H. Kelly

Analysis of the nitrogen ion beam generated in a low-energy plasma focus device by a Faraday cup operating in the secondary electron emission mode

The energy distribution and flux of fast nitrogen particles generated in a Mather-type plasma focus device operating at 0.4 Torr of N/sub 2/ pressure is reported. A Faraday cup operating in the secondary electron emission mode was employed. To determine the total number of beam particles, multiple scattering of the ions was taken into account. It has been possible to register the ion energy up to a lower kinetic energy threshold of /spl ap/50 keV, which is a value much lower than that obtained with a Thomson spectrometer in a previous work.

Coating on metallic samples produced by a small energy plasma focus

A surface treatment of metallic samples irradiated by a nitrogen ion beam generated in a co-axial plasma gun is presented. By placing a titanium insert at the end of the inner electrode of the device, a TiN coating was produced on the surface of the sample. X-ray diffraction, scanning electron microscopy, electron probe x-ray micro-analysis and x-ray photoelectron speectroscopy were employed to study the surface of the treated sample. It was found that the main role of the ion beam was to produce a strong pre-heating of the superficial layers of the substrate, thus favouring the diffusion of the impinging particles.

Nitrogen ion spectrum from a low energy plasma focus device

The nitrogen ion spectrum generated from a low energy Mather-type Plasma Focus device is reported. The main species of ions present are N/sup +1/, N/sup +2/, and N/sup +3/. It is found that its relative concentrations at a distance of several cm from the ion source are in agreement with those predicted for an equilibrium charge-state of the beam, and hence the spectrum of the neutral nitrogen is also derived. By taking into account the lateral spread of the beam due to multiple elastic scattering with the background gas, the derivation of absolute values for the total ion spectrum within the investigated energy range (>170 keV), the total number and total energy of the fast particles has been obtained.

Application of solid-state nuclear track detectors for studies of fast ion beams within PF-1000 and other plasma-focus facilities

Abstract The paper presents results of studies of the fast ion emission from two plasma facilities: PF-1000 and PF-II. The measurements were carried out mainly with solid-state nuclear track detectors (SSNTDs). To register ion mass- and energy-spectra, obtained when primary ion beams are deflected by magnetic and electric fields of a Thomson-type spectrometer, the SSNTDs plates were installed inside the analyzer. To measure ion angular distributions the SSNTDs were located within the main discharge chamber, at different angles to the electrode axis. The SSNTDs were also applied to register ion beam images formed within an ion pinhole camera, and to determine a spatial location of the fast ion sources.

Electrical and plasma characteristics of a quasi-steady sliding discharge

A quasi-steady sliding discharge at atmospheric pressure is generated by combining a surface dielectric barrier together with a DC corona discharge in a three-electrode geometry. The discharge extends along the whole side-length of the electrodes (150 mm) and covers the full inter-electrode gap (30 mm). It is found that this discharge is composed of repetitive streamers that are uniformly distributed along the whole electrode length and that propagate along the inter-electrode gap with an average velocity of ~2 × 107 cm s−1, and with an average electric field of ~120 kV cm−1 and a total particle number of ~5 × 108 at the streamer head. Assuming that the electron distribution function reaches an equilibrium value with the electric field, an electron temperature of 9 eV at the streamer head is obtained. The streamer frequency is around 5 × 104 Hz for a well-developed sliding discharge regime, and the time-averaged electron density amounts to 1.5 × 107 cm−3.

