Variation Of Plasma Potential Research Articles

Variations of plasma potential in RF discharges with DC-grounded electrode

The self-bias voltage Vbias and the plasma potential Vp are the key parameters to control plasma-wall interactions in radio-frequency (RF) asymmetric plasma discharges. Knowing these two parameters allows to monitor the ion energy distribution on the electrode surface. However, in a direct current (DC) coupled plasma, the plasma potential increased to hundreds of volts while no more Vbias developed on the driven electrode. In addition, the plasma potential is strongly impacted by the electrode-wall area ratio. Several analytical or semi-analytical models exist to explain this phenomenon and to approximate the plasma potential, knowing the electrode-wall area ratio considering capacitive sheaths or a combination of resistive and capacitive sheaths. The Vp and several other plasma parameters were investigated experimentally with a Langmuir probe and a Retarding field energy analyser for different electrode/wall area ratios in low-temperature RF plasma. The validity of the models was studied for an extensive range of area ratios with different pressures and RF driving amplitude. Moreover, the presence of strong stochastic heating of the plasma for area ratios below 6 was evidenced by an increased ion flux and an increased electron temperature.

Experimental studies and COMSOL 1-D simulation in Ar capacitively coupled plasmas

This work systematically studies a capacitively coupled plasma (CCP) source using experiments and 1-D COMSOL simulations relevant to Ar plasmas. Two radio frequency compensated Langmuir probes (LPs) and optical emission spectroscopy (OES) were purposefully used to measure the plasma parameters, and the experimental results were compared with those of simulations. We studied the axial variation of plasma parameters using an axial LP between the power and ground electrodes of the CCP at various operating pressures ranging from 10 to 150 mTorr. The electron density showed a gradual increase in its value with rising pressures. In addition, we employed a radial LP at the axial location L = 4 cm from the surface of the power electrode to measure the plasma parameters and compare these data with those of the axial LP and simulations. The variations of plasma potential measured by the radial LP showed an opposite trend of variation to those of simulations and the axial LP at pressures 10–60 mTorr, which is attributed to the plasma diffusion at low pressures. LP and OES measurements and simulation data suggest stochastic heating that generates high electron temperatures at low pressures. In addition, data revealed that the high-density plasma generation at high pressures could be due to the effects of both collisional heating and stochastic heating. Analysis showed that electrons could gain energy from the strong field regime of the sheath closed to the electrodes, which has a similar variation to electron temperature. The results of simulations have shown excellent agreement with experiments, and this work has the basis for plasma applications like plasma-enhanced chemical vapor deposition.

Trade-off in perpendicular electric field control using negatively biased emissive end-electrodes

The benefits of thermionic emission from negatively biased electrodes for perpendicular electric field control in a magnetized plasma are examined through its combined effects on the sheath and on the plasma potential variation along magnetic field lines. By increasing the radial current flowing through the plasma thermionic emission is confirmed to improve control over the plasma potential at the sheath edge compared to the case of a cold electrode. Conversely, thermionic emission is shown to be responsible for an increase of the plasma potential drop along magnetic field lines in the quasi-neutral plasma. These results suggest that there exists a trade-off between electric field longitudinal uniformity and amplitude when using negatively biased emissive electrodes to control the perpendicular electric field in a magnetized plasma.

Electric potential in partially magnetized E × B discharges

Ability to control the electric potential inside the plasma is promising for various applications of E × B plasma because the trajectory of particles, which meets the desired purpose, is mainly determined by the drift motions in the external electric and magnetic fields. In the E × B source, it is well known that the equilibrium states and plasma density and temperature gradient can be sources of plasma instability, which, in turn, affects global plasma parameters. However, the effect of instability on the electric potential has not been verified so far. In this work, correlation of the plasma potential and instability is investigated through simplified-circuit modeling. The consideration of the transverse conductivity of electrons across the magnetic field reflecting the turbulence collision frequency well explains the plasma potential closer to the anode potential despite the presence of negatively biased cathodes. Eventually, this result indicates that the instability can restrict the variation of plasma potential in partially magnetized plasma.

