Chi-Shung Yip

Experimental test of instability enhanced collisional friction for determining ion loss in two ion species plasmas a)

Recent experiments have shown that ions in weakly collisional plasmas containing two ion species of comparable densities nearly reach a common velocity at the sheath edge. A new theory suggests that collisional friction between the two ion species enhanced by two stream instability reduces the drift velocity of each ion species relative to each other near the sheath edge and finds that the difference in velocities at the sheath edge depends on the relative concentrations of the species. It is small when the concentrations are comparable and is large, with each species reaching its own Bohm velocity, when the relative concentration differences are large. To test these findings, ion drift velocities were measured with laser-induced fluorescence in argon-xenon plasmas. We show that the predictions are in excellent agreement with the first experimental tests of the new model.

Verifying effects of instability enhanced ion–ion Coulomb collisions on ion velocity distribution functions near the sheath edge in low temperature plasmas

Experiments have shown that ion velocity distribution functions (ivdfs) with non-Maxwellian tails created by ion–neutral collisions and ionizations along pre-sheaths in weakly collisional plasmas can be thermalized into Maxwellian distributions near the sheath edge. A recent theory suggests that ion–ion collisions enhanced by ion-acoustic instabilities can rapidly thermalize ions near the sheath edge into Maxwellian distributions and that increasing either electron temperature or neutral pressure of a plasma suppresses the growth of instabilities and eliminates the thermalization process. Measurements of ivdfs by laser induced fluorescence showed qualitative agreement between experimental data and a marginal stability curve inferred from the new theory.

Manipulating the electron distribution through a combination of electron injection and MacKenzie’s Maxwell Demon

Experiments on electron heating are performed in a biased hot filament-produced argon plasma. Electrons are confined by multi-dipole magnetic fields on the radial wall of the cylindrical chamber but not the planar end walls. Electron heating is provided by a combination of cold electron injection (Hershowitz N and Leung K N 1975 Appl. Phys. Lett. 26 607) and a MacKenzie Maxwell Demon (Mackenzie K R et al 1971 Appl. Phys. Lett. 18 529). This approach allows the manipulation of the electrons by introducing a depleted tail into the electron energy distribution function or by removing a depleted tail. It is found that the injected electrons mimic and thermalize with the electron species with the closest average energy or temperature. The effect of the injected electrons is optimal when they mimic the secondary electrons emitted from the wall instead of the degraded primary electrons. Both approaches combine to achieve increases in electron temperature Te from 0.67 to 2.8 eV, which was not significantly higher than using each approach alone.

Laser-induced fluorescence measurements of argon and xenon ion velocities near the sheath boundary in 3 ion species plasmas

The Bohm sheath criterion is studied with laser-induced fluorescence in three ion species plasmas using two tunable diode lasers. Krypton is added to a low pressure unmagnetized DC hot filament discharge in a mixture of argon and xenon gas confined by surface multi-dipole magnetic fields. The argon and xenon ion velocity distribution functions are measured at the sheath-presheath boundary near a negatively biased boundary plate. The potential structures of the plasma sheath and presheath are measured by an emissive probe. Results are compared with previous experiments with Ar–Xe plasmas, where the two ion species were observed to reach the sheath edge at nearly the same speed. This speed was the ion sound speed of the system, which is consistent with the generalized Bohm criterion. In such two ion species plasmas, instability enhanced collisional friction was demonstrated [Hershkowitz et al., Phys. Plasmas 18(5), 057102 (2011).] to exist which accounted for the observed results. When three ion species are...

Measurements of the ion drift velocities in the presheaths of plasmas with multiple ion species

We have used a variety of diode lasers to perform laser-induced fluorescence (LIF) measurements that were the first to test the Bohm Criterion for multiple ion species plasmas. These measurements led to the discovery that a collective effect, ion-ion instability enhanced friction, is important for sheath formation in multiple ion species plasmas. New LIF schemes were pioneered in order to complete these experiments with diode lasers. With respect to a new LIF scheme for KrII, accessible to diode lasers, challenges remain fielding an ion flow diagnostic in low temperature plasmas.

