Electron energy probability function and L-p similarity in low pressure inductively coupled bounded plasma

Particle-In-Cell (PIC) simulations are carried out to investigate the effect of discharge length (L) and pressure (p) on Electron Energy Probability Function (EEPF) in a low pressure radio frequency (rf) inductively coupled plasma (ICP) at 13.56 MHz. It is found that for both cases of varying L (0.1–0.5 m) and p (1–10 mTorr), the EEPF is a bi-Maxwellian with a step in the bounded direction (x) and non-Maxwellian with a hot tail in the symmetric unbounded directions (y, z). The plasma space potential decreases with increase in both L and p, the trapped electrons having energies in the range 0–20 eV. In a conventional discharge bounded in all directions, we infer that L and p are similarity parameters for low energy electrons trapped in the bulk plasma that have energies below the plasma space potential (eVp). The simulation results are consistent with a particle balance model

Near-field diffraction of protons by a nanostructured metallic grating under external electric field: Asymmetry and sidebands in Talbot self-imaging

Self-imaging in near-field diffraction is a practical application of coherent manipulation of matter waves in Talbot interferometry. In this work, near-field diffraction of protons by a nanostructured metallic grating under the influence of (a) uniform, (b) spatially modulated, and (c) temporally modulated electric fields are investigated. Time-domain simulations of two-dimensional Gaussian wave packets for protons are performed by solving the time-dependent Schr\"odinger's equation using the generalized finite difference time domain (GFDTD-Q) method for quantum systems. Effects of strength ($E_0$) and orientation ($\theta$) of the uniform electric field on the diffraction properties, such as fringe pattern, intensity of the peaks, fringe shift, and visibility, are investigated. The results show that the Talbot fringes shift significantly in the transverse direction even for a small change in the applied electric field ($\Delta E_0$ $=0.1$ V/m) and its orientation ($\Delta \theta$ $=0.1^o$). The potential barriers arising from a spatially modulated electric field are observed to cause significant distortions in the Talbot patterns when the modulation length ($\lambda'$) is equal to the de Broglie wavelength ($\lambda_{dB}$). Sidebands are observed in the Talbot pattern due to the efficient transfer of energy from the oscillating field to the wave packet when the frequency of oscillation ($\omega$) is of the order of $\omega_0$ ($=2\pi/T_0$), where $T_0$ is the interaction time. This study will be helpful in uniform electric field-controlled precision metrology, developing a highly sensitive electric field sensor based on Talbot interference, and precisely aligning the matter wave optical setup. Furthermore, the sidebands in the Talbot fringe can be used as a precise tool as momentum splitter in matter wave interferometry.Comment: 14 pages, 13 figure

Investigation of effect of excitation frequency on electron energy distribution functions in low pressure radio frequency bounded plasmas

Particle in cell (PIC) simulations are employed to investigate the effect of excitation frequency ω on electron energy distribution functions(EEDFs) in a low pressure radio frequency (rf)discharge. The discharge is maintained over a length of 0.10 m, bounded by two infinite parallel plates, with the coherent heating field localized at the center of the discharge over a distance of 0.05 m and applied perpendicularly along the y and z directions. On varying the excitation frequency f (=ω/2π) in the range 0.01–50 MHz, it is observed that for f ≤ 5 MHz the EEDF shows a trend toward a convex (Druyvesteyn-like) distribution. For f > 5 MHz, the distribution resembles more like a Maxwellian with the familiar break energy visible in most of the distributions. A prominent “hot tail” is observed at f ≥ 20 MHz and the temperature of the tail is seen to decrease with further increase in frequency (e.g., at 30 MHz and 50 MHz). The mechanism for the generation of the “hot tail” is considered to be due to preferential transit time heating of energetic electrons as a function of ω, in the antenna heating field. There exists an optimum frequency for which high energy electrons are maximally heated. The occurrence of the Druyvesteyn-like distributions at lower ω may be explained by a balance between the heating of the electrons in the effective electric field and elastic cooling due to electron neutral collision frequency νen ; the transition being dictated by ω ∼ 2πνen .S. B. gratefully acknowledges support from an Endeavour Research Fellowship of the Australian Government for carrying out this work

Observation of ion heating during stimulated Buneman instability in a temporally growing plasma

Observations and conclusive measurements of ion heating during stimulated Buneman instability (BI) are reported in a temporally growing plasma, created by short-pulse $(20\ \mu \text{s})$ electromagnetic waves. The instability lasts for $\sim 5\ \mu \text{s}$ during the pulse on time and arises when the applied electric-field amplitude $E_0$ supersedes the threshold critical electric field $(E_{cr})$ for free acceleration. During the initial period of plasma development after the launch of the pulsed waves, the maximum growth of BI is registered at $\sim 0.3\ \mu \text{s}$ . The analysis of time-resolved measurements of electron and ion energy distributions indicates an anomalous collision frequency and a hot ion energy tail that persists for more than 700 ns. The energy from the instability is responsible for bringing about the intense and rapid ionization observed upon its termination

Stopping potential and ion beamlet control for micro-resistive patterning through sub-Debye length plasma apertures