Charged particle transport across an obstacle in a non-flowing partially magnetized plasma column

A wake is created in a plasma when a macroscopic body blocks the flow of charged particles from entering in to a downstream plasma region. The phenomena leads to a strong depletion in charged particle density behind the obstacle. In this paper, charged particle transport inside an ionization free region behind a macroscopic obstacle has been investigated for the case of a non-flowing, partially magnetized plasma column. Surprisingly, it is found that the transport of hotter electron population inside the void region is enhanced due to the application of axial magnetic field. Furthermore, the radial plasma density and potential variation inside the obstructed region show an opposing trend than the region outside the obstacle. A phenomenological model is given to explain the mechanism behind observing these trends.

Prediction of Axial Variation of Plasma Potential in Helicon Plasma Source Using Linear Regression Techniques

Analytical expressions are used frequently for the determination and analysis of plasma parameters. Instead of relying on analytical expressions, the proposed method uses regression techniques supplemented with experimental data for the selected parameters (plasma potential). In the machine learning domain, this is equivalent to the creation of the training data set, building and training the model, and authenticating the result over a range of desired physical parameters. An experimental dataset is built using two axially movable Triple Langmuir Probe (TLPs) which measure the electron temperature, electron density, and electric potential of a plasma. The presented work is a first step towards developing an inclusive model with detailed kinetic simulations capable of characterizing the HELicon Experiment for Negative ion source (HELEN-I) with a single driver. Plasma potential is measured at different axial locations (z) by keeping pressure fixed at 6 mTorr.

Turbulence and perpendicular plasma flow asymmetries measured at TJ-II plasmas

Dedicated experiments have been carried out for a systematic comparison of turbulence wavenumber spectra and perpendicular rotation velocity measured at poloidally separated positions on the same flux-surface in the stellarator TJ-II. The rationale behind this study is twofold, namely, validation of the spatial localization of instabilities predicted by gyrokinetic simulations in stellarators and validation of the electrostatic potential variation on the flux surface as calculated by neoclassical codes and its possible impact on the radial electric field. Perpendicular wavenumber spectra and perpendicular rotation velocity profiles have been measured using Doppler reflectometry in two plasma regions poloidally separated as both positive and negative probing beam angles with respect to normal incidence can be selected. A systematic comparison has been carried out showing differences in the perpendicular wavenumber spectrum measured at poloidally separated positions on the same flux-surface, that depend on plasma density, heating conditions and magnetic configuration. The asymmetry found in the standard magnetic configuration under some plasmas conditions, reverses in the high iota configuration. The different intensity in the density fluctuation spectra can be related to the poloidal localization of instabilities found in gyrokinetic simulations. Differences in the radial electric field profile are also found that could be explained to be due to on-surface plasma potential variations.

Langmuir probe investigation of transient plasmas generated by femtosecond laser ablation of several metals: Influence of the target physical properties on the plume dynamics

Langmuir probe measurements were performed on transient plasmas generated by femtosecond laser ablation of several metallic targets (Al, Cu, Mn, Ni, In, Te, W). The analysis of current-voltage characteristics at various delays after the laser pulse gave access to the temporal evolution of ion density, electron temperature and plasma potential. The time-of-flight profile of the current recorded by the probe was also discussed in terms of a shifted Maxwell-Boltzmann velocity distribution, considering both thermal and drift velocities. The plasma parameters derived by these approaches were correlated with the electrical conductivity of the investigated metals. Assuming a direct dependence between the probe ionic signal and the charge carrier mobility in the target, a logarithmic fit was proposed for the plasma potential variation with electrical conductivity, whereas a derivative of this function was applied for the electron temperature. The saturation charge derived from the time-integrated probe ionic signals was influenced by the electrical conductivity of the target and also by the atomic weight of the metal. A steep increase of the thermal and drift velocities with conductivity was observed.

