Design verification of the gyrotron diamond output window for the upgrade of the ECRH system at W7-X

The 10 MW electron cyclotron resonance heating (ECRH) system at the stellarator Wendelstein 7-X (W7-X) currently relies on the successful operation of continuous wave (CW) 1 MW, 140 GHz gyrotrons which have chemical vapor deposition (CVD) diamond output windows cooled by the industrial silicon oil Dow Corning 200(R) 5 cSt. The window features a 1.8 mm thick diamond disk brazed to two copper cuffs with an aperture of 88 mm, which are then integrated in a steel housing. In the context of the upgrade of the ECRH system towards higher microwave power, this gyrotron design has been significantly advanced to fulfill the requirement of 1.5 MW CW operation, still at 140 GHz. A prototype of this new gyrotron is under development at Thales, France. This paper reports the computational fluid dynamics (CFD) conjugated heat transfer and structural analyses of the diamond window performed using the commercial code ANSYS V19.2 to investigate its performance at 1.5 MW operation. Furthermore, sensitivity studies were also carried out with respect to the absorbed power in the disk and the mm-wave beam radius at the window location. These analyses showed that the window design of the existing 1 MW gyrotrons still works quite well at higher power operation, thus verifying the performance of the window. Even in the worst case scenario of 1.5 kW absorbed power, the maximum temperature of 215 °C at the disk center can be safely accepted, being below the conservative limit of 250 °C for CVD diamond. In addition, the non-axial symmetric thermal gradients due to the geometry of the cooling channels lead to thermal stresses in the disk and the cuffs. However, they are much lower than the limits. The copper cuffs experience plasticity deformation in the region of the interface with the diamond disk up to a value of about 1.5 mm

Towards a 1.5 MW, 140 GHz gyrotron for the upgraded ECRH system at W7-X

For the required upgrades of the Electron Cyclotron Resonance Heating system at the stellarator Wendelstein 7-X, the development of a 1.5 MW 140 GHz Continuous Wave (CW) prototype gyrotron has started. KIT has been responsible to deliver the scientific design of the tube (i.e. the electron optics design and the RF design), with contributions from NKUA and IPP. The prototype gyrotron has been ordered at the industrial partner, Thales, France, and is expected to be delivered in 2021. In parallel, a short-pulse pre-prototype gyrotron has been developed at KIT, to provide the means for a first experimental validation of the scientific design in ms pulses, prior to the construction of the CW prototype. This paper reports on the status of the 1.5 MW CW gyrotron development, focusing on the scientific design and its numerical and experimental validation

Demonstration of reduced neoclassical energy transport in Wendelstein 7-X

Research on magnetic confinement of high-temperature plasmas has the ultimate goal of harnessing nuclear fusion for the production of electricity. Although the tokamak1 is the leading toroidal magnetic-confinement concept, it is not without shortcomings and the fusion community has therefore also pursued alternative concepts such as the stellarator. Unlike axisymmetric tokamaks, stellarators possess a three-dimensional (3D) magnetic field geometry. The availability of this additional dimension opens up an extensive configuration space for computational optimization of both the field geometry itself and the current-carrying coils that produce it. Such an optimization was undertaken in designing Wendelstein 7-X (W7-X)2, a large helical-axis advanced stellarator (HELIAS), which began operation in 2015 at Greifswald, Germany. A major drawback of 3D magnetic field geometry, however, is that it introduces a strong temperature dependence into the stellarator’s non-turbulent ‘neoclassical’ energy transport. Indeed, such energy losses will become prohibitive in high-temperature reactor plasmas unless a strong reduction of the geometrical factor associated with this transport can be achieved; such a reduction was therefore a principal goal of the design of W7-X. In spite of the modest heating power currently available, W7-X has already been able to achieve high-temperature plasma conditions during its 2017 and 2018 experimental campaigns, producing record values of the fusion triple product for such stellarator plasmas3,4. The triple product of plasma density, ion temperature and energy confinement time is used in fusion research as a figure of merit, as it must attain a certain threshold value before net-energy-producing operation of a reactor becomes possible1,5. Here we demonstrate that such record values provide evidence for reduced neoclassical energy transport in W7-X, as the plasma profiles that produced these results could not have been obtained in stellarators lacking a comparably high level of neoclassical optimization

Impact of Magnetic Field Configuration on Heat Transport in Stellarators and Heliotrons

We assess the magnetic field configuration in modern fusion devices by comparing experiments with the same heating power, between a stellarator and a heliotron. The key role of turbulence is evident in the optimized stellarator, while neoclassical processes largely determine the transport in the heliotron device. Gyrokinetic simulations elucidate the underlying mechanisms promoting stronger ion scale turbulence in the stellarator. Similar plasma performances in these experiments suggests that neoclassical and turbulent transport should both be optimized in next step reactor designs

