US 3265967 A
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Description (OCR text may contain errors)
MICROWAVE PLASMA DEN sYs E provided.
3,265,967 f T I`sIIT-Y.Mi-:ASUREMENT I .Mark A. He'ald, Swarthmore, Psi., 'assignor to the United States of America as rcpresented by the United States Atomic Energy Commission continuation of application.' ser. No; 153,271, No'v. v16,
1961. This applicationlune 8, 1965, Ser. No. 462,457
3 Claims. (Cl. S24-58.5)
This application is a continuationl of application Serial No. 153,271 filed November 16, 1961, now abandoned,v
entitledv Microwave Plasma Density Measurement System' y This invention vrelates generally to a' microwave measuring system and more particularly toan' improved micro.
wave measuring system for determining the density of a plasma of lions and electrons. i In the electronic and atomic energy tields, it has been necessary. to produce plasmas of ions and electrons and to measure vthe plasma characteristicsincluding the plasma density. As described in Controlled Thermonuclear Rennaedsas 1 Patent oice actions by Glasstone and' Lovberg',` microwave systems have been advantageously .employed for measuring plasma density due. to the fact that the probing microwaveshave been of such low power that they have caused practical-ly no disturbanceof the conditionsjn the' plasma. However,
has beendiflicult or impossible .to provide `a microwave system' for producing continuousl foutpu't. .signaldirectly proportional tothe plasmadensity below its critical density where-the plasma becomes opaque to microwaves.
lthas been discovere-d in accordance with this invention, that these problems areovercorne and that -a continuous output signal can be provided that vanies directly with the plasma density below the critical plasma density, w-hile providinga stable reference signal when. the plasma highly attenuatesf microwaves passing therethrough,'by animproved phase detection system. More particularly, this invention 'in one embodiment contemplates having a frequency modulated signal sourcehaving a highly frequency sensitive microwave bridge with crystal detectors for generating avcarrierA frequency which is phase modulated by the-time varying plasma sample vin onearm of the bridge,
a frequency modulation receiver comprisinga differential amplifier, a limiter-,fa discriminator and a video amplifier connected in a chain to the crystal detectors 'of the bridge, .and an electronic integrating circuit' .that is. operable-with vthe'chainseriati'rri to produce a voltage that varies linearly with the phase modulation o f V the said signal produced .by said detectors. With the proper selection of components and frequencies Ias described in moredetail hereinafter, the desired output andpl-asmadensity determinations arev v'The above Aand further novel features of the invention will appear more fully-from the following detailed descriponly. i
In the drawings'where like parts are marked alike:`
. FIG.'1 is a block diagramof a preferred circuit in accordance' with the present invention for, accomplishing plasma density measurements.
.Referring now to FIG..l,`rneasur ing circuit 1-1 measures the density'of a plasma,le.'g., 'the free elec-tron concentral tion of a plasma in containerl, accordance with this' invention. The' plasma container 13 may b'e a plasma gen- .'erator .such as -a Philips Ion Gage, a gas discharge tube,
a thermonuelearreactor tube sueltas is described in U.S.
. Patent No. 2,910,414 orContr'o lled'Thermonuclear Reactions by Glasstone and Lovberg whereA plasma densities may be below or above 1013 electrons/cm?, or any other plasma container such as a ballistic tunnel for the study The plasma of plasmacaused by high velocity projectiles. in chamber 13 is indicated generally at 15.
Measurement circuit l11 preferably includes a irst section 1'7 having a frequency modulated signal source 19, a highly frequency-sensitive microwave bridge 21, in one arm 23 of which is located the time varying dielectric plasmasample 15, and microwave detecting means 25. Frequency modulated signal source 19 includes a suitable power -source 27 connected to a conventional klystron or backward-wave oscillator 29 capable of producingmicrowaves at a frequency j, of about 35,000 megacycles per second. A vmodulation oscillator 31 -produces a simple sine-wave intelligence signal of second frequency f2 of about 30 megacycles per second which frequency modulates the rstor carrier waves. A radio-frequency transmission line- 33, commonly referred to as `a microwave waveguide, has an appropriate size to deliverv the flmicrowaves through -isolator 35 to directional coupler 37 and plasma 15 in container 13 produces with path -39 an interference signal at junction 47 where the two paths 39 and 41 reunite.
-Waveguide 49 .makes transmission path 39 veryv much longer than reference path 41 and this transmission path 39 contains suitable directive antennas or horns 51 and 53 between which container 13 holds the time varying dielectric plasma sample whose density is to be measured.
