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.

Abstract This paper reports the main design keys and the challenging issues of the Heating and Current Drive System (HCD) of the Divert Tokamak Test (DTT) facility that is under construction at ENEA site in Frascati with the aim to perform studies on the power exhaust in a flexible and easily modifiable environment. The selected HCD systems for DTT are those with the most consolidated technology and expected to be relevant for the future reactor. The status of each system is reported, both in terms of design and procurement, which are well advanced for the system required in the first phase of the DTT exploitation: electron and ion cyclotron resonant heating. The third system is neutral beam injector, based on negative ion acceleration, which will be installed in DTT in a second phase, after the first five years of operation. The full heating power will be reached with the doubling of the radiofrequency power to reach the 45 MW at plasma needed to test the divertor with a power density at reactor level.

Gyrotrons are high-power microwave sources, readily applied in diverse fields ranging from fusion research and thermal treatment of materials to advanced spectroscopic techniques. Here, we provide the cavity design of a 198 GHz gyrotron to generate microwaves with kW level power and a frequency bandwidth of several hundreds of MHz. The design focuses on the interaction cavity, while all other components of the vacuum tube remain unchanged from a currently operational 198 GHz, 60 W frequency-agile gyrotron. The proposed cavity geometry allows for two operating points, one optimized for frequency agility, and the other for high power. Cavity interaction code simulations show that frequency tunability over a range of 400 MHz can be reached using a smooth transition between the first and second axial mode excitations. In addition, these simulations indicate that up to 7 kW microwave power can be obtained at the high-power operating point. Furthermore, the ohmic load deposited on the walls of the interaction cavity was investigated for high-power operation. The expected microwave power and frequency agility of the cavity are promising for applications such as dynamic nuclear polarization (DNP) spectroscopy and electron paramagnetic resonance (EPR) experiments.

Efficiency is a key factor for high-power gyrotrons, particularly in future fusion power plants, where a large number of gyrotron units will be required for plasma heating. A single-stage depressed collector enhances efficiency from approximately 35% to 50% by applying a decelerating potential to the spent beam electrons before they enter the collector. Experimentally, a well-defined threshold exists for the applied deceleration potential, beyond which efficiency decreases and instabilities arise. In this work, we conduct a systematic study of electron beam behavior under very high decelerating potentials by simulating the beam within the complete gyrotron geometry. This approach enables a comprehensive investigation of reflected electron behavior and its impact on cavity interactions. The findings provide insights into the limitations of single-stage depressed collectors. Moreover, quantifying the level of reflected current that does not significantly affect gyrotron performance is a key parameter for the design and development of high-efficiency multi-stage depressed collectors.

Tokamak a configuration variable (TCV), recently celebrating 30 years of near-continual operation, continues in its missions to advance outstanding key physics and operational scenario issues for ITER and the design of future power plants such as DEMO. The main machine heating systems and operational changes are first described. Then follow five sections: plasma scenarios. ITER Base-Line (IBL) discharges, triangularity studies together with X3 heating and N2 seeding. Edge localised mode suppression, with a high radiation region near the X-point is reported with N-2 injection with and without divertor baffles in a snowflake configuration. Negative triangularity (NT) discharges attained record, albeit transient, beta(N) similar to 3 with lower turbulence, higher low-Z impurity transport, vertical stability and density limits and core transport better than the IBL. Positive triangularity L-Mode linear and saturated ohmic confinement confinement saturation, often-correlated with intrinsic toroidal rotation reversals, was probed for D, H and He working gases. H-mode confinement and pedestal studies were extended to low collisionality with electron cyclotron heating obtaining steady state electron iternal transport barrier with neutral beam heating (NBH), and NBH driven H-mode configurations with off-axis co-electron cyclotron current drive. Fast particle physics. The physics of disruptions, runaway electrons and fast ions (FIs) was developed using near-full current conversion at disruption with recombination thresholds characterised for impurity species (Ne, Ar, Kr). Different flushing gases (D2, H2) and pathways to trigger a benign disruption were explored. The 55 kV NBH II generated a rich Alfvenic spectrum modulating the FI fas ion loss detector signal. NT configurations showed less toroidal Alfven excitation activity preferentially affecting higher FI pitch angles. Scrape-off layer and edge physics. gas puff imaging systems characterised turbulent plasma ejection for several advanced divertor configurations, including NT. Combined diagnostic array divertor state analysis in detachment conditions was compared to modelling revealing an importance for molecular processes. Divertor physics. Internal gas baffles diversified to include shorter/longer structures on the high and/or low field side to probe compressive efficiency. Divertor studies concentrated upon mitigating target power, facilitating detachment and increasing the radiated power fraction employing alternative divertor geometries, optimised X-point radiator regimes and long-legged configurations. Smaller-than-expected improvements with total flux expansion were better modelled when including parallel flows. Peak outer target heat flux reduction was achieved (>50%) for high flux-expansion geometries, maintaining core performance (H-98 > 1). A reduction in target heat loads and facilitated detachment access at lower core densities is reported. Real-time control. TCV's real-time control upgrades employed MIMO gas injector control of stable, robust, partial detachment and plasma beta feedback control avoiding neoclassical tearing modes with plasma confinement changes. Machine-learning enhancements include trajectory tracking disruption proximity and avoidance as well as a first-of-its-kind reinforcement learning-based controller for the plasma equilibrium trained entirely on a free-boundary simulator. Finally, a short description of TCV's immediate future plans will be given.