Electron Bernstein Wave Heating and Emission in the TCV Tokamak

Electron cyclotron resonance heating (ECRH) of high-density tokamak plasmas is limited because of reflections of the waves at so-called wave cutoffs. Electron Bernstein wave (EBW) heating (EBWH) via a double mode conversion process from ordinary (O)-mode, launched from the low field side, to extraordinary (X)-mode and finally to Bernstein (B)-mode offers the possibility of overcoming these density limits. In this paper, the O-X mode conversion dependence on the microwave injection angle is demonstrated experimentally. The dependence on the injection angle is studied in high-density plasmas in H-mode, in the presence of magnetohydrodynamic activity, edge-localized modes, and sawteeth. The results of localized heat deposition at an overdense location are presented, demonstrating EBWH for the first time via the O-X-B mode conversion process in a standard aspect-ratio tokamak. The results of global and local power deposition are compared with raytracing calculations. Moreover, a temperature increase due to EBWH is observed. Initial EBW emission measurements with a newly installed ECRH reception launcher are presented. The inverse double mode conversion process B-X-O is observed by measuring the emission for several frequencies at an optimum angl

Reduced field Scenario with X3 heating in W7-X

In the present work, an ECRH scenario with reduced magnetic field 1.75 T is considered. For 140 GHz, this field corresponds to X3 heating. The high mirror-ratio magnetic configuration, B01/B00 ≃ 0.24, was considered as one from most attractive for long-pulse operation with low bootstrap current. Since X3 wave mode can be effectively absorbed only in sufficiently hot plasmas, a preheating stage is necessary, and the requirements for target plasmas suitable for starting X3 have been studied. Different ways to establish target plasmas are also discussed, in particular, augmenting X3 heating with X2 beams at 105 GHz

Is thermal treatment a concern for the nutritional quality of flaxseed, chia and sunflower seeds?

Oilseeds production has increased due to several food industry applications to answer consumers demand for foods with potential health benefits. Most of these benefits are related to the fatty acids profile, since oilseeds are particularly rich in polyunsaturated fatty acids that decrease the risk of several chronic diseases. From the food industry perspective, their application in the enrichment of breads, cakes, cookies and cereal bars, is a challenge. All these products are submitted to different processing methods, including heat treatment, being therefore essential to evaluate their impact on the nutritional value, namely in the fatty acid profile and oxidative stability of oilseeds. In 2016, samples of flaxseeds (Linum usitatissimum L.), chia (Salvia hispanica) and sunflower (Helianthus annuus L.) seeds were obtained from supermarkets in the Lisbon region. The samples were subjected to heat treatment (180 °C) for 10, 20, 30 and 60 minutes. Oilseeds fat was extracted with petroleum ether and for the methylation of the fatty acids a cold transesterification was performed using n-heptane and a methanolic solution of potassium hydroxide (2 M). Chromatographic separation of fatty acid methyl esters was then performed using a gas chromatograph coupled to flame ionization detector. For all the analysed oilseeds, the major fatty acids were polyunsaturated. Nonetheless, for chia and flaxseeds the major polyunsaturated fatty acid was alpha-linolenic (omega 3) fatty acid, while for sunflower seeds the major fatty acid was linoleic acid (omega 6). Foods containing high levels of polyunsaturated fatty acids are more susceptible to lipid oxidation, and some of the conditions that can trigger the oxidation process are the presence of oxygen, exposure to light, and/or heat treatment. In this work, after applying heat treatment on the different types of seeds, it was possible to conclude that no considerable changes were observed in the fatty acid profile of chia, sunflower and flaxseeds. This could be due, in part, to the presence of antioxidant compounds, such as phytosterols and tocopherols, but also due to the temperature of the heat treatment.National Institute of Health Dr. Ricardo Jorge, for financial support to Project “PTranSALT” (2012DAN828). FCT, FSE and MEC, for the financial support of Tânia Gonçalves Albuquerque PhD fellowship (SFRH/BD/99718/2014)N/

Loads due to stray microwave radiation in ITER

High-power microwaves generated by gyrotrons will be extensively used in ITER for a variety of purposes such as assisting plasma breakdown, plasma heating, current drive, tearing mode suppression and as a probing beam for the Collective Thomson Scattering diagnostic. In a number of these schemes absorption of the microwaves by the plasma will not be full and in some cases there could be no absorption at all. This may result in a directed beam with a high microwave power flux or – depending on location and plasma conditions – an approximately isotropic microwave power field. The contribution of electron cyclotron emission to these power densities is briefly discussed. Exposure to in-vessel components leads to absorption by metals and ceramics. In this paper microwave power densities are estimated and, following a brief review of absorption, thermal loads on in-vessel components are assessed. The paper is concluded by a discussion of the current approach to control such loads