The ASDEX Upgrade electron cyclotron resonance heating system [1] uses multi-hole waveguide couplers integrated into the matching mirrors ‘M2’ for power monitoring. While the full Gyrotron output power (105 or 140 GHz, typ. 1 MW per unit) is essentially optically reflected at the M2 mirror, a small fraction of the order of several milliwatt is diverted by the coupler and transmitted via fundamental mode waveguide towards a beam lead Schottky diode, which converts the millimetre wave electric field amplitude into a voltage signal. The system is calibrated by a calorimetric measurement, where short pulses of the full Gyrotron power are used to heat water. The calibration of power monitor designs from the years 2009 until 2012 showed variations up to 25% on different experiment days, which is clearly above the uncertainty of the calorimetric calibration procedure. Using the latest power monitor design, these variations seem to be significantly reduced, even on the timescale of eight years, where hardware was unchanged. In this latest design, both the waveguide coupler and the low power transmission and detection scheme including flange interfaces were revised. The improved longevity seems to be mainly caused by improvements in the mechanical precision, stability and rigidity, which is backed by laboratory measurements.

Microwave interferometry is a reliable, well established, and non-perturbing method to measure the line-integrated electron density of a non-uniform plasma through the phase shift of a wave that propagates the plasma medium. In this paper we combine the phase shift and the attenuation of the wave to experimentally extract both, the line-integrated density and the electron-neutral collision frequency of an atmospheric plasma torch. In addition, a novel method to obtain the 2D spatial plasma density profile of the torch is demonstrated by measuring the microwave power, without any information of the phase. The receiving antenna of the interferometer is moved perpendicularly to the axis of the torch and measures the spatial distribution of the microwave power. The wave is scattered by the plasma and the scattering profile depends on the plasma density profile. Direct comparison of this scattering profile with 3D full-wave simulations provides information on the electron number density profile of the plasma torch.

After a long device enhancement phase, scientific operation resumed in 2022. The main new device components are the water cooling of all plasma facing components and the new water-cooled high heat flux divertor units. Water cooling allowed for the first long-pulse operation campaign. A maximum discharge length of 8 min was achieved with a total heating energy of 1.3 GJ. Safe divertor operation was demonstrated in attached and detached mode. Stable detachment is readily achieved in some magnetic configurations but requires impurity seeding in configurations with small magnetic pitch angle within the edge islands. Progress was made in the characterization of transport mechanisms across edge magnetic islands: Measurement of the potential distribution and flow pattern reveals that the islands are associated with a strong poloidal drift, which leads to rapid convection of energy and particles from the last closed flux surface into the scrape-off layer. Using the upgraded plasma heating systems, advanced heating scenarios were developed, which provide improved energy confinement comparable to the scenario, in which the record triple product for stellarators was achieved in the previous operation campaign. However, a magnetic configuration-dependent critical heating power limit of the electron cyclotron resonance heating was observed. Exceeding the respective power limit leads to a degradation of the confinement.

Collective Thomson Scattering (CTS) diagnostics measure the scattering spectrum of monochromatic incident radiation off collective fluctuations in the plasma. In this contribution, we present the first results from the upgraded CTS diagnostic at Wendelstein 7-X (W7-X) operating in the frequency range between 172 and 176 GHz. This frequency range allows for minimization of noise originating from the electron cyclotron emission in the plasma. Consequently, the good signal-to-noise ratio allows measurements of fast ions or bulk plasma parameters with higher temporal resolution compared with the previously used 140 GHz system.

A Michelson Interferometer is in use at Wendelstein 7-X (W7-X) to probe the Electron Cyclotron Emission (ECE) spectrum [1], [2], [3]. During the past operational campaign (OP2.1), 2nd and 3rd harmonic ECE power density spectra have been routinely recorded in the presence of X2and O2-mode Electron Cyclotron Resonance Heating (ECRH). However, combination of the particular notch filter arrangement and high transmission line losses have thus far prevented overall calibration using a hot source cold source exposure at the input antenna. As an alternative, the response of the individual components is measured and summed. While reasonable numbers on electron temperature are obtained in X2-mode polarisation, interaction between front-end components is neglected and large error bars must be assumed. But the information on the individual components, together with synthetic modelling and data from experiment (OP2.1), has been used to design a new front-end with improved S/N. This optimisation is discussed in this paper with focus on notch filter selection, a new transmission line (Tx-line) and a novel combined quasi-optical taper / polarizer tuner.

