The Effect of Rotating Cylindrical Langmuir Probes in Magnetron Plasmas

Using a cylindrical Langmuir probe, the plasma properties (ion density, electron temperature, floating and plasma potentials) in a magnetron sputter source have been investigated, along one particular line‐of‐sight, but for different probe‐orientations with respect to the B‐field. The plasma in the region hosen for observation is haracterised by electrons, which are magnetised (Larmor radius r_le < both the electron mean‐free‐path λ_e, and plasma extension L) and ions, which are not (their Larmor radius r_I > L, λ_I). Through the development of a simple expression for the electron saturation current at different probe angles relative to the local B‐field, it is possible to correct for the diminished electron currents due to their restricted transport across the field. The results indicate that the measured ion density, electron temperature, floating and plasma potentials are unaffected by the probe orientation, however the electron saturation current is attenuated when the probe is aligned along the B‐field. A simple model for the collection of electrons indicates that classical electron diffusion may not operate in the magnetron, with cross‐field electron transport dominated by anomalous, possibly Bohm, diffusion.     

Potential Formation in a Bounded Plasma System which is Terminated by an Electron Emitting Floating Collector Studied by a Particle‐in‐Cell Computer Simulation

Potential formation in one‐dimensional bounded plasma system terminated by a floating, electron emitting collector is studied by particle‐in‐cell (PIC) computer simulation. Attention is focused to the case of rather strong space charge limited emission. Formation of a potential well (virtual cathode) in front of the collector is observed. As emission increases the floating potential of the electrode and the potential of the bottom of the potential well both increase. The floating potential increases faster than the virtual cathode potential and consequently the depth of the potential well in front of the collector increases also. As long as the emission is not to large (up to approximately 40 times the critical emission) the relation between the depth of the potential well and the normalized emission follows a simple logarithmic formula. For larger emissions the depth of the potential well is larger than predicted by the model. It seems that at very large emission the floating potential of the collector might even exceed the zero reference potential of the source electrode. Such phenomenon has been reported by [A. Marek et al. Contrib. Plasma Phys., 48 , 491 (2008)], where it was observed that the floating potential of a strongly emissive probe exceeded the plasma potential determined from the knee of the current‐voltage characteristics when the same probe was used as a Langmuir probe. But before this actually happens the simulation breaks down. When positive ions start to be repelled by the positive collector back towards the source the system becomes unstable so that a steady state can not be reached and no results can be read from the output of the simulations. That electron emission may destabilize the sheath in front of it, was found also in Hall thrusters, see e.g. [Daren Yu et al. Phys. Plasmas, 15 , 104501, 2008] and [F. Taccogna et al Appl. Phys. Lett., 94 , 251502, 2009]. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Double‐probe measurement in recombining plasma using NAGDIS‐II

We have studied the validity of the double‐probe method in recombining plasmas. Electron temperature (T_e) measured with a double probe was quantitatively evaluated by taking into account the influences of plasma potential fluctuation, plasma resistivity, and electron density fluctuation on the current–voltage characteristics. Differential potential fluctuation and plasma resistivity between two electrodes have a minor effect on T_e especially when the inter‐distance is small (typically 1 mm). Scattering of measured T_e due to the density fluctuation was sufficiently suppressed by making the data acquisition time long (typically 4 s) and taking the average. There is a good agreement between T_e measured with the optimized double‐probe method and that with laser Thomson scattering diagnostics.     

Langmuir Probe in Non‐Maxwellian Plasma: Correlation Between the Electron Energy Distribution Function,the Ion Energy at the Sheath Edge and the Floating Voltage

This work is devoted to the study of the Langmuir probe in non‐Maxwellian plasma, assuming mono‐energetic singly charged ions and a collisionless sheath. Using a general analytical equation for the Electron Energy Distribution Function (EEDF), we study the effect of the EEDF profile on: a) The ion energy at the sheath edge of a negatively biased collector, b) the I‐V probe characteristic and c) the floating voltage (V_p‐Vf). Different methods are used and compared to determine these parameters or characteristics. A correlation is given between the floating voltage, the ion energy at the sheath edge and the EEDF profile. The study is also extended to distribution functions with several components. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Floating double probe in non‐Maxwellian plasmas: Determination of the electron density and mean electron energy

A theoretical study of the floating double probe based on the Druyvesteyn theory is developed in the case of non‐Maxwellian electron energy distribution functions (EEDFs). It is used to calculate the EEDF in the electron energy range larger than –e(V_f ? V_p) from the I–V double probe characteristics. V_f and V_p are the floating and plasma potential, respectively. The analytical distribution function corresponding to the best fit of EEDF in the energy range larger than e(V_f ? V_p) allows the determination of the total electron density (n_e) and the mean electron energy (<?_e>). The method is detailed and tested in the case of a theoretical Maxwell–Boltzmann distribution function. It is applied for experiments that are performed in expanding microwave plasmas sustained in argon. Analytical EEDFs determined by this method are compared with those measured by means of single probes under the same experimental conditions. A good agreement is observed between single and double probe measurements. Results obtained under different experimental conditions are used to define the best conditions to obtain reliable results by means of the double probe technique.     

