Rahul Banka Ion Wake Characteristics as a Function of Experimental Conditions - draft Title Evolution of Ion Wake Characteristics with Experimental Conditions

2025-04-29 0 0 999.81KB 16 页 10玖币

侵权投诉

Rahul Banka

Ion Wake Characteristics as a Function of Experimental Conditions- draft

Title: Evolution of Ion Wake Characteristics with Experimental Conditions

Authors: Rahul Banka1, Katrina Vermillion1, Lorin Matthews1, Truell Hyde1, and Lenaïc

Couëdel2,3

1Center for Astrophysics, Space Physics, and Engineering Research (CASPER), Baylor

University, Waco, TX, USA

2Physics and Engineering Physics Department, University of Saskatchewan, Saskatoon, Canada

3CNRS, Aix-Marseille Université, PIIM UMR 7345, Marseille, France

Abstract: Two-dimensional microparticle crystals can be formed in the sheath of a gas discharge

plasma. Ions from the bulk plasma are accelerated in the sheath electric field, flowing past the

grains to create a positive ion wake downstream from the grains. Interaction between the ion

wake and neighboring grains creates additional coupling between oscillation modes and can

trigger mode-coupling instability (MCI). Recent experiments have shown that at a fixed

discharge power there are threshold pressures above and below which the monolayer always

crystallizes or melts, respectively. The melting is due to MCI being triggered in the crystal

monolayer, while the crystallization is due to the suppression of MCI by neutral damping in the

fluid monolayer. The relationship between the discharge parameters and ion wake characteristics

is unknown. A molecular dynamics simulation of ion dynamics and dust charging is used to self-

consistently determine the dust charge and ion wake characteristics for different experimental

conditions. It is found that the ion wake is strongly dependent on discharge pressure but not

affected much by the discharge power.

1) Introduction:

Complex plasmas are ionized gas that include micron-sized grains. The dust grains

generally become negatively charged and can self-organize into crystalline structures [1–5].

Since these dust grains are easily imaged, complex plasma crystals can be used to study events

such as phase transitions at the kinetic level [6–9]. Experiments investigating phase transitions

with complex plasmas are often performed in a modified GEC rf reference cell. The dust grains

levitate above the lower electrode where the electrostatic force acting on the dust grains in the

sheath above the lower electrode balances with gravity. While the confinement in the vertical

direction is strong, the dust grains are not as strongly confined horizontally, allowing them to

disperse uniformly in a 2D plane forming a hexagonal lattice. Ions from the bulk plasma are

accelerated by the sheath electric field and flow past the dust grains creating a region of excess

ion density downstream from the grain called the ion wake [10–15].

Ion wakes act as an independent body in the interaction between two dust grains, causing

these interactions to be nonreciprocal, meaning the attractive force applied by the wake of one

grain on a second grain is not necessarily equal to the attractive force applied by the wake of the

second grain to the first [16]. This non-reciprocal attraction between the positive ion wake and a

negatively charged neighboring dust particle can cause the in-plane (horizontal) and out-plane

(vertical) oscillation modes to couple, which in turn creates an unstable hybrid mode. Thus, the

energy transferred to the dust monolayer from the ions flowing past the dust grains can trigger

the Mode-Coupling Instability (MCI) causing microparticles to gain energy. In a crystalline state,

the energy transferred to the dust grains can lead to the breaking of symmetry and cause the

structure to melt [16]. When in a fluid state, the dust particle energy (or temperature) has been

Rahul Banka

Ion Wake Characteristics as a Function of Experimental Conditions- draft

observed to continue to increase, suggesting that MCIs continue to act after the crystal has

melted unless suppressed by damping[17].

