US3617804A - Continuous plasma light source - Google Patents

Abstract

A light source apparatus for producing intense monochromatic light from a low-temperature plasma is disclosed. The light source apparatus comprises a housing member for confining a gas at subatmospheric pressure, the housing containing output means disposed to transmit light from the confined gas. At least one anode and one cathode are positioned within the gas in electrical circuit relationship with each other. First control means are provided for controlling the voltage applied to the anode and second control means independently control the temperature of the cathode by regulating the power supplied to it. The pressure of the gas is controlled by a third control means. An intense monochromatic light output is achieved by confining the gas in said chamber at a controlled predetermined reduced pressure, independently controlling the temperature of the electron emitting cathode by means of said second control means and applying a predetermined controlled low voltage to said anode by said first control means. An intermediate mode current is drawn from said cathode and produces in the confined gas a region having a high density of metastable atomic states. A lowtemperature, high-density plasma is continuously produced in said region and intense monochromatic light is emitted as a result of the recombination of ions and electrons in said plasma.

Description

United States Patent [72] Inventors Thomas O. Paine Administrator of the National Aeronautics and Space Administration with respect to an invention of; Willard F. Libby; Carl A. Jensen; Lowell L. Wood [21] Appl. No. 866,442 [22] Filed Oct. 14,1969 [45] Patented Nov. 2, 1971 Continuation of application Ser. No. 479,357, Aug. 12, 1965, now abandoned.

[54] CONTINUOUS PLASMA LIGHT SOURCE 1 Claim, 7 Drawing Figs.

[52] U.S.Cl 315/326,

315/1 1 1,315/358, 313/231, 313/186,313/212, 313/224, 331/945 [51] lnt.Cl ..H0lj6l/l6, H01] 61/28, HOlj 61/78 [50] Field oiSearch 313/231, 12; 315/108 [56] References Cited UNITED STATES PATENTS 2,929,952 3/1960 Gianniniet a1. 313/231 2,972,698 2/1961 Dana et al 313/231 3,172,000 3/1965 Roseneret a1. 313/12 3,292,037 12/1966 Herglotz 315/108 OTHER REFERENCES Wiley & Sons 1927 Tien, MacNair & Hodges: Electron Beam Excitation of Gas Laser... ln Physical Review Letters Jan., 6 1964 Vol. 12 No. 1

Elenbaas: Fluorescent Lamps And Lighting pp. 144, 145 Philips 1962 Primary Examiner- Herman Karl Saalbach ABSTRACT: A light source apparatus for producing intense monochromatic light from a low-temperature plasma is disclosed. The light source apparatus comprises a housing member for confining a gas at subatmospheric pressure, the housing containing output means disposed to transmit light from the confined gas. At least one anode and one cathode are positioned within the gas in electrical circuit relationship with each other. First control means are provided for controlling the voltage applied to the anode and second control means independently control the temperature of the cathode by regulating the power supplied to it. The pressure of the gas is controlled by a third control means. An intense monochromatic light output is achieved by confining the gas in said chamber at a controlled predetermined reduced pressure. independently controlling the temperature of the electron emitting cathode by means of said second control means and applying a predetermined controlled low voltage to said anode by said first control means. An intermediate mode current is drawn from said cathode and produces in the confined gas a region having a high density of metastable atomic states. A low-temperature, high-density plasma is continuously produced in said region and intense monochromatic light is emitted as a result of the recombination of ions and electrons in said plasma.

6A5 IN ANODE VOLTAGE.

CONTQOL l l m 7 1'7 CATHODE POWEQ SUPPLY AND TEMP.

CONTROL GAS OUT TO PUMP AND PRESSURE CONTROL CONTINUOUS PLASMA LIGHT SOURCE This is a continuation of Ser. No. 479,357 filed Aug. 12, 1965 now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a method and apparatus for producing both noncoherent and coherent intense monochromatic light from a continuous low-temperature plasma. More particularly, this invention relates to a method and apparatus for the production of radiation in the form of intense monochromatic light from a continuous low-temperature plasma generated in a region having a high density of atoms in metastable atomic states and inwhich the neutralization of ions by recombination with electrons proceeds at a very rapid rate. By selecting the gas it is possible to produce light at many different wavelengths. The use of helium to give 584-A ionizing light is an example of an interesting case. The method and apparatus are applicable both to noncoherent light sources and to coherent light sources of the commonly called lasers.

