Ion flow and sheath physics studies in multiple ion species plasmas using diode laser based laser-induced fluorescence

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. D

Laser-induced fluorescence measurements of argon ion velocities near the sheath boundary of an argon–xenon plasma

“…To generate a two-ion species plasma, neutral xenon is added to the argon plasma. Contrary to the previous Ar-He experiment where the argon was a heavier ion species [12,13], the argon is now a lighter ion species. Figure 8 shows the plasma potential profile and calculated argon rms velocity as a function of the distance z in the argon-xenon plasma.…”

“…The first experimental hints of this came from Hala's preliminary ion acoustic wave (IAW) measurements suggesting that in Ar-He plasmas, argon ions reach the sheath edge with the velocity close to the ion sound speed of the bulk plasma, which is intermediate between the two individual ion sound speeds, the argon ions being the slow ions and the helium ion being the fast ions [11]. That the argon ions reach the sheath edge travelling much faster than its Bohm velocity was later directly corroborated with laser-induced fluorescence (LIF) measurements using diode lasers [12,13]. However, the helium ion velocity distribution functions (ivdfs) could not be directly examined using LIF techniques.…”

Collisionless “thermalization” in the sheath of an argon discharge

“…13,14 This instability-enhanced collisionality was used successfully to explain, for instance, the Bohm Criterion for multiple ion species plasmas. 10,11,15 Our own results show that the collisionless thermalization that we observe in the sheath actually already begins in the presheath, although it is much more prominent in the former than in the latter. Indeed, we observed that around 10%-15% of the loss of asymmetry in the IVDF (as quantified precisely in Sec.…”

“…Among plasma diagnostics, LIF is a privileged technique to obtain the IVDF with good spatial and velocity resolution without perturbing the plasma. [7][8][9][10][11][12] The results of Claire et al 5 show that the IVDF, after having developed a prominent asymmetric tail in the presheath, becomes again symmetric in the sheath. Here, we will show that this phenomenon can be explained using purely collisionless arguments, without invoking either ordinary or instability-enhanced 13,14 collision rates.…”

Exact Solution for the Generalized Bohm Criterion in a Two-Ion-Species Plasma

“…One is that all ions reach the sheath edge with the same velocity called the ''system sound velocity,'' and the other is that each ion species has its own C s . This problem has been investigated both theoretically [16,17] and experimentally [14,[18][19][20][21]. Franklin showed that in the presence of uniform ionization all ions left with their own C s [17].…”