  Experiments have been conducted in the DIII-D tokamak to explore the in-situ
growth of silicon-rich layers as a potential technique for real-time
replenishment of surface coatings on plasma-facing components (PFCs) during
steady-state long-pulse reactor operation. Silicon (Si) pellets of 1 mm
diameter were injected into low- and high-confinement (L-mode and H-mode)
plasma discharges with densities ranging from $3.9-7.5\times10^{19}$ m$^{-3}$
and input powers ranging from $5.5-9$ MW. The small Si pellets were delivered
with the impurity granule injector (IGI) at frequencies ranging from 4-16 Hz
corresponding to mass flow rates of $5-19$ mg/s ($1-4.2\times10^{20}$ Si/s) at
cumulative amounts of up to 34 mg of Si per five-second discharge. Graphite
samples were exposed to the scrape-off layer and private flux region plasmas
through the divertor material evaluation system (DiMES) to evaluate the Si
deposition on the divertor targets. The Si II emission at the sample correlates
with silicon injection and suggests net surface Si-deposition in measurable
amounts. Post-mortem analysis showed Si-rich coatings containing silicon
oxides, of which SiO$_2$ is the dominant component. No evidence of SiC was
found, which is attributed to low divertor surface temperatures. The in-situ
and ex-situ analysis found that Si-rich coatings of at least $0.4-1.2$ nm
thickness have been deposited at $0.4-0.7$ nm/s. The technique is estimated to
coat a surface area of at least 0.94 m$^2$ on the outer divertor. These results
demonstrate the potential of using real-time material injection to form
Si-enriched layers on divertor PFCs during reactor operation.