ORIGIN OF INVENTION The invention described herein was made in the performance of work under a NASA contract and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat. 435; 42USC 2,457).

2. Description of the Prior Art As has been set forth by two of the inventors in an article entitled Intense 584A Light From a Simple Continuous Helium Plasma, which was originally published in Volume 135, No. -A of The Physical Review" at pages A-l ,247 through A-l ,252 on Aug. 31, 1964, the importance of ionizing radiation in inducing chemical reactions is widely recognized. An intense monochromatic source of light in the far ultraviolet region is highly desirable in space research to study the chemical effects of solar ionizing ultraviolet radiation. Such a source, of course, also has many other scientific and industrial applications which are wellknown to those skilled in the art. It should be noted, for example, that the particular source described herein is also suitable for use as a vacuum ultraviolet laser when hydrogen or helium three is used in combination with helium four as will become apparent from the discussion below. Of course, the more common laser wavelengths may also be induced by an appropriate selection of gases and conditions, i.e., temperature, pressure, current, voltage, and magnetic field intensity.

OBJECTS AND SUMMARY OF THE INVENTION It is therefore an object of this invention to provide a method and apparatus for producing intense monochromatic light from a continuous plasma source.

It is a more particular object of this invention to provide a method and apparatus for producing a monochromatic source of ultraviolet light of very short wavelengths having an intensity per unit of bandwidth which is greater than has heretofore been available. Depending on the environment this light may or may not be coherent such as laser light is.

Briefly stated in very general terms these objects are achieved by producing, in a continuous plasma formed by a gas confined at reduced pressure, region having a high density of atoms in metastable atomic states and/or as ions. Upon recombination of the ions and electrons in the plasma high energies are released in the form of intense radiation. The ions are usually formed as a result of high density of metastable states. The metastable states and/or the ions may be made to form population inversions which may lead to the production of coherent electrogmagnetic radiation.

In The International Dictionary of Physics and Electronics" published by D. Van Nostrand Company, New York, 1961, at page 741 there is given a discussion of the metastable state which defines the sense in which the term is used herein. Broadly speaking, this state is one in which a system has acquired energy beyond that for its most stable state yet has become relatively stable in this high energy condition. Familiar daily examples of this phenomenon are the fact that water at atmospheric pressure may be heated several degrees above its normal boiling point and yet not boil until the system is disturbed from some external source. In the metastable state the water has received energy beyond that normally required for liquid-vapor equilibrium and yet has not become unstable. The system will, however, flash into steam when it is disturbed. This term has similarly been used in atomic and nuclear physics to designate excited atomic states from which all possible quantum transitions to lower states are partially or completely forbidden transitions by the appropriate selection rules. The method and apparatus for producing a plasma having a region of high density of metastable atomic states and/or ions (which may be considered as a variety of a metastable state) to generate an intense ultraviolet or other light will be described in detail below.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings, wherein like reference characters refer to like parts throughout:

FIG. 1 is a diagrammatic sketch of the arrangement of the parts of the source particularly showing their arrangement with respect to a spectrograph by means of which the characteristics of the source were analyzed.

FIG. 2 is a sectional view of one exemplary embodiment of the light source arranged to produce noncoherent light.

FIG. 3 is a graph showing the light intensities from transitions in the predominant gas observed at various points along a line extending through the anode and heated cathode wires of the source. A similar curve, but with its intensity maximum usually near the anode as is the case with hydrogen, may be drawn for each of the minority gases which may be present.

FIG. 4 is a graph showing a plot of the light intensity as a function of anode current at two typical observation points.

FIG. 5 is a graph showing light intensity as a function of anode voltage at three different observation points. Again, in the case of minority gases, this function may have the opposite slope, i.e., increase with increasing voltage.

FIG. 6 is a graph showing the light intensities at two different wavelengths as a function of gas pressure in the source.

FIG. 7 is a graph showing a plot of the logarithm of the state density divided by state multiplicity with respect to state energy for the plasma in various regions of the device. The slope of the graph line affords a measure of plasma temperatures.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Turning now to the drawings and particularly to FIGS. 1 and 2, there is illustrated a low-power device for producing plasmas of approximately l,660 K. and number densities of 10 cm. over a volume of several cubic centimeters. The device when used in a manner to be described below produced an observed intensity of ultraviolet light of the 584-A ZP-l S line of approximately 4X10 photons per second cubic centimeter. This light was found to be monochromatic at the indicated wavelength in the far ultraviolet.