43 years of fun basic plasma physics experiments

This paper presents an overview of some of the most enjoyable low-temperature plasma experiments I have carried out with my students over the course of my career. It begins with the connection between solitons and the Schrodinger equation, and continues with the discovery of cylindrical solitons. From moving structures, we progress to stable ones: sheaths and double layers. How can we measure double-layer potentials? This question leads to the development of emissive probe techniques. From there, we move on to the Bohm criterion and the first measurements of the full plasma potential variations of sheaths and presheaths in a single-species plasma, and later in two-ion-species plasmas. Finally, our recent measurements of double layers and presheaths in uniform helicon plasma are presented.

On uniform plasma generation for the large area plasma processing in intermediate pressures

Radial plasma discharge characteristics in the range of 450 mm were studied in a dual inductively coupled plasma (ICP) source, which consisted of a helical ICP and the side type ferrite ICPs. Since the energy relaxation length is shorter than the distance between each of the ferrite ICPs in an intermediate pressure (600 mTorr), local difference in the plasma ignition along the antenna position were observed. In addition, large voltage drop in the discharge of the ferrite ICPs causes an increase in the displacement current to the plasma, and separate discharge mode (E and H mode) according to the antenna position was observed. This results in non-uniform plasma distribution. For the improvement in the discharge of the ferrite ICPs, a capacitor which is placed between the ends of antenna and the ground is adjusted to minimize the displacement current to the plasma. As a result, coincident transitions from E to H mode were observed along the antenna position, and radially concave density profile (edge focused) was measured. For the uniform density distribution, a helical ICP, which located at the center of the discharge chamber, was simultaneously discharged with the ferrite ICPs. Due to the plasma potential variation through the simultaneous discharge of helical ICP and ferrite ICPs, uniform radial distribution in both plasma density and electron temperature are achieved.

Decay of a finite-sized transient photoplasma in an electrostatic field

Photoplasma is produced through multi-step resonant photoionization method by shining the laser pulses onto a collimated atomic beam. It has finite size having a sharp density gradient at its boundary. It is created within the duration of laser pulse (~10 ns) while it lasts for few tens of micro-seconds. During its decay in an external electrostatic field, the photoplasma passes through various physical phenomena happened along the direction of the electric field. The transient responses of photoplasma to the external electric field and its temporal evolutions are studied using a one dimensional model based on standard particle-in-cell (PIC) technique. The various processes like relaxation of mono-energetic electrons, spatial and temporal variations in plasma potential, plasma-sheath formation, charge particles distribution near the plasma-sheath boundary, motion of plasma-sheath boundary, expansion of the finite-width photoplasma and collections of charge particles at the boundaries (i.e. electrodes) are investigated and discussed.

Inboard and outboard radial electric field wells in the H- and I-mode pedestal of Alcator C-Mod and poloidal variations of impurity temperature

We present inboard (HFS) and outboard (LFS) radial electric field (Er) and impurity temperature (Tz) measurements in the I-mode and H-mode pedestal of Alcator C-Mod. These measurements reveal strong Er wells at the HFS and the LFS midplane in both regimes and clear pedestals in Tz, which are of similar shape and height for the HFS and LFS. While the H-mode Er well has a radially symmetric structure, the Er well in I-mode is asymmetric, with a stronger ExB shear layer at the outer edge of the Er well, near the separatrix. Comparison of HFS and LFS profiles indicates that impurity temperature and plasma potential are not simultaneously flux functions. Uncertainties in radial alignment after mapping HFS measurements along flux surfaces to the LFS do not, however, allow direct determination as to which quantity varies poloidally and to what extent. Radially aligning HFS and LFS measurements based on the Tz profiles would result in substantial inboard-outboard variations of plasma potential and electron density. Aligning HFS and LFS Er wells instead also approximately aligns the impurity poloidal flow profiles, while resulting in a LFS impurity temperature exceeding the HFS values in the region of steepest gradients by up to 70%. Considerations based on a simplified form of total parallel momentum balance and estimates of parallel and perpendicular heat transport time scales seem to favor an approximate alignment of the Er wells and a substantial poloidal asymmetry in impurity temperature.

