Study of wall re-deposition on DC-grounded ITER-relevant mirrors with RF plasma in a first mirror unit

Abstract

In ITER, several first mirrors (FMs) are expected to be DC-grounded with the water cooling lines being implemented as a quarter wavelength (λ/4) RF-filter. DC-grounding of the FMs can significantly increase the plasma potential V p, which could trigger an increased wall sputtering and associated re-deposition on the FMs during plasma cleaning. To understand the scope of this impact, helium discharges were excited with DC-grounded FMs in an ITER-sized mock-up of a first mirror unit (FMU) using wall materials with different sputtering energy thresholds (E th). Additionally, a part of the FM was electrically isolated from the RF to study its impact on the erosion/re-deposition properties on the surface. The E th of the wall materials, as well as its native oxide layers, had a significant influence on the re-deposition observed on the FMs. With high E th where walls were unsputtered, both the DC-grounded and electrically isolated parts of the FM were free of deposits. However, with low E th where the walls were sputtered, there was a net wall re-deposition on the DC-grounded parts of the FM, while electrically isolated parts were still relatively clean. Further, to study the impact of floating wall components, Cu walls in the FMU were isolated from the ground. Here the walls developed a floating potential V f and the ion energy at the walls was lowered to e(V p − V f). The floating walls, in this case, were relatively unsputtered and the FMs experienced a net cleaning with total reflectivity of the mirror preserved at pristine mirror levels. This work shows that electrically isolating the FM as well as the wall surface minimizes wall re-deposition in presence of λ/4 filter and therefore are promising techniques for effective FM cleaning in ITER.

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

“…For several experiments, this DC bias was shortcircuited by the addition of a λ/4 filter at the powered electrode. The details of the λ/4 filter (or notch-filter) are presented in [16]. Consequently, a CCRF discharge is excited while DC grounding the powered electrode at the same time.…”

“…Further, we performed plasma sputtering experiments in the FMU in the DC-coupled scheme (with λ/4 filter), to study the wall sputtering and deposition on M1 and M2 during plasma discharge in magnetic fields. Adding a λ/4 filter leads to DC-grounding of M1 (|V DC | = 0 V) and a considerable rise in the plasma potential V p [12,16]. M2 was kept electrically grounded as well.…”

“…This is typically in the range of ∼10 eV, which is considerably low to cause any surface sputtering of the Si samples. As a result, they accumulate all the deposits that reach its surface without getting sputtered back, giving insight into the wall sputtering and total rate of deposition [16]. For this study helium was used as the discharge gas, as it reportedly leads to wall depositions on FMs during plasma cleaning with λ/4 filter in absence of magnetic field [16], allowing us to make an effective comparison when doing similar experiments in 3 T field.…”

“…However, this also leads to an increase of the V p to several hundred volts [12][13][14][15], which in turn increases the sputtering of wall surfaces surrounding FMs. The sputtered wall material in turn gets deposited on the FMs making the plasma cleaning process less effective [16]. Hence CCRF plasma cleaning with a λ/4 filter requires additional mitigation strategies to achieve FM cleaning [16,17].…”

“…The sputtered wall material in turn gets deposited on the FMs making the plasma cleaning process less effective [16]. Hence CCRF plasma cleaning with a λ/4 filter requires additional mitigation strategies to achieve FM cleaning [16,17].…”