US3417287A

US3417287A - Low power high frequency discharge plasma generator

Info

Publication number: US3417287A
Application number: US582699A
Authority: US
Inventors: Murayama Seiichi
Original assignee: Hitachi Ltd
Current assignee: Hitachi Ltd
Priority date: 1965-10-08
Filing date: 1966-09-28
Publication date: 1968-12-17
Anticipated expiration: 1985-12-17
Also published as: DE1598570C3; DE1598570B2; DE1598570A1

Description

Dec. I 7, 1968 SEHCHI MURAYAMA 3,417,287

LOW POWER HIGH FREQUENCY DISCHARGE PLASMA GENERATOR Filed Sept. 28, 1966 2 Sheets-Sheet 1 2 PFi/OR ART F/G 6 PRIOR ART 1) A 4 A 6-5 .h

3\ A K 4 Y L INVENTOR 851mm MuRnYnnq ATTORNEY 1968 SEHCHI MURAYAMA 3,417,287

LOW POWER HIGH FREQUENCY DISCHARGE PLASMA GENERATOR 2 Sheets-Sheet 2 Filed Sept. 28, 1966 INVENTOR 850cm muaammn ATTORNEY United- States Paten 3,417,287 LOW POWER HIGH FREQUENCY DISCHARGE PLASMA GENERATOR Seiichi Murayama, Hachioji-shi, Japan, assignor to Hitachi, Ltd., Tokyo, Japan, a corporation of "Japan Filed Sept. 28, 1966, Ser. No. 582,699 Claims priority, application Japan, Oct. 8, 1965, 40/ 81,593 8 Claims. (Cl. 315-111) ABSTRACT OF I THE DISCLOSURE A high frequency discharge generator including a coaxial waveguide wherein the outer conductor thereof has an inner diameter which is less than 30 mm., the outer conduct-or of the coaxial waveguide extending beyond the end of the inner conductor so as to cover a high frequency plasma produced at the top of the inner conductor, and

characterized in that carrier gas and a sample to be an- This invention relates to improvements in a high fre-' quency discharge apparatus and it is an object of the invention to stabilize discharge, to improve excitation efficiency of a sample and to provide a high frequency discharge apparatus having a structure convenientfor observation'of a flame.

Spectroscopic analysis of a sample based on the principle of high frequency discharge has been conventionally performed by use of an apparatus as shown in FIG. 1. When microwave power is supplied from the left of a coaxial waveguide 1, a strong high'frequency electric field is induced at an electrode 4 placed at the top of the inner conductor of a discharge coaxial waveguide 2 so that a discharge plasma 6 appears at the instant when a discharge gas is introduced from a discharge gas inlet 5.

In this case, a sample to be analyzed is dissolved into a solution, atomized with a sprayer and introduced together with the discharge gas from the discharge gas inlet 5. Since the sample thus introduced luminesces in the plasma flame 6, the sample may be analyzed by observing the luminescence. In such a conventional apparatus, the size of the discharge coaxial waveguide 2 is adapted to that of an output flange of a magnetron, which is a microwave generator, and so there has been used, for instance, a high frequency discharge apparatus comprising an outer conductor having an inner diameter of about 76.2 mm. and an inner conductor of outer diameter of about 33.3 mm. as shown in FIG. 1, or at least a rather large high frequency discharge apparatus comprising an outer conductor of inner diameter of about 58.8 mm. and an inner conductor of outer diameter of about .16.9 mm. Accordingly, it hasbeen a defect of such a conventional appa ratus that a microwave source capable of producing rather large power above 1 kw.) is required. For instance,-in case whererare gas, such as argon, is used as the discharge gas and the power is low, the core of a flame swerves and becomes unstable as shown in FIG. 2. Moreover, the greater the power becomes, the more the electrode 4 will be exhausted to make discharge unstable. Since the electrode material will be mixed into the plasma flame, the spectra of the elements composing the electrode material will be observed. The above facts constitute great drawbacks of such an excitation source for tion, there is provided a high frequency discharge apparatus composed so that'all the sample introduced may pass through a plasma flame of'a high temperature suificient tomake the sample luminesce.

