Ryuta Ichiki

Experimental observation of dominant propagation of the ion-acoustic slow mode in a negative ion plasma and its application

Different characteristics of ion acoustic waves were experimentally observed in two types of Xe+–F− double plasmas at different electron temperatures. For the lower electron temperature (around 0.15 eV), the slow mode, which had been considered not to dominate the wave propagation, was found to be dominant rather than the fast mode, which was observed to be dominant for the higher electron temperature (around 1.5 eV). According to the previous numerical investigation [Phys. Plasmas 8, 4275 (2001)], the new wave characteristic appeared when the ratio of negative ion mass to positive ion mass and to the ratio of electron temperature to ion temperature are lower than certain critical values. Further, a method of evaluating both the positive ion temperature and the negative ion temperature in a negative ion plasma by observing the dominant slow mode is described. Using this method, the positive and negative ion temperatures in the former plasma were estimated to be 0.075 eV at the highest and 0.1 eV at the lo...

Measurements of the negative ion density in SF6/Ar plasma using a plane electrostatic probe

A new method to estimate the negative ion density in reactive gas plasmas with a Langmuir probe is proposed. This method has the advantage that the negative ion density is evaluated only by taking the ratio of the ion saturation–electron saturation current ratio obtained from the I–V curve of the Langmuir probe measured in an electronegative-gas mixture plasma to that measured in a reference noble gas plasma. The negative ion density in a SF6/Ar double plasma is estimated utilizing this method. Furthermore, the negative ion density measured with this method is confirmed to agree with that calculated from the measured phase velocity of the ion acoustic wave (fast mode) in the SF6/Ar double plasma, where positive and negative ion masses are obtained from the spectrum analysis with a quadrupole mass spectrometer.

Ion acoustic waves in one- and two-negative ion species plasmas

Ion acoustic waves in multi-ion plasmas including two negative ion species are investigated both numerically and experimentally. Numerically, the kinetic dispersion relation in two-negative ion plasmas is investigated. There are three modes of the ion acoustic waves in two-negative ion plasmas. In an Ar+–F−–SF6− plasma, only one of the three modes is dominant, regardless of the values of the electron and the ion temperatures. In a Xe+–F−–SF6− plasma, on the other hand, two modes can be important for a certain range of the electron–ion temperature ratio. The results also imply the possibility of the coexistence of the fast mode and the slow mode in one-negative ion plasmas. Experimentally, ion acoustic waves are observed in an Ar+–F−–SF6− plasma and are found to show a mode transition that agrees with the theoretical prediction for one of the three ion acoustic modes.

On reflection of ion acoustic waves

Reflection of ion acoustic waves from a bipolar potential structure was studied in a double plasma device. The existence of a reflected signal, even when no external bias was applied on the bipolar reflector, could be detected, although with much lower velocity than the incident wave. Application of proper bias on such a reflector resulted in an additional reflected image with greater velocity than the reflection obtained without bias. The experiment was also extended to study negative ion plasma waves.

Improvement of Compact Electron-Beam-Excited Plasma Source for Increased Producible Plasma Density

Multiple modifications of graphite electrodes of a compact electron-beam-excited plasma (EBEP) source to enhance the cooling efficiency of the electrodes resulted in an increase in the maximum value of producible EBEP density. In addition, experiments on surface nitriding of steel samples in nitrogen EBEP demonstrated that the improved compact EBEP source with modified electrodes outperforms one with conventional electrodes, thereby demonstrating the significant advantage of using the improved source for a wide range of industrial and scientific applications.