Much information has been obtained about the properties of plasmas from studies attempting to control thermonuclear energy. One of the principle results of these studies is the knowledge that in a moderately dense helium plasma the rate of neutralization of I'Ie by the reaction.

2e+I-Ie =He+e is very rapid. Alsopgood methods are available for the measurement of plasma temperatures and densities. With these temperatures and densities known, the rate of the firifidyll ii bin t n samba sa s le and compared with the observed ultraviolet intensities stated above. M.

In FIGS. 1 and 2 there is shown a heated cathode wire 10 and an anode wire 11 which are positioned in spaced parallel relationship to each other and which are mounted and encaged in a large metal canister or other container 12. The container 12 is provided with a gas inlet line 13 and a gas outlet line 14 which are respectively connected from and to any convenient vacuum pump or other gas source adapted to maintain the interior of the container at a reduced and definite pressure. It is preferred to use a pump to keep the gas flowing in order to clean generated impurities out of container 12. However, it will be understood that this is not an essential or critical feature of the device.

The container 12 is also provided with a viewing slit 15 which has its longitudinal axis extending parallel to the electrode wires 10 and 11. The direction of view through the slit 15 is indicated by the arrow 16 whereas the direction of electron flow between the electrodes 10 and i l is indicated by the arrow 17 which lies in the plane of these two electrodes. It will be noted that the direction of view as indicated by the arrow 16 is at right-angles to the plane of the electrodes as indicated by arrow 17 and that the slit 15 is so positioned that it views the light generated in a region adjacent to the cathode 10.

It will of course be understood that any suitable optical system, such as a diffraction grating, or other utilization device may be positioned to actually observe or view along the direction indicated by arrow 16. The actual device used and its relationship to thestructure shown in FIG. 2 will depend upon the intended application and use and does not per se form a part of this invention.

In the diagrammatic sketch of FIG. I the apparatus is shown as it was connected to make certain measurements which will be described below. In that arrangement the slit 15 was positioned adjacent to a differential pumping chamber 18 which was provided on its remote side with a second slit 19 leading to the grating in a chamber of a conventional spectrograph 20. As is well-known in the art, such a differential pumping technique is commonly used to permit measurements to made with the spectrograph at low-pressures in order to avoid or minimize absorption by gas escaping through the slit from the plasma light source with its attendant adverse effect on accuracy of the measurement.

In the particular exemplary embodiment shown in the drawings the hot cathode l consisted of a thoriated tungsten wire connected to a conventional power supply 21 which was so connected as to provide a controlled cathode temperature.

The anode 1] consisted or a tantalum wire connected to a controllable source 22 of anode voltage, The gas inlet 13 and gas outlet 14 were connected to a controllable gas source and a vacuum pump 22 which provided a controllable pressure in the chamber 12 of approximately 300 microns of helium.

Typical operating conditions for the device were 300 microns helium pressure and 30 volts applied to the anode with l ampere of current flowing between the anode and cathode.

The exact dimensions and geometry of the light source device are not critical, but the exemplary embodiment preferred for the measurements made here and described below had the following arrangement: The cathode was 3 cm. long and was formed of 0.0l0-inches thoriated tungsten positioned parallel to and 1 cm. from a 3 cm. long 0.020-inches tantalum wire serving as the anode. The viewing slit was 2.5-cm. from the cathode and itself had a width of 150 microns. The helium pressure maintained in the chamber 12 was 300 microns.

The device has three current modes depending on cathode temperature, anode voltage and helium pressure. In the low current mode the device operates as a vacuum diode tube. The intermediate current mode of a hundred or more milliamperes to a few tens of amperes is the condition in which the device operates as desired and as described herein. In the high current mode the device shows negative resistance and operates as a low-pressure arc discharage device in which a current of more than 100 amperes is drawn. In the intermediate current mode the internal resistance of the device is positive and affords stable operation without the use of an external resistance. In the high current mode an external resistance is needed for stable operation.

The desired intermediate-current mode of operation of the device is defined not only by the above-noted current flow of more than milliamperes but less than 100 amperes, but also by the fact that this current mode is achieved by maintaining the tungsten cathode at on the order of 2,5 00 K. or more, by applying an anode voltage in the range of 20 to 100 volts for helium gas at a pressure of approximately 300 microns and the electrode spacing, type and-geometry described here. If the anode voltage exceeds 100 volts the device goes into the high current mode under the conditions described here. The onset of the high current mode will depend on the nature of the use, the system geometry, and other system parameters such as the pressure. More generally the intermediate current mode is that set of operating conditions under which the device carries many times the space charge limited current of a similar device operating in a vacuum, but the resistance of the device is still positive. The high current or are state, on the other hand, is defined as the negative resistance region of operation of the device.