5 As is well known, only a charged particle, for example, an electron or an ion, can receive energy directly from a high frequency electric field. Such charged particles receive energy from the electric field and transfer it to neutral atoms by way of collision. In order for discharge to continue, there must be a balance between the energy which a charged particle receives from an electric field and that which the charged particle gives to a neutral atom through collision. In high frequency discharge'under ordinary atmospheric pressure (in the vicinity of the normalstate), the energy an electron gains from an electric field is much larger than the energy received by an ion from an electric field. Therefore, it is sufficient to take only electrons into account for consideration of energy balance. The energy W which an electron of mass m and charge e gains from an electric field per unit time, is

. expressed in the form,

where E is an effective strength of a high frequency electric field and 'y is a collision frequency. When 7,, is expressed'in terms of a mean free path h and electron temperature T Equation 1 becomes,

v vsmucre' where k denotes a Boltzmann constant.

Now, as is known, the energy which an electron gives .35 to a neutral atom per single collision is equal to,

2 e M 'Ye Since W =W in an equilibrium state, there follows from Equation 2,

Now, the temperature of a chemical reaction flame used for general spectroscopic analysis is at most 3000 K. and it is known that it is impossible to analyze a substance having a high excitation energy, for instance zinc, by using such a flame of chemical reaction as an excita- 1 ti'on source. Therefore, the temperature of 3000 K. can- 6 not be sufiicient as an excitation source for analysis. On g the other hand, since, as is known, a substance like zinc may be easily excited in an arc discharge whereby the temperature of 5000 K. is available, it may be said that the temperature of 5000 K. is suflicient for excitation of a sample. However, a higher temperature is not always suitable for analysis, for the higher the temperature rises, the more the background of continuum will be included in the spectrum, and, consequently, the S/N (signal-tonoise) ratio becomes accordingly worse. When the spectral line intensity is measured with a wave length-scanning monochromator, the signal-to-noise ratio is defined as a ratio of the spectral line intensity to the background con-- W 3 1 Jamar. 2

M As

tinuum intensity. Thus, it is concluded that the temperature of at least 4000 K. is necessary. Now, by setting T =4000 K. in Equation 4, the effective value of a high frequency electric field strength E will be obtained. Discharge gas most suitable for an excitation source for analysis is one which has a small number of spectral lines constituting a background and which is cheap and easy to obtain and so usually argon gas is employed. In this case, a sample is introduced into the plasma in the form of an aqueous solution and accordingly substances other than argon, such as OH molecules, are present in the plasma, the partial pressure of which is considered to be about A of that of argon. Under such a condition, the collision with atoms like argon having a small collision cross section due to Ramsauer-Townsend effect may be neglected. Since at high temperature most of the water molecules are considered to be dissociated into OH molecules, R is estimated from the collision cross section of an OH molecule to be 7\ 1.4 10 [m.] if the partial pressure of OH molecules is assumed to be atm.

From this,

E=5.7 10 [volt/rn.] (5) As has become evident from the foregoing discussion, it is necessary to make all the samples pass through the plasma having an electric field stronger than that given by Equation 5. Thus, the electric field strength at the inner surface of an outer conductor where an electric field is Weakest must be larger than the value given by Equation 5. Since an electric field strength in a coaxial waveguide depends only on the power supplied and the diameter of the coaxial waveguide, the maximum diameter of the coaxial waveguide may be determined from the minimum condition of electric field strength given by Equation 5 if the diameter of the coaxial waveguide and the electric power are known. When the device is to be used as an excitation source for spectroscopic analysis, other conditions, for instance stability of discharge, must be fulfilled. These conditions depend on a high frequency. electric power. When the power is too small, the light runs short of energy and the discharge becomes unstable. In the present invention, measurement has been made over a wide range of electric power from a few tens of watts to a few hundreds of watts and the minimum usable electric power has turned out to be about 100 w. When a rectangular waveguide is connected to a coaxial waveguide, the characteristic impedance of the coaxial waveguide is usually set to be 5052. Under said condition, the maximum inner diameter of the outer conductor may be calculated as follows:

Effective voltage: (50X 100) ==7l [v.1 Etfective current: 100/50 1.4 [amp.]

