Removing W-contaminants in helium and neon RF plasma to maintain the optical performance of the ITER UWAVS first mirror

Ushakov, A.G.; Verlaan, Ad; Ebeling, Rob; Rijfers, André; O’Neill, R.C.; Smith, M.; Stratton, B.; Koster, N.B.; List, Jos van der; Gattuso, Anthony; Lasnier, C.J.; Feder, R.; Maniscalco, Matthew; Verhoeff, Peter

doi:10.1016/j.fusengdes.2018.02.082

Cited by 14 publications

(8 citation statements)

References 6 publications

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“…The new RF-based cleaning system developed for the US-lead Upper Port Wide Angle Viewing System was capable for removing 10-20 nm of tungsten from the SC Mo mirror. No mirror damage has occurred after cleaning and the surface roughness of the mirror did not increase [38].…”

Section: Mirror Surface Recovery (Msr) 231 Finding An Optimum Sc Orie...mentioning

confidence: 93%

Diagnostic mirrors for ITER: research in the frame of International Tokamak Physics Activity

et al. 2019

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Nucl. Fusion 59 (2019) 066029 (10pp) baffled cassette with mirrors was exposed at the main wall of JET for 23,6 plasma hours. No significant degradation of reflectivity was measured on mirrors located in the ducts.Predictive modeling was further advanced. A model for the particle transport, deposition and erosion at the port-plug was used in selecting an optical layout of several ITER diagnostics. These achievements contributed to the focusing of the first mirror research thus accelerating the diagnostic development. Modeling requires more efforts. Remaining crucial issues will be in a focus of the future work of the FM SWG.

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Section: Mirror Surface Recovery (Msr) 231 Finding An Optimum Sc Orie...mentioning

confidence: 93%

Diagnostic mirrors for ITER: research in the frame of International Tokamak Physics Activity

et al. 2019

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“…For the ETS mirrors with the area of approximately 600 cm 2 , that would lead to high powers. From other experiments [2,17,18] the surface power density could be estimated as 0.5-1 W cm −2 , that was sufficient to generate ion fluxes of 5•10 18 ions•m 2 s −1 with energies 50-150 eV to remove Al/Al 2 O 3 films (considered as Besubstitutes for experiments where beryllium could not be used) with a rate of 1-2 nm h −1 .…”

Section: Configuration With Notch Filtermentioning

confidence: 99%

“…In laboratory setups, the efficiency of plasma cleaning was demonstrated and analyzed in relation to various configurations of first diagnostic mirrors and for assorted deposits to be cleaned [3,[14][15][16]. In earlier experiments, feasibility of plasma cleaning was discussed for the Ultra-Wide Angle Visible Spectroscopy diagnostic system [2,17,18]. The electrical circuit of the plasma cleaning system was modeleld for the Visible Spectroscopy Reference System [5,19].…”

Section: Cleaning Discharge Requirementsmentioning

confidence: 99%

Radio-frequency plasma to clean ITER front-end diagnostic mirrors in geometry of Edge Thomson Scattering system

Ushakov¹,

Veldhoven²,

Rijnsent³

et al. 2022

Phys. Scr.

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The ITER Edge Thomson scattering (ETS) system provides electron temperature and density profile measurements in the ITER tokamak for plasma control. In collection optics, the front-end metallic first and second mirrors are expected to experience contamination with beryllium and tungsten degrading performance. Plasma cleaning based on a low-pressure radiofrequency discharge is expected to sputter contaminants. In the ETS, a water-cooled first mirror is combined with a powered electrode. Water cooling was realized as a notch filter for the driving frequency, and the electrode was grounded for a DC- voltage. A new breadboard reproducing the ETS mirror geometry was developed for experimental modelling. The notch filter uses equivalent cables. To understand the cleaning effect, ion energies and fluxes were measured in 40-50 MHz discharges in argon and helium at 1-10 Pa with and without the notch filter. Without a notch filter with minimum power losses, 2-4·1018 ions·m-2s-1 ion fluxes with energies of 100-140 eV were produced. With the notch filter, the peak ion energies were 30-50 eV. The power in the plasma was lower than 50 W with ion fluxes of – 1.5-1.9 1018 ions·m-2s-1. 53-m-long cable feeds equivalent to those of a real system were studied at 13-50 MHz and produced significant power loss: more than 95% at 40-50 MHz. A pre-matching element is essential to reduce power losses. A lower radiofrequency may be less sensitive to pre-matching and may be considered for mirrors without water cooling. A 40 MHz discharge is proposed for cleaning due to better stability, higher ion energies and possibly better uniformity. Powers in plasma in the range of hundreds of Watts may be needed to achieve ion fluxes suitable for cleaning. This requires more powerful components and shall be addressed in later experiments. The results are important for plasma cleaning in ETS and in other diagnostics where water-cooled first mirrors are used.

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“…This results in ion acceleration towards the FM with an energy of e(V p − V DC ), where V p is the plasma potential, and leads to sputtering of surface deposits given the ion energy is greater than the sputtering energy threshold of the deposits. Plasma cleaning of FM deposits via CCRF discharges has been a subject of intense investigation in recent years [7][8][9][10].…”

Section: Introductionmentioning

confidence: 99%

Effect of 3 T magnetic field on RF plasma sputtering in an ITER-relevant first mirror unit

Soni

Iyyakkunnel²,

Steiner³

et al. 2022

Nucl. Fusion

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The first mirror (FM) cleaning operations in ITER are expected to be executed in presence of ∼ 3T magnetic field. In the RF plasma cleaning configuration, this would have a significant influence on the plasma properties, ion energy, angle of incidence as well as flux spatial distribution. To this end, RF discharges were excited in an ITER-sized mock-up of a first mirror unit (FMU) consisting of a powered first mirror M1 and a grounded second mirror M2 placed in a homogeneous 3T magnetic field. The plasma discharge was confined in a beam extending in the direction of the magnetic field, consequently wetting a limited portion of the FMU walls. In the DC-decoupled scheme (without λ/4 filter), this considerably influenced the self-bias voltage VDC that develops on M1. Changing the angle α between M1 normal and magnetic field, modified the plasma wetted wall area Ag and the resulting VDC varied by over two orders of magnitude. Plasma exposure experiments were also done in the DC-coupled scheme (with λ/4 filter), wherein the angle and wetted surface determined the area of the wall sputtered. Increasing α led to an increase in the sputtered wall area Ag, and consequently the wall deposition on grounded M2. However, in all the cases M1 was entirely clean with exception of edge deposits in some. In contrast, both M1 and M2 are coated with wall deposits in absence of a magnetic field and similar plasma exposure. The results show that plasma cleaning with a λ/4 filter in a 3T magnetic field at ITER could potentially prevent the parasitic wall deposition on FMs. The results also highlight the importance of FM orientation in the magnetic fields and the wetted area in their plasma cleaning in both DC-coupled as well as decoupled schemes within the ITER diagnostic systems.

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Removing W-contaminants in helium and neon RF plasma to maintain the optical performance of the ITER UWAVS first mirror

Cited by 14 publications

References 6 publications

Diagnostic mirrors for ITER: research in the frame of International Tokamak Physics Activity

Diagnostic mirrors for ITER: research in the frame of International Tokamak Physics Activity

Radio-frequency plasma to clean ITER front-end diagnostic mirrors in geometry of Edge Thomson Scattering system

Effect of 3 T magnetic field on RF plasma sputtering in an ITER-relevant first mirror unit

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