The unique emission characteristics of the device in the intermediate current mode state are believed to be related to the fact that it is operated with a very hot cathode which provides abundant electrons and hence high currents at relatively low anode voltages. There is thus provided an abundant supply of low-energy electrons. This in turn results in a dense low temperature plasma and an excited atomic state region in which metastable atomic states occur abundantly. In this region there is rapid production of and recombination of ions and electrons and each recombination in turn eventually produces a photon. Thus, the photon flux density from transition going to the ground state of the principal gas under conditions which do not radically shift or broaden the light emitted by the atoms excited directly or via the metastable states, but not the ions, is proportional to the recombination rate which is high in the present high-density plasma arrangement. On the other hand, the line width or bandwidth of the emitted radiation is an exponential function of the temperature. Hence, the low-temperature plasma produced under the above-noted conditions will result in a monochromatic emission of high intensity, which is able to escape complete resonance trapping in its own parent gas.

As will be explained in detail below, the region around the electrodes has been observed to have a high density of metastable atomic states which, of course, are predecessors of the ions. Such an inverted population which holds here for the ions as well as for the metastable states is known to afford the basis for laser action. The device may be used with natural gases such as helium, neon, krypton, xenon, hydrogen, and isotopes of these gases. Gases other than natural helium have been used both alone and by injecting them into the chamber in the presence of or in combination with helium. When, for example, hydrogen at a pressure which is a small fraction of that of the helium, is injected in the presence of the inverted helium population, an inverted population of hydrogen states occurs. This results in the emission of light characteristic of hydrogen, notably the 1,216-A Lyman alpha line in the vacuum ultraviolet. When an arrangement, such as mirrors in an optical cavity, is provided, which provides or tends to provide a preferred direction for the stimulation of radiation, laser action results and coherent light will be produced. Such arrangements are shown in FIGS. 8 and 9 which will be described in detail below.

It has been found that substantially the same operating conditions will optimize the light intensity and the number of metastable states. In previously known plasma generating devices only two variables have been available for control to produce inverted populations: (1) the gas pressure and (2) the electric field strength. In the present device in addition to controlling these two factors, the heated cathode also affords control over the cathode temperature to permit the generation of low energy electrons. The current and the plasma temperature in the device of the present invention are thus necessarily a function of the cathode temperature, the field of strength and the pressure rather than simply the field strength and pressure alone.

Thus, in the above-described particular exemplary embodiment of FIGS. 1 and 2, it was found that if the pressure drops to leas than 50 microns or if the cathode temperature is too low to cause significant emission, or if the anode voltage falls to less than about 22 volts, the device drops into the low current mode, The transition is gradual in the case of varying cathode temperature, but very abrupt with variation in pressure or anode voltage, A drop of less than one-tenth of a volt on the anode will cause the current to change from many amperes to a few milliamperes. With very pure helium and welloutgassed electrodes the process is reversible at the same voltage; otherwise there is a hysteresis effect and higher starting voltage is necessary, although again the current goes from milliamperes to amperes in about one-tenth volt.

Within the intermediate current mode range, anode current is determined primarily by cathode temperature. Anode voltage has little effect once it has been raised past the striking level. Separation between cathode and anode has been increased up to 7 cm. with no appreciable increase in anode voltage required. The limiting factor on the current is the amount of heat the anode can dissipate before it melts, or the heat the lead in conductors can dissipate.

In the intermediate current mode the source resistance is positive and therefore no resistance is required in series with it to give stable operation. With a very stable low output impedance power supply there was no evidence of oscillation up to frequencies of 100 megacycles per second. The high current mode occurs when anode potentials are increased to several hundred volts. This is typical low-pressure arc and a resistor must be placed in series with the source to obtain stable operation.

The intensity of 584-A light is greatest from the cathode region. This light intensity as a function of distance from the anode is shown in the graph of FIG. 3 in which the light or line intensity if plotted in relative or nondimensional units whereas distance from the anode is plotted in millimeters. The millimeter position marked filament position refers to the cathode In the measurements shown in this graph the slit was sampling light from a region 0.2 millimeters wide. The measurements shown in FIGS. 3,4 and 5 were made with 300 microns helium pressure, 30 volts on the anode, and 0.6 amperes current.

