Contains report on one research project.U.S. Atomic Energy Commission (Contract AT(30-1)-3980

Oddou, C.

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PLASMA DYNAMICSVII. PLASMAS AND CONTROLLED NUCLEAR FUSIONA. Waves and RadiationAcademic and Research StaffProf. G. BekefiProf. S. C. BrownR. J. BeckerA. J. CohenProf. W. M. ManheimerProf. B. L. WrightJ. J. McCarthyGraduate StudentsL. LitzenbergerL. P. Mix, Jr.L. D. PleasanceC. OddouW. J. MulliganE. N. SpithasJ. H. Vellenga1. ANOMALOUS CYCLOTRON RADIATIONA characteristic of anomalous cyclotron radiation in weakly ionized Ramsauer gasesis its appearance in a restricted range of pressure and current.1,2 Note that therange also depends on the microwave band that is utilized. It has been shown 1,2 that,in Xe gas, at a pressure of approximately 0.9 Torr, the maximum power appears at100 mA discharge current for S-band frequencies (3000 MHz) and for 0.2 Torr, 600 mAin X-band (9000 MHz). In our experiments this maximum has been observed at ~400 mAin the C-band (5500 MHz) at a pressure p ~ 1. 7 mm Hg in the same gas as shown inFig. VII-1. Although the plasmas used in these experiments are produced differently'*d6 -30'\201(.A)t \\I  \ \\\ •\\IIAFig. VII-1. Experimental results of the anomalous cyclotron radiation poweremitted as a function of the current discharge at 1.7 mm Hg pressure.This work was supported by the U.S. Atomic Energy Commission (ContractAT(30-1 )-3980).QPR No. 95- -- C(VII. PLASMAS AND CONTROLLED NUCLEAR FUSION)and are of different geometry, they are all low-voltage arc-type discharges. This leadsus to believe that a 2 = /w2 is an important parameter of the phenomena. Also theprange of pressures, over which anomalous radiation is observed, is independent of theemitted microwave frequency, as can be seen from earlier results1, 2 and that presentedin Fig. VII-2.40dB -/I./\ SFig. VII-2. Experimental results of the anomalous cyclotron radiation power emit-ted as a function of Xe gas pressure. (Discharge current = 400 mA.)The collisional instability theory 4 in a tenuous magnetoplasma (p << 2 ) predictsthe amplification factor (attributable to the negative absorption) at W = Wb is proportionalto W2 /v. The conditions under which the phenomena are observed do not correspond toPa tenuous plasma and hence the dispersion of the medium must be taken into account. AsQPR No. 95I / ,e,. - -• \//o/(VII. PLASMAS AND CONTROLLED NUCLEAR FUSION)a consequence, there might be collective effects and we would expect to see similar phe-nomena associated with a hybrid frequency wH = ( +  1/2 (which is a resonant fre-quency for the extraordinary wave considered here) rather than with the cyclotronfrequency. But it has been shown 5 that even for a nontenuous medium, anomalous effectscaused by collisions are important only at the cyclotron frequency ob This is a conse-quence of the strong resonant behavior of the right polarized wave conductivity, a- .5rWe have applied the results of fluctuation theory 5 to the case of negative absorptionin order to calculate the total absorption o aEr 2+YjE 12 (where Er and E arethe right and left components of the electric field of the extraordinary wave). In Fig. VII-3the negative absorption is maximum at the cyclotron frequency (P = wb/w= 1) in the case.75 .80 .85 .90 .95 1.00 1.05 1.10 1.15 1 20 1.25P _Wb/.Fig. VII-3. Calculated extraordinary wave total absorption as a functionof p = Wb/w for the following Xe discharge parameters:a = p /w = 0. 2, y = v(v)/w = 0. 1 (v/vT)4. , f = fo v e-of a relatively dense medium (a = op/w = 0.2). Figures VII-4 and VII-5 display thebehavior of this maximum negative absorption when the density and the collision fre-quency are varied. These curves show a maximum in the amplification factor (ornegative absorption) because in this calculation the positive left polarized componentis taken into account. Since the emitted intensity is expected to be closely relatedto the amplification factor, the experimental results shown in Figs. VII-1 and VII-2 arein qualitative agreement with the theoretical predictions.QPR No. 95Fig. VII-4.Theoretical variation of the extra-ordinary wave maximum negativeabsorption at P = wb/w = 1 as a func-tion of a = p/w (electron velocity2 -4distribution f = f v e , collisionofrequency v(v) such that y = v(v)/w =0. 1 (v/vT)4. 5).0.1 0.2 0.3 0.4 0.5 0.6 0.7a P-0.3-0.2z-0,1O10 2.10-1 3.10-1 4.10- 1Y(VT) /(T)//Fig. VII-5. Theoretical variation of the extraordinary wave maximum negativeabsorption at P = wb/w = 1 as a function of y(vT) = V(VT)/w (electron42 -vvelocity distribution f = f v e , a = w /( = 0. 2, collision fre-quency v(v) such that y(v) = v(v)/w = Y(vT)(v/vT) 4. 