Surface modification produced by a nitrogen operated plasma focus device: the role of the ion beam in the heating of a substrate

Abstract The role of the nitrogen ion beam generated with a small energy plasma focus (PF) device in the thermal processing of an austenitic stainless steel substrate is discussed. A numerical solution of the heat equation which takes into account the temperature variations of the thermal coefficients of the material is presented. By using several characteristics of the beam determined in previous works, it is found that the energy content of the beam is not enough to promote a strong heating of the outer layers of the substrate, which is required to explain the introduction of foreign particles to depths well beyond the ion range in the material, and also the martensitic transformation of steel up to a depth of ≈0.6 μm found in this work. The surface treatment is thus attributed to a plasma bubble generated by the disruption of the plasma column, and some evidence of its presence is obtained by employing a Faraday cup (FC). When the numerical model is used with an input energy density corresponding to the experimental value, and with a delivery time equal to the temporal width of the bubble, the evolution of the temperature profiles along the substrate depth shows a melting front reaching the proper depth to explain the penetration of Ti and N atoms found in a previous work, and the martensitic transformation depth presented in this work.

Schlieren technique applied to the arc temperature measurement in a high energy density cutting torch

Plasma temperature and radial density profiles of the plasma species in a high energy density cutting arc have been obtained by using a quantitative schlieren technique. A Z-type two-mirror schlieren system was used in this research. Due to its great sensibility such technique allows measuring plasma composition and temperature from the arc axis to the surrounding medium by processing the gray-level contrast values of digital schlieren images recorded at the observation plane for a given position of a transverse knife located at the exit focal plane of the system. The technique has provided a good visualization of the plasma flow emerging from the nozzle and its interactions with the surrounding medium and the anode. The obtained temperature values are in good agreement with those values previously obtained by the authors on the same torch using Langmuir probes.

On the physical origin of the nozzle characteristic and its connection with the double-arcing phenomenon in a cutting torch

The nozzle current-voltage characteristic for a cutting arc is presented in this work. The measurements are reported using a high energy density cutting arc torch with a nozzle bore radius of 0.5 mm. The arc current was fixed at 30 A while the plenum pressure and the oxygen gas mass flow rate were varied in the range of 0.55–0.65 MPa and 0.32–0.54 g s−1, respectively. The results show a very low electron density and the lack of electron attachment at the plasma boundary layer. No ion saturation current was found. For the smallest mass flow rate value gas breakdown was found for a biasing nozzle potential close to that of the cathode, but no evidence of such breakdown was found for the larger mass flow rate values. Using an expression for the ion speed at the entry of the collisional sheath formed between the nonequilibrium plasma and the negatively biased nozzle wall together with a generalized Saha equation coupled to the ion branch of the characteristic, the radial profile of the electron temperature, t...

Characterization of plasma-gas interactions in a copper cathodic arc operated in oxygen

Several results on the plasma-neutral gas structure generated in a dc cathodic arc are presented. The arc is operated at a current level of 100 A, with a copper cathode, and with oxygen gas at a pressure of 0.5 Pa. The employed diagnostics include spherical Langmuir probes and a calorimetric technique. The plasma potential, electron temperature, ion density, and ion kinetic energy are inferred at different axial positions in the discharge chamber. In order to explain the obtained experimental results, a simplified one-dimensional stationary model to describe the plasma-neutral gas structure is developed. A good agreement is found between the experimental data and the theoretical predictions when charge-exchange between metallic ions and neutral gas molecules and lateral diffusion loses of energetic particles are included in the theoretical treatment.

On the Use of Sweeping Langmuir Probes in Cutting Arc Plasmas—Part I: Experimental Results

The first study of Langmuir probes applied to cutting arcs using a sweeping-probe system is presented. It is found that, under a relatively broad range of experimental conditions (changes in the probe material, in the probe radii, or in the sweeping frequency of the probes), no probe damage is registered, notwithstanding the large value of the power flux present with these arcs. In practice, probes with radii down to 63 mum and with sweeping rotation frequencies down to 8.7 s-1 (probe transit time of ap140 mus through the arc) were used without noticeable alterations. In the measurements of the ion current collected by negatively biased probes, the following two unexpected features are found: the lack of a current plateau in the ion branch of the I-V probe characteristic and the independence of the signal amplitude on the probe radius. According to the experimental evidence, as well as several estimations, we have neglected electron emission of the probe surface as a relevant mechanism in modifying the ion branch of the characteristic. On the contrary, some arguments on which a collection model will be based are presented.