Contains reports on one research project.Joint Services Electronics Programs (U. S. Army, U. S. Navy, and U. S. Air Force) under Contract DA 36-039-AMC-03200(E

Ingraham, J. C.

Novenski, A. E.

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XII. GASEOUS ELECTRONICSAcademic and Research StaffProf. S. C. Brown Prof. J. C. Ingraham W. J. MulliganProf. W. P. Allis J. J. McCarthy A. E. NovenskiGraduate StudentsA. J. Cohen J. E. McClintock T. T. Wilheit, Jr.G. A. Garosi L. D. Pleasance B. L. WrightA. LOW-FREQUENCY OSCILLATIONS IN AN ELECTRON-CYCLOTRONRESONANCE DISCHARGEAn experiment has been initiated to study the low-frequency plasma waves that areexcited in a volume of plasma produced by microwave breakdown at the electron-cyclotron frequency. The experiment will be described and preliminary results quoted.The plasma is contained in a cylindrical quartz tube of 1. 3-cm inner diameter whichis aligned parallel to a DC magnetic field. The tube is also inserted through the centerof the broad face of a rectangular 1 X 2 in. brass waveguide so that the tube axis is at anangle of 5. 70 relative to the waveguide axis. During the experiment the tube is filledwith helium gas at pressures of 0. 005-0. 100 Torr and a microwave signal of approxi-mately 1 watt power and 6000--Mc frequency is transmitted through the plasma. Forvalues of the magnetic field sufficiently close to the electron-cyclotron resonance valuea plasma is produced in the volume of gas within the waveguide. The resultant electron10 11 -3density is estimated to be between 10 and 10 cm , and the electron temperature tobe between 1 and 10 eV.The magnetic field is varied manually across the range of values (AB-100 gauss) forwhich a plasma can be produced, and the amplitudes of the microwave power transmittedthrough and reflected from the plasma are monitored on an oscilloscope. For severalsmaller ranges of values of the magnetic field (AB-2 gauss) within the 100-gauss rangeof magnetic-field values, the microwave amplitudes are modulated at definite frequen-cies from 30 kc to 1000 kc, thereby indicating the presence of a low-frequency wave inthe plasma. In some cases more than one modulation frequency is present at the sametime. In between the magnetic field values at which the microwave signal is modulatedat well-defined frequencies the modulation is still present but it is not possible to definea definite frequency (or frequencies) of oscillation. Figure XII-1 shows photographs oftypical amplitude modulations of the transmitted microwave signal as displayed on theoscilloscope.Figure XII-2 displays typical data taken for a fixed microwave power (1.22 watts)*This work was supported by the Joint Services Electronics Programs (U. S. Army,U.S. Navy, and U.S. Air Force) under Contract DA 36-039-AMC-03200(E).QPR No. 82 123(XII. GASEOUS ELECTRONICS)and frequency (6025 Mc). Electron-cyclotron resonance is indicated by a vertical lineat the appropriate magnetic field and the ion-cyclotron frequency is plotted as a lin-early increasing function of magnetic field.(a)Fig. XII- 1.(a) 715-kc oscillation for 6025-Mc excitation,p = 0. 05 Torr, time-scale 2 i sec/division.(b) 100-kc signal for 6025-Mc excitation, p =0.015 Torr, time-scale 5 iLsec/division.(c) 100-kc and 675-kc signals for 6025-Mcexcitation, time-scale 10 psec/division.(b)(c)The oscillation frequencies occur below the ion-cyclotron frequency. At higher pres-sures only the higher frequency oscillations occur. The 667-kc and 500-kc frequenciesrecurred most often throughout the experiments.Fluctuations in the light emitted from the plasma, as a function of position along thelength of the plasma, were observed by using a photomultiplier and fiber optic lightguide that made it possible to view the plasma light emitted through a longitudinal slitcut along the middle of the broad waveguide face. No variation of the light signal ampli-tude along the plasma length was observed which indicated the absence of standing wavesparallel to the tube axis having wavelengths shorter than twice the tube length.Further measurements of the relative phase of light emitted from two differentpoints along the tube length or from two different points around the tube circumferenceare planned. Measurements of this kind are necessary before a meaningful analysis ofthe nature of these waves can be made. Furthermore, it must be determined whether theQPR No. 82 124PRESSURE 105microns800700605o -'400 +000 rPRESSURE 50 micro++ +- ++H-00PRESSURE 15micron800700600500a ooS300 -2o -+ +++ ++ +0+ +0 +2073.6 2150.4 2227.22112C0 2188.8 2265.6MAGNETIC FIELD (gau)PRESSURE 75 icrons-+ + ++ ±(b)PRESSURE 30 crn-B--+ + + +++ + + +0(d)+Fig. XII-2.A display of typical data at six dif-ferent helium pressures, klystronfrequency 6025 Mc, and klystronpower 1.22 watt. The graphs showthe frequency of oscillation and themagnetic field at which it was ob-served. The magnetic field cor-responding to electron-cyclotronresonance is indicated by a verti-cal line, and the frequency corre-sponding to ion-cyclotron frequencyis labeled vB+. The symbols "(0'or "@" adjacent to data points in-dicate that the frequency wasobserved only in the transmittedmicrowave signal or in the reflectedmicrowave signal.++ + +2073.6 2150.4 2227.22112.0 2188 8 2265.6MAGNETIC FIELD (gauss(f)QPR No. 82800700o 400300200300200100 -125(XII. GASEOUS ELECTRONICS)strong coupling of the microwave ionizing energy to the plasma alters the dispersionrelation of these waves from what it would be in a similar plasma but with the micro-wave power absent.We shall now calculate some typical frequencies for plasma waves that could propa-gate within the plasma column. If the electron temperature is 105 K, an ion-acousticwave propagating parallel to the magnetic field with 60-cm wavelength (twice the plasmalength) will have a 30-kc frequency corresponding roughly to the lowest observed frequen-cies. For this same plasma the low-frequency ion-cyclotron wavel propagating at anangle 0 with respect to the magnetic field has a frequency equal to the ion-cyclotron fre-quency reduced by the factor cos 0, and is nearly independent of the wavelength (providedthe wavelength is less than the circumference of the plasma tube under the present condi-tions). This wave would have frequencies within the range of the experimental data.Finally, the electrostatic drift wave,2 driven by the electron density gradient perpendi-cular to the magnetic field, has a frequency given bySkTe 71 an) ?I e= 1 2n2Zr eB n ar X5K 1 ao 2.4If we take T 10 K, = 2rR, B = 2000 gauss, and R = 0. 65 X 10 - 2e n ar Rmeter, we obtain f= 400 kc, which also falls within the limits of the observed frequencies.J. C. Ingraham, A. E. NovenskiReferences1. T. E. Stringer, "Plasma Physics," J. Nucl. Eng. Part C , 89 (1963).2. F. F. Chen, Phys. Fluids 8, 912 (1965).QPR No. 82 126

Gaseous Electronics

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Gaseous Electronics

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