Contains research objectives and reports on two research projects.Joint Services Electronics Programs (U. S. Army, U. S. Navy, and U. S. Air Force) under Contract DA 28-043-AMC-02536(E

George, E. Victor

Raff, G. J.

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XV. GASEOUS ELECTRONICSAcademic and Research StaffProf. E V. George Prof. S. C. Brown J. J. McCarthyProf. G. Bekefi W. J. MulliganGraduate StudentsJ. GoldenG. J. RaffRESEARCH OBJECTIVESOur general objectives are concerned with the study of atomic and molecular pro-cesses and with the transport of charged particles in weakly ionized collision-dominatedplasmas. We are devoting our efforts to those problems in gaseous electronics that arerelevant to the physics of gas lasers. These devices have numerous applications.We are expanding our studies of gas lasers to the development of techniques formeasuring gas-discharge parameters such as collision cross sections as a functionof energy, by using the lasing mechanism as a probe. In particular, we are inves-tigating the observed time delay between the beginning of the excitation current andthe output laser pulse. It is thought that this delay is a measure of the excitationcross section. These delays have been seen in both ion and He-Ne lasers.We have recently developed a theory for extending the useful range of micro-wave cavities for obtaining electron densities by cavity frequency shifts. This tech-nique is being used in the studies of ion lasers both in the pulsed and cw regimes.With the advent of the high-pressure CO 2 laser our attention has been focusedon studying the plasma formation. At present, the power output of these lasers islimited because the plasma is extremely nonuniform; only a few per cent of the gasmedium is actually excited.We are also looking into problems of striations, sound propagation, and modelocking, which have practical applications in producing better and more stable gaslaser devices.E. V. GeorgeA. TIME -DEPENDENT CHARACTERISTICS OF PULSED ION LASERSThe phenomenon of current -controlled time delays in the laser output of pulsed Argonand Xenon ion lasers has been previously reported.1, 2 Briefly, several particular lasertransitions occur only after delay times of 10 -40 ls from initiation of the excitationcurrent pulse. The value of the delay time depends upon the strength of the pulsecurrent, with shorter delays corresponding to higher currents. This phenomenon isobserved for typical gas pressures in the range 20-100 mTorr and at dischargecurrents from 5-30 A in a 2-mm I. D. bore discharge tube.In order to develop the theory of the time-delay characteristics, the spontaneousThis work was supported by the Joint Services Electronics Programs (U. S. Army,U. S. Navy, and U. S. Air Force) under Contract DA 28-043-AMC-02536(E).QPR No. 100 119(XV. GASEOUS ELECTRONICS)emission of several of the relevant transitions were observed. Figure XV-1 gives apartial energy level diagram for singly ionized Argon. Both transitions 5145 A and3/25/27/29/23576.6 A1/23/25/2 4p 4 D07/24d 4F4880 A 5145 A2 1/2-3/2731 A0S723 AI1/2 ION GROUND STATE3p5 2P0 3/2 ION GROUND STATEFig. XV-1. Partial energy level diagram of Ar II.4880 A, which terminate on the 4s 2P3/2 level, display time-delay behavior. The inten-sities of the spontaneous emissions for the transitions 5145 A, 4880 A, 3576 A, and3520 A were measured as a function of the peak value of the discharge current pulse.The photomultiplier output of the 4880 A spontaneous emission for three values of pulsecurrent is illustrated in Fig. XV-2. In all of the measured Argon transitions, the spon-taneous emission intensity was found to be proportional to the square of the pulse cur-rent.thConsider the rate of population of the j excited level in a singly ionized gas. Therate equation may be expressed3:dN.9 2dt- (-A.+d.n )N. + S .N n + Sijn 2 ,dt- J OJ 0lJwhere N. is the density of excited ions, N is the neutral atom density, and n is theelectron density. The density of singly ionized particles is assumed to be equal to theelectron density. Soj and Sii are the production rate coefficients for the state j by elec-tron collision with neutral atoms and with singly ionized particles, respectively. A. is theJeffective radiative decay rate of the j state, and d. is the destruction rate coefficientof the state by electron collision.QPR No. 100j120(XV. GASEOUS ELECTRONICS)The differential equation has the solutionn N S.+nS- o oj3 -(A.+dn)tN A. + d.n 1- e (1)If the destructive collision rate is negligible compared with the spontaneous emissionrate, A. >> d.n , and the excitation of neutral atoms to the j ion state (one-stepJ ]-excitation process) dominates over the formation of ions from neutrals with subsequentexcitation of the ions (two-step process), the solution reduces to2n2SIj 1  -(Aj +d.n )t(2)N.(t) - 1 - e (2)J A (Note that to the order of approximation which may justify neglecting d.n in thedenominator of the first factor, the destructive rate coefficient cannot be dropped fromthe exponential.If it is further assumed that the electron density is proportional to the dischargecurrent (this seems to be the case based on preliminary results of electron density mea-surements reported in Sec. XV-B), the measured quadratic dependence of sponta-neous emission upon current conforms to Eq. 2.Fig. XV-2. Upper trace: current pulses, 4 A/div.Lower trace: spontaneous emission ofthe 4880 A Ar II line at 30 mTorr pres -sure, 20 mV/div. The order of theemission traces corresponds to theorder of the current traces. Timebase: 10 ý's per major division.Figure XV-2 also shows that there is no delay time between the current pulse andspontaneous emission independent of the amount of current. This was the case for allAr II transitions measured and was a somewhat surprising result. This evidence indi-cates that for the rate coefficients (A.