The helium 584-A light intensity is also roughly proportional to current flow and inversely proportional to anode voltage as is shown in FIGS 4 and 5 respectively. Again in both graphs the intensity is normalized or stated on a relative and hence nondimensional basis.

Also, as can be seen in FIG. 6 a maximum of 584-A intensity is obtained at around 300 microns of helium pressure. In FIG. 6 the anode current is stated in amperes, the helium pressure in microns, and relative light intensities are stated with a fixed voltage of 30 volts on the anode and 405 watts on the cathode filament. Curves are given for the intensity of the 584-A helium line and for the 1,216-A hydrogen Lyman alpha line which occurs when hydrogen gas is introduced into the system. The two curves to the left show the intensity of the helium 584-A light and the Lyman alpha 1,216-A' light, respectively, with respect to helium pressure. The curve to the right gives the source current with respect to pressure for the given anode voltage and filament temperature. Note the sharp shoulder for all three curves at about 30 microns pressure at the transition between the intermediate current mode and the low current mode.

The spectrograph with which the foregoing observations were made was a McPherson Model 225. With the differential pumping slit, which was attached to the spectrograph, 2.5 cm. from the source the spectrum consists only of the 584A 2 I S resonance line in the extreme ultraviolet. Even the 537-A 3P-l S line is usually too weak to observe. At 0.6 amperes the intensity of the 584-A line is 4 X10 photons per second per cm. near the cathode region. This intensity was determined by using a quantum with a gold photocathode. The quantum efficiency of such a photocathode is known. The grating efficiency for the spectrograph was obtained by measuring the intensity of the line before and after reflection from the grating with this photocell.

For this measurement a 2,000A thick aluminum foil was used in front of the photocell to filter out hydrogen Lyman alpha radiation of 1,216-A and to stop ions and electrons when measuring the intensity of the direct beam. The transmissions of the filter to 584-and l,2l6-A radiation were measured. Thin aluminum foils transmit quite well in the extreme ultraviolet up to wavelengths of about 1,000-A at which point their optical density increases until a 2,000-A thick foil is essentially opaque in the near ultraviolet and visible. At the same time the quantum efficiency of the gold photocathode decreases rapidly above 1,000-A. The combination of the two effects can be used to make a very sensitive photocell in the vacuum ultraviolet, which is essentially insensitive to light whose wavelength is greater than 2,000-A. With this information and a calculation of the light passed by the spectrograph entrance slit the absolute intensity of the 584-A line can be determined. The quantum efficiency of the gold photocell is probably not known to better than a 50 percent accuracy. The value chosen was on the low side so that the intensity could in fact be as much as 50 percent greater than stated herein.

The intensity of the Lyman alpha light depends mainly on the helium/hydrogen ratio and the anode voltage as well as on the purity of the helium gas and the cleanliness of the vacuum system. It is generated when impurity molecules containing hydrogen give electrons to the He ions because of the higher ionization potential of helium, or when hydrogen receives energy from collisions of the second kind with metastable helium atoms. When using mass-spectrograpic grade helium or helium evaporated from the liquid, the Lyman alpha intensity was about the same as the 584-A intensity in terms of numbers of photons per second. It is for this reason that we believe anode cooling would reduce the Lyman alpha dramatically.

There is a weak doublet of unknown origin at l,300-A and a few other very weak lines below 2,000-A. Above 2,000-A the other helium emission lines and a few impurity lines appear. The spectrum resembles that produced by a radiofrequency discharge source. No evidence of light form the l-Ie molecule ion is seen. (See R. W. Motley and A. F. Kuckes, Proceedings of the Fifth International Conference on Ionization Phenomena in Gases, Munich, 1962 (North-Holland Publishing Co., Amsterdam I962), Vol. P. 65l). The source is clean in several regions where I-Ie bands would appear, especially in the 600900A region and also for the 5,133-A band, and they are not observed.

Various different electrode geometries have been investigated as well as varying numbers of electrodes. The intensity of 584-A radiation remains roughly the same for a given current. Since the light comes, we believe from bound states formed by ion-electron recombination, the ion densities are not appreciably altered by the number or arrangement of electrodes; nor does the ion density vary rapidly in going from cathode to anode as FIG. 3 shows; however, accurate interpretation of this variation must be done in terms of both electron temperature and ion densities. For the studies described in connection with FIGS. 1 and 2 the simple twowire arrangement previously described is best. This is because the entrance slit of the spectrograph may be arranged so that its axis is parallel to the two wires and the light can then be sampled from a thin plane-shaped region which is perpendicular to the plane of the two wires.

