No abstract

In modern magnetic fusion devices, such as tokamaks and stellarators, radio frequency (RF) waves are commonly used for plasma heating and current profile control, as well as for certain diagnostics. The frequencies of the RF waves range from ion cyclotron frequency to the electron cyclotron frequency. The RF waves are launched from structures, like waveguides and current straps, placed near the wall in a very low density, tenuous plasma region of a fusion device. The RF electromagnetic fields have to propagate through this scrape-off layer before coupling power into the core of the plasma. The scrape-off layer is characterized by turbulent plasmas fluctuations and by blobs and filaments. The variations in the edge density due to these fluctuations and filaments can affect the propagation characteristics of the RF waves-changes in density leading to regions with differing plasma permittivity. Analytical full-wave theories have shown that scattering by blobs and filaments can alter the RF power flow into the core of the plasma in a variety of ways, such as through reflection, refraction, diffraction, and shadowing [see, for example, A. K. Ram and K. Hizanidis, Phys. Plasmas 23, 022504-1-022504-17 (2016) and references therein]. There are changes in the wave vectors and the distribution of power-scattering leading to coupling of the incident RF wave to other plasma waves, side-scattering, surface waves, and fragmentation of the Poynting flux in the direction towards the core. However, these theoretical models are somewhat idealized. In particular, it is assumed that there is step-function discontinuity in the density between the plasma inside the filament and the background plasma. In this paper, results from numerical simulations of RF scattering by filaments using a commercial full-wave code are described. The filaments are taken to be cylindrical with the axis of the cylinder aligned along the direction of the ambient magnetic field. The plasma inside and outside the filament is assumed to be cold. There are three primary objectives of these studies. The first objective is to validate the numerical simulations by comparing with the analytical results for the same plasma descriptiona step-function discontinuity in density. A detailed comparison of the Poynting flux shows that the numerical simulations lead to the same results as those from the theoretical model. The second objective is to extend the simulations to take into account a smooth transition in density from the background plasma to the interior of the filament. The ensuing comparison shows that the deviations from the results of the theoretical model are quite small. The third objective is to consider the scattering process for situations well beyond a reasonable theoretical analysis. This includes scattering off multiple filaments with different densities and sizes. Simulations for these 2 complex arrangement of filaments show that, in spite of the obvious limitations, the essential physics of RF scattering is captured by the analytical theory for a sin...

Abstract-Europe is devoting significant joint efforts to develop and to manufacture MW-level gyrotrons for electron cyclotron heating and current drive of future plasma experiments. The two most important ones are the stellarator Wendelstein W7-X at Greifswald and the tokamak ITER at Cadarache. While the series production of the 140 GHz, 1 MW, CW gyrotrons for the 10 MW ECRH system of stellarator W7-X is proceeding, the European GYrotron Consortium (EGYC) is presently developing the EU-1 MW, 170 GHz, CW gyrotron for ITER. The initial design had already been initiated in 2007, as a risk mitigation measure during the development of the advanced ITER EU-2 MW coaxial-cavity gyrotron. The target of the ITER EU-1 MW conventional-cavity design is to benefit as much as possible from the experiences made during the development and series production of the W7-X gyrotron and of the experiences gained from the earlier EU-2 MW coaxial-cavity gyrotron design. Hence, the similarity of the construction will be made visible in the present article. During 2012, the scientific design of the ITER EU-1 MW gyrotron components has been finalized. In collaboration with the industrial partner Thales Electron Devices (TED), Vélizy, France, the industrial design of the technological parts of the gyrotron is being completed. A short-pulse prototype is under development to support the design of the CW prototype tube. The technological path towards the EU ITER-1MW gyrotron and the final design will be presented. . Both experiments are relying on electron cyclotron resonance heating (ECRH) as the main heating method for steady state operation, while in addition it is planned for ITER to apply electron cyclotron resonance technique for current drive (ECCD). ECRH & ECCD offer the compatibility to the various physics demands, such as controlled plasma start-up, steady state plasma control, and performance optimization by plasma profile shaping. It offers excellent coupling to the plasma, remote launching and very good localization of the absorbed power. Index Terms-PlasmaThe construction of the stellarator W7-X is almost completed and the device is approaching the commissioning phase [3]. W7-X operation will be supported by a 10 MW continuous wave ECRH system working at 140 GHz in 2 nd harmonic X-or O-mode. To date, the ECRH-system of W7-X is in stand-by with already 5 out of 10 gyrotrons operational. The series production of the W7-X 1 MW, CW gyrotrons [4,5] for the 10 MW ECRH system is proceeding.The European gyrotron development for ITER started with an advanced 170 GHz, 2 MW coaxial-cavity design. RF tests with an industrial CW prototype were done at the European test facility at EPFL-CRPP Lausanne in December 2011 [6]. The prototype did show an excellent voltage stand-off. It was directly possible to excite the nominal operating TE 34,19 -mode. The output RF-beam intensity was in good agreement with the expected one. Without further optimization, the RF output power reached the level of almost 2.1 MW in short-pulse (1 ms) operation with sin...

For the development of a DEMOnstration Fusion Power Plant the design of auxiliary heating systems is a key activity in order to achieve controlled burning plasma. The present heating mix considers Electron Cyclotron Resonance Heating (ECRH), Neutral Beam Injection (NBI) and Ion Cyclotron Resonance Heating (ICRH) with a target power to the plasma of about 50MW for each system. The main tasks assigned to the EC system are plasma breakdown and assisted start-up, heating to L-H transition and plasma current ramp up to burn, MHD stability control and assistance in plasma current ramp down. The consequent requirements are used for the conceptual design of the EC system, from the RF source to the launcher, with an extensive R&D program focused on relevant technologies to be developed. Gyrotron: the R&D and Advanced Developments on EC RF sources are targeting for gyrotrons operating at 240GHz, considered as optimum EC Current Drive frequency in case of higher magnetic field than for the 2015 EU DEMO1 baseline. Multipurpose (multi-frequency) and frequency step-tunable gyrotrons are under investigation to increase the flexibility of the system. As main targets an output power of significantly above 1MW (target: 2MW) and a total efficiency higher than 60% are set. The principle feasibility at limits of a 236GHz, conventional-cavity and, alternatively, of a 238GHz coaxial-cavity gyrotron are under investigation together with the development of a synthetic diamond Brewster-angle window technology. Advanced developments are ongoing in the field of multi-stage depressed collector technologies. Transmission Line (TL): Different TL options are under investigation and a preliminary study of an evacuated quasi-optical multiple-beam TL, considered for a hybrid solution, is presented and discussed in terms of layout, dimensions and theoretical losses. Launcher: Remote Steering Antennas have been considered as a possible launcher solution especially under the constraints to avoid movable mirrors close to the plasma. With dedicated beam tracing calculations, the deposition locations coverage and the wave absorption efficiency have been investigated, considering a selection of frequencies, injection angles and launching points. An option for the EC system structure is proposed in clusters, in order to allow the necessary redundancy and flexibility to guarantee the required EC power in the different phases of the plasma pulse. Number and composition of the clusters are analysed to have high availability and therefore maximum reliability with a minimum number of components.