Utilization of outer-midplane collector probes with isotopically enriched tungsten tracer particles for impurity transport studies in the scrape-off layer of DIII-D (invited)

Abstract

Triplet sets of replaceable graphite rod collector probes (CPs), each with collection surfaces on opposing faces and oriented normal to the magnetic field, were inserted at the outboard mid-plane of DIII-D to study divertor tungsten (W) transport in the Scrape-Off Layer (SOL). Each CP collects particles along field lines with different parallel sampling lengths (determined by the rod diameters and SOL transport) giving radial profiles from the main wall inward to R-Rsep ∼ 6 cm. The CPs were deployed in a first-of-a-kind experiment using two toroidal rings of distinguishable isotopically enriched, W-coated divertor tiles installed at 2 poloidal locations in the divertor. Post-mortem Rutherford backscatter spectrometry of the surface of the CPs provided areal density profiles of elemental W coverage. Higher W content was measured on the probe side facing along the field lines toward the inner target indicating higher concentration of W in the plasma upstream of the CP, even though the W-coated rings were in the outer target region of the divertor. Inductively coupled plasma mass spectroscopy validates the isotopic tracer technique through analysis of CPs exposed during L-mode discharges with the outer strike point on the isotopically enriched W coated-tile ring. The contribution from each divertor ring of W to the deposition profiles found on the mid-plane collector probes was able to be de-convoluted using a stable isotope mixing model. The results provided quantitative information on the W source and transport from specific poloidal locations within the lower divertor region.

Recent DIII-D progress toward validating models of tungsten erosion, re-deposition, and migration for application to next-step fusion devices

“…The 2016 DIII-D Metal Rings Campaign (MRC) represented the first large-scale use of high-Z plasma facing components (PFCs) in DIII-D. The hardware and diagnostic tools developed for the MRC have been described in previous works [40][41][42] and are only briefly re-described here. As shown in figure 4(a), two toroidally symmetric rings of isotopically distinct W-coated tiles were installed in the DIII-D lower, outer divertor.…”

“…Two sight lines for a new UV survey spectroscopy (200-400 nm) diagnostic were also installed, allowing for real-time monitoring of a number of WI and WII spectral lines [44]. Sets of graphite rods were installed close to the outboard midplane in the far SOL as collector probes to measure the fluence of tungsten in the far-SOL [41], similar to the approach on ASDEX-Upgrade [45]. Two new Langmuir probes (LPs) were installed at the major radius of the shelf ring, which in addition to the existing LP array and Divertor Thomson Scattering (DTS) system, allowed for detailed characterization of the divertor and SOL background plasmas during MRC experiments.…”

The role of B T-dependent flows on W accumulation at the edge of the confined plasma

“…To this end, DIII-D executed the MRC where two rings of toroidally symmetric, isotopically distinct W-coated tiles were installed in the lower, outer divertor, figure 1 [25]. Double-sided graphite CPs were inserted at the OMP on the MiMES reciprocating drive [26,27] where they remained for a set number of discharges. To avoid re-erosion of deposits on the CPs, they were inserted no deeper than to ∼7 cm outside the separatrix and were connected along field lines that limited on protruding regions of the vessel wall, placing them in what is defined as the wall-SOL [28], see figure 1.…”

“…Studies have already shown the valuable insights to SOL impurity transport/leakage provided by CPs during MRC. In [26,30] it is shown that by using the LAMS data and a stable isotope mixing model (SIMM), the fractional contribution from each W ring to the total W measured on each CP could be measured. The potential effect of the strength of radial transport on the asymmetry in total deposited W between the two CP sides is shown in [27].…”

“…This probe was inserted ∼1 cm further into the plasma than the probe inserted for unfavorable-B T , providing a single RBS data point in the UDL region. LAMS data are not available due to a destructive chemically based ICP-MS method [26] that had been used at this location before LAMS measurements were made. 3DLIM reproduces the RBS observation that the deposition profiles flatten off in the UDL region due to a longer connection length.…”