Munemasa Machida

Mode transitions and hysteresis in inductively coupled plasmas

Optical emission spectroscopy as a noninvasive plasma diagnostic was employed to study mode transitions and hysteresis in an inductively coupled plasma in Ar and Ar∕N2 mixtures. Using selected Ar lines, basic plasma parameters, relevant to the analysis of the mode transitions, were evaluated. Small changes of the electron energy distribution function in the vicinity of the mode transition were detected. The role of metastable Ar atoms in mode transitions and in a hysteresis was clarified. Enhanced production of metastables in the hysteresis region as well as faster transitions in plasmas with higher influence of metastables were observed.Optical emission spectroscopy as a noninvasive plasma diagnostic was employed to study mode transitions and hysteresis in an inductively coupled plasma in Ar and Ar∕N2 mixtures. Using selected Ar lines, basic plasma parameters, relevant to the analysis of the mode transitions, were evaluated. Small changes of the electron energy distribution function in the vicinity of the mode transition were detected. The role of metastable Ar atoms in mode transitions and in a hysteresis was clarified. Enhanced production of metastables in the hysteresis region as well as faster transitions in plasmas with higher influence of metastables were observed.

Transfer of a cold atmospheric pressure plasma jet through a long flexible plastic tube

This work proposes an experimental configuration for the generation of a cold atmospheric pressure plasma jet at the downstream end of a long flexible plastic tube. The device consists of a cylindrical dielectric chamber where an insulated metal rod that serves as high-voltage electrode is inserted. The chamber is connected to a long (up to 4 m) commercial flexible plastic tube, equipped with a thin floating Cu wire. The wire penetrates a few mm inside the discharge chamber, passes freely (with no special support) along the plastic tube and terminates a few millimeters before the tube end. The system is flushed with Ar and the dielectric barrier discharge (DBD) is ignited inside the dielectric chamber by a low frequency ac power supply. The gas flow is guided by the plastic tube while the metal wire, when in contact with the plasma inside the DBD reactor, acquires plasma potential. There is no discharge inside the plastic tube, however an Ar plasma jet can be extracted from the downstream tube end. The jet obtained by this method is cold enough to be put in direct contact with human skin without an electric shock. Therefore, by using this approach an Ar plasma jet can be generated at the tip of a long plastic tube far from the high-voltage discharge region, which provides the safe operation conditions and device flexibility required for medical treatment.

Surface Modification of Polycarbonate by Atmospheric-Pressure Plasma Jets

Plasmas have been frequently employed to modify material surface properties; however, many of these processes are not suitable for treating sensitive materials and biological objects that cannot withstand vacuum and/or high temperature. Atmospheric-pressure plasma jets (APPJs) using inert gases can be easily adapted for this purpose since they exhibit high plasma stability, efficient reaction chemistry, and low temperature. In this paper, an APPJ with simple configuration was designed and applied for surface treatment of polycarbonate (PC) films. Optical emission spectroscopy was used to identify the excited plasma species presented inside the device and in the plasma plume. Material characterization was carried out by water contact angle measurements and atomic force microscopy. The interaction of the plasma jet with the PC samples resulted in a rougher surface and enhanced wettability.

Overview of Recent Results of TCABR

An overview of recent results obtained in TCABR is presented. Experiments on Alfven wave heating have been carried out in both low and high density regimes. Controlling the density rise usually observed in Alfven heating experiments, it was possible to get a clear confirmation of electron temperature increase in low‐density discharges. In the high density regime, the Alfven wave absorption occurs at mode numbers quite different from those for low density. Detailed experiments have been carried out on the transition between low and high‐density confinement regimes, triggered by electrostatic polarization at the plasma edge. The results indicate that the flatness of the density profile and the decrease of edge recycling depend strongly on the level of MHD activity during transition. A preliminary analysis of the electromagnetic emission associated with the relaxation instability in the new regime of runaway discharges discovered in TCABR shows that the observations are coherent with theoretical models. The heat transport in the presence of large magnetic islands has been investigated, in the collisional regime, and found to be properly described by the Fitzpatrick model. Finally, two diagnostic techniques have been further improved, the determination of the position of the local Alfven resonance by microwave reflectometry and the determination of the temperature and density at the plasma edge by the method based upon the uniqueness of the particle confinement time, determined from the hydrogen Balmer series emission.

Vacuum ultraviolet and visible spectroscopy diagnostics on the NOVA-UNICAMP tokamak

Three visible and one VUV spectrometers covering four toroidal positions have been set to study He and H plasma created by NOVA-UNICAMP tokamak. Ion temperatures have been measured at the beginning of the tokamak discharge by Doppler broadening of C, O and He lines. The time evolution of carbon line emissions, with different degrees of ionization, showed to have opposite behavior between high and low plasma ionization condition. The high density operation and 2 kHz oscillation present throughout discharge, caused by periodical touching of the plasma with NOVA-UNICAMP tokamak limiter, restrained the increase of ion temperature.

