Error fields in the Wendelstein 7-X stellarator

Lazerson, S.; Bozhenkov, S.; Israeli, Ben; Otte, M.; Niemann, H.; Bykov, V.; Endler, M.; Andreeva, T.; Ali, Adnan; Drewelow, P.; Jakubowski, M.; Sitjes, A. Puig; Pisano, F.; Cannas, B.

doi:10.1088/1361-6587/aae96b

Cited by 43 publications

(63 citation statements)

References 36 publications

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“…The observed erosion pattern is in rough agreement with the energy deposition with symmetrized trim coil currents to different TDU target modules determined by thermocouples (TCs) on the rear side of the target modules (see Fig. 11 in [23]. Higher temperature rises of the TCs were observed for 2u and 5u, lower temperature rises for 1l, 4u, 5l: This is in line with the observed net erosion pattern where higher erosion is observed in 2u and 5u and smaller erosion in 1l, 4u, 5l.…”

Section: Total Carbon Erosionsupporting

confidence: 76%

“…8). According to the TC data from [23] TM1h to TM5h (see Fig. 2) receive the highest power fluxes in the horizontal targets during standard configuration discharges, while the fluxes to TM6h and TM7h are already considerably smaller.…”

Section: Total Carbon Erosionmentioning

confidence: 96%

“…Carbon erosion has been measured in poloidal direction on 14 horizontal target elements (see Table 2), which (based on the assumption of toroidal symmetry within each target module) can be extrapolated to 14 horizontal target modules. For the remaining 36 horizontal target modules the erosion can be extrapolated based on the TC data from [23] with symmetrized trim coil currents assuming that the carbon erosion is proportional to the temperature rise of the TCs. For all 10 horizontal targets this results in 34.5±8.4 g of net carbon erosion.…”

Section: Total Carbon Erosionmentioning

confidence: 99%

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Material erosion and deposition on the divertor of W7-X

et al. 2020

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The net erosion and deposition pattern of carbon from the Test Divertor Unit (TDU) of the stellarator W7-X was determined. Special target elements with marker layers consisting of about 300 nm molybdenum and 5-10 µm carbon on top were used during the operation phase OP 1.2a. The thicknesses of the marker layers were determined by elastic backscattering spectrometry (EBS) using 2.5 MeV protons before and after plasma exposure and laser-induced breakdown spectroscopy (LIBS) on selected target elements after exposure. Scanning electron microscopy (SEM) was used for investigating the surface morphology before and after exposure. Massive erosion of up to 20 µm carbon was observed at the strike line, in total 48±14 g carbon were eroded from the 10 TDUs. The erosion was laterally nonuniform on the micro-scale. Strongly eroded surfaces were considerably smoother as compared to the original material. Only very little deposition of carbon is observed on the TDU: This means that the TDU is a large net erosion source.

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Section: Total Carbon Erosionsupporting

confidence: 76%

Section: Total Carbon Erosionmentioning

confidence: 96%

Section: Total Carbon Erosionmentioning

confidence: 99%

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Material erosion and deposition on the divertor of W7-X

et al. 2020

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“…The effect of error fields on stellarator configurations has been a subject of study since the measurement of magnetic islands in Wendelstein 7-AS by Jaenicke et al (1993) highlighted that the assumption of flux surfaces in a stellarator experiment is incorrect. Error fields have been studied in the Columbia Non-neutral Torus stellarator configuration (Hammond et al 2016), and in the island divertor in Wendelstein 7-X (Lazerson et al 2018). A recent paper by Zhu et al (2019) addresses the issue of the identification and removal of the error fields responsible for island size using a Hessian matrix approach and making the simplifying assumptions that first derivatives are zero and that variations in the magnetic coordinates can be ignored.…”

Section: Introductionmentioning

confidence: 99%

An adjoint method for determining the sensitivity of island size to magnetic field variations

2021

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An adjoint method to calculate the gradient of island width in stellarators is presented and applied to a set of magnetic field configurations. The underlying method for calculation of the island width is that of Cary & Hanson (Phys. Fluids B, vol. 3, issue 4, 1991, pp. 1006–1014) (with a minor modification), and requires that the residue of the island centre be small. Therefore, the gradient of the residue is calculated in addition. Both the island width and the gradient calculations are verified using an analytical magnetic field configuration introduced by Reiman & Greenside (Comput. Phys. Commun., vol. 43, issue 1, 1986, pp. 157–167). The method is also applied to the calculation of the shape gradient of the width of a magnetic island in a National Compact Stellarator Experiment (NCSX) vacuum configuration with respect to positions on a coil. A gradient-based optimization is applied to a magnetic field configuration studied by Hanson & Cary (Phys. Fluids, vol. 27, issue 4, 1984, pp. 767–769) to minimize stochasticity by adding perturbations to a pair of helical coils. Although only vacuum magnetic fields and an analytical magnetic field model are considered in this work, the adjoint calculation of the island width gradient could also be applied to a magnetohydrodynamic (MHD) equilibrium if the derivative of the magnetic field, with respect to the equilibrium parameters, is known. Using the island width gradient calculation presented here, more general gradient-based optimization methods can be applied to design stellarators with small magnetic islands. Moreover, the sensitivity of the island size may itself be optimized to ensure that coil tolerances, with respect to island size, are kept as high as possible.

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“…The search for optimized parameters for physics programs in steady-state operation phase OP2 (from2021) resulted in new magnetic configurations, which have to be carefully checked before their use [4], [5]. Successful compensation of main magnetic field errors [6] have been done by five normal conducting trim coils (TCs) installed on the cryostat and accessible for checking and maintenance.…”

Section: Introductionmentioning

confidence: 99%

Cyclic Behavior of Wendelstein 7-X Magnet System During First Two Phases of Operation

Bykov

Zhu

Carls

et al. 2020

IEEE Trans. Appl. Supercond.

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The sophisticated large magnet system of the Wendelstein 7-X (W7-X) stellarator has been operated during first two experimental campaigns at the Max-Planck-Institute for Plasma Physics in Greifswald, Germany, for roughly 13 months. Its 70 superconducting coils (NbTi CCIC) are extraordinary not only due to complex 3D shapes of 50 non-planar coils, but also due to a non-linear support system. In addition, five big resistive coils with the aim to correct W7-X error fields are installed on the outer cryostat and use rubber pads in their supports to compensate thermal expansion of the coils. The structural behavior of the W7-X magnetic system is monitored and evaluated on the basis of the analysis of the signals from the extended set of mechanical and temperature sensors. Several cooldown/warming up and thousands of electromagnetic cycles with different loading patterns and with up to 70% design load magnitude have been performed by the system successfully. The focus of this paper is on the cyclic structural behavior of the W7-X magnet system and the comparison with finite element predictions. Several related issues such as bolts and rubber pads prestress degradation, support slippage development, evolution of mutual coil displacement, loading path dependence of stress levels, sliding weight support (cryoleg) adjustment, and sensor failure are addressed. Lessons learned so far are also briefly summarized.

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Error fields in the Wendelstein 7-X stellarator

Cited by 43 publications

References 36 publications

Material erosion and deposition on the divertor of W7-X

Material erosion and deposition on the divertor of W7-X

An adjoint method for determining the sensitivity of island size to magnetic field variations

Cyclic Behavior of Wendelstein 7-X Magnet System During First Two Phases of Operation

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