Contains reports on two research projects.United States Atomic Energy Commission (Contract AT(30-1)-1842

Domen, J. K.

Hooper, E. B., Jr.

DSpace@MIT

PLASMA DYNAMICSIX. PLASMA PHYSICSProf. S. C. BrownProf. W. P. AllisProf. G. BekefiProf. D. R. WhitehouseDr. G. LampisM. L. AndrewsJ. K. DomenD. L. FlanneryGeorgeGlenn, Jr.Hooper, Jr.JamesonKramerKronquistLlewellyn-JonesMattisonJ. J. McCarthyW. J. MulliganJ. J. Nolan, Jr.L. D. PleasanceG. L. RogoffD. W. SwainF. Y-F. TseR. E. ZelazoA. ELECTRON DENSITY MEASUREMENTS WITH A LASER INTERFEROMETERA laser interferometer to measure small dielectric coefficients has been describedin a previous report. I In the interferometer three mirrors are placed in a straight lineto form two coupled optical cavities, one containing the laser plasma and the other thedielectric that is to be studied. In the present case this is a plasma. The laser itselfthen acts as a detector for the unknown dielectric coefficient. 2The present system uses a compensation technique to study repetitively pulsedplasmas. A calibrated variable dielectric is placed in series with the unknown andadjusted to balance it. Because the plasma is pulsed, a null technique is used in whichthe cavity containing the two dielectrics is compared with the "empty" cavity. In thisFig. IX-1. Schematic diagram of the experiment.This work was supported principally by the United States Atomic Energy Commis-sion (Contract AT(30-1)-1842).QPR No. 78(IX. PLASMA PHYSICS)manner, dielectric coefficients have been measured with uncertainties of approximately-310-3 /LX, where L is the length of the dielectric in centimeters, and X the laser wave-length in microns. For studies of a plasma with the 3. 39 [ line of the helium-neon laserused this corresponds to an electron density of approximately (1013/L) cm - 3 .The output of the laser is detected by a photomultiplier that drives a gated amplifier.The use of the gated amplifier permits a determination of the time dependence of theelectron density of the plasma. In such a manner, plasmas that decay at the exponentialrate of a few microseconds can be studied. As the noise that is present in the interfer-ometer arises primarily from vibrations and from sound in the room, and hence fallsoff rapidly at high frequencies, the sensitivity of the gated instrument is almost that ofthe ungated system.A block diagram of the electronic system is shown in Fig. IX-1. The timing circuitsare essentially the same as those used in the afterglow microwave radiometer, to whichthe reader is referred for details. 3The compensator has been changed from the crystalline quartz previously described.The compensator employing the electro-optic effect in quartz was discarded for tworeasons: very high voltages were required for reasonable phase shifts, and acousticwaves were generated in the quartz by these voltages which led to nonreproducibleresults. The present compensator utilizes an acoustic driver to produce a sound wavetravelling across the beam. The varying density of air then produces a phase shift inthe light.The compensator was calibrated by using the multiple-pass technique of Gerardo andVerdeyden 4 and was measured to be linear to within 2 per cent. We had some difficultywith pick-up of acoustic waves by the laser, but this effect was reduced to acceptedlimits by careful shielding. It was found that if the laser discharge contained striations,they tended to "lock-on" to changes in the laser light intensity. To prevent such stria-tions the laser was operated with both DC current and an RF (30-Mc) voltage applied bymeans of metal strips around the discharge tube.The discharge which is being studied at present with the interferometer is a pulsedP.I.G. discharge. The discharge is shown in Fig. IX-2. The current pulses are ofapproximately 150 amps at 200 volts and of 75-psec length (see Fig. IX-5). The gaspressure is in the range of 0. 