Luminescent target interactions in ICP discharges: a diagnostic method for plasma current density

Recently, it has been observed that during the operation of an inductively coupled plasma (ICP), a luminescent target (BAM, BaMgAl₁₀O₁₇:Eu) can interact with the plasma beam and emit blue light. After excluding the influence of ultraviolet (UV) and electromagnetic wave radiation, the results indicate that the BAM target may undergo luminescent excitation due to collisions with electrons and ions. This led us to investigate the physical mechanism behind this plasma luminescence excitation phenomenon. A spectrometer was used to record the luminescent spectroscopy and peak light intensity. Under excitation by argon plasma, the BAM material emits a continuum spectrum from 400 nm to 550 nm, with the peak light intensity located at 462.58 nm, which is the same as the spectrum excited by UV torchlight. To identify the relationship between the plasma parameters and the luminescent intensity, Langmuir and Faraday probes were employed to determine the local plasma parameters such as electron density, electron temperature, and current density. After normalizing the peak light intensity to the plasma parameters, the most interesting point is that the current density is linearly correlated with the luminescent light intensity. To verify the repeatability and lifetime of the plasma-luminescence interaction, a 600 s lifetime test was conducted in a 200 W ICP discharge environment. The maximum difference for the peak light strength of the luminescent spectrum is 6.5%. From a voltage bias experiment and a theoretical derivation, we initially identified that bombardment by ions plays the dominant role in the luminescence excitation process, which also explains the mechanism by which the current density is proportional to the luminescence intensity. This new finding leads us to reconsider the possibility of applying this plasma luminescence phenomenon to optical plasma diagnostics. The BAM light intensity can potentially be used to predict the current density of a plasma beam for large-area two-dimensional (2D) measurements and can capture high spatial resolution in a single test. We believe that this method may lead to high-efficiency, spatially resolved plasma current density measurement.