Notch filters are a key component in millimeter-wave plasma diagnostics systems for magnetically confined fusion plasmas. They are required to protect sensitive millimeter-wave receivers from stray radiation of electron cyclotron heating systems. These heating systems employ gyrotrons that emit strong millimeter-wave radiation (90 dBm) in one or, in the case of modern step-tunable gyrotrons, several narrowband frequency lines. In this article, we describe the design and performance of notch filters based on waveguide technology, operating in the F-band (90–140 GHz), that reject one or two selectable frequencies in the passband. We also present the solution for a filter where the rejected frequencies are outside of the passband of the plasma diagnostics. This is demonstrated for a filter operating in the W-band (75–110 GHz). To fully protect the diagnostic systems, typical stopbands with not less than 60-dB rejection and at least 500-MHz width are required.

This article presents the design, implementation, and first data of a uniquely flexible, multi-channel, frequency comb Doppler backscattering diagnostic recently made operational in the ASDEX-Upgrade tokamak [A. Gruber and O. Gruber, Fusion Sci. Technol. 44, 569 (2003)]. It uses a double side-band signal fed into a ×6 frequency multiplier to produce a multiple-frequency output spectrum. Seven of these frequencies are simultaneously measured in the receiver via a two-step frequency down-conversion and traditional I/Q demodulation. The frequency comb spectrum is fully tunable to sit anywhere in the W-band. The inter-frequency separation is also uniquely tunable remotely between 0.1 and 6 GHz without any hardware changes. The diagnostic can be operated in both O and X-mode polarizations and at both oblique and normal incidence to the cutoff layer. The time evolution of backscattered signals, in excess of 30 dB, from seven distinct frequencies sampled simultaneously is presented across an L-to-H-mode confinement regime transition.

A Collective Thomson Scattering (CTS) diagnostic system is planned for the International Thermonuclear Experimental Reactor (ITER), which will be a primary diagnostic for measuring the dynamics of the confined fusion born alpha particles in the ITER plasma. The probe beam for this diagnostic comes from a 60 GHz 1 MW gyrotron. Since the measured signal close to the frequency of the probing beam is in the nW range, the receiver has to be protected with a narrow notch filter rejecting the gyrotron frequency. At the same time it also needs protection against stray radiation from the 20 MW ITER Electron Cyclotron Heating System (ECH), operating at 170 GHz.

Reflecting gratings have been installed in the vacuum vessel of ASDEX Upgrade for all beamlines of the electron cyclotron resonance heating system. Potentially unabsorbed millimetre wave power after the first pass through the plasma is redirected towards the plasma centre. This increases the efficiency of heating schemes with reduced single pass absorption like O-2 or X-3. In order to monitor beam position and power, thermocouples were installed into the gratings. A numerical model was developed to evaluate the beam intensity during short pulses from the thermocouple measurement in a non-stationary environment. An experiment was carried out, where only the X-3 resonance is present in the plasma, and the millimetre wave beam shine-through was measured successfully as a function of the central plasma electron temperature. This allows to deduce the X-3 absorption experimentally. Scanning the launching angles, it seems possible to measure the 2D beam cross section after the first pass through the plasma.

Sensitive sub-THz diagnostic systems for magnetically confined fusion plasmas need protection from stray radiation produced by high-power Electron Cyclotron Resonance Heating (ECRH) systems which operate in one or more narrowband frequency lines. These frequencies can be within and sometimes also above the frequency band of the diagnostic system. Notch filters are key components to provide this protection.