010-0.200 torr, and the magnetic field varied from0-800 gauss.The laser intensity as a function of time is shown in Fig. IX-3. Here the slowlyvarying signal is due to the compensator, and the rapidly varying signal to the plasma.The gated amplifier samples this signal twice during each cycle, once during the plasmapulse, and once at the peak of the compensator signal.An expanded picture of the laser intensity during the plasma pulse is shown inFig. IX-4. The electron density as measured by the interferometer is shown in Fig. IX-5.QPR No. 78Fig. IX-2.-,7; zZZD..,.JzLAL<Fig. IX-3.P. I.G. discharge.TIME(2 msec / DIV. )Laser intensity.QPR No. 78BZz.11U-IuTIME( 50 pIsec / DIV. )Fig. IX-4. Discharge current and laser intensity.TIME (/ SEC)Fig. IX-5. Electron density in the plasma.QPR No. 78cJo-Jz-zw0wzzoI-w-Jw..(IX. PLASMA PHYSICS)The decay of the plasma is now being studied in detail. The results thus far indicatethat the decay can be explained entirely on the basis of ambipolar diffusion, if the elec-trons are assumed to have temperatures of several electron volts and the ions to havetemperatures of almost 1 volt. This model is in accord with previous measurementsmade during the pulse on low-pressure P. I. G. discharges.5 Although there is someevidence of instabilities during the current pulse, there is no present indication thatinstabilities affect the decay of the plasma.E. B. Hooper, Jr.References1. E. B. Hooper, Jr., "Laser Interferometer," Quarterly Progress Report No. 75,Research Laboratory of Electronics, M.I.T., October 15, 1964, p. 55.2. D. E. T. F. Ashby and D. F. Jephcott, Appl. Phys. Letters 3, 13 (1963).3. J. C. Ingraham and J. J. McCarthy, Quarterly Progress Report No. 64, ResearchLaboratory of Electronics, M.I. T., January 15, 1962, pp. 76-79.4. J. B. Gerardo and J. T. Verdeyden, Appl. Phys. Letters 3, 121 (1963).5. R. Geller and D. Pigache, "Plasma Physics," J. Nucl. Energy, Part C 4, 229 (1962).B. REVERSAL OF ROTATION AND STEADY-STATE CHARACTERISTICS OFA BEAM-PLASMA DISCHARGEA preliminary report I has been given on a rotation observed in a discharge initiatedby an 800-volt electron gun (oxide-coated cathode, VA-220) with an output ofTO DIFFUSION PUMP AND WELSH PUMP 1397Bt0n L 28 " AGAS OSCILLATORINLET PULSEOSCILLATOR LENGTH- WATER-COOLED COLLECTOR PULSERO COLLECTOR AND SHIELD ASSEMBLYo ELECTRON GUN AND SHIELD ASSEMBLY POWERSPYREX CROSS SUPPLYQ ION GAUGEo ACCESS PORTo FILAMENT TRANSFORMER( 5" DIAMETER STAINLESS-STEEL CHAMBERFig. IX-6. Beam plasma general arrangement.QPR No. 78(IX. PLASMA PHYSICS),NICKEL ANODE approximately 20 ma, operated at pulsedlengths of a few milliseconds to achieve aOXIDE-COATED CATHODE steady state, or on DC.1 The general char-HIGH VOLTAGE ANDHEATER LEADS acteristics of the initiation of this type of3" 1 /4" 3" 1 1 4 plasma have been described by Getty andBORON-NITRIDE SUPPORT 2Smullin.ARMCO IRON POLE PIECES5" 1. ApparatusFig. IX-7. Detail of electron gun Schematic diagrams of the apparatus areassembly. shown in Figs. IX-6 and IX-7. The plasmais produced by electrons drifting through aregion, 28 inches long, between an electron collector and a Pierce gun with an anode-cathode separation of 1/8 inch. Symmetrical Armco iron pole pieces, 3 inches in diam-eter, form magnetic shields for the hollow-cylinder and electron gun. The entire systemis enclosed in a stainless-steel chamber, 5 inches in diameter. Low-value resistorsdetect current flow to various regions of the system, so essentially the entire assemblyis at anode potential. The discharge is operated in an argon atmosphere, generally ata pressure of 10-4 mm, and the argon is pumped continuously across one of the ends ofthe chamber. A solenoid system of 6 coils energized by 3 regulated power supplies per-mits radial access into the chamber, and produces high mirror ratios, yet maintains auniform field that is more than 1 ft long in the central region with a 1-2 per centdecrease off axis, although a uniform field was not always needed for the rotation.2. Reversal of Rotation and Flute ModesPlasma rotation with mode perturbations has been observed in a wide variety ofplasmas.3 A reversal of rotation is reported in a slow magnetic compression where thecolumn rotates in the m = 1 mode. Left-handed rotation occurs when a "hot-ion" plasmais present, but then becomes right-handed as compression continues. 