It should be realized that conditions may vary somewhat from those heretofore described when it independent of desired to produce laser" or coherent light. Experiments, which have been performed but not yet published in any technical paper or other news media, establish that a device such as shown in FIG. 8 may be used to produce very intense beams of coherent light on a continuous basis. In such an arrangement it is desirable to use a plurality of electrodes in order to increase the current capacity and hence the power output of the laser. While it is true that the intensity for a given current is independent of the number of electrodes, it is also true that the magnitude of the current which can be drawn is limited by the heat which the anode can dissipate and that increasing the number of anodes will thus increase the total current capacity of the device.

It has also been shown that this laser light may be easily modulated in intensity at frequencies suitable for communication and that laser action may be produced at exceptionally short wavelengths, such as 1,2 l 6-A hydrogen alpha, or at hellum 584-A wavelengths appropriately shifted by Doppler, Stark, isotope, or other action so as to effectively create an inverted population for one or more particular energies of systems.

Furthermore, it may be noted that the modification of control features such as by the addition of high magnetic fields, may be used to increase the intensity of the light by any of the above-mentioned actions.

PLASMA DENSITY AND TEMPERATURE Work done on the decaying plasma of the B-1 stellarator at Princeton by Hinnov and ll'lirschberg has established a convenient and reliable spectroscopic method for determining electron density and temperature. This procedure was used by Robben, Kunkel, and Talbot to determine the ion densities and temperatures in a plasma-jet wind tunnel. This procedure depends on electron-collision-induced transitions being dominant over purely radiative transitions. When this condition is met a kind of thermal equilibrium between electrons and the bound states near the ionization limit is established, providing the difference in energy between and two neighboring bound states be small compared to kT. Then the densities of these bound states, their energies with respect to the ionization potential, and the electron temperature will be related by the Boltzmann equation.

AEn,m/kT-=In(Nn/Nm (2) AE,,,,,, is the difference in energy between states n and m and ln(N,,/N is the logarithm of the ratio of the densities of the two states.

The densities of these states were determined by measuring the absolute intensities of their spectral lines and dividing these intensities by the corresponding transition probabilities. The transition probabilities were calculated from the same oscillator strengths as used by Robben et al. These are based on theoretical calculations by Bates and Damgaard (See D. R. Bates and A. Damgaard, Phil. Trans. Roy. Soc. (London) A242, 101 (1949)), and (E Trefftz, A. Schluter K. Dettmar, and K. Jorgens, Z. Astrophysc. 44, l( 1957)) by both the Coulomb expansion and variational techniques, which agree quite closely.

The absolute of lines up to principal quantum number, n=9, were measured Unfortunately, the spectrograph used was primarily a vacuum ultraviolet instrument and absolute values of line intensities of lines originating from states higher than n=9 did not prove to be reliable; nor could the point where the lines merged into the continuum be observed, which would have provided a check on calculated electron densities via the Inglis-Teller equation (See D. R. Inglis and E. Teller, Astrophys. .1. 90,439( l939)). Fortunately, the electron temperature was high enough so that the Boltzmann relation held down to states with n=5. The plot of the logarithm of the state density divided by state multiplicity with respect to state energy is shown in FIG. 7 The slope corresponds to a plasma temperature of l,660 K. near the cathode. The extrapolation of the line to E= gives the logarithm of (N,./g,.)e lkT. This is related to electron density by the Saha equation:

where N N and N n are the number densities of the electrons, ions, and bound states, respectively, and the gs are their multiplicities; --E is the energy of the bound state;

g,g,(21rmk/h) =l 2X10 if Tis to be in electron volts.

Since the space-charge limited current of the device is only a few milliamperes in the absence of positive charges, the charge density of the electrons must be essentially balanced by the charge density of the He ions for currents of amperes to flow. Therefore, setting N,=N, is justified and The average ion density near the cathode is then determined to be 8.4000 ions/cm. at 30-V anode voltage and 0.6-A anode current. The temperature near the cathode is l,660 K., and the average intensity of 584-A light is 4X10 photon/sec/cmn The ion density near the anode is 5X10 "cm. at a temperature of z I ,900" K. The intensity of 584-A light from the anode region is =10 photons/sec/cmfi- In order to properly interpret the observed intensity of 584-A radiation, it is necessary to determine the extent to which it is resonance imprisoned. Trapped radiation will be scattered out of the acceptance cone of the spectrograph and will not be seen by it. At BOO-p. helium pressure and room temperature the mean free path of 584-A radiation at the resonance peak is 0.0016 cm. Certainly, then much of it is trapped.