Effect of antenna size on electron kinetics in inductively coupled plasmas

Spatially resolved measurements of electron energy distribution functions (EEDFs) are investigated in inductively coupled plasmas with two planar antenna coils. When the plasma is sustained by the antenna with a diameter of 18 cm, the nonlocal kinetics is preserved in the argon gas pressure range from 2 mTorr to 20 mTorr. However, electron kinetics transit from nonlocal kinetics to local kinetics in discharge sustained by the antenna coil with diameter 34 cm. The results suggest that antenna size as well as chamber length are important parameters for the transition of the electron kinetics. Spatial variations of plasma potential, effective electron temperature, and EEDF in terms of total electron energy scale are also presented.

Effect of recombination and ionization on the analysis of Mach and single probes in un-magnetized and magnetized plasmas of MAP-II and DiPS devices

For the analysis of atomic processes such as recombination and ionization in the divertor plasmas, we have developed a fast-scanning multiple probe system, which is composed of single, Mach and emissive probes along with Thomson scattering system at the material and plasma (MAP)-II divertor simulator with low magnetic field (about 200G). Measured spatial variations of electron density, electron temperature, flow velocity and plasma potential will be given in comparison with those by Thomson scattering method. To analyze the data, a new fluid model on the ion collection to a probing object in un-magnetized plasmas is developed including electron–ion recombination, molecular activated recombination and ionization. This system is applied to another divertor simulator, divertor plasma simulator (DiPS), with higher magnetic field (up to 2kG).

Hollow cathode theory and experiment. I. Plasma characterization using fast miniature scanning probes

A detailed study of the spatial variation of plasma density, temperature, and potential in hollow cathodes using miniature fast scanning probes has been undertaken in order to better understand the cathode operation and to provide benchmark data for the modeling of the cathode performance and life described in a companion paper. Profiles are obtained throughout the discharge and in the very high-density orifice region by pneumatically driven Langmuir probes, which are inserted directly into the hollow cathode orifice from either the upstream insert region inside the hollow cathode or from the downstream anode-plasma region. A fast transverse-scanning probe is also used to provide radial profiles of the cathode plume as a function of position from the cathode exit. The probes are extremely small to avoid perturbing the plasma; the ceramic tube insulator is 0.05cm in diameter with a probe tip area of 0.002cm2. A series of current-voltage characteristics are obtained by applying a rapid sawtooth voltage wave form to the probe as it is scanned through the plasma at speeds of up to 2m∕s to produce the profiles with a spatial resolution of about 0.05cm. At discharge currents of 10–25A from the 1.5-cm-diameter hollow cathode, the plasma density inside the cathode is found to exceed 1014cm−3, with the peak density occurring upstream of the orifice. The plasma potentials on axis inside the cathode are found to be in the 10–20V range with electron temperatures of 2–5eV, depending on the discharge current and gas flow rate. A potential discontinuity or double layer of less than 10V is observed in the orifice region, and under certain conditions appears in the bright “plasma ball” in front of the cathode. This structure tends to change location and magnitude with discharge current, gas flow, and orifice size. A potential maximum proposed in the literature to exist in or near the cathode orifice is not observed. Instead, the plasma potential increases from the orifice exit both radially and axially over several centimeters to values of 5–10V above the anode voltage. The potential and temperature profiles inside the cathode are insensitive to anode configuration changes that alter the discharge voltage at a given flow. Application of an axial magnetic-field characteristic of the cathode region found in ring-cusp ion thrusters increases the plasma density in the cathode plume, but does not significantly change the potential or temperature. Measurements of the plasma profiles and the internal cathode parameters for a hollow cathode operating at discharge currents of up to 25A in xenon are shown and discussed.

