Measurement Of Energy Distribution Research Articles

Probe Measurements in Electronegative Plasmas: Modeling the Perturbative Effects of the Probe‐Holder

AbstractA basic property of an electronegative plasma is its separation into two distinct regions: an ion‐ion region far from boundaries, where the densities of positive and negative ions are higher then electron density, and a near‐boundary electron‐ion region, where negative ions have practically negligible density. This is due to the influence of the ambipolar electric field, which depends on electron (not negative ion) plasma parameters. This electric field “holds off” negative ions from the boundary, as the ions have lower mobility and temperature compared to the electrons. Therefore, negative ions will be repelled by any object inserted into the plasma. This can lead to errors in measurements of negative ion and electron parameters by any invasive method. Numerical modeling of electric probes in an argon‐oxygen plasma clearly demonstrates possible errors of direct measurements of negative ion probe current. This can also affect results from the photo‐detachment method and direct measurements of negative ion energy distribution (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Ion acceleration at different stages of a pulsed vacuum arc

Detailed measurements of ion energy distributions (IEDs) originating at different stages of a pulsed vacuum arc are studied. It is shown that for a variety of cathode materials (Al, Mg, Ti and Zr), the directed energies per charge unit of ion originating at the initial stage of the arc, i.e. in 25 µs after ignition, are close to each other and are, approximately, as much as Edir/Z ≈ 70 eV. A non-Maxwellian shape of these IEDs that is due to the presence of significant ‘tails’ of ions accelerated up to energies of a few hundred eletronvolts is found. In 100 µs after the arc ignition the directed energies relax to values that are close, principally, to values that have been measured earlier elsewhere. It is found that the ‘anomalously’ accelerated ions propagate within a narrow angle that is as much as, approximately, ±15° in relation to the plasma flux axis. These characteristics suggest that beyond the commonly adopted gas-dynamic mechanism of ion acceleration in cathode micro-jets, at the initial stage of a pulsed arc the mechanism of additional ion acceleration is presented, which is due, obviously, to a self-consistent electric field arising in front of the plasma macro-jet.

Analysis of the infrared spectra of the peculiar post-AGB stars EP Lyrae and HD 52961

Aim: We aim to study in detail the peculiar mineralogy and structure of the circumstellar environment of two binary post-AGB stars, EPLyr and HD52961. Both stars were selected from a larger sample of evolved disc sources observed with Spitzer and show unique solid-state and gas features in their infrared spectra. Moreover, they show a very small infrared excess in comparison with the other sample stars. Methods: The different dust and gas species are identified on the basis of high-resolution Spitzer-IRS spectra. We fit the full spectrum to constrain grain sizes and temperature distributions in the discs. This, combined with our broad-band spectral energy distribution and interferometric measurements, allows us to study the physical structure of the disc, using a self-consistent 2D radiative-transfer disc model. Results: We find that both stars have strong emission features due to CO_2 gas, dominated by ^{12}C^{16}O_2, but with clear ^{13}C^{16}O_2 and even ^{16}O^{12}C^{18}O isotopic signatures. Crystalline silicates are apparent in both sources but proved very hard to model. EP Lyr also shows evidence of mixed chemistry, with emission features of the rare class-C PAHs. Whether these PAHs reside in the oxygen-rich disc or in a carbon-rich outflow is still unclear. With the strongly processed silicates, the mixed chemistry and the low ^{12}C/^{13}C ratio, EP Lyr resembles some silicate J-type stars, although the depleted photosphere makes nucleosynthetic signatures difficult to probe. We find that the disc environment of both sources is, to a first approximation, well modelled with a passive disc, but additional physics such as grain settling, radial dust distributions, and an outflow component must be included to explain the details of the observed spectral energy distributions in both stars.

Quantum chemical contribution to electron neutrino mass determination

Theoretical data have been obtained needed to analyze the experimental measurements of the T2 β-decay energy distribution near the end point. The energies of the six lowest 1∑+ states of HeT+ have been computed for internuclear distances 1.0 ≦4 R ≦ 4.0 bohr. The wavefunctions have been used to calculate the transition probabilities to these states from the electronic ground state of T2. Distribution of the probabilities over the final vibrational and rotational states has been evaluated including rotationally predissociating resonances. Using the stabilization method the 1∑+ resonance states of HeT+ above the ionization limit have been investigated. Preliminary potential energy curves for several of these states have been determined for a wide range of internuclear distances.