In accordance wi-th the particular systemof this invention, -it has been found .that the plasma phase-modulates the above-described particular microwaves'passing through transmission or test path 39 and this phase modulation corresponds to the' plasma density. This phase modulation is detected by a balanced mixer comprisingA crystal detectors 215 which produce a signalv at the particular frequency f2 carrying the phase modulation. This signal is applied to a frequency modulation vreceiver 6 1 and an integrating circuit 63` through a coaxial cable 65 and this produces a voltage which varies linearly with the phase-shift produced yby .the plasma sample 15. This voltage is fed to i an oscilloscope 67 or tape-recorder or other data recording instruments soas to indicate the plasma density directly without complicated calculations and to this end residual amplitudernodulation of the microwave oscillator stabilizes vthe output signal-even when the microwaves are highly attenuated by the-plasma or thc plasma density 'is high.
The frequency modulation receiver 61 or phasedetector 61 of this invention has a differential pre-amplifier 69 that amplifes higher and lower frequencies than the frequency f3 within a frequency b and f3 determine-d by the time rate of change of plasma density with the frequency f2 acting as a center frequency. Coaxial cable 65 transmits the fz ysignal to limiter 71 that removes any amplitude modulation from the f2 signal, i.e., retains only phase-modulation of Discriminator 73 derives variations in the form of a video-frequency .voltage -in accordance with the frequency modulation of the center frequency f2. Thereupon, ampliiier 75 boosts this video-frequency voltage and transmits it to an electronic time-integrator 77 which recovers thev phase modulation envelope produced by the plasma and produces a voltage that is proportional to the Batertted Aug-ust 9, 1966 Spenge? callyitime-integrated by intcgrator77 to recover the phasemodulation envelope. v
In review of the above Klystron 29 produces microwaves at frequency f1 and oscillator 31 produces a sine An example of the parameters of the system of this inventionA are: f1'=35,00(l megacycles'per second, f2=30 megacycles per second, f3= 2 megacycles per second, and T :l second. providef1 f2 f3 1/T, Alsov the degree of frequency modulation, M= 'tf/j1, of oscillator 29 andthe lengthL of wa-veguide 49 must vbe related by the equation v M1 approximateiy equals W'whereW is the wavelength corresponding to the frequency f1.
In one embodiment, the crystal detector is a'M-icrowave lAssociates detector model w26-AR, the limiter isl a 30 rnc. LF. amplifierLEL model LF. 20B, the discriminator is a 30 mc. VELS model 99, the video amplifier is a Kicthley mode-H0213 and the integrator isan ELS model 181.4
In operationklystron or backwardwave oscillator 29 of frequency fhpowered by av suitable power supply27,
. is frequency modulated by an oscillator 3.1 of frequency f2. The microwave output power of oscillator 29 is delivered to a radio-frequency transmissionline such as waveguide 33. rWaveguide 33 if of -a size appropriate to ithe `frequency f1 and is used to interconnect all elements I where N represents the number of interference cycles In this example, these frequencies must of thecircuit operating at `the frequency f1, An isolat'or oi padding attenuator 35 preventsany reflected microwave power from altering the frequencyor amplitude of the output of oscillator 29. The microwave power is divided by a hybrid junction or directional couplerffinto two waveguide transmission paths 39 and 41. The power traversing these' two paths is later reunited at the hybrid junction 47. One of these two 'paths is made very much vlonger than the other vby the inclusion of additional waveguide 49;- the additional waveguide may be located in eitherpath 39 or` in path 41, whichever is more convenient.- IPath 39 contains a time-varying dielectric sample 15, the properties of which it is the function'of this circuit to measure. Specifically, the sample may be a timevaryingionizedgas or plasma. The microwave power is 'beamed through the samplebymeans of suitable directive antennas 5 1 and 53. The amplitude of microwave' power reaching junction 1 7 by path 41 is adjustedby means'of levelsetting attenuator 43, The -phase of thefmicrowave signal reaching junction 47 by path 41 is controlled by a'calibrated,.variable phase shifter45,` for purposes 'of .calibrating the output signal from the circuit. The output branches of hybrid junction `47 deliver'the microwave -power to microwave detector crystals 25, which apply an electrical signal in thevic'inity of the mfodulation'ffre- A.quency f2 to the differential pre-amplifier 69. Thecombin-ation of hybrid junction and detector crystals 25 and 251- is sometimes known asa balanced mixer. Tirne variations of the relative phase of the'microwave signals reaching junction 47 by paths 39 and 41, on account of the action of sam-ple 1S or calibrationphase shifter 45, produce a'phasc-modulation of the signal at frequency .f2 applied to the pre-amplifier 69. To preserve this