In this contribution, the tests of the pre-series gyrotron TH1509UA for the Divertor Tokamak Test facility (DTT) at the FALCON test facility are presented. This versatile test bed proves useful for testing continuous wave (CW) high-power gyrotrons, but also serves as a platform for testing components for the transmission line or the Upper Launcher of ITER and DTT. The gyrotron has demonstrated a power level of 1.02 MW at the gyrotron output window, corresponding to 980 kW at the output of the Matching Optics Unit (MOU) with a power variation during the pulse of < 2% after a stabilisation period. Additionally, an efficiency of 40% has been demonstrated during five consecutive 100 s pulses. Compared to the previous version, TH1509U, this gyrotron demonstrates the successful prevention of parasitic mode excitation over a wide range of parameters around the design operating point. The potential for even higher power performance has been shown in short pulses but not explored in long pulses yet, which instead focused on demonstrating compliance with the required specifications for the DTT project.

A numerical model is presented to describe the electron beam neutralization during long-pulse and continuous-wave operation in gyrotrons. This model has been implemented in the electrostatic, self-consistent electron optics code Ariadne. Using this model, the electron beam parameters in the cavity could be determined as a function of the level of neutralization. The electron beam was shown to be partially neutralized in the cavity, and the effect of neutralization on the beam properties was investigated for a cylindrical gyrotron. This study was extended to a coaxial cavity gyrotron, identifying two ion trapping regimes, which only had a minor impact on the beam parameters.

A dual-frequency gyrotron has been developed within the context of the recent Tokamak à Configuration Variable (TCV) upgrade. The gyrotron is designed to generate a 1 MW, 2 seconds RF wave at 84 or 126 GHz. Before integrating the gyrotrons in the TCV tokamak ECRH system, an extensive characterization of their behaviour has been performed. This paper focuses on presenting the results of these experiments at the two operating frequencies. The power measurements are systematically compared with numerical simulations. This comparison highlights the validation of numerical codes and the effect of After Cavity Interaction (ACI), a crucial factor that must be considered for achieving a good agreement between theoretical predictions and experimental results.

This paper presents the new 2D electrostatic particle-in-cell code FENNECS developed to study the formation of magnetized non-neutral plasmas in geometries with azimuthal symmetry. This code has been developed in the domain of gyrotron electron gun design, but solves general equations and can be applied in other domains of plasma physics. FENNECS is capable of simulating electron-neutral collisions using a Monte Carlo approach and considers both elastic and inelastic (ionization) processes. It is also capable of solving the Poisson equation on domains with arbitrary geometries with either Dirichlet or natural boundary conditions. The Poisson solver is based on a meshless Finite Element Method, called web-splines, based on b-splines of any order, and used for the first time in the domain of plasma physics. In addition, the effect of fast ions colliding with the electrodes and causing ion induced electron emission at the electrode surfaces has been implemented in the code. In this paper, the governing equations solved by FENNECS and the numerical methods used to solve them are presented. A number of verification cases are then reported. Finally, the parallelization scheme used in FENNECS and its parallel scalability are presented.

The progress of the European THALES TH1509U 170 GHz 1 MW CW industrial gyrotron program is presented in this report. The test results on the upgraded TH1509U and the compliance with main technical requirements are discussed.

The ionization of the residual gas in gyrotrons causes the generation of positively charged ions and negatively charged electrons. Using the electron optics code Ariadne, it was possible to simulate the behavior of these electrons in the overall gyrotron geometry in the presence of static electric and magnetic fields as well as the electromagnetic field of the nominal transverse electric mode in the cavity. The main observation of this study is that the negatively charged electrons gain enough energy from their passage through the cavity to overcome the decelerating voltage and escape towards the collector. On the other hand, the positively charged ions remain trapped in the cavity region contributing to the neutralization effect.