The Silicon Cube detector

A new experimental device, the Silicon Cube detector, consisting of six double-sided silicon strip detectors placed in a compact geometry was developed at CENBG. Having a very good angular coverage and high granularity, it allows simultaneous measurements of energy and angular distributions of charged particles emitted from unbound nuclear states. In addition, large-volume Germanium detectors can be placed close to the collection point of the radioactive species to be studied. The setup is ideally suited for isotope separation on-line (ISOL)-type experiments to study multi-particle emitters and was tested during an experiment at the low-energy beam line of SPIRAL at GANIL.

Retarding Field Analyzer for Ion Energy Distribution Measurement Through a Radio‐Frequency or Pulsed Biased Sheath

AbstractA compact, floating retarding field energy analyzer for measurement of ion energy distributions impacting an electrode through a radio‐frequency or pulsed bias sheath in a plasma discharge is presented. The analyzer is designed to sit on the electrode surface, in place of the substrate, and wide‐band low pass filters allow it to float at the electrode potential. This avoids the need for modification of the electrode. The capabilities of the analyzer are demonstrated through ion energy distribution and electron energy distribution measurements at the electrode surface in an inductively coupled plasma reactor. For a sinusoidal radio‐frequency driving signal applied to the electrode the analyzer is shown to resolve ions with different mass. When the radio‐frequency power to the plasma pulsed the analyzer is used to resolve the ion energy distributions at different times in the pulse. The high energy tail of the electron energy distribution reaching the electrode surface is also measured. A comparison with a Langmuir probe shows exceptional agreement in the energy region where both devices overlap.

Low-energy particle interaction with plasma-irradiated metal surfaces

In fusion experiments, plasma erodes walls of devices containing various elements including carbon. The eroded carbon penetrates into edge plasma and is transported backwards forming the carbon-adsorbed layers on the surfaces of components like mirrors for optical plasma diagnostics. To characterize the carbon-adsorbed layers by measuring energy distribution of particles reflected from the surface, we have used the experimental setup equipped with a magnetic deflection momentum analyzer and the time-of-flight energy analyzer of neutrals. Ion beams in the energy range 1–2 keV irradiated the samples of Mo and W with carbon deposition on them that were prepared in separate plasma chambers. The beams produced ions and neutrals with characteristic emission angle and energy distribution depending upon conditions of the sample surfaces. Experimental results are compared with the numerical calculation model ACAT (Atomic Collisions in Amorphous Targets) and the evaluations on how the structure of deposition layers may affect the particle reflection on solid surfaces were made.

Hydrogen adsorption on Pd(133) surface

Abstract An approach based on measurements of the total energy distribution (TED) of field emitted electrons is used in order to examine properties of the Pd(133) from the aspect of hydrogen adsorption. The most favourable sites offered to a hydrogen atom to be adsorbed are indicated and an attempt to ascribe the peaks of the enhancement factor R in the TED spectrum to the specific adsorption sites is made.

Hot electron energy distributions from ultraintense laser solid interactions

Measurements of electron energy distributions from ultraintense (>1019 W/cm2) laser solid interactions using an electron spectrometer are presented. The effective hot electron temperatures (Thot) have been measured for laser intensities (Iλ2) from 1018 to 1021 W/cm2 μm2 for the first time, and Thot is found to increase as (Iλ2)0.34±0.04. This scaling agrees well with the empirical scaling published by Beg et al. [Phys. Plasmas 4, 447 (1997)], and was modeled by particle-in-cell simulations.

Nonequilibrium Tunneling Spectroscopy in Carbon Nanotubes

We report measurements of the nonequilibrium electron energy distribution in carbon nanotubes. Using tunneling spectroscopy via a superconducting probe, we study the shape of the local electron distribution functions, and hence energy relaxation rates, in nanotubes that have bias voltages applied between their ends. At low temperatures, electrons interact weakly in nanotubes of a few microns channel length, independent of end-to-end-conductance values. Surprisingly, the energy relaxation rate can increase substantially when the temperature is raised to only 1.5 K.

