US3536948A

US3536948A - High frequency torch discharge plasma generator provided with single electrode of aluminum

Info

Publication number: US3536948A
Application number: US635634A
Authority: US
Inventors: Manabu Yamamoto; Seiichi Murayama
Original assignee: Hitachi Ltd
Current assignee: Hitachi Ltd
Priority date: 1962-07-09
Filing date: 1967-01-27
Publication date: 1970-10-27
Anticipated expiration: 1987-10-27
Also published as: FR1362349A; DE1464755A1; GB1036113A; DE1464755B2

Description

C. 27, 1970 MANABU YAMAMOTO ETAL 5,536,948

HIGH FREQUENCY TORCH DISCHARGE PLASMA GENERATOR PROVIDED WITH SINGLE ELECTRODE OF ALUMINUM Flled Jan. 27, 1967 2 Sheets-Sheet 1 F|G.I F|G.5 FIG.6

HIGH FREQUENCY TORCH DISCHARGE DISCHARGE-MAIN TAINING GAS-NITROGEN ELECTRODE-ALUMINIUM (WATER-COOLING) DIRECT CURRENT ARC DISCHARGE DISCHARGE-MAINTAINING GAS-NITROGEN INVENTORS fa/nag, Yamako mgl BY 1754i E ct. 27., 1970 l MANABU YAMAMOTO EVAL 3,536,948

HIGH FREQUENCY TORCH DISCHARGE PLASMA GENERATOR SINGLE ELECTRCDE OE ALUMINUM PROVIDED WITH 1967 2 Sheets-Sheet 2 Filed Jan. 27,

F l G. 2 F l G. 3

mi A7 48 6 M/ m4. m. m w M w wnO/Jh OO PO 6 ./.Vm 44 44 3 3 .r ww w w w w A t L L L J A .A AZ E Y zmwznmmohmd B oA O. 4 7 2 3 U C l 48` 8 4 48 3 9 TO n 2 OO 6 4 44 3 W WW W United States Patent U.S. Cl. 313-231 2 Claims ABSTRACT F THE DISCLOSURE A high frequency torch discharge plasma generator of the present invention is provided with a single electrode of aluminum which is least erosive by high temperature at the time of applying high frequency, high voltage to the electrode. The apparatus is constructed with a single electrode held at the tip of a supporter having inlet and outlet for cooling water to prevent overheating of the electrode and a shielding tube made of high refractory and insulating material having inlet for a discharge-maintaining gas.

This application is a continuation-in-part of prior application Ser. No. 293,532 filed on July 8, 1963, now abandoned, and entitled High Frequency Torch Discharge Generator.

The present invention relates to an improvement in high frequency torch discharge plasma generator provided with a single electrode.

An essential object of the present invention is to provide a high frequency torch discharge plasma generator provided with only one electrode composed substantially of aluminum and having various advantages such that said single electrode is not eroded at all and the discharge is maintained at very stable state for a long period of time and is not accompanied by any noise.

Said object and the other objects of this invention have been attained by using aluminum as the material of the single electrode of a high frequency torch discharge plasma generator.

The nature, objects and advantages of the present invention will become more apparent by reference to the following description when taken in conjunction with the accompanying drawings in which:

FIG. 1 is a sectional elevation for showing one preferred embodiment of the high frequency torch discharge plasma generator according to the present invention;

FIGS. 2-4 show, respectively, spectra of discharge plasmas of high frequency discharge plasma generators, wherein FIG. 2 relates to the case of using an electrode made of tungsten, FIG. 3 relates to the case of using an electrode made of copper, and FIG. 4 relates to the case of using a tungsten electrode;

FIG. S shows a spectrum of the discharge plasma generated by a high frequency torch discharge plasma gen erator according to this invention, in which single aluminum electrode is used; and

FIG. 6 shows one example of the spectrum of the plasma produced by D.C. arc discharge plasma generator.

A high frequency torch discharge plasma can be generated by disposing an electrode applied with a high voltage of a high frequency in a gas stream. Generally, in such a discharge plasma generator as described above, the most important problem consists in electrode erosion. That is,