phasemodulation vthe pre-amplifier must amplify frequencies in the band widthja on both sides of the centei frequency f2. The transmission of signals near f2' between components operating at that frequency is 'accomplished using exible coaxial cable 65; The output'of pre-amplifier 69 is delivered to the amplifying and limiting circuit 71, which removes any amplitude modulation. The output ofthe limiter 71 -is delivered to a discriminator 73, which Vprovides a video-frequency output in Iaccordance with the frequency-modulation of the signal with center-frequency f2. f signal is amplied-byamplifier 7 5 and electronidispersion, such that wave signal at frequency f2 that modulate the microwaves. Coupler 37 splits the modulated 'microwaves into two paths. The microwaves in the first or transmission path 39 pass from yhorn 51'to horn 53. The microwaves in the second or reference path 41 join with the microwaves from the first path to produce an interference signal at detector 25 that has its 'principal Fourier component at the harmonic of themodulation frequency nearest to N,
arising at the. output junction of the dispersive bridge as a result of the frequencyexcursion of the frequency modulated oscillator (Le. as a result of the frequency modulation ofthe microwave oscillators 29 and 31 together with thcextra wave guide 49 as observed when the sample 1 5 .is static and transparent to the microwaves). This produces aninterference signal having a phase modulation ofthis principal Fou'rier'component, and this phase modulation of this principal Fourier component is recovered by feeding this interference signal to a tuned amplifier passing only this one harmonic, and feeding this am- -plitier output seriatim to a discriminator.integrator chain.
To this end the phase shift caused by plasma 15 in arm 23 produces a phase modulation of this component in accordancel with this invention and thephase modulation is recovered by feeding the output signal from the detector 25 to tuned amplifier 69 which passes only this one harmonic and transmitting this harmonic to a discriminator integrator chain wherein the low frequency response is set by the integration time constant. The high frequency response is limited by the carrier-frequency aspect of the (fundamental)-modulation frequency. l
As will be understood fromA the above, this invention contemplates the interaction with electromagnetic fields (microwaves) and the measurement of the free electron concentration as a functionof the frequency at which this measurement is made. No interference signals which are sinusoidal functions of phase shift are involved in the output. lNeither are resonance frequencies, phasepropor tional display by means off a raster system or unbalance feedbacks to a frequency modulated oscillator involved.
In contrast to the heretofore known system, where it was impossible-to provide a continuous output directly proportional to the plasma density below the critical plasma density and unambiguous when the'density is above critical, it' is possible with the system of this invention to do this by operation of the described system with `a small Nv is approximately -unity and the amplifier is tuned to the fundamental. Thus, the system of this invention has the advantage that it gives direct voltage output and of simple operation at a high modulation frequency.
What is claimed is:
1. A frequency modulated microwave system for meastiring the time varying density of aplasma, comprislng producing frequency modulated microwaves that are split in to a short. path and a long 4path containing said plasma and joined to produce an interference signal carrying a phase modulation correspondingto the phase-shift produced by passing the microwaves through the plasma, said interference signalvhaving its principall Fourier cornponent' at theharrnonic N ofthe modulation frequency where N represents the number of interference cycles arising when said split microwaves are joined as a result of the frequency modulation excursion of the microwaves in passing through said plasma, recovering said Fourier component and integrating it to produce a continuous output signal directly corresponding to said plasma deni' sity when said plasma is transparent, while indicating unambiguously when the plasma is opaque.
2. The system vof claim 1 wherein said modulation is equal to M, said long path is equal vto Lland ML approximatelycquals W vwhere; Wi'is the wavelength of said micro-waves that are modulated.
3'. The system of claim l wherein the microwaves have a frequency of 35,000 megacycles persecond, the modula-v i tion thereof has-a frequency of 30 rnegacycles per second, the time rate of change of the plasma density produces a change in frequencies within a band of frequencies of 2 megacycles per second on either side of said modulation frequency, and said time integration is equal to one second.
References Cited by the Examiner UNITED STATES PATENTS 2,767,373 10/1956 Maggio 324-67 2,798,197v 7/1957 Thurston S24- 58.5 2,971,153
2/,1961 Wharton et al 324-58.S
WALTER CARLSON, Primary Examiner.
A. E. RCHMOND, Assistant Examiner.