Influence of Plasma Resistance and Fluctuation on Probe Characteristics in Detached Recombining Plasmas

In order to find the causes of the strong anomaly of current‐voltage characteristics of Langmuir probe observed in detached recombining plasmas in a linear divertor plasma simulator, NAGDIS‐II, we have investigated plasma resistance along a magnetic field and potential fluctuations in the detached recombining plasmas. Simple calculation on the ratio between the plasma length, at which plasma resistance and resistance of ion sheath formed around a probe tip become equal, and an electron collection length indicates that the evaluation of electron temperature T_e becomes inaccurate at T_e of less than 0.6 eV when plasma density and neutral pressure are 1.0 × 10¹⁸ m^—3 and 10 mtorr, respectively. The potential fluctuation in detached recombining plasmas was found to be so large compared to T_e/e, which can also modify the probe characteristics.     

A Report on H mode in Magnetic Pole Enhanced Inductively Coupled Nitrogen Plasmas

The properties of low pressure magnetic pole enhanced, inductively coupled nitrogen plasma were studied by using electrical probe (Langmuir probe) under the conditions of RF powers in the range of 50‐220 W and pressures of 15‐75 mTorr. The electron energy probability function (EEPF) and electron density (n_e) obtained from the RF compensated Langmuir probe was compared with the theoretical results. The theoretical fits of the EEPF shows that the shapes of EEPF are evolved from generalised distribution to Maxwellian distribution function. It was also observed that at a low power (50 W) the discharge remains in inductive (H‐mode) mode for all the pressures (15‐75 mTorr). At a higher pressure and relatively low RF power, the measured EEPF show a hole near 3eV of energy. The intensities of the emission lines at 337.1nm (Second Positive System) and 391.4 nm (First Negative System) due to C³Π_u → B³Π_g and B²$\textstyle \sum_u^+$ → X² $\textstyle \sum_g^+$ transitions respectively, closely follows the variation of n_e with RF power and filling gas pressure. The stability of the H mode was also investigated using skin depth. Electron temperature and plasma potential indicate that the discharge at higher power (above 50 W) almost remain in H mode. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Control of the Interface Between Electron‐Hole and Electron‐Ion Plasmas: Hybrid Semiconductor‐Gas Phase Devices as a Gateway for Plasma Science

Coupling electron‐hole (e^–‐ h⁺) and electron‐ion plasmas across a narrow potential barrier with a strong electric field provides an interface between the two plasma genres and a pathway to electronic and photonic device functionality. The magnitude of the electric field present in the sheath of a low temperature, nonequilibrium microplasma is sufficient to influence the band structure of a semiconductor region in immediate proximity to the solid‐gas phase interface. Optoelectronic devices demonstrated by leveraging this interaction are described here. A hybrid microplasma/semiconductor photodetector, having a Si cathode in the form of an inverted square pyramid encompassing a neon microplasma, exhibits a photosensitivity in the ～420–1100 nm region as high as 3.5 A/W. Direct tunneling of electrons into the collector and the Auger neutralization of ions arriving at the Si surface appear to be facilitated by an n ‐type inversion layer at the cathode surface resulting from bandbending by the microplasma sheath electric field. Recently, an npn plasma bipolar junction transistor (PBJT), in which a low temperature plasma serves as the collector in an otherwise Si device, has also been demonstrated. Having a measured small signal current gain h_fe as large as 10, this phototransistor is capable of modulat‐ing and extinguishing the collector plasma with emitter‐base bias voltages <1 V. Electrons injected into the base when the emitter‐base junction is forward‐biased serve primarily to replace conduction band electrons lost to the collector plasma by secondary emission and ion‐enhanced field emission in which ions arriving at the base‐collector junction deform the electrostatic potential near the base surface, narrowing the potential barrier and thereby facilitating the tunneling of electrons into the collector. Of greatest significance, therefore, are the implications of active, plasma/solid state interfaces as a new frontier for plasma science. Specifically, the PBJT provides the first opportunity to control the electronic properties of a material at the boundary of, and interacting with, a plasma. By specifying the relative number densities of free (conduction band) and bound (valence band) electrons at the base‐collector interface, the PBJT's emitter‐base junction is able to dictate the rates of secondary electron emission (including Auger neutralization) at the semiconductor‐plasma interface, thereby offering the ability to vary at will the effective secondary electron emission coefficient for the base surface (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Floating Potential of an Electron Emitting Collector that Terminates a Bounded Plasma System