the electrode is placed under an extremely erosive condition on account of it contacting a high temperature ionized gas, and when any electrode erosion occurs, it may cause the electric discharge to be unstable. Accordingly, for example, in the case of plasma jet generator utilizing a direct current arc discharge, a rare gas is used as the discharge-maintaining gas, a tungsten electrode is used as the stick-shaped cathode, and a water-cooled copper electrode is used as the nozzle-shaped anode in order to protect the electrodes from their erosion. ln the D.C. arc discharge plasma generator of the conventional type, wherein two mutually opposing electrodes are provided, and a direct current voltage is applied across these two electrodes so as to conduct a D.C. arc discharge therebetween, it is absolutely necessary to take measures to avoid erosion of the electrodes, which contact this high temperature gas, by means of, for example, using the so-called non-consumable metal of high melting point such as tungsten, etc. as the electrode material, or Water-cooling an electrode made of metal of relatively high melting point. When a low melting metal such as aluminum is used as the electrode material, the electrode is intensely worn out with the result that it is impossible to maintain stable discharge plasma for a long period of time. In fact, in the D.C. arc discharge plasma generator, even when high melting metal such as tungsten, molybdenum, or the like is used for a cathode and an anode (nozzle electrode), there unavoidably occurs wear of these electrodes more or less. And, by this wear of the electrodes, splashes of the electrode material mix in the plasma, which are excited to luminesce in the plasma and cause mal-effect to the luminescent spectrum. Thus, in the D C. arc Vdischarge plasma generator, there has been adopted a method to minimize the erosion or consumption of the electrodes as far as possible by use of high melting metal as the electrode material. However, according to various experiments by the inventors, it h-as been found that the above-mentioned method is not effective for the high frequency torch discharge. That is to say, when the high frequency torch discharge plasma is subjected to a spectroscopic investigation, many spectral lines of tungsten such as shown in FIG. 2 and FIG. 4 would appear even when a tungsten having an extremely high melting point is used as the constituting material of the single electrode of said high frequency torch discharge plasma generator. On the other hand, even when copper having a high heat conductivity is used as the single electrode metal and sufficient water cooling is effected to the copper electrode, many spectral lines of copper appear as shown in FIG. 3. In the high frequency torch discharge plasma generator, therefore, perfect prevention of the electrode from being Worn out is impossible, even if the single electrode is constructed with a high melting metal. According to these facts, it has become clear that there is an essential difference between the high frequency torch discharge and the direct current (D C.) arc discharge, and also between their discharge phenomena themselves. Generally, the plasma produced by the direct current arc discharge assumes a state near thermal equilibrium and the gas temperature and the electron and ion temperature in the said D.C. arc discharge plasma are of from 7,000 K. to 8,000" K., but the gas temperature in the high frequency torch discharge plasma is remarkably low. This fact seems to be correct, because there was a clear difference obtained by spectroscopic investigation in spectrum appearance in both cases of the direct current arc discharge and high frequency torch discharge which were carried out by the use of the same nitrogen as the discharge-maintaining gas. When such a spectrum produced by D.C. arc discharge as shown in FIG. 6 and such a spectrum produced by high frequency torch discharge as shown in FIG. 5 are compared with each other, the intensity of the nitrogen molecular band in the former case decreases more suddenly than that of the latter case. According to this fact, it is true that the gas temperature in the case of high frequency torch discharge plasma is lower than the gas temperature in the case of D.C. arc discharge plasma. However, in the case of high frequency discharge, as shown in FIGS. 2 and 4, tungsten having a very low luminous characteristic can be easily excited. Accordingly, the excitation temperature in the case of high frequency torch discharge plasma can be guessed to relatively high.

Thus, in the D.C. arc discharge plasma generator, consumption of the electrode becomes less as the melting point of the electrode material is high, while the consumption thereof becomes great as the melting point of the electrode material is low. However, on the other hand, the present inventors have confirmed through their experiments that in the high frequency torch discharge plasma generator having only one electrode, there is no relation at all between the melting point of the electrode material and the degree of its consumption, so that the electrode cannot always be prevented from being consumed even by use of material having a high melting point. Based on this discovery, the inventors conducted further experiments to prove that, when aluminum is used as the electrode material for the high frequency torch discharge plasma generator, consumption of the electrode can be perfectly removed.

Therefore, the present invention is to provide a high frequency torch discharge plasma generator provided with only one electrode composed substantially of aluminum and having various advantages such that said electrode is not eroded at all and the discharge is maintained at a very stable state for a long period of time and is not accompanied by any noise.

This invention is based on a new discovery that aluminum is the most appropriate material for the single electrode of the high frequency torch discharge plasma generator, said discovery having been obtained as a result of various experiments relating to various electrode materials. In the high frequency torch discharge plasma generator utilizing a water-cooled aluminum electrode, no erosion of the electrode occurs and it can be confirmed by a spectroscopic investigation that no aluminum line appears as shown in FIG. 5.

In FIG. Il is shown an embodiment of a high frequency torch discharge plasma generator according to the present invention, said generator comprising a single aluminum electrode 1, an electrode supporter 2, a high frequency power source 3, a shielding tube 4. The shielding tube 4 is made of a highly refractory as well as electrically insulating substance such as, for instance, quartz, etc. This tube is `disposed coaxially about the supporter 2 and the electrode 1 fitted at the tip thereof, the rear end (the end to the side of the support 2) of which being closed and the front end (the end to the side of the electrode 1) of which fbeing left open. At the rear end of this shielding tube 4, there is provided an inlet 6 for a discharge-maintaining gas. The supporter 2 is constructed with an electrically conductive material such as, for example, copper in a form of double tube, thereby to provide a flow path of the cooling water leading to an outlet 8.