Experimental study of the pseudo spark-produced electron beam for material processing

The pseudospark-produced electron beam has been experimentally investigated. The measurements of spatial current distribution for the free-running mode (/spl sim/100 Hz) showed spatial instabilities of the beam propagation. The effect of a dielectric channel used for the stabilization of the beam propagation was studied. The beam interaction with metal targets was investigated, and a substantial part (about 50%) of the beam current was found to be reflected from the target inclined at an angle of 30-60/spl deg/ with respect to the beam axis.

Plasma treatment of poly(dimethylsiloxane) surfaces using a compact atmospheric pressure dielectric barrier discharge device for adhesion improvement

In this work we present some results of the treatment of polydimethylsiloxane (PDMS) surfaces using pulsed dielectric barrier discharge plasmas. The results of plasma treatment using different gases and mixtures, argon, argon plus water vapor, helium, helium plus water vapor, nitrogen and nitrogen plus water vapor, were compared testing the adhesion between two PDMS samples for each kind of plasma. We also studied the water contact angle in function of plasma process time of PDMS surfaces with each kind of plasma treatment. The plasma was characterized by optical emission spectroscopy (OES) to identify the emitting species and determine the plasma temperatures. The plasma temperature for each process was estimated comparing the spectrum obtained by OES with the spectrum generated by SpecAir simulation code. Measurements of power delivered to the plasmas were also performed. As the results, all the process using different gases show good adhesion efficacy between PDMS samples when long exposure time (larger than 150 seconds) is applied. However, when only a few discharges are applied to PDMS samples the helium plasma process presented best results. Atomic Force Microscopy (AFM) analysis of PDMS samples treated with helium plasma showed reduction in the surface roughness, which increase the surface contact area and improves the adhesion.Results of the treatment of poly(dimethylsiloxane) (PDMS) surfaces using novel atmospheric pressure pulsed dielectric barrier discharge plasmas are presented. Different gases (argon, helium, nitrogen) as well as their mixtures with water vapor were compared in terms of the improvement of adhesion between two PDMS samples after processing by plasma. The plasma was characterized by optical emission spectroscopy to identify the emitting species and determine the plasma temperatures. For all the gases studied, plasma processing resulted in increase of adhesion between PDMS samples if long exposure time (larger than 150 s) is applied. However, for very short treatment times (20 plasma pulses, total processing time about 3 s) the highest efficiency was found for helium plasmas. Water contact angles at PDMS surfaces as function of plasma processing time was analyzed. Atomic force microscopy analysis was performed to show reduction in the surface roughness after plasma treatment, which is likely to be the responsible for increase of the surface contact area and thus the adhesion between two PDMS surfaces. The role of the two mechanisms in the improvement of adhesion (enhanced wettability and changes in the surface morphology), for different time scales, is discussed. Interestingly, for the minimum processing time (20 plasma pulses), the improvement in adhesion and reduction of surface roughness are observed although the changes in the water contact angle are insignificant.

A Novel Plasma Technique for Surface Treatment: The Plasma Expander

This paper describes a new plasma treatment method: the plasma expander. In this approach, expanding shock waves are generated in a vacuum chamber by pulsed plasmas. Collisions of fast species in the waves modify the properties of solid surfaces exposed to the plasma. The degree of such modification is governed by the energy delivered by the plasma exposure. To confirm the efficacy of this approach, modifications induced in the properties of thin polymer films produced by plasma-enhanced chemical vapor deposition by exposure to nitrogen plasma shock waves were investigated. The films were prepared from benzene radio frequency plasmas and subsequently exposed to different quantities of nitrogen shock waves Nw. The effects of Nw on the wettability, molecular structure, and mechanical properties of the films were studied. Fourier transform infrared spectroscopy revealed that greater Nw resulted in the loss of C-H groups and the rupture of benzene aromatic rings observed in the structure of the as-deposited films. Furthermore, the contact angle strongly increased and the hardness, evaluated by nanoindentation, increased up to fourfold with the increase in the intensity of the treatment.

Multipoint Thomson Scattering Diagnostic For The TCABR Tokamak With Centimeter Spatial Resolution

This paper describes a multi‐point Thomson scattering system that is being developed for the TCABR tokamak based on a signal delay technique, which allows the determination of the electron temperature and plasma density radial profiles, with approximately 1 cm spatial resolution, employing just one spectrometer.

Multiposition Multipass Thomson Scattering Diagnostic for Tokamak NOVA-UNICAMP

In this article, the use of an 8×8-matrix independent 10-stage dynode multichannel photomultiplier for multipass Thomson scattering is described. Light from a pulsed ruby laser is irradiated 12 times in the central region of plasma using the resonator cavity of a multipass system, and integrated scattered light is detected for the first time using the multichannel photomultiplier XP1752 from Philips. The simultaneous measurements of electron temperature and density at four spatial positions, which can be extended by increasing the numbers of Analog-to-Digital Converter (ADC) channels and signal output windows, have been performed throughout full discharge by a shot-to-shot procedure. An electron temperature Te of 50 eV and a density ne of 5×1012 cm-3 were measured for the tokamak NOVA-UNICAMP. Because of the use of a low-power ruby laser (typically 1 J) and scattered light amplification using a multipass system, the level of stray light was kept low and absolute density measurements were made possible by nitrogen Rayleigh scattering.