4The present weakly ionized plasma (less than 1 per cent), characterized by a domi-nant m = 2 flute mode, rotates in a left-handed sense in the range =80-=200 gauss, withthe frequency decreasing from 5 kc/sec. After the stoppage of rotation near a criticalfield of 200 gauss, the rotation becomes right-handed, attaining an asymptotic value of3 kc/sec at high fields (1000 gauss). The m = 4 mode appears intermittently in the right-handed region until a few hundred gauss is attained. The typical dependence of the fre-quency of rotation on the central field for a fixed mirror ratio is shown by the heavy linein Fig. IX-8, with the lighter lines indicating the ranges in which the m = 2 and m = 4modes appear. Though not indicated in the figure, the field was extended to 1500 gauss,with the frequency remaining at the asymptotic value of 3 kc/sec. At the crossover point,the electron cyclotron frequency corresponded to the plasma frequency with an accuracyQPR No. 78(IX. PLASMA PHYSICS)10m= 25IB ( gauss)LH0 RH "- 250 500 750 10006----------------------------- --------------------5 - m=2m=410 m = MODE NUMBERFig. IX-8. Frequency and mode dependence of rotation on centralmagnetic field. Mirror ratio, 2. 2; gun voltage, 820;-4pressure, 1.2 X 104 mm; crossover, =200 gauss. Atcrossover, v = v = 550 Mc/sec.p ecof measuremeht of approximately 15 per cent.Be cause of an error in polarity, the oscillograms in the previous report I actuallydescribed the left-handed rotating region where an outward radial E field is requiredunder the assumption of an E X B drift.A perturbation of the plasma column can arise perpendicular to the field so that theplasma is shaped into a fluted column with an mO dependence, where m gives the num-ber of such flutes. 5 The present plasma has such a flute perturbation with the troughsextending into the luminous core of the discharge. No spiraling effect of the flute couldbe found along the axial direction. 6 Neither was such an effect found along a radial vec-tor; in a two-probe system (separation of probe tips fixed at 3/4 inch) no phase changewas found along a radial vector except between the opposite sides of the boundary of theluminous plasma core where the current to the outermost probe lagged by approximatelyFigure IX-9 is another plot for the low B-field region where a mode change occurredeven in the left-handed region. The signal was too erratic, however, for an accuratedetermination of the mode number just before crossover.The two rotating regions have opposite dependence on mirror ratio as indicated inFig. IX-10, in which, for two subcritical field values, the frequency shows no depend-ence on the mirror ratio and the rotation remains left-handed. Once the critical fieldQPR No. 78m=2m= 3,4 (?)208 B ( gauss)112 144 176Fig. IX-9. Frequency and mode dependence of rotation oncentral magnetic field. Mirror ratio, 2. 18; gunvoltage, 820; pressure, 1.2 X 10 - 4 mm; cross-over, z210 gauss.m=2* 98 GAUSSo 114 GAUSSMIRROR RATIOI LH II1 RH 2 3 4 5 6Fig. IX-10.Dependence of rotation frequency onmirror ratio at constant central mag-netic field.m= 2m=2* 238 GAUSSo 305 GAUSSm =MODE NUMBERQPR No. 78(IX. PLASMA PHYSICS)is exceeded, the rotation remains right-handed; but here the frequency can be changedby varying the mirror ratio. Rotation was detected by a photomultiplier and by a widevariety of resistor grounded probes (Langmuir, antenna-type, and large geometricallyfixed plates tangent to the chamber wall), biased or unbiased.3. Probe MeasurementsThe luminous core of the discharge varies with the