If the inner diameter of the outer conductor is set to be 2R, the electric field strength E at the inner surface of the outer conductor is expressed, from a well-known formula of coaxial tubes, in the form,

E=60 (effective current) 1/R [volt/m.] (6) From

Equations

5 and 6,

R=1.5 1O [m.] (7) From this, the maximum value of the inner diameter of the outer conductor becomes,

Therefore, if the inner diameter of the outer conductor is made smaller than 30 mm., the strength of the electric field becomes larger than the value of Equation 5 and the electron temperature T becomes higher than 4000 K. under working conditions. Thus, it is secured that all the samples pass through a plasma having a temperature higher than 4000 K. to produce light for spectroscopic analysis with a good excitation efficiency.

If a gas other than argon is used, the mean free path x, of a charged particle becomes much smaller than the value with argon and correspondingly the required electric field strength becomes larger than the value of Equation 5. Accordingly, the inner diameter of the outer conductor must be smaller than the value given by Equation 8.

For a better understanding of the present invention, reference will be made to the following description of an embodiment of the invention, taken in conjunction with the accompanying drawings, in which;

FIG. 1 is a longitudinal sectional view of a conventional high frequency discharge generator;

FIG. 2 shows the state of a discharge plasma flame generated by a conventional high frequency discharge generator;

FIG. 3 is a longitudinal sectional diagram of a high frequency discharge generator according to the present invention;

FIG. 4 shows the state of a discharge plasma flame generated by a high frequency discharge generator according to the invention; and

FIG. 5 is a diagram showing spectral lines obtained. by spectroscopic analysis with a high frequency discharge generator according to the present invention.

In an apparatus according to the present invention, microwave power is supplied from the left of a rectangular waveguide 7, shown in FIG. 3. Then a strong high. frequency electric field is produced at an electrode 4 positioned at the top of an inner conductor 3 of a discharge coaxial waveguide 2. When the discharge gas is introduced from a discharge gas inlet 5, a plasma flame 6 is generated. A sample for analysis is dissolved into a solution, atomized with a sprayer and introduced together with the discharge gas from the discharge gas inlet 5. The sample thus introduced emits light in the plasma flame 6 and hence the sample may be analyzed by observing the luminescence. The inner diameter of the outer conductor of the coaxial waveguide must be less than 30 mm. and in FIG. 3 is shown an embodiment wherein the inner diameter of an outer conductor of a coaxial waveguide is 20 mm. Dielectric 8 serves to prevent leakage of the discharge gas into the rectangular waveguide 7 and marks the end of the flow path of the discharge gas. The discharge gas inlet 5 and the dielectric 8 are separated by at least 10 mm. The distance between the discharge gas inlet 5 and the electrode 4 is made larger than 20 mm. The discharge gas is introduced from the discharge gas inlet 5 in a direction tangential to the circumference of the outer conductor 2 of the coaxial Waveguide and is made to rise around the inner conductor 3 spirally. The outer conductor of the coaxial waveguide is extended to the extent that most of the flame is covered to a more or barrier extent and holes or a slit 9 is provided thereon so that an iarbitary point on the flame axis may be observed. The electrode 4 at the end of the inner conductor of the discharge coaxial waveguide is formed of aluminum.

The effects of each part of a discharge apparatus fabricated as described above will be described in detail hereinbelow.

By making the inner diameter of the outer conductor of the discharge coaxial Waveguide less than .30 mm, it becomes possible to cause the discharge plasma flame 6 spread all over the interior of the outer conductor of the coaxial waveguide ias'shown in FIG. 3 even in case of a low electric power of -200 w. Since the outer conductor confines the flame in this case, a stable discharge as shown in FIG. 4 is obtained. Under such a low power, it is possible to generate a plasma flame 6 which does not exhaust the electrode and which is ideal for an excitation source for spectro chemical analysis. Also, when the discharge plasma fiame 6 spreads over the interior of the outer conductor of the coaxial waveguide as shown in FIG. 3, all the introduced sample enters the plasma flame to luminesce. According to a conventional method, however, much of the sample passes outside the plasma flame, and so the excitation efliciency was poor. The outer conductor extended so as to cover the discharge plasma flame 6 prevents the sample from escaping from the plasma flame and accordingly it has the effect of facilitating the absorption of the microwave power in the plasma flame as well as adding to the excitation efliciency. By extending the outer conductor, loss of microwave power due to leakage may be reduced by several tens of decibels and hence it is possible to reduce the danger to the human body caused by microwaves as well as to improve the efliciency of the generator. It has another eifect of preventing noise radiation resulting from discharge.

