Fluid-dynamic and thermo-mechanical analyses of the Monoblock Mitre Bend for the ITER electron cyclotron Upper launcher

The ITER Electron Cyclotron Heating (ECH) system will be used to counteract magneto-hydrodynamic plasma instabilities by aiming up to 20 MW of mm-wave power at 170 GHz. The primary vacuum boundary at the Electron Cyclotron Upper Launcher (EC UL) extends into the port cell region through eight beamlines, defining the so-called First Confinement System (FCS). Each beamline, designed for the transmission of 1.31 MW, is delimited by the closure plate at the port plug back-end and by a diamond window in the port cell. The FCS essentially consists of a Z-shaped set of straight corrugated waveguides (WG) connected by Mitre Bends (MB) with a nominal inner diameter of 50 mm. Due to the space restrictions, the eight MBs at the last FCS section are grouped into two monoblocks. Each Monoblock Mitre Bend (MBMB) consists of a body with corrugated feedthroughs defining a specific angle for each beamline and four mirrors attached by bolted connection to reflect the mm-wave power. The thermal expansion arising from the ohmic losses, the cooling pressure, the bolt pre-tension and the imposed displacements coming from the connection with the transmission lines are the primary design loads for the MBMBs. The fluid-dynamic analyses performed for both upper and lower MBMBs show that highest temperature takes place in the MB mirrors, reaching a maximum value of 203 degrees C at the beam center. The thermomechanical analyses demonstrate that the peak stress also occurs at this location with a maximum value of 324 MPa. These stresses are categorized and compared with the allowable limits defined in the ASME code, what probes that the current design of both upper and lower MBMBs are able to withstand the loads taking place during the mm-wave normal operation.

Prototype mitre bends of the ex-vessel waveguide system for the ITER upper launcher: Thermal hydraulic simulations and experiments with off-center mm-wave beams

René Chavan, Timothy Goodman, Humberto Torreblanca Quiroz, Matteo Vagnoni, Anastasia Xydou

On ITER, long pulse gyrotrons are required as a power source for electron cyclotron heating (ECH) and current drive (CD). The microwaves are guided from the gyrotrons, which are placed far from the Tokamak, into the plasma by transmission lines (TLs) and a launching antenna (launcher). Each of the four ECH Upper launchers features eight waveguide (WG) TLs, with at least 95% of the power from the gyrotrons coupled into in the main HE mode of the TLs. In the ex-vessel portion of the system between the port-plug closure plate and the isolation valve and diamond window, there are miter bends (MBs) that change the direction of the TL by reflecting the millimeter waves (mm-waves) using mirrors; the mirrors must handle 1.31 MW, 170 GHz, 3600 s pulses. Various MBs perform these reflections at an angle of 90 degrees, or nearly 100 degrees. As a result of the ohmic dissipation, an intensive peaked heat flux appears near the center of the MB mirror and thus, a dedicated cooling system is present to ensure the temperature control of the mirror and housing e.g. [1]. The power that is not found in the HE mode can cause the beam to be not perfectly centered, resulting in an off-centered heat flux on the mirror surface; as is the case for the experiments described here. This study presents new finite element modeling of such beams, created in CFX of ANSYS Workbench [2], compared to the experimental findings for pulses of 170 GHz, 0.5 MW and 240 s. Monitor points are placed in the same positions as TCs that have been fitted in the mirror close to the heated surface and direct comparison of the temperature values is performed. Through transient simulation the time constants are calculated and compared with those of the experiments.

2021

Self-tracking Solar Concentration

Volker Günter Zagolla

Solar concentration is intended to decrease the cost of expensive solar cell material by replacing the solar cell with a focusing optical system as the initial receiving aperture. This reduces the amount of solar cell material needed but adds the need of tracking to the entire system in order to keep the smaller solar cell at the focal spot. In state of the art concentrators, precise tracking (less than 1°) is needed in two directions to cover diurnal and seasonal variations. This adds to the cost of the system, diminishing the savings from the smaller solar cell, and consumes energy which in turn decreases the overall system efficiency. A self-tracking concentrator has the potential to increases the tolerances of the system (less precise tracking needed) and cover seasonal variations (only tracking in one dimension remains). Self-tracking makes use of the energy of the spectrum that is typically not utilized in PV, thus self-tracking does not consume an outside surplus of energy. During the thesis such a self-tracking system was developed. This thesis describes two concepts for the realization of a self-tracking concentrator, one of which is then developed into a working demonstration device with an angular acceptance of 32° (±16°). The work covers both the development of these two different mechanisms as well as an analysis of their performance as a single element and a discussion of their performance as part of a much larger device. The first concept, the micro-fluidic concentrator, uses vapor bubbles to efficiently couple light into a liquid waveguide (chapter 3), while the second concept, based on a phase change actuation uses the thermal expansion of paraffin wax to create a coupling feature at a waveguide (chapter 4). Based on the findings, the phase-change concentrator was then expanded into a self-tracking concentrator with a possible effective concentration of 10x (chapter 5). As a result this thesis shows the feasibility of a self-tracking concentrator and describes in detail the processes and techniques for its realization. Future work based on the designs presented in this thesis can optimize the concepts. Simulations show that a system based on this approach can achieve 150x effective concentration by scaling the system collecting area to reasonable dimensions (30 x 1 cm²). An optimized optical system can then also increase the acceptance angle of the self-tracking concentrator to the ±23° needed in order to cover seasonal variations. In addition, the presented concept is based on earth abundant, inexpensive materials, making it technologically and economically viable to extend it to much larger formats.

EPFL2015

Design and Numerical Analyses of the M4 Steering Mirrors for the ITER Electron Cyclotron Heating Upper Launcher

René Chavan, Timothy Goodman, Matteo Vagnoni

Four electron cyclotron heating upper launchers (ECHULs) will be used at ITER to counteract magnetohydrodynamic plasma instabilities by targeting them with up to 24 MW of mm-wave power at 170 GHz. This mm-wave power is injected through eight ex-vessel (EV) waveguide assemblies for each ECHUL to the in-vessel (IV) waveguides. The mm-wave power exiting the eight IV waveguides inside the ECHUL is reflected by three fixed mirror sets and finally aimed by two independent steering mirrors to specific plasma locations. These two steering mirrors have recently experienced an important redesign in order to deal with the new requirements. This article reports the status of the steering mirrors as well as the fluid dynamic and thermomechanical analyses carried out to validate the design for the normal operation (NO) scenario. The fluid dynamic analyses show that the power dissipated in both steering mirrors due to the mm-wave radiation, nuclear heating, and plasma heat flux can be properly removed with an acceptable mass flow generating admissible pressure drop, temperature rise, and corrosion rate values. The results obtained in the thermomechanical simulation, validated using the American Society of Mechanical Engineers (ASME) code, shows that the steering mirror design is capable of withstanding the expected loads taking place during NO.