Potential formation in one‐dimensional bounded plasma system terminated by a floating, electron emitting collector is studied by a fully kinetic one‐dimensional model and particle‐in‐cell (PIC) computer simulation. As the electron emission from the collector is gradually increased, the floating potential of the collector and the electric field at the collector both increase quite strongly. When the critical emission is reached, the electric field becomes zero. If the emission is increased further, the electric field changes sign and becomes positive and a virtual cathode is formed in front of the collector. The floating potential of the collector also increases, but much more slowly, than below the critical emission level. If the ratio between the temperatures of the emitted and of the bulk electrons is high enough, the floating potential of the collector may even exceed the zero potential of the source, but in this case the simulation becomes unstable, when the positive ions start to flow back from the collector towards the source. Simulation results obtained below the critical emission level are in very good agreement with the model, provided that the particle densities obtained from the simulation are normalized correctly. In this case the potential and electric field profiles found from the PIC simulation match almost perfectly to the profiles obtained from the numerical solution of the Poisson equation (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Near-cathode potential drop in a vacuum diode with explosive emission of electrons

The near-cathode potential drop u_K is studied in a high-voltage high-current (U=20–40 kV, i=100–500 A) vacuum diode with a cathode operating under explosive emission of electrons. The magnitude of U_K is estimated from oscillograms of the probe current under conditions of breakdown of the layer between the probe and the cathode-flare plasma. It is shown that u_K is the same order of magnitude as the near-cathode potential drop in vacuum arcs. U_K does not change greatly when there are sharp rises in diode current, indicating that oscillations in diode current are probably related to processes at the boundary for drawing of electrons from the flare plasma.     

Methane Conversion in the Flowing Afterglow of a Dinitrogen Microwave Plasma: Initiation of the Reaction

The reaction of methane in the nitrogen afterglow is studied by combination of direct measurements and computer experiment. The experimental data were obtained by optical emission spectroscopy of the discharge, by gas chromatography of resulting stable products and by probe diagnostics of charged species in afterglow plasma. The simulation was based on a macroscopic kinetic approach covering 24 species and 61 reactions with input data given by the afterglow experiment. In the present stage of the modelling the initiation of methane conversion was studied. It was found that, contrary to active discharge, in the afterglow plasma the active neutral species (mainly excited dinitrogen molecules) are most important for the dissociation of methane into CH_x and H radicals.     

Measurements of Flow Velocity in the Plasma Generator PSI‐2

The plasma flow velocity in the Plasma Generator PSI‐2 has been investigated by using of Mach probe. PSI‐2 is a stationary high‐current arc discharge in which the quasi‐neutral plasma expands along the magnetic field lines. The low‐temperature (T_e < 20 eV), medium density (n_e ∼ 10¹⁸— 10¹⁹ m^—3 ) plasma in the discharge is similar to the plasma in the divertor region of tokamaks. From the ratio of ion saturation currents collected from opposite sides of the probe the flow velocities (Mach numbers) in argon and hydrogen discharges are obtained.     

Sondenmessung der Plasmaparameter und deren Fluktuation in Argon-Niederdruck-Schwachstrom-Säulen im transversalen Magnetfeld

This paper reports on measurements of probe noise and double probe characteristics in argon-low-pressure low-current discharges under the influence of a local transversal magnetic field. Above a critical value of magnetic field intensity B_c there are no moving striations but partially correlated fluctuations. The space distribution of probe noise (ion saturation current and floating potential) and the measured values of electron temperature and density are discussed. At B_c a characteristic value of electron temperature T_e,c ≈ 1,4 eV independent of filling pressure has been found. A first opinion on the character of the fluctuations is given.     

Effect of Discharge Plasma Potential on Diffusion Plasma Parameters Controlled by a Mesh Grid in a Double Plasma Device

The plasma region under investigation is separated from the discharge region by a mesh grid. Plasma potential and electron number densities and electron temperatures under bi‐Maxwellian approximation for electron distribution function of the multi‐dipole argon plasma are measured. The cold electrons in the diffusion region are produced by local ionization. The hot electrons are the ionizing electrons behaving as Maxwellian. The electron trapping process in the discharge region is produced by potential well due to positive plasma potential with respect to the anode and by a repulsive grid. The dependence of ratios of the density of the hot to the cold electrons N_E (=N_eh/N_ec) and hot to cold electron temperature T(=T_eh/T_ec) in the diffusion region on the depth of the potential well has been investigated. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Generation of THz Radiation from Laser Beam Filamentation in a Magnetized Plasma

