| Cathodic arc with highly ionized metal plasma, has been widely used for thin film synthesis, ion implantation and ion injection into accelerators. Up to now, many workers have measured and reported proper parameters for their vacuum arc systems in which DC arc sources with high current (>kA) were used to generate the discharge. However, few reports about pulsed arc sources in low-current discharge can be found. In this thesis, we mainly focus on the basis optical phenomenon and physical characteristics in pulsed low-current vacuum arc plasma, and set up a series of diagnostic methods based on spontaneous emission from the vacuum arc plasma. By carrying out the experiment of emission spectrum, it has been found that the characteristics and the causes of non equilibrium properties in pulsed vacuum arc plasma. Additionally, the influence of different discharge conditions on the plasma characteristics has also been studied. Through the high-speed photography experiment, we obtained the transient processes and discharge types in the pulsed vacuum arc discharge, and the characteristics of the multi-spot discharge and the plasma jet are analyzed. The results of these studies can be concluded as follows:1. The characteristics of pulsed vacuum arc plasma spectrum have been investigated, and the non-equilibrium effects of particles have been studied.Through the multi-channel spectrometer, we measured the emission spectrum of the pulsed low-current vacuum arc discharge. The highest charge of titanium metal ions in this work is+2, and these lines are present in ultra violet band200-300nm. The lines emitted from+1charged ions and atoms are mainly in the band300-490nm and490-525nm, respectively. Some lines of hydrogen atom and the C2molecular Swan spectrum have also been found. Based on the Boltzmann-slope method, the+1charged ions are in in local thermal equilibrium state, but the atoms are in non-equilibrium state. By fitting the C2Swan spectrum using the software Specair, the rotational is far less than the vibrational temperature, so the plasma is in the non-equilibrium state.2. The interpolation algorithm for the discrete-points tables of hydrogen atomic line Stark broadening can be constructed, and the electron density and ion temperature are determined.Based on the basic theory principles and computational results of Griem and Gig-Card calculations, a simple and precise interpolation algorithm for the discrete-points tables can be constructed to obtain the traditional ne-Te diagnostic maps of hydrogen atomic line Stark broadening. The results show that hydrogen alpha line Stark broadening increases with the atomic reduced mass and with the electron temperature decreases. After a little hydrogen isotope gas was injected into the chamber, the convoluted fitting of hydrogen atomic alpha spectral line have been performed to obtain Stark broadening. Based on the computational results of Gig-Card calculation, a corrected method is set up to determine the electron density and ion temperature. The variation of the reduced mass pair due to the non-equilibrium effect contributes to the difference of the results derived from two hydrogen isotope alpha lines.3. A method has been introduced to correct the atomic non-equilibrium effect in vacuum arc plasma, and to determine the relative concentration of atoms and single charged ions.Based on the analysis of the non-equilibrium effects on the particle level population, a modified method for determining the electron temperature is established. The results show that the electron temperature derived from the intensity of atomic and ionic lines is the same, and atoms are in ionizing plasma environment. The ionization temperature has been determined through Saha-Boltzmann plots, and it is found that the ionization temperature is equal to the electron temperature, but electron density from Saha equation is much more than the value from Stark broadening results. It indicates that the main cause of Saha non-equilibrium is the different optical thickness of particles, then we can obtain the relative population of atoms and single charged ions.4. The characteristics of pulsed vacuum arc plasma under different discharge conditions have been investigated, and the influence of different discharge conditions on vacuum arc plasma have been studied. When increasing the discharge current, all the intensity of the atomic and ionic lines and C2molecular Swan (0,0) line have the local maximum value in the increasing trendency. The ionic excitation temperature shows the same trend, but the atom has just a slight variation. The electron density increases linearly with the discharge current. Additionally, the increasement of current lead to the increasing trend of the rotational temperature and the decreasing varation of the vibration temperature, both temperatures have fixed value at last. The obvious Doppler shifts of the lines emitted from different particles show that the directional velocity of all kinds of particles in the plasma increases with arc current. After the injection of the hydrogen isotope gas, it has been found that the ion temperature also has the corresponding local maximum value. Therefore, the variation of the electron temperature and ion temperature is the main cause of the local intensity changes.When the gap between two electrodes becomes large, the line intensity of atoms and ions is weak, but the intensity of hydrogen atomic alpha line becomes more strongger. The electron temperature and electron density and gas rotational temperature becomes larger, but the molecular vibrational temperature decreases. Increasing the discharge current, the intensity of titanium ions, hydrogen alpha line, double charged carbon ions increase at first and then decrease finally, but the carbon atoms and carbon single charged ions increase linearly. The excitation temperature of single charged titanium ion is not affected by the current, and but the excitation temperature of double charged titanium ion appeared to the same variation as its line intensity. The rotational temperature increases with the current till up to the constant value, while the vibration temperature has the same variation of double charged titanium ion. Additionally, the electron density has the same tendency as the hydrogen alpha line intensity. Observation of Doppler shifts shows that more irregular fluctuation of directional speed in vacuum arc discharge with the large gap.The background pressure can directly affects the pulsed discharge processes, and perform different effects on different discharge gaps. For small spacing discharge processes, all the line intensity increases at first and then decreases. For large spacing discharge processes, the intensity of C2molecular Swan (0,0) line has the same variation while the ionic line intensity decreases at first and then increases finally. Regardless of gap size, excitation temperature has the same variation as the ionic line intensity, but the gas rotational temperature is always increasing in small gap and decreasing in large gap. Additionally, the electron density slightly increases firstly and then decreases in the small space, but always reduce in the large space.5. The physical processes of vacuum breakdown have been investigated, and the mechanism of pulsed vacuum arc discharge and the characteristics of plasma jet have been studied.The results of high-speed photography experiment show that, vacuum breakdown processes between two electrodes under high voltage began with the cathodic erosions, the secondary electron on the surface of the insulation material is the main contribution to the formation of the original arc channel. The movement of anode light spot is actually the processes of plasma produced from cathode spot replacing the diffusion plasma channel supplied by the insulation, and it is also the process of decreasing the impedance of the plasma channel. Finally, the stable arc channel is established.The results of high speed photography show that many cathode spots simultaneously supply the plasma in low-current pulsed vacuum arc discharge. The character of surface flashover is the only one plasma channel. Even if many spots appear simultaneously on the electrode surface, the plasma from cathode spots shares one flashover channel between two electrodes. Compared with the changes between the location and light intensity of cathode spots, the averaged life time and the apparent spot velocityof cathode spots have been obtained. Meanwhile, it has been found that the new cathode spot would appear around the ablated surface region.It has been found that the maximum light intensity in the axial distribution of plasma jet decreases with the discharge duration. Meanwhile, the position of the maximum light intensity is leaving away from the edge. It is contributed that cathode spots are moving along the edge at a certain speed and the light intensity in the radial distribution is becoming valley shape. Although the intensity distribution of the initial jet is unhomogeneous, but it may converge in a certain distance and become the symmetric Gauss distribution. |
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