Modern Electron Cyclotron Resonance Heating (ECRH) systems in fusion plasma research take advantage of multi-frequency gyrotrons. This means that the frequency band of some millimeter-wave diagnostics contains more than one narrowband gyrotron-frequency line, which needs to be suppressed. Compact standard-waveguide notch filters, based on coupled waveguide resonators with rectangular cross section, are presented, which can provide very high suppression of several gyrotron frequencies providing low insertion loss in the pass band.

Sensitive millimeter wave diagnostics in magnetic confinement plasma fusion experiments need protection from gyrotron stray radiation in the plasma vessel. Modern electron cyclotron resonance heating (ECRH) systems take advantage of multi-frequency gyrotrons. This means that the frequency band of some millimeter wave diagnostics contains more than one narrow-band gyrotron-frequency line, which needs to be effectively suppressed. A compact standard waveguide notch filter based on coupled waveguide resonators with rectangular cross-section is presented which can provide very high suppression of several gyrotron frequencies and has low insertion loss of the passband.

Comb reflectometers offer the advantage of measuring several radial positions in plasma simultaneously. This allows for the investigation of fast timescales during L-H transitions, I-phases, I-mode bursts, transients during heat wave propagation, etc. A drawback of many present-day systems is that they use a fixed frequency difference between the probing frequencies. Hence, although the central probing frequency can be varied, the probing frequency difference is usually fixed. The new design presented in this work uses an advanced microwave generation and detection scheme, which allows for arbitrary probing frequencies and probing frequency separations.

The electron cyclotron resonance heating (ECRH) system of the ASDEX Upgrade tokomak has been upgraded over the last 15 years from a 2 MW, 2 s, 140 GHz system to an 8 MW, 10 s, dual frequency system (105/140 GHz). The power exceeds the L/H power threshold by at least a factor of two, even for high densities, and roughly equals the installed ion cyclotron range of frequencies power. The power of both wave heating systems together (>10 MW in the plasma) is about half of the available neutral beam injection (NBI) power, allowing significant variations of torque input, of the shape of the heating profile and of Qe/Qi, even at high heating power. For applications at a low magnetic field an X3-heating scheme is routinely in use. Such a scenario is now also forseen for ITER to study the first H-modes at one third of the full field. This versatile system allows one to address important issues fundamental to a fusion reactor: H-mode operation with dominant electron heating, accessing low collisionalities in full metal devices (also related to suppression of edge localized modes with resonant magnetic perturbations), influence of Te/Ti and rotational shear on transport, and dependence of impurity accumulation on heating profiles. Experiments on all these subjects have been carried out over the last few years and will be presented in this contribution. The adjustable localized current drive capability of ECRH allows dedicated variations of the shape of the q-profile and the study of their influence on non-inductive tokamak operation (so far at q95 > 5.3). The ultimate goal of these experiments is to use the experimental findings to refine theoretical models such that they allow a reliable design of operational schemes for reactor size devices. In this respect, recent studies comparing a quasi-linear approach (TGLF) with fully non-linear modeling (GENE) of non-inductive high-beta plasmas will be reported.

The Collective Thomson Scattering (CTS) diagnostic measures the scattering spectrum of incident radiation off collective fluctuations in plasmas. In Wendelstein 7-X (W7-X) the diagnostic uses a 140 GHz heating gyrotron as a source of the probing radiation. At this frequency, the CTS spectra are heavily affected by the electron cyclotron emission, and the microwave beam propagation is restricted at typical W7-X plasma parameters. The diagnostic was successfully commissioned in the last experimental campaign and demonstrated ion temperature measurements. However, the signal-to-noise ratio was too low for measuring other quantities such as the fast-ion velocity distribution function or the fuel ion ratio. Currently, the W7-X CTS diagnostic is undergoing an upgrade to a frequency of 175 GHz. This will increase the sensitivity of the diagnostic, since the noise due to electron cyclotron emission will be reduced, and it will relax the constraints on microwave beam propagation in W7-X. Here we present the salient features of the upgraded CTS system and discuss its prospects for both thermal-ion and fast-ion measurements.