Gyrotrons are essential for electron cyclotron resonance heating in fusion reactors, making efficient operation crucial for advancing fusion energy. Past experiments revealed instability issues due to trapped electrons in the magnetron injection gun (MIG) region, causing undesired currents and operational failures. To address this, tight manufacturing tolerances are required for the MIG geometry [Pagonakis et al., Phys. Plasmas 23, 023105 (2016)]. We present the initial findings of the trapped electrons experiment developed at the Swiss Plasma Center, designed to understand the physics of electron clouds in gyrotron MIGs. T-REX replicates MIG geometries, as well as their typical electric and magnetic fields, and it is supported by 2D particle-in-cell simulations with the FENNECS code [Le Bars et al., Phys. Plasmas 29, 082105 (2022); Le Bars, Ph.D. thesis, EPFL, Lausanne, 2023]. The setup includes two coaxial electrodes in a vacuum chamber atop a superconducting magnet, with a central electrode biased to negative DC voltages and an outer one at the ground, creating a radial electric field (1-2 MV/m) and an axial magnetic field (B < 0.4 T). This setup mimics Penning-Malmberg traps. We present the experimental device and first findings on current distribution and also a qualitative comparison with FENNECS simulations [Le Bars et al., Comput. Phys. Commun. 303, 109268 (2024)]. Planned diagnostics include optical emission spectroscopy, phosphor screen imaging, streak camera imaging, and potentially electric field distribution via the Stark effect. This research aims to enhance gyrotron performance and reliability in fusion energy systems.

Using the new interaction code Phaedra, a thorough and systematic investigation of the operation of a high-power gyrotron cavity in the hard excitation regime took place. The preliminary result of this analysis shows that in the hard excitation regime, the nominal mode can potentially be excited at more than one frequency with a difference in the range of few tens of MHz and with a significant power difference at the same accelerating voltage. Furthermore, improper choice of parameters for the startup simulations can guide to an important overestimation of the stability of the mode at higher voltages.

A model is presented for simulating the electron beam within the entire gyrotron geometry using an electron optics code. The model incorporates the influence of the electromagnetic fields generated by the cavity's operational mode through a simple self-consistent approach. This simulation method proves highly beneficial for investigating the effects occurring within the complete gyrotron structure. One such application includes the examination of ions and electrons, resulting from the ionization of residual gas molecules upon collision with the electron beam. By calculating the trajectories of these particles, interesting conclusions can be drawn. (c) 2024 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/)

We report on the initial validation of the novel code FENNECS, which simulates the spontaneous formation of trapped electron clouds in coaxial geometries with strong externally applied azimuthal flows and in the presence of a residual neutral gas. For this purpose, a realistic gyrotron electron gun geometry is used in the code, and a self-consistent electron cloud build-up is simulated. The predicted electronic current resulting from these clouds that is collected on the gun electrodes is simulated and successfully compared with the previous experimental results for configurations with different externally applied electric and magnetic fields. These different configurations effectively modify the size and depth of the trapping potential wells responsible for the confinement of the electron clouds. This investigation also provides further insight into the link between potential well depth and resulting electronic current.

The progress of industrial gyrotron developments at Thales, that are done in collaboration with its European partners, are presented in this report. Recent test results are discussed and the design status of new industrial units under development is presented.

After an initial testing period at the Karlsruhe Institute of Technology, the upgraded EU 1 MW, 170 GHz continuous wave (CW) industrial prototype gyrotron (TH1509U) for Electron Cyclotron Resonance Heating and Current Drive (ECRH&CD) in ITER has been transferred to the Swiss Plasma Center (SPC) to be tested in an ITER relevant configuration, to reach the performance level required by Fusion for Energy (F4E) to qualify it for the ITER project, and to increase the pulse length to very long pulses.

A 250 GHz quasi-optical ring resonator/pulse compressor is presented. The ring consists of an input coupler and three mirrors. A low-loss semiconductor located in the ring provides the output coupling once irradiated by a laser. In the first tests with a 17 mW, 250 GHz source, 5.8 ns output pulses were generated with a power gain of 16. This is the first demonstration of a ring resonator/pulse compressor with high output power gain in the millimeter wave/THz regime. The resonator has many possible uses including applications at low power in spectroscopy and at high power in testing accelerator structures.

A significant effort has recently been initiated at ETHZ for the development of high frequency, high power, frequency-agile gyrotrons for DNP-NMR spectroscopy. In this context an in house cavity interaction code is under development. In this paper, some details on the development of the new cavity code and the preliminary results will be presented.

A 250 GHz quasi-optical ring resonator consisting of an input coupler and three mirrors has been designed and tested. A low-loss silicon wafer in the ring provides output coupling of the stored power when irradiated by a pulse from a 532 nm laser. The ring created 5.8 ns, 268 mW output power pulses when excited by a 17 mW, 250 GHz continuously operating input source, achieving a power gain of 16. In a fully tuned ring, higher gain is achievable. If the ring was used with a pulsed input source having a pulse length of several times the fill time, the ring could be used as an efficient pulse compressor with similar high gain. The resonator has a wide range of applications, including, at low power, spectroscopy and, at high power, testing of accelerator structures and materials.