Ion velocity-locking in the neighborhood of virtual cathodes via instability enhanced collisional friction

Axial ion motion in virtual cathodes and their connected presheaths are studied with laser-induced fluorescence. Virtual cathodes are formed using small electrodes (A electrode/A loss < 1.7(2.3 m e/m i)1/2) biased at higher than the plasma potential far from the electrode in multi-dipole confined filament discharges of argon gas. The virtual cathodes are electrostatic, with no magnetic fields present near the electrode to confine ions. An emissive probe is employed to measure the full potential profile from the bulk plasma up to the surface of the electrode. A planar Langmuir probe is employed to measure the electron temperature T e, the plasma density n e and to calculate the Debye length. Ions reflected from the electron sheath are only observed when the electrode is biased 1T e/e above the bulk plasma potential. When the electrode is biased more negatively, ions retain most of the kinetic energy from the potential drop along the presheath and sheath structure. When reflected ions are observed, ions retain very little of the kinetic energy from the presheath and sheath potential drop. The Baalrud–Callen–Hegna two-stream instability enhanced collisional friction theory is invoked to explain the phenomenon and provides qualitative agreement with the experimental results.

Effect of a virtual cathode on the I – V trace of a planar Langmuir probe

Experimental and modelling studies were performed to investigate how the presence of a virtual cathode (Forest and Herschkowitz 1986 J. Appl. Phys. 60 1295) near a planar Langmuir probe affects its accuracy in electron temperature measurements. It is found that the effect of the virtual cathodes causes the I–V trace to flatten and falsely create higher electron temperature measurements.

Investigation of possible sheath disappearance near a electrode biased at the plasma potential

Summary form only given. It is well established that when an electrode is biased negative with respect to the plasma potential, an ion sheath forms and when it is biased positive with respect to the plasma potential, an electron sheath forms provided that the electrode is small Aplate/Achamber <; (me/Mi)1/2) [1]. However, when a small electrode is biased at the plasma potential, it is unknown whether an ion sheath, an electron sheath or no sheath will form. Movable small (3-5cm diameter) plates biased at the plasma potential are immersed in a filament discharge in a multidipole chamber. Plasma potential and IVDFs near the plate are measured to determine whether an ion sheath, an electron sheath or no sheath formed. Ion velocities are determined by Laser-Induced Florescence, the electron temperature and electron density are measured by a planar Langmuir probe and the plasma potential is measured by an emissive probe.

The Langmuir's paradox: Can the ion acoustic instability at the sheath edge thermalize the ions too?

Summary form only given. Recently a theoretical prediction was that in single-species plasmas, ion-ion collisional friction is enhanced by the ion acoustic instability. The theory predicted that the instability will not only enhance the thermalization of the electrons, but will also, near the sheath-edge, thermalize the non-Maxwellian tail of the ion velocity distribution function (IVDF), caused by charge exchange in the presheath. The theory also predicted that this instability disappears through collisional damping as neutral pressure of the plasma increases. This experiment aims to verify this theory by measuring the IVDFs near the sheath edge in a multi-dipole chamber discharge in Argon and Xenon gas for a variety of neutral pressures and electron temperatures. The threshold parameters of the phenomenon are explored. The IVDFs are determined by Laser-Induced Florescence, the electron temperature is measured by a Langmuir probe and the plasma potential towards the boundary is measured by an emissive probe.

Effects of wire thickness, neutral pressure and gas composition on the inflection point technique

The inflection point technique in the limit of zero emission determines the plasma potential by fitting a straight line to the graph of the emission current versus the inflection point of the emissive probe I–V traces. The plasma potential is determined by extrapolating the line to the limit of zero emission. The effects of wire thickness, gas composition, neutral pressure and position on the technique were investigated. Experiments were performed in a multi-dipole filament discharge. Wire thicknesses of 0.013, 0.025, 0.05 and 0.1mm were studied. Experiments were done in Argon, Xenon and Helium plasmas with neutral pressures ranging from 0.5mTorr to 3mTorr. Measurements were performed from the bulk of the plasma to its sheath edge near a 10cm diameter negatively biased plate.