An Electron Cyclotron Resonance Heating (ECRH) system employing 8 gyrotrons is in routine operation at the ASDEX Upgrade tokamak. The gyrotrons are of two-frequency type operating at 105 and 140 GHz with a maximum output power of up to 1 MW and 10 s pulse length. The gyrotron output beams are coupled to 8 waveguide transmission lines via quasi-optical Matching Optics Units (MOUs). The oversized corrugated HE 11 waveguides with a diameter of 87 mm are operated at atmospheric pressure with overall lengths between 65 and 102 meters. The number of quasi-optical miter bends per line is between 6 and 8. High mode purity in the transmission lines is critical with respect to both, losses and atmospheric breakdowns. Beam measurements at low power have been performed along the transmission lines and are compared to high power measurements.

An Electron Cyclotron Resonance Heating (ECRH) system employing 8 gyrotrons is in routine operation at the ASDEX Upgrade tokamak at IPP Garching. The gyrotrons are of two-frequency type operating at 105 and 140 GHz with a maximum output power of up to 1 MW and 10 s pulse length. The gyrotron output beams are coupled to 8 waveguide transmission lines via quasi-optical Matching Optics Units (MOUs). The oversized corrugated HE11 waveguides with a diameter of 87 mm are operated at atmospheric pressure with overall lengths between 65 and 103 meters. The number of quasi-optical miter bends per line is between 6 and 8. High mode purity in the transmission lines is critical with respect to both, losses and atmospheric breakdowns in the waveguides. Beam measurements at low power have been performed along the transmission lines and are compared to high power measurements. Near-field calculations and measurements of mm-wave beams radiated from open-ended HE11 waveguides show, that varying intensity patterns determine the distribution of Ohmic loading in compact mm-wave beam launching antennas, where the first mirror is located in the reactive near-field or close to the Fresnel maximum.

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.

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.

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.

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.

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.

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.

High power mm waves for fusion plasma heating need to be elliptically polarised to ensure good absorption in the plasma. In some scenarios, electron cyclotron resonance heating (ECRH) at higher harmonics (X3 and O2) is used, but this has significant shine-through because of low single pass absorption. Grating reflectors at the inboard strike point form a holographic mirror that reflects the beam back into the plasma. This paper investigates the optical properties and ohmic losses of both the polariser and the reflectors with the 3D fullwave code IPF-FD3D. The reflection properties of a reflector for ASDEX Upgrade and the improved ohmic losses of a waveguide polariser were confirmed.

Electron cyclotron heating of fusion plasmas requires to launch a high power millimeter wave beam with a well defined polarization, generally elliptical. This can be achieved with two corrugated mirrors with groove depths of ≈ /8 and ≈ /4. In HE11 waveguide transmission lines these polarizers are incorporated into 90° miter bends and are subject to very high power density. To study their loss behaviour we have inserted into the ASDEX Upgrade ECRH transmission line such polarizers with sinusoidal grooves of periodicity 1.07 mm, depth 0.8 mm (≈ /4-mirror) resp. 0.57 mm (≈ /8-mirror), and a thickness of 5 mm. As material we took stainless steel with higher loss and easily measureable temperature rise, measured with a Pt-100 sensor on the backside of the mirror. The measurements were done with 50 ms pulses of 375 kW at 140 GHz. Between the output of the gyrotron and the mirror under test we have a quasi-optical MOU allowing to feed the waveguide with an arbitrary polarization. We measured the losses of such polarizers and of a plane mirror depending on the angle of a linearly polarized incident electric field, and on the angle of the grooves, both with respect to the incidence plane of the miter bend. These are shown in Fig. 1. For a plane mirror and for = 0° and 90° we have theoretical results

A set of two corrugated polarizer mirrors is typically used in high-power electron cyclotron resonance heating (ECRH) systems to provide the required polarization of the ECRH output beam. The ohmic losses of these mirrors can significantly exceed the losses of plane mirrors depending on the polarization of the incident beam with respect to the orientation of the grooves. Since polarizer mirrors incorporated into miter bends of a corrugated waveguide line are limited in size, active water cooling can become critical in high-power cw systems like the one for ITER. The ohmic loss of polarizer mirrors has been investigated experimentally at high power. A strategy to minimize the losses for given mirror geometries has been found.

The ASDEX Upgrade electron cyclotron resonance heating operates at 105 GHz and 140 GHz with flexible launching geometry and polarization. In 2016 four Gyrotrons with 10 sec pulse length and output power close to 1 MW per unit were available. The system is presently being extended to eight similar units in total. High heating power and high plasma density operation will be a part of the future ASDEX Upgrade experiment program. For the electron cyclotron resonance heating, an O-2 mode scheme is proposed, which is compatible with the expected high plasma densities. It may, however, suffer from incomplete single-pass absorption. The situation can be improved significantly by installing holographic mirrors on the inner column, which allow for a second pass of the unabsorbed fraction of the millimetre wave beam. Since the beam path in the plasma is subject to refraction, the beam position on the holographic mirror has to be controlled. Thermocouples built into the mirror surface are used for this purpose. As a protective measure, the tiles of the heat shield on the inner column were modified in order to increase the shielding against unabsorbed millimetre wave power.

A set of two corrugated polarizer mirrors is typically used in high-power Electron Cyclotron Resonance Heating (ECRH) systems to provide the required polarization of the ECRH output beam. The ohmic losses of these mirrors can significantly exceed the losses of plane mirrors depending on the polarization of the incident beam with respect to the orientation of the grooves. Since polarizer mirrors incorporated into miter bends of a corrugated waveguide line are limited in size, active water cooling can become critical in high-power cw systems like the one for ITER. The ohmic loss of polarizer mirrors has been investigated experimentally at high power using different mirror materials. A strategy to minimize the losses for given mirror geometries has been found. Results for different groove geometries are compared.

The upgraded electron cyclotron resonance heating (ECRH) system at ASDEX Upgrade (AUG) has been routinely used with eight gyrotrons during the last experimental campaign. A further upgrade will replace the existing system of four short-pulse (140 GHz, 2 s, 500 kW) gyrotrons. The final goal is to have around 6.5–7 MW at 140 GHz (or 5.5 MW at 105 GHz) from eight units available in the plasma during the whole AUG discharge (10 s). The system operates at 140 and 105 GHz with X2, O2 and X3 schemes. For B > 3 T also an ITER-like O1-scenario can be run using the 105 GHz option. Four of the eight launching antennas are capable of fast poloidal movements necessary for real-time control of the location of power deposition.