At these ion densities, pressures, and temperatures, the principal broadening mechanism is the Doppler effect due to temperature. Since the helium 2' P atoms which have been formed as a result of ionic recombination will be considerably hotter than the neutral helium atoms, their emitted light will be Doppler shifted further and will be expected to travel further through the surrounding helium gas, which is at room temperature. These excited helium atoms will usually have un dergone only electronic collisions between the time they are formed from the ions and the time they emit 584-A radiation. They will therefore retain the temperature associated with the ions.

The 584-A photons must travel through 2.5 cm. of helium at 300 ,u to reach the differential pumping slit, at which point they will not be significantly further scattered in the spectrograph, and their intensity is measured. If one introduces into the light path an additional amount of helium at 300 u as an absorber, the 584A radiation will be further attenuated. The extent to which it is attenuated will depend on the temperature of the emitting and absorbing atoms, and on the distance which the light has already traveled through the absorbing helium. The exact relation for the intensity reaching a given point in the absorber, with the assumption that any scattered photons are lost, is

IA is the intensity A cm. from the emitter, v the velocity of emitting atom toward or away from observer, 1}, the temperature of the emitting atom, 11, the temperature of absorbing atoms, M the mass of a helium atom, and P equals 600 cm. the absorption coefficient of 300 K. helium at 300- pressure. Thus, it is improper to speak of a mean free path when the absorber is helium, since the absorption is not an exponential function of A.

Observations on the 584-A radiation from the helium plasma give I, ,,/I ,,=0.70. This is what one would expect for T, =l,700l(. For T -300 KI12 5/I2 5 0.l9- Therefore we conclude that the emitting atoms are at 1 ,700 K.

At T.,=l,700 K. l /T =0.l02. The average ion density near the cathode is z lo /emf, however, it is higher, =='.2..5X1Q lcmf, very near the cathode. At 2.5 10- ions/cm the recombination coefficient is ==2.5Xl0-' cmfi/sec, taking the value of Bates et al. (See G. L. Natason, Zh. Tekhn. Fig. 29, 1373 (1959) English trans.: Soviet Phys-Tech. Phys., 4,1263 (1960)) which are one-half as large as those of Hinnov and Hirschberg. If one assumed that all of the recombined atoms eventually emit 584-A radiation, due to electronic mixing between the singlet and triplet states, one would then expect to observe This agrees very well with the observed light intensity and the ion densities calculated above for these regions. On the basis of all of these considerations it is concluded that ionic recombination by reaction (1) is the mechanism of light emission.

However, a further additional point to consider is the extent to which light produced by 2' P helium atoms excited directly from the ground state would be observed. This cross section for electrons (See D. R. Bates, Atomic & Molecular Process (Academic Press, N.Y., 1962, page 262) and O. Thieme, Z. Physik. 8,412 (1932)) of energies less than 30 volt is less than 1X10 cm. Therefore we expect, at 0.6amperes and 300- pressure, that less than 3.6)( 2P excitations would occur in lsecond. Since I /Lofor T =300 K. is less than 0001, one would observe less than 3.6110 584-A photons per second from this mechanism.

Another point is tat the peak in the light intensity is near the cathode. One would not ordinarily think that electrons would achieve the minimum of minimum of 21.5 e.v. so near the cathode when total anode voltage can be as low as 22 volts. However, ions will be concentrated near the cathode and will recombine more rapidly there.

SOURCE MECHANISM Origin of the ions: Since the space-charge-limited current of the device described in the absence of positive charges is of the order of milliamperes, positive ions must be present to neutralize the space charge in order that currents as large as amperes could flow. Because the source can be operated at potentials significantly below the 24.5 volt ionization potential of helium, it is unlikely that ionization directly from the ground state is involved. A two-step process involving excitation to the 2 S metastable state with a maximum cross section of 4X10 cm. at 20.5 e.v., (See G. .l. Schultz & R. E. Fox, Phys. Rev. 106, 1179 (1957)) and subsequent ionization by a second electron with a cross section estimated to be 10 cm. (by comparing HeZ S to lithium (See H. Funk, Ann. Physik 396, 149 (1930)) is possible. Another possibility is the formation of the triplet state followed by triplet-triplet annihilation to produce an ion, electron, and atom in the ground state. The triplet-triplet annihilation cross sections are very large (=10" cm?) (See A. V. Phelphs & J. P. Molnar, Phys. Rev. 89, 1202 (1953)).