In the actual operation, a discharge-maintaining gas such as nitrogen, argon, etc. is introduced from the gas inlet 6 into the supporter Z in such manner that the introduced gas stream may flow surrounding the electrode 1 and jet out to the external atmosphere from the open end thereof. In this state, when a high frequency, high voltage is applied to the tip of the electrode by the high frequency power source, high frequency torch discharge is generated within the discharge-maintaining gas surrounding the electrode 1. By this discharge, the discharge-maintaining gas is excited or ionized to generate high frequency torch discharge plasma.

In this case, cooling water may be introduced from the inlet 7 to prevent overheat of the electrode 1. It is to be noted that, in FIG. l, a gas inlet means to be connected to the gas inlet 6 and a cooling water supply means to be connected to the cooling water inlet 7 are all left out.

In FIG. l, one output terminal of the high frequency power source 3 is connected to the supporter 2 for the electrode l so as to apply high frequency, high voltage to the tip of the electrode 1. However, it should be understood of course that any other expedient is applicable as the means for impressing high frequency, high voltage on the tip of this electrode 1, such as one in which a tank coil is wound around the shielding tube 4, through which a high frequency current is caused to ow; or one in which high frequency power is transmitted through a coaxial cable so as to directly supply the high frequency voltage to the aluminum electrode fitted at the tip of the internal conductor of the coaxial cable; or further one in which a cavity resonator is provided about the shielding tube 4, and a high frequency power is supplied to this resonator, thereby inducing high frequency, high voltage at the tip of the electrode "1.

As an example, when high frequency power of 20 mc., 700 w was applied to the tip of the electrode l, while introducing nitrogen as the discharge-maintaining gas from the discharge gas inlet 6, the gas temperature of the discharge plasma generated was 4,000-5,000 K. It has been confirmed that this temperature is considerably low in the value in comparison with the gas temperature in the discharge plasma obtained from the D.C. arc discharge of 7,000-8,000 K. It has also been verified that, in the above-described example, the exciting temperature in the plasma reached 7,000-8,000 K, which was considerably higher than that in the case of the D.C. arc discharge. Also, no spectral line of aluminum could be detected at all in the light emitted from the discharge plasma. This phenomenon signifies that the aluminum electrode used in the high frequency torch discharge plasma generator having only one electrode is eroded at its least. Accordingly, a stable plasma can be generated for a long period of time, which is advantageous. A particular attention should be drawn to the fact that such effect of preventing the electrode from erosion can be obtained only in the case of using aluminum as the electrode material in the high frequency torch discharge plasma generator, and that enetrely reverse effect is resulted when aluminum is used as the electrode material for conventional apparatus, in which arc discharge is performed between two electrodes.

In the high frequency torch discharge plasma generator according to the present invention, having only one electrode substantially consisting of aluminum, there is no electrode erosion, stable operation of a long period of time is secured, and no noise occurs. Accordingly, said generator can be utilized for various objects, for example, it can be used as an excitation source for the spectroscopic analysis, and for measuring excited states of atoms or molecules, and various physical Values. Particularly, when the present invention is applied to a spectroscopic analysis apparatus, characteristic spectral lines of the species can be detected with a high sensitivity by the process which comprises coverting the species into powdery state in the case of solid material or into mist state in the case of liquid material and then introducing the species of the said converted state into the inlet 6 such as shown in FIG. 1 or into a separate species introducing passage together with a discharge-maintaining gas; and exciting the said introduced mixture in a discharge plasma.

While We have shown and described the preferred embodiment, it Will be apparent that various modifications and changes may be made therein, particularly in the form and relation of parts, without departing from the spirit of our invention as set forth in the appended claims.

What We claim is:

1. A high frequency torch discharge plasma generator comprising: a single aluminum electrode; means to impress a high-frequency voltage on the tip of said electrode; and means for forming a stream of gas covering the tip of said electrode.

2. A high frequency torch discharge plasma generator comprising: a quartz tube; a single aluminum electrode disposed therein; a high frequency power source connected to said electrode; an inlet for a discharge maintenance gas, .said gas streaming about the tip of said elec trode; and cooling means for cooling said electrode.

6 References Cited UNITED STATES PATENTS 2/1963 Gage 313-231 X 8/1966 Lai 313-231 X OTHER REFERENCES Roddy & Green, The Radio-Frequency Plasma Torch, Electronics World, Feburary 1961, pp. 29, 31, and 117.

ROY LAKE, Primary Examiner E. R. LAROCHE, Assistant Examiner U.S. Cl. X.R. 315-111