The terahertz (THz) frequency radiation production as a result of nonlinear interaction of high intense laser beam with low density ripple in a magnetized plasma has been studied. If the appropriate phase matching conditions are satisfied and the frequency of the ripple is appropriate then this difference frequency can be brought in the THz range. Self focusing (filamentation) of a circularly polarized beam propagating along the direction of static magnetic field in plasma is first investigated within extended‐paraxial ray approximation. The beam gets focused when the initial power of the laser beam is greater than its critical power. Resulting localized beam couples with the pre‐existing density ripple to produce a nonlinear current driving the THz radiation. By changing the strength of the magnetic field, one can enhance or suppress the THz emission. The expressions for the laser beam width parameter, the electric field vector of the THz wave have been obtained. For typical laser beam and plasma parameters with the incident laser intensity ≈ 10¹⁴ W/cm², laser beam radius (r₀) = 50 μm, laser frequency (ω₀) = 1.8848 × 10¹⁴rad/s, electron plasma (low density rippled) wave frequency (ω₀) = 1.2848 × 10¹⁴ rad/s, plasma density (n₀) = 5.025 × 10¹⁷cm^–3, normalized ripple density amplitude (μ)=0.1, the produced THz emission can be at the level of Giga watt (GW) in power (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of Excitation and Vibrational Temperature on the Dissociation of Nitrogen Molecules in Ar-N2 Mixture RF Discharge

Non-equilibrium argon-nitrogen mixture plasma generated at 13.56 MHz is characterized by optical emission spectroscopy and Langmuir probe techniques. The excitation and vibrational temperature are studied as a function of argon percentage in the mixture, at 30-Pa filling pressure and input RF powers of 200 and 300 watt, to find out their role in dissociation of N₂ molecules. In this work, the excitation temperature is determined from Ar-I emission line intensities by using the simple Boltzmann plot method and is found to increase with argon mixing in nitrogen plasma. In similar fashion, the vibrational temperature of second positive system has been determined and is found to also have increasing trend with argon addition. The effect of excitation and vibrational temperature on the nitrogen molecular dissociation level is also monitored. It is observed that N/N ₂ ratio increases with increase in excitation and vibrational temperature and falls slightly at the end.     

悬浮型微波共振探针在电负性容性耦合等离子体中电子密度的测量

本文首先利用悬浮型微波共振探针测量了Ar等离子体的电子密度,并与朗缪尔双探针的测量结果进行了比较,表明了微波共振探针在低密度等离子体测量的可行性.对40.68 MHz单射频容性耦合Ar/SF₆和SF₆/O₂等离子体的测量结果表明:电负性气体SF₆掺入Ar等离子体显著降低了等离子体电子密度,但随着增加SF₆的流量,电子密度表现为缓慢下降;而O₂掺入SF₆等离子体中,电子密度则随着O₂流量的增加表现为持续的下降.另外,40.68 MHz/13.56 MHz双频激发的SF₆/O₂容性耦合离子体的电子密度并不随低频功率的变化而变化.本文对上述的实验现象进行了初步的解释.     

The Influence of the Fluctuation Amplitude on the Probe Characteristic

The time-averaged probe current is calculated by using a sampling method for sinusoidal and sawtooth-like oscillations of the space potential. The effect of oscillations on the measured plasma parameters obtained with the aid of the single probe method, double probe method and the method of the second derivative of the probe characteristic is discussed, with the electron saturation current being taken into account. In the ion current the values r_p/λ_D, λ/r_p and M_i characterizing the working regime are varied. The calculated results are checked by corresponding measurements in a beam generated plasma.     

Floating Potentials in the Vicinity of Biased Flush‐Mounted Probes

An array of surface‐mounted probe triplets in the Alcator C‐Mod divertor region [1, 2] demonstrates the influence of a biased probe tip on local plasma potential. For the experiments described here, each poloidally aligned triplet contains one probe which measures the local I—V characteristic with a bias sweep and two passive probes which sample the floating potential. The floating potential at the two passive tips varies as a function of the bias at the swept tip. This dependence is shown to vary according to the local plasma parameters. Variations in both the amplitude and width of the floating potential disturbance on electron temperature and density are resolved and compared with different mechanisms of cross field current transport.     

Spectroscopic Measurement of Electron Temperature and Density in an Argon Plasma Jet Based on Collisional‐Radiative Model

The present paper describes a spectroscopic method or determining electron temperature T_e and density N_e in an argon plasma jet on the basis of a Collisional‐Radiative model of argon. Electron temperature and density in the argon plasma were measured by the method developed, and comparison of them was discussed with those obtained with a Langmuir probe. The results or T_e and N_e obtained by the spectroscopic method agreed roughly with those by the probe.