+d.n_) in the exponential of Eq. 2 the d.n term] ]QPR No. 100 121(XV. GASEOUS ELECTRONICS)is negligible compared with A.; this implies a vast dominance of radiative transi-tions over destructive collisions. Previously reported measurements of the 4954 A,XeIII transition, which show laser time-delay characteristics indicate that thespontaneous emission is also subject to current-dependent time delays, analogousto the laser output. 2 Thus the Xenon transition data suggest the possibility of electrondestruction of the upper laser ion level as a significant mechanism in time -delaybehavior. The Argon measurements, however, raise serious doubts concerning theefficacy of destructive collisions in explaining the time-delay characteristics and, per-haps, favor the theory of gradual freeing of trapped ion resonance radiation on accountof ion temperature increases within a current pulse. 4G. J. RaffReferences1. E. V. George and S. F. Fulghum, Jr., "Time-Dependent Threshold Characteristicsof a Pulsed Xenon Laser," Quarterly Progress Report No. 96, Research Labora-tory of Electronics, M. I. T., January 15, 1970, pp. 113-115.2. G. J. Raff, S. F. Fulghum, Jr., and E. V. George, "Time-Dependent ThresholdCharacteristics of Pulsed Nobel Ion Lasers," Quarterly Progress Report No. 98,Research Laboratory of Electronics, M. I. T., July 15, 1970, pp. 71-73.3. T. Cottrell, IEEE J. Quant. Electronics, Vol. QE-4, p. 435, 1968.4. M. B. Klein, Appl. Phys. Letters 17, 29 (1970).B. ELECTRON DENSITY MEASUREMENTS IN PULSED ARGONION LASERSA program of measurements of the electron densities in pulsed ion lasers, by meansof microwave cavity techniques has begun. The density is calculated from observedshifts in the TE 0 1 1 cavity resonance that are due to the presence of the plasma. Theextension of microwave diagnostics to high-density laser plasmas has been reportedpreviously.IThe measurements are being conducted on a laser tube of.42-cm discharge lengthand 2. 0 mm bore diameter. The current is pulsed at a repetition rate of 30 per second,with peak currents of up to 25 A. Figure XV-3 is a block diagram of the exper-imental apparatus. As the microwave generator is slowly swept through the frequencyrange of the expected shift (5470-5500 MHz), its output is gated on for a duration of1 [s at a rate synchronized to the laser pulse rate. A variable time delay -r betweenthe initiation of the laser pulse and the gate pulse allows the microwave sampling of theplasma at any time within the pulse, or in the afterglow region. The pulsed micro-wave cavity output is then fed to a synchronous detector slaved to the pulse repeti-tion rate where the samples are integrated to yield a continuous signal of the cavityQPR No. 100 122(XV. GASEOUS ELECTRONICS)Fig. XV-3. Apparatus used in measurement of the pulsed plasma discharge.output power. This power level is input to the vertical axis of the chart recorderwhile the microwave generator sweep voltage that is proportional to the micro-wave frequency is fed to the horizontal axis. Wavemeters are used as frequency cali-brators. The frequency shift of the cavity resonance for given plasma conditions ismeasured from that of the resonance without the plasma discharge, and the electrondensity is thereby computed.Preliminary density measurements are reported here.1. Density-Current RelationshipFigure XV-4 shows the sampling gate superimposed upon a discharge current pulseof 6 A. The 37-jis duration of the pulse is typical of all discharges, and thegate delay 7 was held fixed at 20 is. Figure XV-5 shows the results of densitymeasurements for Argon as a function of pulse current for 4 values of filling pres-sure. We hope that recent improvements in the experimental techniques willreduce the scatter in data points. It is fairly obvious from Fig. XV-5, how-ever, that the electron density is linear, with pulse current over substantially the totalcurrent range investigated. The measurements indicate considerably higher values ofdensity than would be expected on the basis of extrapolating similar measurementsperformed on Argon ion cw discharges in the same plasma geometry. The presentpulsed data tentatively indicate a density scaling law ofn 70 X 1011Jp (cm-3),where n is electron density, J is the discharge current density in A/cm 2 , and p isQPR No. 100 123Fig. XV-4.30Z 2010Discharge current pulse for 66 mTorr Argonwith gate pulse superimposed. Vertical:2 A/div. Horizontal: 5 ps/div.100 m Torr806040I I I I I I I I I10I (A)Fig. XV-5. Electron density as a function of pulsed dischargecurrent for 40, 60, 80 and 100 mTorr pressuresof Argon.QPR No. 100I I I I I I I I I I Ir-- -- - 19124(XV. GASEOUS ELECTRONICS)pressure in Torr. The proportionality coefficient is approximately a factor of 35 higherthan that obtained from cw measurements.2 Differences in other plasma parametersbetween the two cases, such as field strength, particle temperatures, and wall tem-perature are being investigated in order to explain the scaling defect.2. Density-Time RelationshipThe time behavior of electron density within the discharge pulse has also beenobserved. For this measurement, the discharge current was held constant at 20 A,while the sampling gate delay was varied from 5 ps to 50 is from the turn-on of the pulse. Measurements in times less than 5 [s from turn-on couldnot be accomplished because of the transient ringing of the microwave cavitywithin this period. For a pressure of 66 mTorr of Argon, we found that theelectron density maintained a value of 2. 6 X 10 cm , with less than 7%measurement variation, for the entire measurable duration of the currentpulse.G. J. Raff, E. V. GeorgeReferences1. L. D. Pleasance and E. V. George, "Microwave Diagnostics of High-Density LaserPlasmas," Quarterly Progress Report No. 98, Research Laboratory of Electronics,M.I.T., July 15, 1970, pp. 73-78.2. L. D. Pleasance and E. V. George, "Microwave Density Measurements in IonLaser Plasmas," Twenty-third Annual Gaseous Electronics Conference, Hartford,Connecticut, October 20-23, 1970.QPR No. 100 125

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