5)QPR No. 95-0.2z0o -0.10 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5SWbFig. VII-6. Calculated extraordinary wave total emission as a functionof p = Wb/w for the following Xe discharge parameters:p v(v) 4a 0. 2; y- - 0. 1 (v/vT) and f = f e0-/o,A I d 100 mA0 I d 200 mAO I d= 300 mA100 200GRID CURRENT (mA)Fig. VII-7. IG(VG) characteristics of the grid (at pfor different anode current Id .= 1.7 Torr of Xe)QPR No. 95(VII. PLASMAS AND CONTROLLED NUCLEAR FUSION)The hypothesis of negative absorption has not yet been confirmed by direct measure-ment of the amplification. In our experiment, an amplitude-modulated microwave signalat w = wb is incident on a cavity-plasma system, and the reflected or transmitted poweris synchronously detected. Variation of the detected signal as a function of the magneticfield shows only positive absorption. This has been mentioned in previous work.1, 6It seems, therefore, that these anomalous radiation results cause us to either doubtexplanations based upon negative absorption or to hypothesize that the occurrence of thephenomena is masked by the poor coupling of the radiation with the plasma. On theother hand, it must be mentioned here that a particular characteristic of the calculatedemission of Xe plasma with no peak in its distribution function (that is, with positiveabsorption only) is a strong enhancement of the emission at = ob . Figure VII-6 showsthe variation of the calculated total emission Tr{ Er 2 + d LE 2}, where (r andSare the right and left components of the microscopic fluctuating current correlationtensor. The enhancement of the emission is of the order of 15 dB, but cannotexplain appearance of a stronger anomalous radiation peak (30 d).Characteristics still unexplained are the appearance of anomalous radiationin bursts correlated with low-frequency oscillations in the tube and the related2520 -152p (Torr)Fig. VII-8. Frequency variation of the grida function of gas pressure.oscillation asQPR No. 95(VII. PLASMAS AND CONTROLLED NUCLEAR FUSION)problem of production of a distribution function with a peak. In the early experi-ments 7 a round grid localized very close (1 mm) to the cathode was designed togive some of the electrons the perpendicular energy required for the creation ofthe peak. But different positions of the grid in the negative glow or in the posi-tive column show the same over-all phenomena. Above a certain value of the gridcurrent strong grid potential oscillations and anomalous microwave radiation appear,and the VG(IG) characteristics of the grid acquire negative slope (see Fig. VII-7).A study is being made to relate this observation of negative resistance to theobservation of anomalous negative conductivity in diodes or triodes filled with gashaving the Ramsauer effect.8 Whether or not this phenomena involves ionization,relaxation oscillation, or oscillation resulting from negative resistance in Ramsauergases cannot be ascertained. Nevertheless, the pressure of Xe gas is seen to playan important role on the resulting oscillation frequencies, as can be seen inFig. VII-8.These considerations permit us to proceed with the future study of anomalousradiation in two ways: (i) an experimental study of the coupling of the plasma radi-ation to observe negative absorption and (ii) a theoretical study of the production ofan electron distribution function with a peak with the related problem of the natureof the low-frequency oscillations.C. OddouReferences1. J. D. Coccoli, "Anomalous Pulsed Emission and Absorption by a Xenon Plasma atthe Electron-Cyclotron Frequency," Quarterly Progress Report No. 72, ResearchLaboratory of Electronics, M.I. T., January 15, 1964, pp. 102-111.2. S. Tanaka, K. Mitani, and H. Kubo, Proc. VIe Conf6rence Internationale sur lesPh6nomhnes d'Ionisation dans les Gaz, Paris, July 8-13, 1963.3. C. Oddou, Quarterly Progress Report No. 93, Research Laboratory of Electronics,M.I.T., January 15, 1969, p. 93.4. S. Tanaka and M. Mitani, Inst. Plasma Phys. J. (Japan) 19, 1376 (1964).5. A. C. Reisz and B. L. Wright, Quarterly Progress Report No. 94, Research Lab-oratory of Electronics, M. I. T., July 15, 1969, pp. 122- 129.6. S. Tanaka, Inst. Plasma Phys. J. (Japan) 21, 2028 (1966).7. J. D. Coccoli, "Further Observations of Anomalous Cyclotron Radiation in Xenonand Krypton," Quarterly Progress Report No. 73, Research Laboratory of Elec-tronics, M.I.T., April 15, 1964, pp. 35-40.8. S. Ohara, Inst. Plasma Phys. J. (Japan) 19, 1925 (1964).QPR No. 95

Plasmas and Controlled Nuclear Fusion

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Plasmas and Controlled Nuclear Fusion

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