T. Scherer

First Operation of a Step-Frequency Tunable 1-MW Gyrotron With a Diamond Brewster Angle Output Window

Experimental results using a step-frequency tunable D-band gyrotron are reported. The short pulse (~3 ms) gyrotron is equipped with an elliptically brazed chemical vapor deposition (CVD) diamond Brewster angle output window. It is designed for the operation in the frequency range from 111.6 up to 165.7 GHz. Operating parameters for ten different frequencies corresponding to an equal number of different cavity operating modes has been measured. A minimum output power of 830 kW and a peak output power of 1.3 MW have been realized. For all frequencies, the parameters of the RF beam generated by the internal quasioptical converter, such as fundamental Gaussian contents and beam waist, are sufficiently good to allow an efficient coupling of the RF power out of the window. This is the first time a diamond Brewster angle window has been used in a high power gyrotron (~1 MW). Such a system offers the path to a simple and compact window solution for high power broadband applications using gyrotrons.

Experimental and theoretical thermal analysis of CVD diamond window units for the ITER upper launcher

An ITER torus window prototype with CVD diamond disks and corrugated waveguides are being investigated by using IR imaging and temperature measuring technique during high power RF microwave loading up to 1 MW at a frequency of 170 GHz at the JAEA gyrotron facility. To evaluate the cooling efficiency of the window design the temperature distribution over the diamond disk area is measured and compared with a theoretical thermal FEM analysis.

ECRH System For ITER

A 26 MW Electron Cyclotron Heating and Current Drive (EC H&CD) system is to be installed for ITER. The main objectives are to provide, start‐up assist, central H&CD and control of MHD activity. These are achieved by a combination of two types of launchers, one located in an equatorial port and the second type in four upper ports. The physics applications are partitioned between the two launchers, based on the deposition location and driven current profiles. The equatorial launcher (EL) will access from the plasma axis to mid radius with a relatively broad profile useful for central heating and current drive applications, while the upper launchers (ULs) will access roughly the outer half of the plasma radius with a very narrow peaked profile for the control of the Neoclassical Tearing Modes (NTM) and sawtooth oscillations. The EC power can be switched between launchers on a time scale as needed by the immediate physics requirements. A revision of all injection angles of all launchers is under consideration for increased EC physics capabilities while relaxing the engineering constraints of both the EL and ULs. A series of design reviews are being planned with the five parties (EU, IN, JA, RF, US) procuring the EC system, the EC community and ITER Organization (IO). The review meetings qualify the design and provide an environment for enhancing performances while reducing costs, simplifying interfaces, predicting technology upgrades and commercial availability. In parallel, the test programs for critical components are being supported by IO and performed by the Domestic Agencies (DAs) for minimizing risks. The wide participation of the DAs provides a broad representation from the EC community, with the aim of collecting all expertise in guiding the EC system optimization. Still a strong relationship between IO and the DA is essential for optimizing the design of the EC system and for the installation and commissioning of all ex‐vessel components when several teams from several DAs will be involved together in the tests on the ITER site.

Preliminary design of the ITER ECH upper launcher

The design of the ITER electron cyclotron launchers recently reached the preliminary design level -the last major step before design finalization. The ITER ECH system contains 24 installed gyrotrons providing a maximum ECH injected power of 20 MW through transmission lines towards the tokamak. There are two EC launcher types both using a front steering mirror; one Equatorial Launcher for plasma heating and four Upper Launchers (UL) for plasma mode stabilization (neoclassical tearing modes and the sawtooth instability). A wide steering angle of the ULs allows to focus on magnetic islands which are expected on the rational magnetic flux surfaces q = 1 (sawtooth instability), q = 3/2 and q = 2 (NTMs).

Cooling design and analysis of the ITER EC Upper launcher

ITER will be equipped with four EC (Electron Cyclotron) upper launchers of 8 MW microwave power each with the aim to counteract plasma instabilities during operation. The launcher antennas will be installed into four upper ports of the ITER vacuum vessel. All in-vessel microwave components of an EC antenna, comprising several sets of mirrors and waveguides are mounted into so-called upper port plugs. These are basically hollow casks which fit into the ports as cantilevered built-in components, forming thus integrated systems which guarantee optimum performance and simplify assembly and maintenance.