While the additional heating can cause a crystallized monolayer to melt, it makes it more

difficult for particles in an initial fluid state to form crystalline structures. Previous experiments

have shown that for microparticle monolayers levitating in the sheath of a radio-frequency

discharge at a fixed discharge power there are two threshold pressures [18,19]: an upper

threshold, , above which the monolayer always has a crystalline structure, and a lower

threshold, , below which the monolayer always undergoes mode-coupling instability (MCI)

causing the monolayer to melt. Between these two pressures, the monolayer can be in either a

crystalline or fluid state. If the monolayer is initially in a fluid state, it will remain as a fluid in

the pressure range between  and  until the pressure is increased to , at which

point it will crystallize. Similarly, if the monolater is initially in a crystalline state, it will remain

a crystal until the pressure is decreased to , at which point it will become a fluid.

In a simplified model, the ion wakes can be thought of as fixed, positive point charges

charge  at a distance  downstream of each dust particle. This model adequately represents the

system if grains remain far enough apart that the wake charge and location relative to the dust

grain are constant [15]. In previous studies, [16, 20–29], the point charge model has been used to

study MCIs; however, the impact of changing discharge parameters, such as rf power and neutral

gas pressure, on the ion wake parameters remains largely unknown.

The molecular dynamics simulation Dynamic Response of Ions And Dust (DRIAD)

[15,30–32] are used to determine dust charge and ion wake characteristics for the different

experimental conditions described in Couëdel and Nosenko article [33], hereafter referred to as

Paper I. Plasma parameters that are unknown or not easily measured such as the sheath electric

field, electron temperature, ion and electron number density, and ion flow speed, are determined

through an iterative approach that optimizes the balance between the resultant electrostatic and

gravitational forces on a dust grain for a given ion flow speed, allowing the wake characteristics

to be obtained as a function of system power and pressure. Discharge parameters for stable

levitating dust grains have been experimentally determined many times in the past, yet the

relationship between these parameters and the characteristics of the ion wake is unknown. As

such, DRIAD is utilized to model the characteristics of the ion wake for different discharge

parameters.

This paper is organized as follows. Section 2 describes the experiment from which the

input plasma parameters are derived. Section 3 describes the numerical model, DRIAD, and the

process by which the additional plasma parameters are obtained. The results of the simulated

discharge conditions are presented in Section 4 along with the calculated wake characteristics

such as the total wake charge and the distance between the dust grain and the ion wake’s center

of charge. Section 5 is a discussion of the results and conclusions.

2) Experimental Background

This section summarizes the results from Paper I [33].

Experiments were performed in a modified GEC cell. The experimental setup is depicted

in figure 1. These experiments were performed using argon gas between 0.5 and 2 Pa, and rf

Rahul Banka

Ion Wake Characteristics as a Function of Experimental Conditions- draft

power between 5 and 25 W, as measured. A dust monolayer was suspended in plasma consisting

of spherical melamine-formaldehyde (MF) microparticles with a diameter of   .

Threshold values of  and  were found for various rf powers (experimental

procedures can be found in Paper I [33]). Two sets of experiments were performed. The first set

of experiments explored power ranges from 25 W to 16 W. The second set of experiments,

exploring the power range 16W to 7W, utilized the same dust monolayer as the first set of

experiments but was performed about an hour later. During this time it is assumed the MF dust

grains were etched, reducing their size at a rate of ~1.25 nm/min [34], implying that for the

second set of experiments the dust grains were slightly smaller. The expected size difference is

about 2% and is ignored in this study. The stability of a crystalline monolayer was shown to

increase with rf power and argon pressure. However in Paper I, the MCI thresholds is shown to

be strongly dependent on interparticle interactions and wake parameters leading to the study

presented in this paper [33].

Tables I and II provide the neutral gas pressure, power, and measured effective grain

charge (taken to be the excess charge on the dust grain relative to the ion wake) for the 24

experimental conditions from the first and second sets of experiments, respectively.

Fig.1 Schematic of the experimental setup. Reproduced from Paper I [33].