Images and Energy Distributions of Electrons Emitted from a Diamond pn-Junction Diode

A diamond pn-junction diode electron emitter was examined in an emission spectro-microscope. An electron emission efficiency of ∼10-2 to the diode current was confirmed at a forward diode current larger than ∼40 μA. Measured electron emission images showed that electron emission indeed occurs at the terrace and up to the terrace boundary of p-type diamond electrode. Measured energy distribution curves of emitted electrons showed that electrons are indeed injected to the conduction band of p-type diamond electrode and emitted from a negative electron affinity surface of p-type diamond. [DOI: 10.1380/ejssnt.2009.660]

Measurements of angular and energy distributions of prompt neutrons from thermal neutron-induced fission

The experimental setup and methodology used to measure prompt neutron angular and energy distributions from thermal neutron-induced fission are described. The neutrons are detected using two scintillation detectors, while the fission fragments are detected by multi-wire proportional detectors in conjunction with the TOF technique. To separate events corresponding to neutrons and γ-quanta, a double discrimination by the pulse shape and the time-of-flight is applied. Some preliminary results of an experiment performed with the 235U target are presented and briefly discussed. The yield of “scission” neutrons has been estimated in the framework of a simple evaporation model and was found not to exceed 5% of the total neutron yield. Including an assumed of anisotropy of the fission neutron angular distribution in the center-of-mass system of fission fragments into the model calculation leads to an increase in the “scission” neutron yields inferred from the data.

Ion kinetic energy distribution in a pulsed vacuum arc with a straight magnetic filter

In vacuum arcs of interest for film deposition the ion kinetic energy is of importance because it influences the coating properties. In this kind of discharge, the ions come out from the cathode spots with a high kinetic energy (20–150 eV). In the present work, we present measurements of vacuum arc ion energy distributions in a pulsed vacuum arc with a straight magnetic filter. A retarding field analyser (RFA) was used to perform the measurements that were carried out with a variable magnetic field strength (of the order of 10 mT). Since the interpretation of the results obtained from the RFA lies in the knowledge of the plasma and floating potential values, we have employed also Langmuir probes for determining those quantities. The obtained results for the ion kinetic energy are similar to those reported by other authors, but they were also found to be independent of the magnetic field strength. The electron temperature was also found to be independent of the magnetic field strength and of the axial position along the filter, indicating the absence of collisions.

Further progress in the characterisation of complex radiation fields

One of the topics which forms part of CONRAD project addresses the problems related to the dosimetry of complex-mixed radiation fields at workplaces. This topic was included in work package (WP) 6. WP 6 was established to co-ordinate research activities in two areas:the development of new techniques and the improvement of current techniques for characterisation of complex workplace fields (including high-energy fields and pulsed fields): measurement and calculation of particle energy and direction distributions (Subgroup A); and model improvements for dose assessment of solar particle events (Subgroup B). In both cases in order to aid the research, WP 6 increases the efficiency of resource utilisation, and facilitates the technology transfer to practical application and for the development of standards. This contribution presents a general overview of activities of SG A; specific results related to the benchmark experiment at GSI Darmstadt are presented separately, and will be published in other way. As far as the results acquired in the frame of the SG B activities, these are presented in the meeting held as part of EURADOS AM 2008.

Spatial distribution and near-field coupling of surface plasmon polariton Bloch modes

The Bloch nature of surface plasmon polaritons in a one-dimensional plasmonic crystal is directly observed using ultrafast and phase-sensitive near-field microscopy. Individual Bloch harmonics within one Bloch mode are separated and visualized. Different harmonics within the same Bloch mode turn out to have a different lateral amplitude distribution. The coupling between the different Bloch harmonics and a near-field probe is studied by comparing the measured energy distribution over the different harmonics to the theoretically expected distribution. It is found that higher order harmonics couple more efficiently to the probe than lower order harmonics.

