Bulletin of the American Physical Society
69th Annual Gaseous Electronics Conference
Volume 61, Number 9
Monday–Friday, October 10–14, 2016; Bochum, Germany
Session ET2: Sheaths/Plasma PhysicsFocus
|
Hide Abstracts |
Chair: Benjamin Yee, Sandia National Laboratories Room: 2a |
Tuesday, October 11, 2016 11:00AM - 11:15AM |
ET2.00001: (Student Award Finalist) Formation of Anode Spots in Low Pressure Plasmas Brett Scheiner, Edward Barnat, Matthew Hopkins, Scott Baalrud, Benjamin Yee When small electrodes are biased sufficiently above the plasma potential, the rate of electron impact ionization of neutrals can increase near the electrode. At neutral gas pressures of \textasciitilde 1-100mTorr, it has been previously observed that if this ionization rate is sufficiently high a double layer forms near the electrode. Sometimes this double layer will move outward, separating a high potential plasma, attached to the electrode surface, from the bulk plasma. This phenomenon is known as an anode spot or fireball. Using observations from the first 2D particle-in-cell simulations of the anode spot, a model has been developed describing this formation process. In this model ionization leads to the buildup of an ion rich region adjacent to the electrode, which modifies the potential structure in a way that traps electrons near the electrode surface. This establishes a quasineutral plasma near the electrode. When the density of this plasma is large enough, a pressure imbalance across the double layer leads to its expansion from the electrode surface. Observations from simulations, along with the presented model, are found to be consistent with time resolved measurements of the electron density from laser collision induced fluorescence, and with plasma emission measurements. [Preview Abstract] |
Tuesday, October 11, 2016 11:15AM - 11:45AM |
ET2.00002: First experimental studies of ion flow in 3 ion species plasmas at the presheath-sheath transition Invited Speaker: Greg Severn The Bohm sheath criterion is studied with laser-induced fluorescence (LIF) in three ion species plasmas using two tunable diode lasers. KrI or HeI 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 (IEF) was demonstrated [Yip et al. Phys. Plasmas, 2010] to exist which accounted for the observed results. When three ion species are present, it is demonstrated under most circumstances the ions do not fall out of the plasma at their individual Bohm velocities. It is also shown that under most circumstances the ions do not fall out of the plasma at the system sound speed. Results are consistent with the presence of instabilities.\\ \\Author gratefully acknowledges collaborators Dr. Noah Hershkowtiz, Dr. Chi-Shung Yip, Dept. of Engineering Physics, Univ. Wisconsin-Madison, and Dr. Scott Baalrud, Dept. Physics, Univ. Iowa. [Preview Abstract] |
Tuesday, October 11, 2016 11:45AM - 12:00PM |
ET2.00003: Observation of Ion-neutral Collision Effect on Two-Ion-Stream Instability near Sheath-Presheath Boundary Nam-Kyun Kim, J. Song, H.-J. Roh, Y. Jang, S. Ryu, G.-H. Kim The ion velocity normal to the sheath-presheath boundary in weakly-collisional Ar/Xe mixture plasmas was measured by using LIF measurement. This investigation would give an answer to the old debate topic in the sheath community, whether each ion enters the sheath with their \textit{own Bohm velocity}, $C_{\mathrm{B}}=(T_{\mathrm{e}}$/$M_{\mathrm{i}})^{\mathrm{1/2}}$. In collisionless two-ion-species plasmas, Barrud and Hershkowitz concluded that the two-stream instability limits their velocities to become \textit{the common system sound speed}, $C_{\mathrm{s}}=(n_{\mathrm{1}}T_{\mathrm{e}}$/$n_{\mathrm{e}}M_{\mathrm{1}}+n_{\mathrm{2}}T_{\mathrm{e}}$/$n_{\mathrm{e}}M_{\mathrm{2}})^{\mathrm{1/2}}$. This instability is activated when the relative velocity becomes a critical velocity. In practices, the collisionless condition is not achievable. In this study, the ion-neutral collision effect on the instability was investigated with increasing the pressure of the Ar/Xe mixture gas in the range of 0.5 - 2 mTorr. Plasma is generated in a DC multi-dipole source in which $n$(\textit{Ar}$^{\mathrm{+}})$/$n$(\textit{Xe}$^{\mathrm{+}})$ is controlled to be 1. Results show that the instability is grown at $p$ \textless 2 mTorr and the ion drift velocities at the sheath edge are close to $C_{\mathrm{s}}$. At 2 mTorr, the ions reach their individual $C_{\mathrm{B}}$ at the sheath edge because the instability is not grown, observing that the characteristic length of the instability is a function of the ion-neutral collisions. The details will be discussed in the conference. [Preview Abstract] |
Tuesday, October 11, 2016 12:00PM - 12:15PM |
ET2.00004: Boundary conditions for electropositive and electronegative radio-frequency sheaths Mark Sobolewski Plasma sheaths play a dominant role in determining ion bombardment energies. To optimize plasma processes, sheaths must be understood and carefully controlled, which requires predictive models. One very efficient approach is to only model the sheath, excluding the bulk plasma. This approach, however, requires boundary conditions at the plasma/sheath boundary. Models that use the step approximation for electron density require initial ion velocities. More exact models with Boltzmann electrons (and, for electronegative discharges, negative ions) require the electron temperature (and the temperature and relative density of negative ions). It is often assumed that these boundary conditions have negligible effects on ion energies, but, for certain conditions in radio-frequency sheaths, this is not true. Analytic models as well as numerical simulations show that, at low frequencies ($\leq$ 1 MHz) and high bias voltages, the amplitude of the low-energy peak in ion energy distributions (IEDs) at the electrode is very sensitive to the boundary conditions. By measuring IEDs and sheath voltage waveforms, we obtain the most appropriate values of the boundary conditions for electropositive (Ar) as well as electronegative (CF$_4$) discharges and insight into their presheath dynamics. [Preview Abstract] |
Tuesday, October 11, 2016 12:15PM - 12:30PM |
ET2.00005: Laser-Collisional Induced Fluorescence Measurements of a Magnetosheath in a Biasable Ring Cusp Source Neil Arthur, John Foster This work involves exploring active control of the plasma potential and density spatial distribution in a mulitpole ion source through the active biasing of individual magnetic cusps. Cusp bias can be achieved by applying voltage to the magnetic surface generating the cusp. By controlling the current flow through the cusps, active altering of the primary electron containment length, at least at low voltages, is possible. Electrostatic probes do not work well in the presence of magnetic field and within sheaths. The Laser-Collisional Induced Fluorescence (LCIF) diagnostic is enabling in that it allows for imaging of the electron distribution in the magnetosheath. LCIF is used to quantify the response of electron density to active cusp biasing. We hope to gain insight into the physical processes occurring at the magnetic cusps and elucidate how those processes impact not only the plasma conditions in the bulk plasma, but also source efficiency and stability. The goal is to determine how bulk plasma properties change in response to modifications to current collection at the cusp magnetosheath. If the plasma properties vary with electric field in the cusp, then magnetic cusps must be considered as active rather than passive collectors. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700