR on the wave dispersion is considered. Second, particle in cell simulations for uniform deviations are conducted. Third, the influence of nonuniform randomly distributed manufacturing tolerances is investigated. It is found that uniform deviations along the HCIR strongly degrade gain and bandwidth, which results in small acceptable tolerances for these deviations. However, these constitute systematic errors, which can be compensated by adjusting the manufacturing process. In contrast, nonuniform deviations along the HCIR with Gaussian random distribution, e.g., caused by vibrations during manufacturing, do not exhibit the same characteristics. These are investigated by PIC simulation of HCIRs with random deviations along their outer wall. Their effect on gain and bandwidth has been shown to be smaller compared to uniform deviations, which allows for increased tolerances for nonuniform deviations.

For the operation of a helical-waveguide gyro-TWT a circularly polarized microwave, which is co-rotating with respect to the beam electrons, is required in the interaction region. Hence, the linearly polarized input wave has to be converted into a circularly polarized one. A polarizer based on a truncated circular cross-section at 263 GHz (G-band) is presented. The final polarizer is able to produce a circularly polarized wave at its output with an axial ratio of below 1.22 and a bandwidth of 20 % around its center frequency. Additionally, a reflection better than -30 dB and an insertion loss of below 0.3 dB are achieved. Furthermore, it is found that the bandwidth does not dependent on the taper length.

The first 140 GHz 1.5 MW CW (1800 s) industrial gyrotron (TH1507U), manufactured by THALES for the W7-X stellarator, has established a worldwide record in its category by demonstrating 1.3 MW output power in long pulse operation. The main features of the industrial unit and the achieved performances are briefly discussed.

In the frame of a EUROfusion Enabling Research (ENR) Project, KIT and NKUA have been working on new technical concepts for highly efficient, megawatt-class fusion gyrotron systems that will operate at the 2nd-harmonic of the electron cyclotron frequency. This will reduce the required magnetic flux density in the gyrotron cavity by a factor of two and therefore will lead to more compact and cost-efficient gyrotron systems at DEMO-relevant and sub-THz frequencies.