Contains reports on two research projects

Allis, W. P.

Brown, S. C.

Looney, D. H.

McCarthy, J. J.

Phelps, A. V.

Rose, D. J.

DSpace@MIT

II. MICROWAVE GASEOUS DISCHARGESProf. 5. C. Brown J. J, McCarthyProf. W. P. Allis A. V. PhelpsD. H. Looney D. J. RoseA. 100-MEGACYCLE BREAKDOWNA large reentrant cavity, resonant at 100 Mc, has been constructed toextend the measurements of high-frequency gas-discharge breakdown in hydro-gen and to check the theory in the region of high electric fields. Thisparticular frequency was chosen as a compromise between the ease of obtain-ing high r-f power and the physical limitation of the size of the cavityinvolved.From the equation (1)2  dh QPI/owe find that the electric field at the center of the cavity can be found ifwe know the tuning rate, dh/dv, the unloaded Q, QU, and the power absorbed,P. The method of obtaining dh/dv has been described in a previous report(1) and the value was checked by assuming the cavity to be a capacitance-loaded resonant transmission line and calculating dN/dv directly. The powerabsorbed by the cavity was measured by a Micromatch Model M252 wattmeter,capable of measuring both incident and reflected power and, indirectly, theabsorbed power. By means of this wattmeter the cavity was matched at reso-nance by adjusting the coupling to the cavity for practically zero reflectedpower. The unloaded Q of the cavity can be found if the susceptance B ofthe cavity is known at a frequency slightly off resonance as well as theconductance G of the cavity, known from the condition of match at resonance.A measurement of the power incident on and reflected from the cavity slightlyoff resonance suffices to determine a circle of constant reflection coeffi-cient on an admittance chart from which B can be determined. The unloadedQ will then be given by the expressionBf0QU = 2 Afwhere f0 is the resonant frequency and Af the small change in frequency madeto measure B.From a knowledge of the tuning rate and the unloaded Q, which areproperties of the cavity, and the power required to initiate breakdown, theelectric field for breakdown was calculated and compared to existing data-17-(II. MICROWAVE GASEOUS DISCHARGES)obtained at microwave frequencies and by Thompson (2) and Githens (3) atlower frequencies. This comparison is shown in Fig. II-1. It is possibleto obtain a wide range of values for EA in this experiment and these valueswill be extended further in the near future. It is also intended to measurebreakdown in helium contaminated with a little mercury vapor for which asimple theory may be used.Fig. II-1 Effective breakdownvoltages as a function ofE/p calculated frombreakdown measurementsof numerous workers.References(1) 3. C. Brown, M. A. Biondi, M. A. Herlin, E. Everhart, D. E. Kerr,R.L.E. Technical Report No, 66, M.I.T. (1948),(2) J. Thompson, Phil. Mag. 22, 1 (1937).(3) 3. Githens, Phys. Rev. Zj, 822 (1940).-18-(II. MICROWAVE GASEOUS DISCHARGES)B. MEASUREMENT OF THE PROPERTIES OF LOW-DENSITY MICROWAVE GAS DISCHARGESA cavity method has been developed for measuring the properties of lowdensity discharges. The method is applicable as long as the absolute magni-tude of the electron current density is much less than the displacement cur-rent density.Analysis shows (4) (5) that, in the region of the cavity resonanceinvolved, one may replace the cavity and discharge by a lumped constantcircuit. Such a circuit for a cavity with single ouput is shown in Fig. 1I-2,where go is the series conductance of the line, loops, etc;(_W coois the susceptance of the empty cavity; a is the radian frequency; so is thecavity resonant frequency; g is the conductance due to all internal lossesexcept the discharge; gd + jbd is the discharge admittance. The dischargeadmittance can be expressed as{E2dvgd + Jbd e 2dv00 E2dvwhere o is the complex conductivity of the discharge.gs Since all the parameters of theAAA . .circuit except the discharge admittancecan be determined (5) from measurementon the empty cavity, two measurementsnn the loaded evitv will suffice to de-Fig. II-2 Equivalent circuit of cavity termine gd and b d . These might be thewith discharge. db standing-wave ratio R at the emptydRresonant frequency u, R at o cu, or  at q. The discharge admittance isthen found by straightforward an lysis. These three methods are useful if theloaded resonant frequency a)o is so far removed that the oscillator cannotreach it, but the accuracy of the measurements is not great. If possible, thenew resonant frequency o and the db standing-wave ratio Ro at m should bemeasured. The former determines bd directly, for the shift of the resonanceis given by ~o - o = - bdo/p; the latter measurement determines gd"Most accurate results are obtained by plotting a resonance curve for theloaded cavity. If this is done the maintaining field can be calculated atonce. In addition, various checks can be made, such as the constancy of B.j -- g <\o /g I-19-(II, MICROWAVE GASEOUS DISCHARGES)For any of the measurements it is necessary that frequency excursionsbe made at constant electron density. A simple method of monitoring thedensity consists of measuring the light intensity of the discharge. As thefrequency is varied over the range of interest, the input power is adjustedso that the intensity is kept constant. Since the spatial and velocity dis-tributions of the electrons do not change appreciably over the small rangeof frequency, the method is quite reliable,If power measurements on the loaded cavity are made at frequencies otherthan the loaded resonant frequency, the power absorbed must be corrected tothat absorbed at resonance, in order to use standard formulas involving theQU of the cavity. At constant electron densityo g'g (1+ gwhere P is the power absorbed at w, Po is the power absorbed at to, andg' g+ gd.If the maintaining field is independent of density, the discharge isstable in the cavity only as long as dPd/dH ('/Pd) < 1, where Pd is thepower absorbed in the discharge, and H is the average density. If theseries loss is neglected, analysis shows that the discharge is stable ifA(l + g + AP) + B - )> 0where gd = Ad, bd - BO. The constants A and B are functions of the fieldand electronic modes, and are independent of the cavity size and electrondensity. Since A > 0, B < 0, it may be seen that the discharge is alwaysstable for co- co. The frequency of extinction above ' can be estimatedby noting that P(ac/o - cD/W) = g + AWn at the half-power points of theloaded resonance curve.The cavity will tend to undercouple with increasing discharge power.The db standing-wave ratio decreases from Ro, its value at the empty reso-nance, to R1, the value at the loaded resonance. It can be shown thatRo - R' = 20 log (1 + AQU)where QU is the unloaded Q of the empty cavity.These techniques are being applied to the study of maintaining electricfields of microwave gas discharges. Preliminary measurements of gas-20-(II. MICROWAVE GASEOUS DISCHARGES)discharges in hydrogen at densities of the order of 108 electrons/cc indi-cate that the method is practicable.References(4) J. C. Slater, Rev. Mod. Phys. 18, 441 (1946).(5) D. J. Rose, D. E. Kerr, M. A. Biondi, E. Everhart, S. C. Brown,R.L.E. Technical Report No. 140, M.I.T. (1949).-21-

Microwave Gaseous Discharges

http://dspace.mit.edu/bitstream/1721.1/50841/1/RLE_QPR_016_II.pdf

Microwave Gaseous Discharges

Abstract

Similar works

Full text

Available Versions

DSpace@MIT