The source of Lyman alpha radiation is charge or energy transfer from ions or metastable atoms to impurities containing hydrogen. Thus, the maximum observed Lyman alpha intensity of =10 photons per second is of interest in indicating the total possible rate of production of ions and metastable atoms. However, this Lyman alpha intensity was observedby injecting H gas into the system. This caused the He584-A line to almost disappear, so that the source mechanism may have been modified somewhat.

OTHER GENERAL APPLICATIONS By a reversal of the technique used for measuring plasma temperature and density the system becomes a convenient and precise laboratory instrument for the measurement of oscillator strengths and dipole moments in excited states of more complicated atoms such as the heavier noble gases.

Operation at higher pressures will produce a continuous and intense source of atomic ions such as I-le together with information about their production cross sections and recombination coefficients.

At thls point one ought to emphasize the simplicity of the device. This plasma source can be constructed and operated on the most modest budget. Temperature, density, and physical dimensions of the plasma can be varied over wide ranges. Density is varied by adjusting the anode current. Temperature is determined by the anode voltage and the gas pressure. The device may be made quite large so that a fairly extensive plasma is created. And the operation is continuous, so that measurements on it may be made at a convenient pace and without the necessity of rapid-response instrumentation. All this combined with the fact that plasmas of appreciable densities are not really readily available to most researchers at any cost make this plasma source most attractive for a variety of problems ranging from studies of recombination rates to Stark broadening.

These uses, of course, are in addition to the primarily intended function of the device as a source of ultraviolet light or ionizing radiation. In this function, particularly where supplied with hydrogen for emission of the hydrogen Lyman alpha line or helium three, or other system modification such as high recombination rate and Stark broadening by sudden applications of large magnetic fields, as noted above, the device is also suitable for use as a laser.

While preferred embodiments of the method and apparatus have been described in detail above, together with specific characteristics and parameters thereof, and while a discussion of the underlying mechanism of operation of the devices in accordance with our present best understanding has been presented, it is to be understood that this has been done only by way of illustration and example and is not intended as a limitation on the scope of the invention which is defined by the following claims.

What we claim is;

l. A method of producing intense, monochromatic very short wavelength ultraviolet light at a wavelength of 584-A from a continuous, low-temperature plasma comprising the steps of:

a. confining helium gas at a controlled predetermined reduced pressure of 300 microns;

b. independently controlling the temperature of an electron emitting cathode positioned in said gas to a constant temperature of at least 2,500" K. to produce an abundant supply of low-energy electrons; and

. applying a predetermined controlled low voltage of from 22 volts to volts to an anode positioned in said gas in an electrical circuit relation with said cathode in which the internal resistance of the circuit is positive so as to draw an intermediate mode current of from 0.1 amperes to less than 100 amperes from said cathode without arcing in order to produce in said confined gas a region having a density of metastable atomic states above 10 cm. and to thus produce from them a low temperature, highdensity plasma in said region, said intense monochromatic light at a wavelength of 584-A being emitted as a result of the recombination of ions and electrons in said plasma.

Claims (1)

1. A method of producing intense, monochromatic very short wavelength ultraviolet light at a wavelength of 584-A from a contiNuous, low-temperature plasma comprising the steps of: a. confining helium gas at a controlled predetermined reduced pressure of 300 microns; b. independently controlling the temperature of an electron emitting cathode positioned in said gas to a constant temperature of at least 2,500* K. to produce an abundant supply of low-energy electrons; and c. applying a predetermined controlled low voltage of from 22 volts to 100 volts to an anode positioned in said gas in an electrical circuit relation with said cathode in which the internal resistance of the circuit is positive so as to draw an intermediate mode current of from 0.1 amperes to less than 100 amperes from said cathode without arcing in order to produce in said confined gas a region having a density of metastable atomic states above 1013 cm. 3 and to thus produce from them a low temperature, high-density plasma in said region, said intense monochromatic light at a wavelength of 584-A being emitted as a result of the recombination of ions and electrons in said plasma.

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