central field with a slight depend-ence on the mirror ratio, being approximately 1 inch in diameter at 200 gauss, and1/4 inch at 400 gauss for a mirror ratio of 2. Particularly, the region from the luminousboundary to the chamber wall was investigated with a Langmuir probe. The averageelectron energy = e-Vp + T (where Vp and T are the space potential and electrontemperature averaged over the azimuthal direction for some particular radial position)was found to be a constant through this region, and 6 changed only with changes of pres-sure, magnetic field, mirror ratio or gun voltage.Experimental evidence indicates that the rotational reversal is associated with areversal of the radial E field, and the direction is consistent with an E X B drift. Therotation reverses with a reversal of the axial B field. A Langmuir probe measuredaverage plasma parameters along a radius at 1/2 inch intervals with a condenser usedin the probe circuit to average azimuthal fluctuations caused by the perturbed plasmaboundary. Such probe measurements were most reliable in the range below the criticalmagnetic field where the rotation was always left-handed and the plasma density outsidethe luminous core was -5 X 10 9 /cc. (The density tripled when the gun voltage was raisedfrom 800 volts to 1400 volts.) The subcritical field extended from -80 gauss to=200 gauss. A radially outward E field wasfound in this region to be approximately+20.0 - 60 volts/m just outside the luminous coreboundary and approximately 10 volts/m near+19.5 -the cylindrical wall. The radial E field didnot change significantly as the B field was+19.0 - increased; this indicates that the frequency- 'followed an E/B dependence. If the rotationalT frequency based on E X B were calculated for+7.0 - the field just outside the luminous core, theVF calculated frequency would be almost an order0 I I of magnitude higher than the observed fre-0 0.5 1.0 1.5 2.0 2.5r (inches) quency. Under the assumption that the ionFig. IX- 1. An average plasma and temperature is 0.1 ev, the inclusion of the ionfloating potential plot. pressure term7 gives a further, but smallQPR No. 78(IX. PLASMA PHYSICS)contribution to the left-handed rotation.For this plasma the radial electric field cannot be estimated from the easily obtain-able floating potential, since the space and floating potentials were found to have oppositegradients in the region of interest. Radial temperature and density gradients were pres-ent with the average electron temperature ordinarily falling from 4 volts near the lumi-nous boundary to 2 volts near the chamber wall, and the average density decreasingexponentially by a factor of three over this distance. Figure IX-11 is a typical averagespace potential Vp plot from which the radial E field is estimated, and the floating poten-tial VF for a subcritical magnetic field is given. The azimuthal fluctuations were verypronounced when VF maintained an almost constant value along a radial vector. Forsubcritical fields, the plasma density remained at a rather constant value at some par-ticular radial distance in the region outside the luminous core. But, as this value wasexceeded, the density for some fixed radial position decreased linearly with the magneticfield, and the ion current to the chamber wall rapidly declined (see Fig. IX-12). Probemeasurements thus became less reliable for estimating the space potential, especiallyat larger radial distances for fields above the critical field, but they did indicate that a5.84.2I I I '70 110 150 190 230B ( gauss )270 310 350Fig. IX-12. Decrease of plasma density with increasing magneticfield at two fixed radial positions. Gun voltage, 800;mirror ratio, 2. 1.QPR No. 780.840.680.520.360.200.16 . . . . . i I I I I' " ' '(IX. PLASMA PHYSICS)4B ) 3/20 2 4 6 8 10 1cmFig. IX-13. Current-phase relations illustrating right-handedrotation. Magnetic field, 402 gauss; mirror ratio, 4.reversal of the radial E field occurred at the rotational reversal.Figure IX-12 indicates the type of response in the right-handed rotating region. Herethe mode is M = 2, and the frequency 1. 