Abstract
Diode lasers have proved to be a valuable light source for laser-induced fluorescence (LIF) measurements for plasma science since the early 1990s, and they have recently improved the state of the art of measuring ion flow from ion velocity distribution functions (ivdfs) at the sheath–presheath boundary in single and multiple ion species plasmas. In the case of a low temperature two ion species plasma (ArI + HeI), we were the first to show experimentally that ion species may reach the sheath edge flowing at a very different speed than that expected from the single species Bohm Criterion (ArII ions exceed the individual Bohm flow speed by almost a factor of 2 at the sheath edge). Simulation results are found to agree. Diode laser technology relevant to LIF measurements in multiple ion species plasmas is discussed with the aim of addressing outstanding problems in sheath formation in such plasmas.
Highlights
The flow of multiple ion species onto plasma boundaries is an important feature of many plasma systems
We have demonstrated unambiguously that in such a two ion plasma, the ArII ions do exceed their individual Bohm speed at the sheath edge, and approach the ion sound speed of the ArII + HeII plasma system, as calculated in the bulk plasma [16]
Our confidence in our technique and our estimate of the fiducial point in the I2 fluorescence spectrum was confirmed by our calculation of the vrms as a function of position, showing that it passes through the Bohm velocity, within error, just at the sheath edge (Fig. 2)
Summary
The flow of multiple ion species onto plasma boundaries is an important feature of many plasma systems. We have demonstrated unambiguously that in such a two ion plasma, the ArII ions do exceed their individual Bohm speed at the sheath edge, and approach the ion sound speed of the ArII + HeII plasma system, as calculated in the bulk plasma [16] This was the first experimental work that measured ion flow at the sheath edge in multiple ion species plasmas using LIF which combined plasma potential measurements, and which served to locate the ivdfs relative to the spatial sheath structure. Was not the first to employ an LIF diagnostic to measure ion flow at the sheath edge: the work on a single ion species plasmas began with Gulick et al [17], who found that in a high density magnetized ECR plasma (ne = 3 Â 1011 cmÀ 3, kTe 10 eV) that the ions did not reach the Bohm speed at the boundary. We wish to discuss experimental aspects of these results not previously reported, in connection with the diode laser based LIF diagnostic, and we introduce first results of particle-in-cell (PIC) simulations which agree with those results
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