Hairpin resonator probe measurements in RF plasmas

This work investigates the use of hairpin probes in plasma where RF plasma potential is present. The microwave resonance of the hairpin is used to determine electron density. Two types of hairpin probe were used. One type was dc coupled: its dc potential could be varied while monitoring its resonance frequency and collected current. The other probe was designed to be fully floating, being (dc) isolated from ground and able to float with RF variations in the plasma potential. Additional measurements of the RF plasma potential and its effect on the dc floating potential of the former probe were made using a wire loop probe. The resonant frequency of the dc coupled probe at zero current (nominal floating potential) was less than that determined from the fully floating probe. This is attributed to the wider sheath around the former caused by RF plasma potential across it. The presence of the electron-free sheath around the wires of the hairpin is included in the analysis that links the resonant frequency to the electron density in the bulk plasma. When the dc coupled probe was biased at the true floating potential (determined from independent loop probe measurements) its resonant frequency was closer to, though still consistently higher than, that of the floating probe. This work shows that RF potential across the probe sheath affects the resonance of a hairpin probe and should be accounted for when using hairpin probes in discharges where RF plasma potential variations are even as low as a few times the electron temperature (in volts).

Pulsed dc operation of a Penning-type opposed target magnetron

This article describes the medium frequency pulsed dc power operation of a Penning-type (opposed target) magnetron to sputter copper in an argon atmosphere. The current–voltage characteristics of the magnetron are detailed in dc and pulsed dc conditions. The variations in optical emission from the excited species in the plasma are measured as a function of time during the applied pulses. The intensity of the argon emissions closely follows the variations of the target voltage but the different behavior of the copper emissions shows some evidence of self-sputtering of the target by copper ions. The energy and flux of the ions arriving at the substrate are measured as a function of time and it is shown that during pulsed operation, as the pulse off-period is extended, there are large increases in ion energy and ion flux. The initial overshoots in the target voltage cause a rapid expansion of the plasma sheath at the analyzer probe and wide variations in plasma potential leading to an initial pulse of high energy ions during the first 500ns. The plasma potential then settles to ∼40eV and there is a second peak in ion flux at this energy with a peak at ∼4000ns due to the increased ion drift resulting from electron extraction from the target.

Radio-frequency plasma potential variations originating from capacitive coupling from the coil antenna in inductively coupled plasmas

The radio-frequency plasma potential in a stove top inductively coupled plasma source is measured by a capacitive probe. The experimental results are compared to a crude circuit model which accounts for capacitive coupling between the rf coil and the bulk plasma. The capacitive coupling model has three terms: the dielectric window capacitance, the sheath capacitance between the dielectric window and the bulk plasma, and the bulk plasma to ground sheath capacitance. The crude circuit model predictions are verified by quantitative comparison with the measured rf plasma potential in the bulk argon plasma at pressures from 1 to 20 mTorr and radio-frequency (13.56 MHz) plasma power levels from 60 to 1000 W. Finally, the measured ion energy spectrum, as determined by a retarding potential analyzer, agrees with rf plasma potential measurements over the entire range of experimental conditions.

Hot spot formation on electron-emissive target plate with plasma potential variation across magnetic field

Nonlinear dynamic behavior of hot spot formation on tungsten (W) material surfaces is numerically investigated by introducing a transient heat pulse into the local area of the hot W sheet immersed in high heat flux plasma. It is found that the formation and development of hot spot depend not only on plasma parameters (plasma heat flow, plasma potential variation) and the energy and time scale of heat pulse but also sensitively on electron-emission characteristics of the tungsten surface as well as the size of hot spot in a nonlinear way.

On the heating mode transition in high-frequency inductively coupled argon discharge

In high-frequency inductively coupled argon discharges with a planar-type coil the phenomena of discharge mode transition (E–H mode transition) are investigated. Experimental observation is done at the low pressure of 10 mTorr and the high frequency of 19 MHz over a range of rf power, 40–525 W. First of all, the discharge mode transition is observed through a change of luminous intensity. This transition is found to occur at the relatively high power of about 280 W compared with the mode transition in a 6.5 MHz discharge. Also, some distinctive features are compared to low-frequency discharges during this transition. In particular, during the E–H mode transition the apparent changes of plasma potential are observed and the sudden variation of plasma potential is proposed as an important factor that indicates the change of power coupling. The features of the discharge mode transition in high-frequency discharge are discussed by considering the power coupling at each mode by measurements of the electron energy distribution functions.