3) DRIAD and Use of Experimental Data for Model Inputs

The molecular dynamics simulation DRIAD was used to model the dynamics of the ions

and dust and self-consistently calculate the dust charge [15,31,32]. The charge on a dust grain is

determined from the electron and ion currents to the dust grain surface. The electron current is

calculated using orbit motion limited (OML) theory, where the electrons are assumed to be

Boltzmann distributed and are not directly modeled. The ion current to the dust grain depends

on the ion flow speed as well as the dust charge and is calculated by counting the number of ions

with charge that cross the collection radius of a dust grain, . The charge collected per time

step is then   . The ions are simulated within a cylindrical region where the z-axis is

oriented along the direction of the sheath electric field. In this work, the wake region of a single,

fixed dust grain was studied, where the dust grain is placed at the center of the cylindrical

simulation region with a height of 10 and radius of 1 .

Rahul Banka

Ion Wake Characteristics as a Function of Experimental Conditions- draft

Simulation parameters were based on the 24 combinations of RF power and neutral gas

pressure listed in Tables I and II. The neutral gas density was calculated from the neutral gas

pressure at a gas temperature = 300 K,  , where  is the Boltzmann constant.

The electron temperature,  eV, and electron density,   cm, are provided

for a neutral gas pressure P = 0.66 Pa and RF power of 20 W, based upon Langmuir probe

measurements performed in the same experimental setup. Using these values as a guide, further

input parameters needed for the simulation were determined using a uniform density discharge

model for the bulk plasma [35] to calculate the expected values for the electron temperature in

each case. The electron and ion densities,   , were then determined based on the electron

density measurements at various rf powers and pressures as reported in Figure 2 of Paper I [33].

The initial electric field for each case was determined by considering the maximum expected

potential difference between the plasma bulk and the rf electrode based upon the maximum peak-

to-peak rf voltage, which is a function of rf power and neutral gas pressure. These values were

determined for each of the 24 cases and used as input parameters for the first iteration of DRIAD

simulations. The results were checked for consistency by requiring the effective electrostatic

force   to be within 10% of the gravitational force  acting on the dust grain. Based

on the results of the comparison, the electric field and ion flow velocity values were adjusted for

the next iteration of DRIAD simulations. This iterative process continued until the force balance

matched to within  and the input ion flow velocity estimated from the electric field matched

the output ion flow velocity to within 13%. The electric field values determined by the iterative

method correspond to the electric field determined from PIC simulations at a distance 10-14 mm

above the lower electrode, as shown in Figure 2, which is consistent with experimentally

observed dust levitation heights of ~10 mm [36]. The parameters for , , and  are given in

Tables I and II for each of the cases.

Fig 2. Results for the axial electric field determined from PIC simulations at rf powers and neutral gas pressures

comparable to the conditions used in the 24 cases presented in this work.

文档加载中……请稍候！
如果长时间未打开，您也可以点击刷新试试。

下载文档到电脑，查找使用更方便

10 玖币 0人已下载

立即下载

摘要：

RahulBankaIonWakeCharacteristicsasaFunctionofExperimentalConditions-draftTitle:EvolutionofIonWakeCharacteristicswithExperimentalConditionsAuthors:RahulBanka1,KatrinaVermillion1,LorinMatthews1,TruellHyde1,andLenaïcCouëdel2,31CenterforAstrophysics,SpacePhysics,andEngineeringResearch(CASPER),BaylorUniv...

展开>> 收起<<

Rahul Banka Ion Wake Characteristics as a Function of Experimental Conditions - draft Title Evolution of Ion Wake Characteristics with Experimental Conditions.pdf

共16页,预览4页

还剩页未读，继续阅读

声明：本站为文档C2C交易模式，即用户上传的文档直接被用户下载，本站只是中间服务平台，本站所有文档下载所得的收益归上传人(含作者)所有。玖贝云文库仅提供信息存储空间，仅对用户上传内容的表现方式做保护处理，对上载内容本身不做任何修改或编辑。若文档所含内容侵犯了您的版权或隐私，请立即通知玖贝云文库，我们立即给予删除！

Rahul Banka Ion Wake Characteristics as a Function of Experimental Conditions - draft Title Evolution of Ion Wake Characteristics with Experimental Conditions

相关推荐

开通VIP享超值会员特权

作者详情

相关内容

热门标签

举报选择: