724 PROCEEDINGS OF THE IEEE, VOL. 59, NO. 5, MAY 1971 Ion Laser Plasmas
WILLIAM B. BRIDGES, FELLOW, IEEE, ARTHUR N. CHESTER, MEMBER, IEEE, A. STEVENSHALSTED, MEMBER, IEEE, AND JERALDV. PARKER, MEMBER, IEEE
Invited Paper
Abstract-The typical noble gas ion laserplasma consists of a cold cathode), pulsed dc dischargesusing electrodeless high-current-density glow discharge in a noble gas, in thepresence of a magnetic field. Typical CW plasma conditions are current densities magnetic induction coupling, pulsed and CW RF discharges of 100 to 2000 A/cm2, tube diameters of 1 to 10 mm, filling pressures of using induction coupling with electric fieldsboth longitudi- 0.1 to 1.0 torr, and an axial magnetic field of the order of lo00 0. nal and transverse to the long dimension of the plasma, Under these conditions the typical fractional ionization is about 2 percent and the electron temperature between2 and 4 eV. Pulsed ion microwave cyclotron resonance discharges, beam-generated lasers typically use higher current densities and lower operating plasmas, and pinch-generated plasmas (both z- and &pinch pressures. types). In this brief review it would be impossible to cover This paper discusses the properties of ion laser plasmas, in terms of both their external discharge parameters and their internal ion and the detailed characteristics produced byeach excitation excited state densities. The effect these proporties have on laser op- method. Representative examplesmust necessarily be eration is explained. Many interesting plasma effects, which are im- chosen. It is fortunate, however, that the method of produc- portant in ion lasers, are given attention. Among these are discharge nonuniformity near tube constrictions, extremely high ion radial drift ing the plasma seems to have little effect on the laser prop velocities, wall losses intermediate between ambipolar diffusion and erties; CW ion lasers exhibit roughly the same overall free fall, gas pumping effects, and radiation trapping. The current characteristics whether produced by a hot cathode dc dis- status of ion laser technology is briefly reviewed. charge or an RF induction excited system. Pulsed lasersare I. INTRODUCTION likewise relatively independent of the excitation means. In much of the discussion following, the argon ion laser [l I- HE ION LASER is one of the most important prac- [3] will be taken as the example, since the largest body of tical sources of visible and ultraviolet coherent light information exists for this most popular laser type. Tpresently known.In addition, it incorporates a gaseous For a typical ion laser the most important physical plasma exhibiting a number of interesting properties besides processes producing the laser action are as follows. In the laser action. This paper briefly reviews some theof principal glow discharge, approximately 1 percent of the atoms are characteristics of ion laser plasmas. ionized. Electrons with a mean energy of 2 to 4 eV collide Section I summarizes the operating principles and output with ground state ions and produce excited ions, including Characteristics of the most important ion lasers. Section I1 ionic states that constitute the upper and lower laser levels. presents a basic description of the CW argon ion laser The excitation cross sections and radiative lifetimes are such plasma as an example, covering mainly those aspects that that the steady-state population density of the upper laser are reasonably well understood at present. Section I11 levels inthe discharge is much larger than the density ofthe discusses some ofthe important plasma phenomena which lowerlaser levels. This population inversion produces also occur in ion laser plasmas but which are understood optical gain and laser action, typically on radiative transi- only qualitatively. This discussion includesboth technologi- tions between several pairs of upper and lower levels. The cal problems and their underlying physical effects. laser output wavelengths depend on the choice of gas, the operating conditions, or the use ofwavelength-selective A. Characteristics of Ion Lasers laser mirrors. An ion laser plasma generally consists of a long narrow The great practical importance of ion lasers derivesfrom cylindrical highdensity glow discharge. The optical radia- several useful characteristics. The optical gain of the ion tion from the discharge is directed repeatedly through the laser plasma is very high, typically> 1 dB/m (23 percent/m) glow region bymirrors placed at each end of the tube. The [3]-[7] and as high as 13 dB/m in argon [8]; this makes it positive column of the discharge plasma constitutes the veryeasy to securelaser oscillation evenwhen mirrors laser medium and amplifies the radiation. Many different and windows are not of the highest optical quality. In addi- excitation techniques have been used to produce a glow tion, ion lasers produce numerous laser lines that lie in or discharge of the proper characteristics. Among these are near the visible portion of the spectrum; at present, 437 pulsed and CW dc discharges using electrodes (both hot and different wavelengths havebeen observed, originating from 29 different elements, and ranging in wavelength from Manuscript received January 16, 1971. This invited paper is one of a 0.2358 to 1.555 pm [9].Finally, the most important laser series plannedon topics of general interest-The Editor. lines (and more than half of all observed lines) employ W. B. Bridges, A. N. Chester, and J. V. Parker are with the Hughes only noble &es in the discharge tube, and the chemical Research Laboratories, Malibu, Calif. 90265. A. S. Halsted iswith the Electron Dynamics Division, Hughes Aircraft inertness of these gases simplifies many aspects of tube Company, Torrance, Calif. 90509. technology. BRIDGES et al. :ION LASER PLASMAS 125
TABLE I OPERATINGCONDITIONS OF TYPICAL ION LASERS
Tube Tube DiameterLengthDischarge Type of Laser (mm) (cm) Current (A) Gas FillPressure
~ ~ ~ ~~~~ Pulsed, noble 2-10 10-200 100-1OOO 10-50mtorr gas CW, noble gas 1-12 10-50 10-100 50-1000 mtorr CW, He-Cd 3 150 0.040.10 2-10 mtorr Cd plus 2-8 torr He
The operating conditions of the most usefulion lasers are Fig. 1. A high-performancemetal-ceramic argon ion laserdischarge summarized in Table 1. The simplest type of ion laser is tubewith its associated magnet. Metal cooling fins surroundthe ceramic section. (Hughes Electron Dynamics Division.) that operated with a low-dutycycle pulsed discharge, generally with pulses 1-100 ,us in duration and repetition rates up to a few hundredpulses per second. A typical The outputcharacteristics of the most extensivelystudied pulsed tube consists of a long glass, quartz, or ceramic practical ion lasers are summarized in Table 11. The argon capillary with an anode anda cold cathode located in side- ion laser, with blue and green output, has undergone the arms at each end of the tube. An external optical resonator most advanced developmentto date. Thenewest amongthe is usually employed,with Brewster’s angle windows sources listed is the xenon IV laser,’ which demonstrates a mounted on the discharge tube to minimize reflective loss. high-power pulsed mode of operation thatis, so far, unique The laser output generally follows the currentpulse. How- among ion lasers. The high electrical efficiencies (exceeding ever, if the currentdensity and thepulse length become too 0.1 percent) and the availability of multiwatt output at a great, laser action may be quenched by loss of gasdensity great variety of wavelengths (of whichonly a few are listed in the capillary (caused by thermal driveout and gas here) make ion lasers important sources of coherent radia- pumpingeffects) or by radiation trapping of the lower tion in the visible and near UV. laser level, as discussed later. In addition, at long pulse The effect of length and tube diameter on laser output lengths the capillary may be thermally destroyed unless power and efficiency is illustrated in Fig. 2 for the blue- adequately cooled. green lines ofthe argonion laser. In this figure the CW laser The second type of ion laser listed in Table I is the CW output power per unit length of discharge column is plotted noble gas ion laser. The earliest CW ion lasers used a water- as a function of the parameter JR, where J is discharge cooled quartz capillary to permit high average input power current density and R is the inner tube radius. For most to the discharge. Gas pumping caused by electron and ion curves, the externally applied magnetic field and the gas drift motion in the positive columnled to substantial filling pressure have been adjusted to maximize the laser pressure gradients along the tube. To keep the discharge output at each value ofcurrent used. The curves are marked maintained at a convenient voltage it was found desirable to indicate the tube diameter, which ranges from 1.2 to 12 to equalize the pressure at the cathode and anode by con- mm. The abscissa is marked to indicate approximate input necting them with a gas return path, which consisted of a power per unit length of discharge, since the input power glass tube with sacient conductance to permit easy gas can be correlated with the quantity JR in a manner roughly flow but long enough to prevent a discharge from striking independent of tube radius. The lines of constant electrical through it. A photograph of such a tube appears in [lo]. efficiency indicate the ratio of laser output to electrical input The addition of a magnetic field (about 1 kG) with field for the positive column only; thusthe indicated efficiencies lines parallel to the optical axis increases the laser output, of Fig. 2,unlike those of Table 11, do notinclude electrode particularly from tubes of less than 6-mm diameter, and a potential drop, electrode heater power(if needed), or magnetic field is usually used with such tubes. The higher electromagnet power (ifneeded). The great variation that is current densities and extended lifetime currently demanded apparent in the curves of Fig. 2 represents, in part, the of ion lasers have led to the development of highly engi- gradual improvement of ion laser technology, with respect neered metal-ceramic tubes of the type shown in Fig. 1. to both electrical efficiency and power output from a given The third major type of ion laser, typified by the CW length tube. helium-cadmium laser, operates at modestpower levels The highest reported power per unit length is 105 W/m and does not require special cooling. The most popular from a 12-mm tube [12], and the highest efficiency is 0.16 version of this laser [ll] resembles in construction the percent from a 6.35-mm tube [13]. Large diameter tubes familiar helium-neon lasers except for the addition of a sidearm containing cadmium metal. The vapor pressure of The roman numeral following the element name or symbol refersto cadmiumin the discharge is controlled by heating this the ionization state of the species from which the spectroscopic line arises. The numeral is one larger than theionization. Thus “Xe3+”is equivalent sidearm. Both zinc and selenium ion lasers have.been made to “Xe IV” since the&st spectrum of xenon, Xe I, arises from the neutral in this form to date(see tabulation andreferences in [9]). atom. 726 PROCEEDINGS OF THE IEEE, MAY 1971
TABLE I1 ION LASER PERFORMANCE: 1970 STATE OF THE ART
Output PowerperOverall Electrical Lasing Species LasingStrongestMode Wavelengths (pm) Output Power Unit Length Efficiency (A)
Ar II cw 0.4880, 0.5145 0.16'120 W' W/mb 105 Kr I1 cw 0.6471 - 10 W' W/m8 5 -0.01'** Cd I1 cw 0.4416 0.2 we W/me 0.14 0.09' cd I1 cw 0.3250 0.02 W' -0.01'J W/mc 0.014 AI 111 cw 0.3638, 0.3511 0.01'5 WL W/m' 5 Xe IV cw 0.4954-0.5395 0.5 Wh 1 W/mh ~ ~~ ~ a K. Banse, H. Boersch, G. Herziger, G. Schafer, and W. Seelig, 2. Angew. Phvs.. vol. 26, 1969, pp. 195-200. H. Boersch, J. Boscher, D. Hoder, and G. Schiifer, Phys. Left.,vol. 31A, 1970, pp. 188-189. J. R. Fendley, Jr., Proc. 4th DOD Con$ Laser Technology (San Diego, Calif., Jan. 1970), vol. 1, pp. 391-398 (unpublished). W. B. Bridges and A. S. Halsted, Hughes Res. Labs., Malibu, Calif., Tech. Rep. AFAL-TR-67-89, May 1967 (unpublished);DDC accession no. AD-814897. J. P. Goldsborough, Appl. Phys. Lett., vol. 15, 1969, pp. 159-161. W. T. Silfvast, Appl. Phys.Letr., vol. 13, 1968, pp. 169-171. The efficiency of 9.02 percent given in Silfvastis a misprint and should instead be 0.09 percent. W. B. Bridgesand G.N. Mercer, Hughes Res. Labs., Malibu, Calif., Rep. ECOM-0229-F, 1969Oct. (unpublished); DDC accession no. AD-861-927. h- , "CW operation of high ionization statesin a xenon laser,"IEEEJ. Quantum Elwtrom (Corresp.), vol. QE-5,Sept. 1969, pp. 47&477. ' W. W. Simmons, private communication, July 31, 1970. j Pulse efficiency is defined as peak laser output power, divided by the product of peak tube current and initial applied voltage. ' J. R. Fendley, Jr., private communication, Jan. 1970. (- 10-12 mm) have some advantages-they require less or no external magneticfield for efficient operation; their larger wall area eases the thermal load on wall materials; and the larger optical beam size reduces the flux density .that must be withstood by optical components such as 50 windows and mirrors. However, many applications require that the laser operate in a single transverse optical mode, E \ 3 so that the laser output is completely phasecoherent across I the beam front. This is very dif6cult to achieve in such a I- o z Iarger diameter tube [14] and limits the usefulness of this W J type of ion laser. An additional disadvantage of the larger c_ IO z 3 bore tubes is that the laser gain and output power decrease E a with increasing tube diameter [6], [17] ; thus many lines a5 W will not oscillate at all in a larger diameter tube, and those ab that will oscillate require higher quality windows and mirrors for efficient operation. The relative merits of large and small diameter ion-laser tubes may be summarized as follows.When maximum power per unit length isdesired on the strongest laser transitions (i.e., the Ar 11, Kr 11, and Xe IV lines of Table 0.5 11), the large diameter tubes are an appropriate choice. When minimum beam divergence (from single transverse mode operation), less output power (- 1 W), or other laser transitions are required, a small diameter tube is usually the best choice. When maximum overall electrical efficiency is required, the choice of tube geometry will depend upon I I I Ill,, I I1 0.11 ' IIIJ 5 IO 50 100 500 the wavelengthsdesired and other operational require- JR, A/cm ments. I I I I I I IIII I I I111111 I I I Not surprisingly,the ranges of plasma characteristics that I 5 10 50 100 500 APPROXIMATE INPUT POWER, kW/m have been most fully explored correspond to the operating Fig. 2. Summary of reported power output characteristics of CW Ax I1 conditions of practical devices, as summarized in Tables I ion lasers from the following sources: Fig. 4in [ 161; Fig. 1 in [53]; Fig. and I1 and in Fig. 2. We will proceed now to give a more 13in[17];Fig.5in[54];Figs.10and14in[32];Fig.6in[]3];Fig.1in complete description of what is known about the ion laser [12]; and Table I (4-mm by 18-in tube) in [SI. Indicated electricaleffi- ciency is calculated for positive column only, using axial electric field itself. Our discussion will center mainly on the most highly data from Fig. 32 in [17]; Figs. 4 and 14 in [32]; and eq. (3.1) in [26]. developed ion laser, the blue-green lines of Ar11. BRIDGES et al. :ION LASER PLASMAS 727 (4 (b) (4 (4 Fig. 3. Energy level diagrams for the blue and green transitionsin Ar 11, showing various excitation schemes (from [17]). B. Excitation Mechanisms of Ion hers An additional excitation process wasproposed by Labuda The original model of laser excitation in noble gas ions et al.[19], withionic metastables serving as the intermediate was proposed by Gordon et al. [15], [16], and is shown species rather than the ion ground state, as shown in Fig. schematically in Fig. 3(a). In Gordon’s model, the upper 3(c). Absorption measurements [19] show that these meta- laser level is excitedby collisions betweenan electron and a stable levels are highly populated. The ionic metastables ground state ion (process 2); the ion was presumably pro- are produced either by electron collisionwith the ionic duced by the collision of an electron and a neutral atom in ground state lprocess 1 in Fig. 3(c)] or (more probably) by the ground state (process 1). Radiative cascade from higher cascade from higher lying states (process 2) ; these higher states and de-excitation of the upper laser level byelectrons lying states exhibit populations N,a J2,as shown by spon- are neglected. The lower laser level is rapidly depopulated taneous emission data. However, because the metastables by radiation (in the vacuum W at ).x740 A for the 4s are primarily destroyed by electron collisions rather than by states of Ar 11). This depopulation is assumedto occur much spontaneous emission, it can be shown that the metastable more rapidly than any collisional electron excitation (dotted population N, will be proportional to J rather than J2 (see arrow). Finally, one assumes approximate charge neutrality [16] or [17]). The upper laser level is then populated by yet in the plasma and current-independent electron tempera- another electron collision (process 3), and the relation N, ture. Then the upper laser level population N, turns out to cc J2 obtains as before. vary as Our understanding of the upper level excitation processes remained essentially as described above until the measure- N, a npi w n,‘ a J2 (1) ments made by Rudko and Tang [20]. They found that a where ne, ni,and J are the electron density, ion density, and significant fraction of the upper laser level population was discharge current density,respectively. Such a quadratic created by radiative cascade from higher lying states [see current dependence has been observed inspontaneous emis- Fig. 3(d)]. By summing the spontaneous emission intensi- sion measurements over a wide range of gas pressures,cur- ties of all spectral lines terminating on a given upper laser rents, and tube dimensions typical of CW ion lasers (see, level (for example,4p 2D&2,the upper level of the 0.4880-pm for example, [17] and [18]). However, in pulsed operation, transition), and then comparing this with the sum of in- radiation trapping of the lower level population can upset tensities of those lines originating from that level we may the simple current dependence of (1). (See Section 111-C for determine the relative pumping rate due to cascade from a discussion of this effect.) the higher lying states. In a 1-mm-diameter tube operated An alternate excitation method involving a single electron without a magneticfield, Rudko and Tang found that collision was proposed by Bennett et al. [3] and is shownin approximately 50 percent of the 4p ,D!&, level population is Fig. 3(b). This excitation process apparently occurs prin- created by cascade. Similar measurements were made by cipally in low-pressure discharges excited short by pulses. A Bridges and Halsted [17], who obtained a value of 23 per- discharge with very highE/p is required to produce an elec- cent for the cascade contribution to this same level in a 3- tron temperature sufficiently high to achieve a significant mm-diameter tube operated without a magnetic field and single step excitation rate. This mode of operation is also 22 percent for the cascade contribution to the 4p 4D!&, characterized by a unique distribution of spectral output. (0.5145-pm) upper level.Since the populations N, from For example, under these conditions, 0.4765 pm is the which the cascade takes place also exhibit a quadratic de- strongest laser line in Ar 11. Such a spectral distribution is pendence on J, the same quadratic dependence N2 a J2 re- neverobserved in CW operation, where the 0.4880-pm sults again. Note that the upper laser levelpopulation due to and 0.5145-pm lines alwaysdominate. Moreover, the single cascade cannot be separated from that due totwo-electron- step excitation process (assuming constant electron tem- collision processes by its current dependence alone. It may perature) would lead to a spontaneous emission rate from be seen from the foregoing discussionthat the characteristics the upper laser level which varies linearly with discharge of ion lasers are a direct consequence of the collision and current, which isnot observed in CW operation. radiation processes occurring within the ion laser plasma. 728 PROCEEDINGS OF THE IEEE, MAY 1971 11. FUNDAMENTALS 11. OF THE IONLASER PLASMA TABLE I11 In this section we will present a brief physicaldescription SELECTED EXAMPLEOF AN ION LASERPLASMA of the basicion laser plasma, excludingspectroscopic Laser species argon properties. Many interesting problems suchas gas pumping, Discharge tube diameter 2mm throat erosion, etc., which are associated with ion lasers Discharge current 5A External pressure 600 mtorr will be discussed later in Section 111. Although such prob- Electric field 6.3 V/cm lems are all connected with the plasmaproperties, they are Current density 160 A/cm2 also dependent on the overall geometry; however, in this Power dissipation 31 W/cm Electron temperature (double probe)1.9 eV (22 000°K) section we will treat the local discharge properties in the Ion temperature (Doppler width) 0.17 eV (2000°K) laser bore without regard to geometric properties other than Gas temperature (Doppler width) 0.15 eV (1 700°K) tube radius. On this basis the ion laser discharge can be Electron density (stark broadening) 3.4 x 1013 m-3 well characterized as given by the following independent Neutral density (X-ray absorption) 5.8 x 1015 m-3 Fractional ionization (calculated) 0.64 x parameters. Ion drift velocity (DoDDler shift) 1.2 X 104 ~m/s Electrondrift v;l&G(calculat&) 2.9 x IO7 cm)s R Tube radius, cm. Plasma frequency 52 GHz J Discharge current density, A/cm2. Debye shielding length 1.8 x 10-3 mm p Externally measured gas pressure, torr. Ion-atom mean free path 0.2 mm B Axial magnetic field, G. Electron-atom mean free path 0.8 mm-12 mm(function of energy) Ignoring spectroscopic quantities, such as level populations, the only macroscopic dependent property in this descrip ternal pressure which yieldsmaximum laser output. Several tion is the axial electric fieldE, and various derived quan- observations concerning the properties tabulated in Table tities (e.g., power input/length=IE). In addition, there are I11 are in order. many microscopic properties such as species number den- 1) The Debye length is -0.001 times the tube diameter, sity and temperature. whichimplies that charge neutrality isclosely obeyed The remainder of this section is organized as follows. except in thesheath region near the wall. First, we will examine a particular ion laser discharge in 2) The discharge, which is often referred to as an arc, is some detail to establish the nature of the plasma under more closely related to a glow discharge despite the high discussion. Second, we will review the experimental results power input. The large difference between gasand electron obtained when the external conditions are changed (e.g., temperature illustrates this very clearly. varying J or R), including a brief discussionof the theoreti- 3) The plasma frequency is in a difficult measurement cal approaches that have beentaken in attempting to explain region. It is too high for RF or microwave measurements these observations. and toolow for optical measurements. As a result, electron The particular example we will consideris a 2-mm- densities have been primarily measured by Stark broaden- diameter smooth quartz capillary tube with a continuous dc ing of the H, line using smallamounts of added hydrogen. discharge current of 5 A. This is not typical of present-day 4) The plasma is dominated by boundary phenomena. ion lasers, which commonly operate at currents of 20-30 A Electron and ion mean free paths arecomparable to the tube or more in bores constructed of smooth wall ceramics, diameter. Ion loss isentirely to thewalls and the mechanism refractory metal disks, or graphite segments.However, controlling ion motion is intermediate between collision- there are several justifications for using this particular less free-fallmotion and ambipolardiffusion. example. Most important is the availability ofa reasonable The combination of 1) boundary effects and 2) the lack of body of data taken by R. C. Miller and coworkers at Bell simplifying assumptions concerning particle motion is a Telephone Laboratories [21]. The low current chosen for fundamental cause of the present inadequate theoretical thesemeasurements was dictated by the useof quartz understanding of ion laser plasmas. We will return to this capillaries, which fail rather quickly at high currents. Also, point after concluding our discussion of the experimental although 5 A is not a state-of-the-art ion laser discharge, data. it is well overthreshold for laser action, so that the plasma Having establishedthe nature of the ion laser plasma,we conditions are not atypical of ion laser discharges. Finally, can now proceed to a discussion of the effect of varying comparisons among many ion laser devices, employing a discharge diameter, gas pressure, current density, and mag- wide variety of construction techniques ranging from solid netic field. The amountof experimentalinformation on CW dielectric wall capillariesto stacks of thin metal disks, have ion laser plasmas is very small. Measurements by Kitaeva, always demonstrated essentially identical laser and electri- Osipov, and Sobelev (KOS [22]) at the Physics Institute of cal performance for the same diameter discharge. the USSR Academy of Sciences, and by Miller, Labuda, Although the available measurements limit our discus- and Webb (MLW [21]) at Bell Telephone Laboratories, sion primarily to the argon plasma, similar plasma prop provide the bulk of the available information. The data of erties are to be expected in krypton, xenon, and other ion KOS are based on spectroscopic measurements of longi- laser species. tudinal and transverse linewidths for various ion and neu- Table I11 summarizes the information known about the tral lines of argon. Two different bore diameters (2.8 and 2-mm-diameter 5-A argon discharge operating at an ex- 1.6 mm) were used, and measurements were made over a BRIDGES et al. :ION LASER PLASMAS 729 TABLE IV TABLE V ION Ah?) ATOMTEMPERATURES AS A FUNCTION OF CURRENT DENSITY ELECTRON TEMPERATUREAND DENSITY AS A FUNCTION OF CURRENTDENSITY AND FRESSURE Ti (“K) T. (OK) J (Aim2) 1.6 mmp 2 mmb 2.8 mm’ 1.6 mm’ 2 mmb 2.8 mm’ p=1.0 p=0.62 p=0.37 p=0.21 1601800 2550 3900°K 1900 1250 J(A/cm2) T toe torf to+ torf 200 2600 200 4400°K1400 1700 2300 300 3800 300 33005600°K 1600 1800 50 27 oo(r 6.3 400 3450 2350 000’ 23100 4.3 500 4800 3000 4800 500 150 37ooob 5.7 2.4 1.8 1 .o 200 50 ooob 6.9 3.3 2.3 1.4 a V. F. Kitaeva, Yu. I. Osipov, and N. N. Sobolev, “Spectroscopic 250 60 ooob 8.9 4.1 2.8 1.6 studies of gas discharges used for argon ion lasers,” ZEEE J. Quantum 300 70ooob 4.6 3.1 1.8 Electron., vol. QE-2, Sept. 1966, pp. 635-637. 350 82 ooob 4.8 3.2 1.9 R. C. Miller and C. E. Webb, Bell Telephone Labs., Murray Hill, N. J., private communication. * R. C. Miller and C. E. Webb, Bell Telephone Labs., Murray Hill, V. F. Kitaeva, Yu. I. Osipov, P. L. Rubin, and N. N. Sobolev, “On N. J., private communication.(Te measured bydouble probe; Ne measured the inversion mechanism in the CW argon-ion laser,” ZEEE J. Quantum by stark broadening. Tube diameter 2 mm.) See also eq. (3.2) in [26]. Electron., vol. QE-5, Feb. 1969, pp. 72-77. V. F. Kitaeva, Yu. I. Osipov, and N. N. Sobolev, Zh. Eksp. Teor. Fir.. vol. 4, 1966, p. 213. (T. deduced from T,; Ne calculated fromconduc- tivity formula derivedby V. N. Kolesnikov, doctoral dissertation, Physics range of current densities from 100 to 500 A/cm’ and gas Inst., USSR Academy of Sciences, 1962. pressures from 100 to 700 mtorr. Measurements by MLW have utilized a variety of techniques, including spectro- ture for Jc 100 A/cm’. Indirect measurements of electron scopic measurements, double probes, and X-ray absorption. temperature by KOS arebased on the theory of Kagan and However, the range of current density investigated by MLW Perel’ [28] which predicts the relation is somewhat restricted, so that intercomparison of data be- tween the two investigators is difficult. We willnot discuss q = 0.56 T, + 0.13 T, (3) pulsed ion laser plasmas as a separate category, even between the effective transverse temperature q,the neutral though some pulsed plasma measurements have been at- gas temperature T,, and the electron temperature T,. They tempted [23]-[25], since sufficientdata arenot yet available. find an increase of T, proportional to J up to the highest Ion and Atom Temperatures: Ion and atom temperatures current density measured, 350 A/cm*. They speculate that are deduced directly from Doppler linewidths and as a this linear dependence will continue to 550 A/cm’ where result are rather reliable. The data in Table IV, plotted the temperature would be N 11 eV. Although the ranges of against current density, are taken from bothKOS and measurement do not overlap, the result of these two investi- MLW. The consistency is relatively good,and Chester [26] gations do not appear compatible. Further work on this has suggestedthe empirical formula subject is clearly needed. It is clear that the electron temperature decreases with TJ300 = 1 0.9 X lo-’ JR”’ (2) + increasing pressure. The most rapid variation occurs in the for the atom temperature. Since data areavailable for tubes range 0.1 Neutral Atom Density: The neutral atom density is very E ;3.0 difficult to measure in small capillary discharges. Absorp- n tion of X-rays passingalong the length of the capillary was 5 2.5 employed by MLW to deduce total atom+ ion densities, av) 82.0 but the results were reliable only at pressures greater than z F those employed in laser discharges. They observed a de- Z 1.5 0 crease in neutral atom density with increasing atom tem- a v) perature as would be expected from simple gas law con- u. 1.0 0 siderations. However, the measured densities were con- k 0.5 sistently 20-40 percent higher than those calculated from 9 I I the gas lawrelation 501 I I I ' I I I I I 0 02 0.60.4 0.8 1.0 1.41.2 1.6 1.8 2.0 2.42.2 MAGNETICFIELD, kG Fig. 4. Variation of the intensity of the spontaneous endlight with axial magnetic field fora number of Ar I1 ion laser line and neutral ArI lines This is not surprising considering the strong dynamic (from [17]). forces acting on the gas (see the discussion on gas pumping which follows) and the small conductance of the discharge laser output versus current which yield the same functional tubes. Miller [31] has suggested that the theory of thermal dependence with a magneticfield as without, except at transpiration might be more appropriate to this situation proportionately lower discharge current. The effect of the since the atomic mean free path is comparable to the tube magneticfield, however, is somewhat more complicated radius for the smaller bore lasers. The gas law relation has than this, as illustrated in Fig.4. The spontaneous emission not been tested experimentally inlarger bore lasers where it at constant current from ion lines is observed to increase is most likely valid. Nonetheless, is it used by most of those 2.5 to 3 times with increasing magnetic field, while spon- who advance theoretical explanations of ion laser plasmas. taneous emission from neutral atom lines is observed to Both KOS and Herziger and Seelig [27] use it to obtain the decrease. The neutral level populations are saturated and neutral number density needed in the electric conductivity their intensity is a function of electron temperature and not calculation, and Lin and Chen [30] have used a more com- electron density (cf. [33]). The observed decrease inneutral plete form in which the external pressure is set equal to the line emission implies a decreasing electron temperature. total pressure of atoms, ions, and electrons. The ion level populations are therefore determined by the Electric Field Gradient: The axial electric field measured interaction of ion density and electron temperature. The under condition of optimum' laser action (pRzO.1 torr decrease in electron temperature is also borne out by an ob- cm) [32] is observed to obey the relation [17], [26], [32] served decrease in the axial electric field strength with in- creasing magnetic field [ 171. E3= 6.5 V (5) There is an optimum value of magnetic field which de- for 0.5 A. Gas Pumping in Zon Lasers I I I I I I One of the interesting and practically important char- 0.4 i acteristics of the high-current-density low-gas-pressure ion laser discharge is the rapid pumping of gas from one end of the tube to the other. In the first CW ion lasers it was found that after tens of seconds ofoperation the filling gas would be pumped from the cathode and bore to the anode to such a degree that laser action would cease and the dis- charge wouldextinguish. A convenient solution to this problem is to provide some form of gas return path external to the discharge which connects the anode andcathode ends and allows an external equalizing flow of neutral gas to occur [lo]. A gas return path having approximately the same diameter as the laser bore, but somewhat greater length to prevent electrical breakdown, is found to be adequate to achieve maximum laser output and adequate t discharge stability. -0.1 I I I 1 1 I 0 5 10 15 20 25 30 Even with a high-conductance external gas return path, DlSCrtARGE CURRENT,A very large anode to cathode pressure differentials may still Fig. 5. Anode-to-cathode pressuredifferential versus discharge current occur. Fig. 5 shows the measured anode to cathode pressure in a typical argon ion laser discharge for different valuesof gas Ming differential Ap as a function of discharge current for a pressure (from [ 171). number of different values of gas filling pressure. These data were taken on a quartz tube filled withargon, having a pumping. The characteristics measured at different posi- 3-mm-diameter by 46-cm-long bore [17]. At 25 A and the tions along the bore can exhibit markedly different de- optimum gas fill of 0.25torr andmagnetic field of 1.05kG, pendences because of gas pumping effects. The practical this tube produced a laser output of 3 W. The notation importance ofgas pumping effectsin ion lasers has C, = CEindicates that the calculated gas conductance of the prompted new theoretical analysis which has extended the external gas return pathwas equal to that of the bore when early treatments of Langmuir [37] and Druyvesteyn [38]. the gas in both was at room temperature. Similar pressure Halsted [ 171 and, in a more thorough analysis, Chester differentials and current dependence were measured in [26], [39] have analyzed gas pumping in the gas pressure tubes of different diameters, and filledwith krypton or range of interest in ion lasers, including in their analyses xenon [ 171. provisions for an external gas return path. The pressure differential Ap is observed to be strongly A general explanation of the cause of gaspumping can be dependent on the gas pressure and discharge current. At 3 given qualitatively as follows.While ions and electrons torr, the pressure differential increases linearly up toabout within the positive column gain equal but opposite mo- 5 A, and then saturates. All previous experimental studies mentum from the axial electric field, greater momentum is of gas pumping reported in the literature [34]-[36] were delivered by the electrons to the neutral gas than by the made at low discharge currents (< 1 A). Accordingly, only ions, or equivalently, greater momentum is transferred to the initial linear increase of Ap with current predicted by the walls by the ions than by the electrons. This difference theory was observed. can be analyzed by consideration of the differing longi- At the low gas pressuresthat areoptimum for laser action tudinal and axial drift rates, mean freepaths, and recombi- (0.16 to 0.25 torr in a 3-mm tube), Ap first increasesand then nation rates [40] ofthe two species. A detailed discussion of decreases sharply with increasing current, and actually can these effects is contained in [39]. Suffice it to say here that becomenegative as shownin Fig. 5. Previous to these recent theory gives an accurate explanation of the pressure measurements on ion lasers, no such behavior had been differences experimentally observed inion lasers. reported. The magnitude of Ap should be noted; in a typical A basic appreciation of the factors iduencing gas pump laser, Ap across the bore can be as great or greater than the ingin ion lasersmay be obtained by considering the initial fill pressure, depending on the discharge current, schematic representation of the laser bore and external pressure, and size ofthe gas volumes attached at anode and return path shown in Fig. 6. Acting within the bore of the cathode ends. tube we have two pumping sources Qpumpand Qion. From It is extremely important that anyone performing mea- these two sources a pressure differential Ap develops which surements on ion lasers appreciate the strong dependence causes a gas flow through the vacuum conductances of the of pumping rate and Ap on discharge current, magnetic external gas return tube CE and the bore C,. Physically, field, and filling pressure. Often measurements of line in- Qion results from the axialdrift of ions in the positive tensities, electron temperature, plasma density, etc., as a column. Sincethese ions become neutral atomsat the function of Z, B, or p are taken at a fixed position along the cathode, this ion flow is equivalent to an equal amount of bore. Obviously such measurements willbe strongly af- gas flow. The source Qpump,as referred to above, is less ob- fected by changes in the neutral gas density caused by gas vious physically and results from the net transfer of mo- 732 PROCEEDINGS OF THE IEEE, MAY 1971 BORE flow in such tubes if adequate off-axis flow is to be pro- CATHODE vided. ANODE Ion laser tubes of advanced design (cf. Fig. 1) now use off-axis holes withina solid conduction-cooled beryllia bore to provide a gas return path. Thegas in these off-axispaths is at the wall temperature of approximately 100°C, and hence provision for adequate gas return path conductance is easily made. B. Conditions at Laser Bore Constrictions Physical damage to the inside of the walls confining the PCATHODE PANWE discharge, primarily in the vicinityof the cathode-end throat orbore constriction, often limits the maximum laser EXTERNAL GAS RETURN PATH input power. In glass or quartz tubes, discoloration and Fig. 6. Schematic representation of the laser discharge tube showing the erosion are observed inthis region and in metal segment or forces acting on the gas, and the bore and external gas return path disk-bore tubes, severe heating and sputtering damage is conductances. noted. Quantitative data showing an example of localized power dissipation are presented in Fig. 7. The bore of the mentum to the gas by electrons, and thus is always anode tube shown was constructed using the common ion laser directed. Note that if Qpumpjust equals Qion, Ap across the technique of electricallyisolated radiation-cooled bore seg- tube will be zero. Under these conditions, there must still be ments, supported in a quartz vacuum envelope, to confine a flow of neutral gas toward the anode equal to Qion. This the discharge. Information on conditions in the throat may flow is driven by net electron collisions with neutrals even be deduced from measurements of the temperature of the in the absence of a neutral pressure gradient. When Qpump radiation-cooled segments. The bore segments inFig. 7 are is greater than Qion,the anode pressure will increaseuntil Ap drawn to scale so that the temperature reading of a par- is sufficient to cause an anode-to-cathode flow through the ticular segment is plotted directly below the segment. Seg- bore and external return path equal to Qpump - Qion. ment 8, the last tapered segment in the throat, operated at As reflected inthe schematic diagram, Ap is found to vary the highest temperature. At 1200"C, this segment radiates inversely with the quantity (C,+ C,). This relationship is (and therefore absorbs from the discharge) more than verifiedexperimentally [17], provided one accounts for double this power of a segment at 1050°C. From such the strong dependence of the gas conductance of the bore measurements as these, one may conclude that localized on the temperature of the neutral gas. The high temperature power dissipation is more severe the more abrupt the throat of the gas in the bore (typically > 1ooo"C at the higher dis- transition, and that power dissipation is also a strong func- charge currents of practical interest) is the cause for the tion of the position of the throat relative to the converging decrease in Ap with increasing discharge current shown in axial magnetic field (cf. [17]). Fig. 5. This occurs both because of the decrease in gas The conditions of localized heating occur in the throat number density in the bore, which reduces the pumping because of the existence ofa double sheath which forms to force, and because of an increase in the viscosity ofthe gas, equalize the random plasma electron currents passing be- which reduces the effectiveness of the pumping force in tween the large and thesmall diameter regions of the bore. moving gas from cathode to anode. A general model of conditions in the vicinity ofa discharge The strong dependence of gas conductance on neutral constriction has been described by Langmuir [41] and gas temperature has an importantrole in the design oftubes others [42]. To understand why a sheath must form, con- having internal rather than external gas return paths. It sider that the large and small diameter regions at the con- may be shownthat for viscous flow the conductance varies striction are characterized by electron densities and tem- as the 7/4 power ofthe gas temperature. Thus the conduc- peratures ne and T,. The random electron current in each tance of a path containing gas at 1OOO"C is only 1/13 ofits region istherefore proportional to n,T;/'. Since the electron value at room temperature. The trend in compacttube design density and temperature are greater in the small diameter is to try to include the gas return path within the bore struc- region than in the large, the random electron currents must ture to avoid the awkwardness and fragility of an external be greater in the small bore region. Across the constriction, by pass tube such as that described in [lo]. In tubes using the net electron current flow must be equal (ignoring for the radiation-cooled metal or graphite disks [17], [18], designersmoment the directed discharge current), and therefore a often attempt to employ off-axis holes inthe segments as a sheath of potential V, must form at the constriction to gas return path. Because the gaspassing through these reduce the greater outward flow of electrons from the small segments ishot, typically 1000°C, thesepaths are not nearly bore, as shown in Fig. 8. If the directed discharge current is as effective in reducing the pressure differential in the bore accounted for, the effect is to increase the sheath height at as an external path at room temperature, with the result the cathode-end bore constriction and to reduce the sheath that very large cross-sectional areas must be devoted to gas height at the anode end. BRIDGES et al. : ION LASER PLASMAS 133 I I I I I I I I I I I SEGMENT NO. 1 2 3 4 5 6 7 8 9 OII I213 14 15 16 17 :E -THROAT -b. UNIFORM BORE fi...... if ...... -J**: I 2 3 4 5 DISTANCE ALONG AXIS, in Fig. 7. Variations of the temperature along the bore of a radiationcooled segmented graphite bore ion laser, showing the dependenceon the electric field strength in the throat region. Bore segments are drawnto scale at the topof the figure (from [43]). DOUBLE SHEATH I POTENTIAL Fig. 8. Schematic representation of the potential and the electric field distributions in the vicinity of an abrupt discharge constriction. 7 34 PROCEEDINGS OF THE IEEE, MAY 1971 the anode and cathode has beenlargely determined by 140 empirical design.Especially at high discharge current densities, and in CW krypton and xenon ion lasers which 120 s are subject to discharge instabilities, the design of the throat J100 c canvery much affect the maximum current and power 80 output that can be obtained. A fuller discussion will be 0 W found in [17] and [43]. .1=5A n 1=7& P =0.08Torr 40 C. Radiation Trapping _--I x) Another interesting phenomenon that plays a role in typi- 0 cal ion laser plasmas isthe trapping orreabsorption of spon- DISTANCE FROM CATHWE,cm taneously emitted radiation. We mentioned earlier that the lower laser levels (the 4s levels shown inFig. 2, in the case of the Ar I1 laser) are populated by electron collision, by spon- r1 I II taneous cascade from higher levels, and by the laser transi- y 3- I1 I ?E I I-- tion itself. Despite these populating processes, an inversion 1 -L--J------L ------’11 - @.2- I II between upper and lower laser levels can be maintained I TI I aa , ------J ’li - because the lower level has a very rapid radiative decay to g 7; the ion ground state in the vacuum ultraviolet (723 A for 9. il - UNIFORM BORE -4 the Ar I1 4s 2P3i2-+3p5’P3/2 transition). The ratio of A co- I I I I I I 1 13 x) 30 40 50 60 7o efficients for this process to the laser A coefficients istypical- DISTANCE FROM CATTHOOE ,cm ly large : Fig. 9. Wall potential (upper curve) and longitudinal potential gradient (lower curve) as a function of distance along the discharge columnof an aircooled quartz discharge tube (from [17]). We havedirectly measured the variation in wall potential and potential gradient in an aircooled quartz bore laser as calculated by Statz et al. [44,with corrections]. using a combination ofked and movable wall probesand a However, radiation trapping can reduce the effective high-impedance voltmeter. The measured variation in wall decay rate and increase the lowerlevel population. The potential and voltage gradient along the wall is shown in 723-A photons are absorbed by ground state ions before Fig. 9, and the potential on axis is sketched assuming ap- they can escape from the plasma, thus creating new 4s 2P3iz propriate values for electron temperature in the large and excited lower levels. These, in turn, reradiate, and so forth. small diameter regions. The measured variation in potential The net effect is that the lower level is depopulated by a and electric field is in good agreement with the behavior diffusion of radiation to the edge of the plasma rather than predicted by the simple model. The sheath heights at the by direct radiation to the outside world. The theory of this cathode and anodeconstrictions were approximately + 10 mode of radiation transport is well known fromthe classical V and -4 V, respectively, being increased and reduced work of Holstein [45], [46]. It is difficult to apply Holstein’s slightly by the directed discharge current, and, as expected, theory quantitatively because of uncertainties in the ion were independent of current to first order. Because of the laser plasma parameters. Reasonablygood qualitative presence of the double sheath, electrons will be accelerated agreement canbe obtained for some of the more pronounced through the sheath from the cathode region and will raise phenomena, however. the electron energy, ionization rate, and wall sheath height One prominent feature of pulsed ion lasers that results in the throat region. This is believed to account for the from radiation trapping is the occurrence of a “dead,” or increased heat dissipation and sputtering damage in the negative gain interval near the beginning of the light out- throat region. put pulse. This deadregion develops gradually with in- In low-pressure mercuryglow discharges, it has been creasing current, as shown in Fig. 10. The top trace shows claimed that double sheaths of the type described above the 0.488-pm laser output asa function of time for a 16.7-A produce very strong low-frequency oscillations, presumed current pulse. The corresponding current pulse isnot shown to be ion waves [42]. Under certain discharge conditions we in Fig. 10, but it begins at t =0, rises to its peak value by have observed laser modulation at discrete frequencies in t = 10 ps, remains essentially flatto t = 40 ps, and then drops the range of 150 to 220 kHz [43]. This modulation also to zero at t = 55 p. For the small value of peak current used appears as modulation of the spontaneous sidelight emis- in the toptrace, 8 ps of the 10-ps rise time is neededto reach sion. It has been determined that this modulation is strong oscillation threshold. As the peak current is increased to in the cathode and cathode throatregions of the discharge, 25 A, this initial delay is decreased, and a slight dip in the but notin the uniform bore. This suggests that the modula- output develops at about t= 10 ps. With a peak current of tion may be due to waves excited by oscillations excited in 33.3 A (third trace from the top) the dip is more pronounced, the double sheath as reported in [42]. However, this must almost reaching zero output att = 10 ps. At still higher cur- remain as speculation pending further experiments. rents, the outputpulse has separated into two discrete parts The exact choice of shape for the bore constrictions at with a zero output region in between. Transmission mea- BRIDGES et al. :ION LASER PLASMAS 135 50mAl67A 75mA25A where K, and K, involve only atomic constants (Einstein coefficients, excitation cross sections, wavelength, etc.) in- IOOmA33.3A dependent of J. A two-step excitation process has been assumed in (7), giving a J2 dependence of gain. For a cylin- drical discharge configuration, Holstein gives the trapping 125mA41.7A factor as 1.60 v= ‘ koR[.rrIn (k,R)] ‘I2 150mA30A where k, is the absorption coefficient of the trapped line and R is the discharge radius. If we ignore the slowly vary- 175mA50.3A ing logarithmic term and note that k, - nJAvd, where n, is the ion ground state density and Avd is the Doppler line- width of the absorbing vacuum W transition, then 200mA66.7A 0 20 40 60 80 TIME, pa Moreover, if -pJR (cf. [21] quoted 1261) for the Fig. 10. Time variation of the 0.4880-pm Ar I1 output in a small pulsed n, - ne in ion laser (2-mm diameter by 30 cm) as the peak discharge current is plasma region of interest here, then increased. go = KlJ2 1 - ~ surements by a number of workers [6], [23], [47], [48] have ( confirmed that this dead period is actually absorbing, that is, the lower laser level is temporarily more highly popu- where K, absorbs the remaining constants all assumed to be lated than the upper level. relatively insensitiveto p, J, R, and Avd. A similar evolution of a dead period can take place if the Equation (10) now indicates what happens as tkpulse peak discharge current is &xed and the gas pressure is in- current builds up : the gain risesas J’ until the second term creased. In this case, the width of the dead period increases in parentheses becomes signscant. When J is large enough in both directions in time until the entire output pulse is to make this term unity, the gain drops to zero and then be- “swallowed up.” Likewise, at appropriately adjusted cur- comes negative(absorption) on further increase in J. How- rent and pressure, a similar sequence of time variations can ever, the Doppler linewidth also increases as the discharge be seen as a function of radial position in the discharge. heats up, a process that lags the time variation of J to some The output near the discharge wall shows no dead region, extent. When the ion temperature is sufficiently highso that while a progressively wider dead region develops toward the increase in Av, reduces this term below unity, the laser the tube axis. It is possible to adjust current and pressure so gain goes positive and oscillation returns. Increasing either that the laser oscillates onlynear the tube walls, producing the tube pressure or the tube radius makes the term larger ring-like output modes as reported by Cheo and Cooper in magnitude and lengthens the time required. [49], provided the laser cavity will allow such high-order It is also clear from (10) that it is possible to increase transverse modes. p or R sufficiently so that no increase in Avd will suffice, All these characteristics wererecognized early in the and this also has been observed. Klein [23] has measured history of ion lasers; the temporal and spatial variations the time developmentof the vacuum W transition and Avd, were mentioned in the initial paper on the argon ion laser simultaneously with the laser output from a pulsed Ar I1 [1].3The htexplanation of these observations in terms of laser, and is able to conlirm these processes qualitatively. radiation trapping wasgiven by E. I. Gordon (private The argument given has said nothing explicitly about the communication) in June, 1964. This argument has been variation of radiation trapping withmagnetic field. We further confirmed fora pulsed laser by Klein[23], and may would expect fromthe discussion in SectionI1 that the addi- be summarized briefly as follows (see [17] or [23] for more tion of a magnetic field would be equivalent to an increase detailed developments). According to Holstein, the effect in J without an increase in ion temperature. Thus the mag- of radiation trapping may be expressed by modifying the netic field should add to the power output when the J2 total A Coefficientof the radiating level (in this case the variation dominates but detract from the laser output where lower laser level N,) by a trapping factor 7 so that A, is trapping has becomesignificant. Gorog and Spong [50] replaced by yA,. have investigatedthe output power and mode patterns as a The gain coefficientof an ion laser can then be written function of both B and p. Their results are also consistent with (10). Increasing either B or p sufficiently caused the formation of ring modes and a drop in laser power, but The reference to this phenomenon in the text of [l, p. 1301 sounds a littlestrange, however, since the accompanying oscilloscope tracewas over a reasonable range of values, an increase in B could be deleted in the interestof a shorter paper. compensated by a decrease inp or J and vice versa. 136 PROCEEDINGS OF THE IEEE. MAY 1971 Despite the qualitative agreement with this simple pic- [15] R. C. Miller, E. F. Labuda, and E. 1. Gordon, presented at the 1964 ture of radiation trapping, there is clearly muchmore to be Conf. Electron Device Research, Cornel1 Univ., Ithaca, N. Y. [16] E. F. Labuda, E. I. Gordon, and R. C. Miller, “Continuous-duty learned before truly quantitative predictions can be made. argon ion lasers,” IEEE J. Quantum Electron.,vol. QE-1,Sept. 1965, The problem of ionization to higher states has been raised pp. 273279. by Cottrell [5] and these complicate any simple picture. [17] W. B. Bridgesand A. S. Halsted, ”Gaseous ionlaser research,” HughesRes. Labs.,Malibu, Calif., Tech. Rep. AFAL-TR-67-89, Dealing with the problems of highly ionized species will be May 1967 (unpublished); DDCaccession no. AD-814-897. necessary to explain the behavior of the strong Xe IV lines, [18] W. B. Bridges and G. N. Mercer, “Ultraviolet ion lasers,’’ Hughes for example. It is possible that radiation trapping of several Res. Labs., Malibu, Calif., Tech. Rep. ECOM-0229-F, Oct. 1969; DDC accession no. AD-861-927. cascading processes will haveto be treated. [19] E. F. Labuda, C. E. Webb, R. C. Miller, and E. I. Gordon, “A study of capillary discharges in noblegases at high current densities,” Iv. SUMMARY presented at the 1965 18th Gaseous Electronics Cod, Minneapolis, Minn. We have attempted to introduce the reader to the basic [20] R. I. Rudko and C. L. Tang, “Excitation mechanisms in the Ar I1 properties of ion laser plasmas and torelate these properties laser,” Appl. Phys. Lett.,vol. 9, July 1, 1966, pp. 414. to the mechanisms responsible for the principal laser char- [21] R. C. Miller, E. F. Labuda, andC. E. Webb, unpublisheddata 1964- 1967, private communication. acteristics. Necessarily, we have had to omit many topics [22] V. F. Kitaeva, Yu. I. Osipov, and N. N. Sobolev, “Electron tempera- of interest to those engaged directly in ion laser researchand ture in the electric discharge used for the argon ion laser,” JETP development. For example, we have not gone into the Lett., vol. 4, Sept. 15, 1966, pp. 146-148. [23] M. B. Klein, “Radiation trappingprocesses in the pulsed ion laser,” details of discharge tube construction, including the choice Ph.D. dissertation, Univ. of California, Berkeley, Mar. 1969 (un- of materials and configurations. Even though such details published). play important roles in obtaining reliable long-life laser [24] -, “Time-resolved temperature measurements in the pulsed argon ion laser,” Appl. Phys. Lett.,vol. 17, July 1, 1970,pp. 29-32. operation, they do not affect the first-order plasma prop- [25] S. Hattori and T. Goto, “Excitationmechanism and plasmaparame- erties. Further details on a variety of construction tech- ters in pulsed argon-ion lasers,” IEEE J. Quantum Electron., vol. niques and technical properties are contained in [ 16 I- [ 181, QE-5, NOV.1969, pp. 531-538. [26] A. N. Chester, “Experimental measurements of gas pumping in an [43], and [51], among others. For a general review of CW argon discharge,”Phys. 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Mercer, “Gaseous ion laser PROCEEDINGS OF THE IEEE, VOL. 59, NO. 5, MAY 1971 737 research,” Hughes Res.Lab., Malibu, Calif., Tech.Rep. AFAL-TR- mechanism for quenching and ring-shaped beam formation in ion 68-227, July 1968; DDC accession no. AD-841-834. lasers,” Appl. Phys. Lett.,vol. 6, May 1, 1965, pp. 177-178. [44] H. Statz, F. D. Horrigan, and S. H. Koozekanani, “Transitionproba- [50] I. Gorog and F. W. Spong, “High pressure,high magnetic field bilities for some Ar I1 laser lines,“ J. Appl. Phys., vol. 36, July 1965, effectsin continuous argon ion lasers,” Appl. Phys. Lett., vol. 9, pp. 2278-2286. The values for transitions to the ground state are July 1, 1966, pp. 61-63. underestimated by a factor of 5, as pointed out by Rubin; see [52]. [51] K. G. Hernqvist and J. R. Fendley, Jr., ”Construction of long life A correction was publishedby Statz and coworkers inJ. Appl. Phys., argon lasers,” ZEEE J. Quantum Electron.,vol. QE-3, Feb. 1967, pp. vol. 39, July 1968, pp. 40454046. 6672. [45] T. Holstein, “Imprisonment of resonance radiation in gases, Part I,” [52] V. F. Kitaeva, A. N. Odintsov, and N. N. Sobolev, “Continuously Phys. Rec., vol. 72, Dec. 15, 1947, pp. 1212-1233. operating argon ion lasers,” Sou. Phys.-Usp., vol. 12, May-June [46] -, “Imprisonment of resonance radiation in gases, Part 11, Phys. 1970, pp. 699-730. Rev., vol. 83, Sept. 15, 1951, pp. 1159-1168. [53] R. Paananen, “Continuously-operatedultraviolet lasers,” Appl. [47] E. I. Gordon, E. F. Labuda, R. C. Miller, and C. E. Webb, “Excita- Phys. Lett., vol. 9, July 1, 1966, pp. 34-35. tion mechanisms of the argon-ion laser,” in Physics of Quantum [54] K. Banse, H. Boersch, G. Herziger, G. Schaffer, and W.Seelig, Electronics, P. L. Kelly, B. Lax, and P. E. Tannenwald, Eds. New “Hochleistungs-Ionenlaser,”Z. Angew. Phys., vol. 26, Feb. 4, 1969, York: McGraw-Hill, 1966. pp. 195-200. [a] P. 0.Clark, W. B. Bridges, and A. S. Halsted, 1965 (unpublished). [55] I. Gorog and F. W. Spong, “Experimental investigation of multi- [49] P. K. Cheo and H. G.Cooper, “Evidence for radiation trapping as a watt argon lasers,” RCA Rev., vol. 28, Mar. 1967, pp. 38-57. A Surface-Charge Induction Motor SOON DAL CHOI, MEMBER,IEEE, AND DONALD A. DUNN, SENIOR MEMBER,IEEE Abstract-A motor thatuses anelectric field interaction withsur- motorsand generators and dc magnetohydrodynamic face charges induced on the surface of a lossy dielectric rotor as the force mechanism is described. A theory of force generation and generators that require contact to be made between the efficiency for a transverse-magnetic wave interaction witha moving working fluidor rotor andthe stator [6]. conducting medium and a space-harmonic analysis of a practical cir- Historically, interactions between rotating electric fields cuit to carry such a wave are pmsonted. A comparison is made be- tween the results of this analysis and test results on a three-phase and dielectric solids and fluids date back to the work of 60-Hz motor using a Bakelite or a P1exigl.r rotor. Satisfactory agrw- Arno [7]in 1892. Theories of the force on spherical dielectric mont betwwn theory and experiment is obtained in most r.rp.ar. bodies in quadrupole fields were developed by Born [8] Motor operation was obsorvod in both diroctions of rotation, as a rwult of interaction with the first backward space harmonic. Om- and Lertes [9]and experimental devices withthis configura- erator operation is also theoretically possible and the theory for a tion were built by Grossetti [lo] to test the properties of single-harmonic-wave generator is praentod. Generator operation dielectric materials.Pickard [11 ] has made further contribu- was notoburvod, however, in the device that wntested. tions to the theory. A patent on a device usingthis principle I. INTRODUCTION has recently been issuedto Fisher et al. [12].The history of PUMPING effect in slightly conducting liquids due this subject has been reviewed by Melcherand Taylor [13]. toan electrohydrodynamic traveling-wave inter- The basic mechanismby which a force is generated involves A action with induced surface charges on the liquid the creation of surface chargeson the dielectric rotor which interface has been demonstrated by Melcher [I 3. The have a relaxation time determined by the properties of the mechanism by which his pump operated is directly anal- dielectric material. The surface-charge pattern has the same ogous to that of the conventional induction motor [2]and wavelength as the electric field that induces the charges the ac magnetohydrodynamic pumpor generator [3]. and in the steady state is shifted in space phase with respect Thesedevices may be contrasted with iondrag effect to the electric field by a phase angle that is determined by pumps [4]and related devices [5]such as conventional dc the relaxation time. Thus there is an optimum dielectric relaxation time for a given applied frequency. Manuscript received February 24, 1970; revised September 29, 1970. The present study of the surface-charge induction motor This work was supported by the U. S. Army, Navy, and Air Force under followsdirectly from Melcher’s [l ] work. A relativistic Contract 225(83) with Stanford University, Stanford, Calif. analysis of such a device with a finite air gap has been S. D. Choi is with the Jet Propulsion Laboratory, Pasadena, Calif. D. A. Dunn is with the Institute for Plasma Research, Stanford Uni- carried out following previous workby DUM,Wallace, and versity, Stanford, Calif. Choi [14]which assumed an infinitesimal air gap. A space-