Propagation characteristics of ion acoustic waves in an Ar/SF6 plasma

There has been a great interest in wave phenomena in a negative ion plasma. Usually sulfur hexafluoride (SF6) gas is used to produce a negative ion plasma, because SF6 gas has a large electron attachment cross-section for electron energies less than 1 eV. Furthermore, the negative ion concentration, r, can be controlled by partial pressure of the gas. However, the introduction of SF6 gas produces several ion species in discharge plasmas. In fact, there are many discussions on ion species in discharge plasmas. In the most of the experiment on the ion acoustic wave in a negative ion plasma, only F− or only SF6 was assumed as the negative ion species. 1, 2) On the other hand, in many experimental studies on the ion acoustic soliton in Ar/SF6 plasmas it was assumed that the light components, that is, F− contributes more effectively to the ion acoustic wave, 4) and the results of these experiments were consistent with this assumption. Cooney et al., however, obtained the soliton velocity that shifted gradually from the theoretical value for Ar and F− to that for Ar and SF6 as r increased. 5) Although they mentioned that this was perhaps due to an increase in the relative concentration of SF6 to F − as r increased, there was no evidence for the phenomenon. On the other hand, it was recently observed by the mass spectrometry that the relative concentration of SF6 to F− increases with increasing r in the double plasma (DP) device. Thus, in order to confirm whether the variations in the relative concentrations of ion species affect the propagation characteristics of the ion acoustic wave or not, we detected dominant ion species in an Ar/SF6 plasma with a quadrupole mass spectrometer (QMS) and measured the phase velocity of the ion acoustic wave for different values of r. While much work on the ion acoustic wave in two-positive ion plasmas have been done, 8) the study on the ion acoustic wave in two-negative ion plasmas hardly has been carried out. The purpose of this paper is to observe the dependence of the ion acoustic wave on ion species, especially negative ion species. The experiments were carried out using a multidipole DP device. The diameter and the length of the device

Estimate of the negative ion density in reactive gas plasmas

The negative ion density in a SF6/Ar double plasma is estimated. Here, the density ratio of negative ions to positive ions α is evaluated from the reduction rates of the ion and electron saturation current. Furthermore, the negative ion density obtained with this method is confirmed to agree with that calculated from the measured phase velocity of the ion acoustic wave (fast mode) when α<0.6, where the positive and negative ion mass are obtained from the spectrum analysis with a quadruple mass spectrometer (QMS) system. The negative ion density in fluorocarbon ECR plasma is also estimated by means of the Langmuir probe method. It is found that α in fluorocarbon ECR plasma is less than 0.5.

Comments on the Reflection/Excitation Properties of a Bipolar Potential Structure Used in Ion Wave Experiments

A series of experiments on the reflection of ion waves from a bipolar potential structure inserted in a double plasma machine is described. It was observed that the bipolar potential structure could also excite additional ion waves depending upon the bias voltages that were employed. The measured signals are interpreted to be pseudo-waves and ion acoustic waves.

Ion-burst method for positive and negative ion species measurements

Abstract A simple method for positive and negative ion species measurements utilizing ion bursts is presented. In this ion-burst method, the ion bursts are excited by a potential applied to a mesh grid immersed in the plasma. Since the velocity of the ion burst is dependent on the excitation voltage and the ion mass, the ion species can be determined from measurement of the ion-burst velocity. In the DC discharge of an Ar/SF6 gas mixture, the positive ion species are found to be Ar+, SF3+ and SF5+, and the negative ion species are found to be F− and SF6−. These results are compared with the spectrum analysis using a quadrupole mass spectrometry system and show qualitative agreement with them.

Properties of linear ion acoustic waves in negative ion plasmas

Summary form only given, as follows. It is now well known that there exist two modes of ion acoustic waves in plasmas including two ion species. This theory also holds when one of the two is a negative ion species, although characteristics of the modes are strongly changed by the negative ions. As in such negative ion plasmas, the two modes are customarily called the fast and the slow modes, after the properties of their velocities. On the other hand, the features of their Landau damping rates which had been uncovered by theoretical and experimental investigations were as follows: the slow mode always has much higher damping rate than the fast mode to be usually trivial physically, resulting in the domination of various ion wave phenomena by the fast mode. However, we have recently found that the slow mode can possess lower damping rate instead of the fast depending upon the mass ratio of the negative to the positive ion species and the temperature ratio of electron to ion. These parameters have respective critical values, below which the slow mode can be dominant. These values are calculated using kinetic approach. To confirm this novel wave phenomenon, we performed ion wave experiments.