7 kc/sec. Inasmuch as the density in the regionoutside the luminous core is now only approximately 108/cc, the probes are biased at+22 volts, because of the low gain of the four-trace preamplifier units. For this oscil-logram the current to the wall was sampled by a system of copper plates, 3/4 inchsquare, tangent to the chamber wall and grounded through resistors. The plate positionsare indicated in Fig. IX-13, and the numbers refer to the corresponding traces (uppertrace is number 1). Positive current reads upward.4. Radial Ion FlowThe discharge is characterized by ion flow to the chamber wall with an associatedincrease of electron current along the discharge axis. There is some indication, thoughinconclusive, that the radial flow is by ordinary diffusion. With a condition that the con-ductivity across the magnetic field is negligible in the axial rate, the diffusion equationis solved for the steady-state, low-pressure plasma, and the radial dependence of thedensity is predicted 8 asn(r) = K(r/q),where K is a Hankel function, and q is the e-folding density length. With the assump-tions used, the expression for q isLr.qC1where L is the interaction length, r. is the ion Larmor radius, and X. is the ion mean-Ic 1free path. Experimentally, the average density in the region outside the luminous coreis exponential as predicted. The calculated value of q is approximately 1 cm, aQPR No. 78(IX. PLASMA PHYSICS)reasonable value for the plasma.For the present apparatus, with interaction length L = 70 cm, argon pressure-41. 2 X 10 mm, B = 160 gauss, electron temperature 4 ev, and with the assumption thatthe ion temperature is 0. 1 ev, the relation X. < L < , where X is the electron mean-1 e efree path, holds; however, L/Xki 2, and R/r.ic 4 with R = 5 cm, the radial plasmadistance. These ratios are rather small for determining whether a collision-dominateddiffusion is present.Ordinary radial diffusion as determined by the ion diffusion coefficient may beexpected from a "short-circuiting" mechanism, 9 whereby large radial E fields are pre-vented by the grounding of chamber and end walls as in the present apparatus. A ratherlarge radial E field is present, however, and the short-circuiting effect may be inhibitedby an end-wall potential sheath, as the space potential is high (=+20 volts).J. K. DomenReferences1. J. K. Domen, "Steady-State Beam Plasma Discharge," Quarterly Progress ReportNo. 75, Research Laboratory of Electronics, M.I. T., October 15, 1964, pp. 51-54.2. W. D. Getty and L. D. Smullin, J. Appl. Phys. 34, 3421 (1963).3. W. Bernstein, et al., Phys. Fluids 3, 1019 (1960);N. Rostoker and A. C. Kolb, Phys. Rev. 124, 965 (1961);N. D'Angelo and R. W. Motley, Phys. Fluids 6, 422 (1963);H. Bodin and A. Newton, Phys. Fluids 6, 1338 (1963);K. Bol, Phys. Fluids 7, 1855 (1964);B. Hartenbaum, Ph. D. Thesis, Department of Mechanical Engineering, M. I. T., 1964;H. Lashinsky, Phys. Rev. Letters 12, 121 (1964);K. I. Thomassen, Phys. Rev. Letters 14, 587 (1965);D. L. Morse, Phys. Fluids 8, 516 (1965).4. W. A. Perkins and R. F. Post, Phys. Fluids 6, 1537 (1963).5. M. N. Rosenbluth and C. L. Longmire, Ann. Phys. 1, 120 (1957); W.. A. Perkinsand R. F. Post, Phys. Fluids 6, 1537 (1963).6. B. B. Kadomtsev and A. V. Nedospasov, J. Nucl. Energy Part C 1, 230 (1960).7. L. Spitzer, Jr., Physics of Fully Ionized Gases (Interscience Publishers, Inc.,New York, 1962), p. 31.8. G. Guest and A. Simon, Phys. Fluids 5, 503 (1962).9. A. Simon, Phys. Rev. 98, 317 (1955); F. Boeschoten and F. Schwirzke, NuclearFusion 2, 54 (1962); F. Boeschoten, J. Nucl. Energy Part C 6, 339 (1964).QPR No. 78

Plasma Physics

http://dspace.mit.edu/bitstream/1721.1/54112/1/RLE_QPR_078_IX.pdf

Plasma Physics

Abstract

Similar works

Full text

Available Versions

DSpace@MIT