UWAVS first mirror after long plasma cleaning: Surface properties and material re-deposition issues

Ushakov, A.G.; Verlaan, Ad; Ebeling, Rob; Wu, Chien-Ching; O’Neill, R.C.; Smith, M.; Stratton, B.; Koster, N.B.; Gattuso, Anthony; Lasnier, C.J.; Feder, R.; Maniscalco, Matthew; Verhoeff, Peter

doi:10.1016/j.fusengdes.2019.02.128

Cited by 11 publications

(10 citation statements)

References 9 publications

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“…This paper presents preliminary results of cleaning the CXRS FM of EAST, including cleaning uniformity, possible cleaning damage to the FM, and new findings like redeposition during the repetitive cleaning cycles. These results are very different from some similar investigations [15,27] in the lab considering a real tokamak first mirror and large-scale mirror size. This study hopes to provide potential insights and reference into the development of the in situ cleaning of largescale FMs in EAST and possibly in ITER in the future.…”

Section: Introductioncontrasting

confidence: 99%

Study on plasma cleaning of the large-scale first mirror of the charge exchange recombination spectroscopy diagnostic on EAST

Peng

Yan

Chen

et al. 2019

Plasma Sci. Technol.

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In the Experimental Advanced Superconducting Tokamak (EAST), the reflectivity of the charge exchange recombination spectroscopy (CXRS) first mirror (FM) was dramatically dropped down to 20% of the original value after the operation of two EAST experimental campaigns from 2014-2015, leading to degradation of the signal intensity of the CXRS diagnostic to an unacceptably low level. The radio frequency (RF) plasma cleaning of the CXRS FM with a dimension of 303×81×76 mm 3 and a small curvature of 0.008 mm -1 was performed to remove deposits to recover the reflectivity. After 168 h cleaning by RF plasma, the maximum specular reflectivity of the FM could reach 92% of the original value at 532 nm, making the cleaned CXRS FM eligible to be reused for the CXRS diagnostic in the 2016 EAST campaign. Dedicated tests of sputtering polished mirror samples were performed to explore the cleaning uniformity and possible damage to the mirror surface. The specular reflectivity did not show obvious dependence on locations along the surface with the same cleaning time. The measured surface roughness gradually increased with sputtering time. The reflectivity remained almost unchanged regardless of different sputtering times and locations, indicating negligible damage to the FM surface even after 100 h sputtering. The recontaminated CXRS FM in the 2016 EAST campaign was firstly cleaned for 81 h, and the least reflectivity recovery for areas with relatively thick deposits was only 40%. After continuing cleaning to 147 h, redeposition of the sputtered residual deposits on the FM surface was observed. In the future for in situ cleaning of the FMs in EAST and ITER, deposits should be removed timely when they are very thin taking into account a very long cleaning time and presumable redeposition of thick and nonuniform deposits.

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Section: Introductioncontrasting

confidence: 99%

Study on plasma cleaning of the large-scale first mirror of the charge exchange recombination spectroscopy diagnostic on EAST

Peng

Yan

Chen

et al. 2019

Plasma Sci. Technol.

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“…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%

“…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%

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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“…If the cleaning energy is too high, it may cause a change in surface morphology or damage the function [20]. Low energy and prolonged cleaning may cause surface re-deposition [21]. Therefore, it is of outstanding academic and practical significance how to clean the surface quickly and reduce the damage to the surface.…”

Section: Introducementioning

confidence: 99%

Study on plasma cleaning of surface contaminants on pure copper

Wang

et al. 2023

Mater. Res. Express

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To demonstrate the relationship between the type of working gas and the efficiency of plasma cleaning, the kinetic energy of the incident ions was controlled by adjusting the bias, and the effect of different ion sources on the sputtering yield under the same bias was compared. The plasma flow generated by glow discharge was used to clean the sample surface to evaluate the plasma cleaning efficiency. Scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), X-Ray Powder Diffraction (XRD), Ultraviolet-visible Spectrophotometer (UV) and Surface Profiler were used to characterize the surface morphology, element types, phase composition, specular reflectance and three-dimensional (3D) morphology before and after plasma cleaning. The results show that the sputtering yield increases with the increase of bias, and when the bias value is lower than 400 V, the mass of the samples has almost no change before and after cleaning. It takes more energy to remove the surface oxide than the pure metal copper. The results also show that argon plasma has a better effect on removing pollutants on the sample surface then nitrogen plasma. However, long time of plasma cleaning is often accompanied by re-deposition. The changes in copper reflectance induced by plasma cleaning are also discussed.

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UWAVS first mirror after long plasma cleaning: Surface properties and material re-deposition issues

Cited by 11 publications

References 9 publications

Study on plasma cleaning of the large-scale first mirror of the charge exchange recombination spectroscopy diagnostic on EAST

Study on plasma cleaning of the large-scale first mirror of the charge exchange recombination spectroscopy diagnostic on EAST

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

Study on plasma cleaning of surface contaminants on pure copper

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