Focused multiple ion beamlets from a microwave plasma source is investigated for localized micron-scale modification of substrates in a patterned manner. Plasma electrodes (PE) with an array of through apertures having aperture diameters of the order of plasma Debye length are investigated for generating the beamlets. Extraction through sub-Debye length apertures becomes possible when the PE is kept at floating potential. It is found that the current – voltage characteristics of the extracted beamlets exhibits interesting features such as a space-charge-limited region that has a different behaviour than the conventional Child-Langmuir’s law and an extraction-voltage-limited region that does not undergo saturation but exhibits a Schottky-like behaviour similar to that of a vacuum diode. A switching technique to control the motion of individual beamlets is developed and the stopping potential determined. The beamlets are thereafter used to create localized micro-resistive patterns. The experimental results are compared with simulations and reasonably good agreement is obtained

Fluctuations in electron cyclotron resonance plasma in a divergent magnetic field

The dependence of fluctuations on electron-neutral collision frequency (Ven) and the radial location is investigated in an electron cyclotron resonance plasma in a divergent magnetic field region for a set of magnetic fields. Results indicate that the fluctuations depend strongly on the collision frequency. At lower magnetic fields and Ven, the fluctuation levels are small and are observed to peak around 3–5 cm from the central plasma region. Coherent wave modes are found to contribute up to about 30% of the total fluctuation power, and two to three harmonics are present in the power spectra. There are two principal modes present in the discharge: one appears to be a dissipative mode associated with a collisional drift wave instability initiated at a lower pressure (collision frequencies) (∼0.5mTorr) and is stabilized at a higher pressure (≳3mTorr). The other mode appears at intermediate pressure (≳1.75mTorr) and possesses the signature of a flute instability. The fluctuation levels indicate that flute modes are predominant in the discharge at higher pressures (>1.75mTorr) and at higher values of the magnetic field (∼540 Gauss)

Quasisteady state interpulse plasmas

The generation of quasisteady state plasmas in the power off phase, by short pulses [pulse duration (Τp) - 0.5-1.2 µ s] of intense (60-100 kW) microwaves in the X band (9.45 GHz) is observed experimentally. The steady state is sustained from a few to tens of microseconds and depends upon the ionization processes in the interpulse phase and the characteristic diffusion length. The results are explained by a model, which considers the electron acceleration effects by the large amplitude of the field, the energy losses, and the characteristic electromagnetic field decay time. The effects of wave frequency, microwave power density, and particle diffusion on the steady state are investigated. A striking difference with conventional afterglows of pulsed discharges is pointed out

Surface wettability of an atomically heterogeneous system and the resulting intermolecular forces

We present the effect of 0.5 keV Ar+ beam irradiation on the wetting properties of metallic thin films. Observations reveal a transition from hydrophilic to hydrophobic nature at higher beam fluences which can be attributed to a reduction in net surface free energy. In this low-energy regime, ion beams do not induce significant surface roughness and chemical heterogeneity. However, they cause implantation of atomic impurities in the near surface region of the target and thus form a heterogeneous system at atomic length scales. Interestingly, the presence of implanted Ar atoms in the near surface region modifies the dispersive intermolecular interaction near the surface but induces no chemical modification due to their inert nature. On this basis, we have developed a theoretical model consistent with the experimental observations that reproduces the effective Hamaker constant with a reasonable accuracy

Agreement analysis between three different short geriatric screening scales in patients undergoing chemotherapy for solid tumors

Background: Comprehensive geriatric assessment (CGA) in routine practice is not logistically feasible. Short geriatric screening tools are available for selecting patients for CGA. However none of them is validated in India. In this analysis we aim to compare the level of agreement between three commonly used short screening tools (Flemish version of TRST (fTRST), G8 and VES-13. Methods: Patients ≥65 years with a solid tumor malignancy undergoing cancer directed treatment were interviewed between March 2013 to July 2014. Geriatric screening with G8, fTRST and VES-13 tools was performed in these patients. G8 score ≤14, fTRST score ≥1 and VES-13 score ≥3 were taken as indicators for the presence of a high risk geriatric profile respectively. R version 3.1.2 was used for analysis. Cohen kappa agreement statistics was used to compare the agreement between the 3 tools. p value of 0.05 was taken as significant. Results: The kappa statistics value for agreement between G8 score and fTRST, between VES-13 and fTRST and between VES-13 and G8 were 0.12 (P = 0.04), 0.16 (P = 0.07) and 0.05 (P = 0.45) respectively. It was found that maximum agreement was observed for VES-13 and fTRST. The agreement value of VES-13 and fTRST observed was 59.44 %(39.63% for high risk profile and 19.81% for low risk profile). The agreement value of G-8 and fTRST was 39.62% (2.83% only for high risk profile and 36.79% for low risk profile). The lowest agreement was between G8 and VES-13, 35.84% (7.54% for high risk detection and 28.30% for low risk detection). Conclusion: There was poor agreement (in view of kappa value been below 0.2) between the 3 short geriatric screening tools. Research needs to be directed to compare the agreement level between these 3 scales and CGA, so that the appropriate short screening tool can be selected for routine use