The ITER ECH & CD Upper Launcher: Steps towards final design of the first confinement system

The ITER Electron Cyclotron Heating and Current Drive (ECH&CD) Upper Launcher, whose preliminary design was approved in 2009, is on its way towards the final design. The design work is being done by a consortium of several European research institutes in tight collaboration with F4E. The main focus is the finalization of the design of all components for the First Confinement System (FCS), which forms the vacuum and Tritium barrier. The FCS comprises structural components as well as the external waveguide components in the port cell. Structural components of the FCS include the flange seal, backend frame and closure plate. The external waveguide components include the isolation valve, CVD diamond windows, miter bends and straight waveguides. Because finalizing of the design of these components is directly influenced by the layout of many in-vessel components, the design work includes also further development of the entire launcher. This paper summarizes the most recent status of the design work on the structural components of the launcher FCS, which are the support flange, the socket, the closure plate and feed-throughs for waveguides and cooling pipes. The design work includes the engineering layout of these components in accordance with system requirements, load specifications and Quality and Safety classification. An outline of the overall design of the launcher will be presented. The design progress was based on a set of related analyses, of which particular results are given. Also the integration of the associated mm-wave components, assembly strategies, neutronic aspects and the design of the shielding components will be described.

The ITER EC-H&CD Upper Launcher: FEM analyses of the blanket shield module with respect to surface and nuclear heat loads

In the frame of the new grant signed in November 2011 between Fusion for Energy (F4E) and the ECHUL-CA consortium, the development process of the Electron Cyclotron Heating and Current Drive (EC H&CD) Upper Launcher (UL) in ITER has moved a step towards the final design phase. The Blanket Shield Module (BSM) is a plasma facing component located at the tip of the launcher. The structure consists of a first wall panel (FWP) and a shell both with embedded cooling channels. A flange on the rear part allows the BSM to be connected by bolts to the main frame of the UL. Being a plasma facing component, the BSM is subjected to severe heat loads due to both thermal and nuclear irradiation. The current baseline value of surface heat load during normal plasma operation is 0.5 MW/m2, while the volumetric nuclear heating is responsible for a total generation of about 160 kW. The temperature gradients resulting from the abovementioned heat loads have been assessed by FEM analyses. The temperature distributions are then transferred to a structural model for calculation of the induced thermal stresses. The surface heat load is applied to the FWP as a constant flux. The nuclear loads, instead, were assessed by MCNP calculations and are provided by means of a mesh tally with a grid step of 1 cm. The results have shown that the temperature reaches 260 °C at the FWP and at the flange of the BSM. As a consequence of large temperature gradients, high stresses (in the order of 200 MPa) are also induced at the inner cooling channels of the BSMs structure.

Multi-frequency ECRH at ASDEX Upgrade, status and plans

The multi-frequency Electron Cyclotron Heating (ECRH) system at the ASDEX Upgrade tokamak employs depressed collector gyrotrons, step-tunable in the range 105–140 GHz. The system is equipped with a fast steerable launcher allowing for remote steering of the ECRH beam during the plasma discharge. The polarization can be controlled in a feed-forward mode.

Dielectric RF properties of CVD diamond disks from sub-mm wave to THz frequencies

ITER torus windows with CVD diamond disks for high power heating applications (170 GHz, 1–2 MW) are being investigated by different low- and high power measurement setups in the frequency range of 90 to 170 GHz [1,2]. To understand the loss mechanisms in diamond material the determination of the frequency dependence of dielectric constant (ε = ε′ −jε″) and loss tangent (tan δ) at higher frequencies up to several THz is essential. It is well known from the experience with other window materials for high power fusion applications (ECRH) like silicon or sapphire electrons and phonons are responsible for microwave losses. In diamond the sp2-carbon content and surface roughness determines surface losses. Additionally, the electronic surface states for different chemical finishing of the diamond disks can be studied in the THz region.

Investigations of dielectric RF properties of ultra low loss CVD diamond disks for fusion applications

To determine the high-power long pulse RF loss properties of CVD diamond window disks for the ITER ECRH Upper Launcher a Fabry-Perot measurement setup has been used [1]. The dielectric loss is strongly influenced by the surface chemistry of the diamond. Special surface engineering techniques are available due to the reduction of the electrical surface conductivity and therefore of the RF losses.