Energy distribution of plasma-assisted electron and ion emission from TGS single crystals

Electron and ion emission accompanying non-thermal plasma processes, produced at the surface of TGS single crystals under driving ac electric field exceeding 10 3 V/cm, have been carried out. These plasma-assisted emission of electrons and ions were examined by means of time and energy distribution measurements. The intensity of registered charges (electrons and ions) displayed on the 2 ms time scale are represented by two distinct peaks. Time dependent energy spectrum of charges, detected under our experimental conditions, involves electrons and ions with maximum energy up to 30–40 eV for first peaks and up to 70–80 eV for second one. Additionally, the energy of electrons is focused at about 10–15 eV for first and second peaks and about 60–70 eV for second ones; the ion energy spectrum for both peaks exhibits only distinct low energy maximum focused at about 5–15 eV.

Ion energy distribution of an inductively coupled radiofrequency discharge in argon and oxygen

We report an experimental investigation of the ion energy distribution in an inductively coupled electron cyclotron wave resonance (ECWR) discharge with a superimposed static magnetic field. The inductively coupled discharge is sustained by applying a 13.56MHz radiofrequency (RF) power to an aluminium single-turn coil located inside the vacuum chamber. The source region was separated by a grid from the diffusion region. Ion energy distribution (IEDF) measurements employing an energy-dispersive mass spectrometer or plasma process monitor (PPM) whose entrance opening was 15cm away from the grid were performed in the diffusion region. The IEDF is composed of two peaks; a low-energy peak due thermalized ions and a high-energy peak due to ions coming directly from the source region without undergoing thermalization. The energetic difference between the groups thus reflects the plasma potential difference between the source region and the diffusion region. The pronounced intensity variation of the high-energy peak with increasing pressure is caused by charge-changing collisions yielding a depletion of the high-energy ions with increasing effective path length.

Sputtering of organic molecules by clusters, with focus on fullerenes

Energetic cluster beam bombardment of organic samples involves a number of physical and chemical effects that are interconnected in a complex manner. In this labyrinth for the scientist's mind, molecular dynamics (MD) simulation constitutes our Ariadne's thread, because it provides the time-resolved microscopic view of the desorption process that is needed to interpret experiments. By combining molecular ion yield and energy distribution measurements with MD simulations, it is possible to explore desorption and ionization from thin organic films and polymers. Recent results show that the overall dynamics and, to a large extent, the details of the cluster-induced sputtering process are well described using a simpler coarse-grain prescription instead of a full atomistic model, allowing us to speed up computations and leap towards larger systems and higher projectile energies. In this article, we first describe some of the characteristic mechanisms of energetic cluster and especially fullerene projectile bombardment of organics. Then, open issues arising from recent experimental investigations are addressed, such as the importance of specific chemical reactions and ionization processes that are outside the scope of the model. Finally, the discussion highlights cases where the predictive nature of the simulations should be used to facilitate the work of experimentalists, such as testing the properties of new cluster projectiles.

Critical distance for secondary ion formation: Experimental SIMS measurements

We have performed direct experimental measurements of the critical distance for the secondary ion formation process. To this end, we compared the experimentally measured energy distribution of secondary Si − ions with the theoretical energy distribution (Sigmund-Thompson relation) of secondary Si atoms. Our model states that the maxima positions of these two energy distributions differ by the Coulomb interaction potential between the outgoing ion (Si − in our case) and a charge with the opposite polarity formed at the surface after electron transition between the outgoing Si atom and the surface. Quite a reasonable value was obtained for the critical distance, but with a large scatter in experimental data. The conclusion has been made that the experimental technique should be improved to get more precise values of the critical distance, which is of high importance for practical purposes.

Retarding field analyzer for ion energy distribution measurements at a radio-frequency biased electrode

A retarding field energy analyzer designed to measure ion energy distributions impacting a radio-frequency biased electrode in a plasma discharge is examined. The analyzer is compact so that the need for differential pumping is avoided. The analyzer is designed to sit on the electrode surface, in place of the substrate, and the signal cables are fed out through the reactor side port. This prevents the need for modifications to the rf electrode--as is normally the case for analyzers built into such electrodes. The capabilities of the analyzer are demonstrated through experiments with various electrode bias conditions in an inductively coupled plasma reactor. The electrode is initially grounded and the measured distributions are validated with the Langmuir probe measurements of the plasma potential. Ion energy distributions are then given for various rf bias voltage levels, discharge pressures, rf bias frequencies--500 kHz to 30 MHz, and rf bias waveforms--sinusoidal, square, and dual frequency.