8 gyrotrons are in routine operation in the Electron Cyclotron Resonance Heating (ECRH) system at the ASDEX Upgrade tokamak. All gyrotrons are step-tunable operating at 105 and 140 GHz with a maximum output power of about 1 MW and 10 s pulse length. The system includes 8 transmission lines, mainly consisting of oversized corrugated waveguides (I.D. = 87 mm) with overall lengths between 50 and 70 meters including quasi-optical sections at both ends. The transmission lines are operating in air and transmission at maximum power levels was limited by arcing in the past. Several measures led to a significant improvement of the power handling capability of the transmission lines.

Collective Thomson scattering (CTS) is a microwave diagnostic allowing measurements of a number of plasma parameters such as the bulk ion temperature, the plasma composition, drift velocities and fast ion velocity distribution function. A CTS system has been successfully installed and commissioned on the Wendelstein 7-X (W7-X) stellarator. The measured spectra are analyzed by the means of the CTS forward model eCTS and the Minerva scientific framework enabling the use of Bayesian inference of relevant plasma parameters. Here we discuss the options for further optimization of the CTS diagnostic and focus on two topics of importance for the inference of bulk ion temperature values from CTS spectra: influence of impurities on the CTS spectra and the width of the notch filters that are employed to protect the receiver from high-power radiation. In addition to that we discuss the possibility of effective charge measurements by CTS. We explore the existence of an optimal notch filter width.

The ECE diagnostic at W7-X in its standard mode of operation measures in X2 mode polarization with a 32 channel radiometer in the frequency band around 140 GHz for central magnetic field 2.5T. The radiometer is calibrated by a noise source and the overall system absolutely calibrated by means of a hot-cold source placed outside the torus in front of a Gaussian telescope optics with identical geometry and transmission line as it is installed for the measurements in the plasma vessel. The system is supplemented with a 16 channel zoom device with 4 GHz span for higher frequency resolution at a suitable radial range and a Michelson interferometer for the characterization of higher harmonics sharing the same line of sight.

The electron cyclotron resonance heating system at ASDEX Upgrade (AUG) is currently being extended to eight similar Gyrotrons in total. Each Gyrotron operates at 105 and 140 GHz and is designed for up to 1 MW millimetre wave output power. A substantial part of the AUG program will focus on experimental conditions, where the plasma density may be above the X-2 cut-off density at 140 GHz. In order to cope with the high density, the heating system will operate in the O-2 mode scheme with potentially incomplete absorption in the first pass. Reflecting gratings installed into the heat shield on AUG's inner column allow for a controlled second pass of the beam's unabsorbed fraction. Thermocouple measurements serve to control the beam position on the grating. The beam geometry is being finalized for the launchers #1-4. Beam propagation is simulated with the TORBEAM code and previous high density experiments are used as a database. The geometry is optimized using three criteria: central deposition, high absorption and robustness of the beam dump after the second pass. The experimental conditions, and the plasma electron density in particular, may vary such that the Gaussian beam parameters of the incoming beam on the grating deviate from the design values. It is proposed to model the effect of the grating with an equivalent ellipsoidal mirror. Laboratory measurements are shown, which support this model.

The European Gyrotron Consortium (EGYC) is developing the European 1 MW, 170 GHz Continuous Wave (CW) industrial prototype gyrotron for ITER in cooperation with Thales Electron Devices (TED) and Fusion for Energy (F4E). This conventional, hollow-cavity gyrotron, is based on the 1 MW, 170 GHz Short-Pulse (SP) modular gyrotron that has been designed and manufactured by the Karlsruhe Institute of Technology (KIT) in collaboration with TED. Both gyrotrons have been tested successfully in multiple experiments. In this work we briefly report on the results with the CW gyrotron at KIT and we focus at the experiments at the Swiss Plasma Center (SPC). In addition, we present preliminary results from various upgrades of the SP tube that are currently tested at KIT.