The discharge gas is introduced from the discharge gas inlet 5 into the interior of the outer conductor in a direction tangential to the circumference of the outer conductor and forms a spiral flow. In this case, unless the distance between the discharge gas inlet 5 and the dielectric barrier 8 forming the end of the flow path of the discharge gas is more than mm, the spray introduced from the discharge gas inlet 5 turns to water drops on the dielectric '8 to cause loss of a sample. Also, unless the distance from the discharge gas inlet 5 to the electrode 4 ,is more than 20 mm., a uniform spiral flow may not be obtained and the discharge becomes unstable.

The luminous state of a sample in the discharge plasma 6 depends appreciably on the position in the plasma flame. For a sample to emit light, three processes, is. (1) evaporation of a spray, (2) dissociation of molecules composing a sample material, (3) excitation of the dissociated element are required in advance. Since the ease with which these processes occur depends on the kind of a sample, the point in the flame where a maximum S/N ratio is obtained in the observation of the spectral lines of sample elements differ from sample element to sample element. Generally, it often happens that the central point of the flame, more than mm. above the end of the electrode, constitutes an optimum observation point. In the present apparatus, holes or a slit 9 is provided so that an arbitary point on the axis of the flame may be observed and the apparatus is composed so that a position where a maximum S/N ratio is obtained for any element may be observed.

FIG. 5 shows an experimental result obtained with the present apparatus when an aqueous solution of zinc of 5 p.p.m. (parts per million) (ZnSO aqueous solution) is atomized with a sprayer and introduced with discharge gas (argon) into a plasma flame. The light from the plasma flame is lead into a monochromator and the output therefrom is received by a photomultiplier tube The current running\ through the photomultiplier tube is recorded in a recorder while scanning the wavelength of the monochromator. It is concluded from' this result that the spectral line of Zn 4810 A. may be distinguished from background noises even when the concentration of zinc solution is much lower than 5 p.p.m.

If the minimum detectable concentration is defined as a concentration which can emit the spectral line twice as intense as background noises, it is concluded that the minimum detectable concentration of the present apparatus is 0.3 p.p.m. :for zinc. This sensitivity is much better than that of the conventional apparatus, whose minimum detectable concentration or zinc is 5 p.p.m.

Iclaim:

1. A high frequency discharge generator comprising:

plasma generating means for generating a discharge plasma flame in the form of a coaxial waveguide including inner and outer conductors,

power supply means for applying high frequency elec= trical energy to said coaxial waveguide, and

gas supply means for supplying a gas between the in ner conductor and the outer conductor of said coaxial waveguide,

said outerfconductor having an inner diameter of less than 30 mm. and extending beyond said inner conductor at the end thereof at which the plasma flame is generated, so that a stable flame is generated at low power.

2. A high'frequency discharge generator as defined in claim 1 further including a dielectric barrier in said coaxial waveguide between said gas supply means and said power supply means preventing passage of said gas.

3. A high frequency discharge generator as defined in claim 2 wherein said gas supply means includes an inlet aperture in said outer conductor positioned at least 10 mm. from said dielectric barrier.

4. A high frequency discharge generator'as defined in claim 3 wherein said inlet aperture is positioned more than 20 mm. from said one end of inner conductor.

5. A high frequency discharge generator as defined in claim 2 wherein said gas supply means introduces gas intov said coaxial waveguide in a tangential direction to the inner circumference of said outer conductor so as to produce a spiral rising of said gas around said inner con: ductor.

6. A high frequency discharge generator as defined .in claim 1 wherein said one end of said'inner conductor is provided with an electrode tip made from aluminum.

7. A high'frequency discharge generator as defined in claim 6 wherein the gas supplied by said gas supply means is argon.

8. A highfrequency discharge generator as defined in claim 7 wherein the portion of said outer conductor ex. tending beyond said inner conductor is provided with a viewing aperture, said power supply means introducing electrical energy to said waveguide at the other end thereof.

References Cited UNITED STATES PATENTS 3,280,364 10/1966 Sugawara 313-231 X 3,353,060 11/1967 Ya'ma-moto et a1. a..." 315 111 FOREIGN PATENTS 1/1963 France.

U.S. Cl. X.R.