Contains reports on five research projects

Badessa, R. S.

Bates, V. J.

Brown, D. A.

Funkhouser, A. T.

Searle, Campbell L.

Steelman, J. E.

Stroke, G. W.

Yaffee, M. A.

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VI. MOLECULAR BEAMSProf. J. R. Zacharias V. J. Bates E. L. KamenProf. J. G. King D. A. Brown F. J. O'BrienProf. C. L. Searle R. Golub J. E. SteelmanDr. G. W. Stroke W. D. Johnston C. O. ThornburgR. S. Badessa R. WeissA. CESIUM BEAM TUBE INVESTIGATIONWhen the X-band excitation frequency of the atomic clock is at exact cesium reso-nance, theoretically a null should occur at the output of the 100-cps amplifier. In prac-tice, we found that instead of there being a true null, the 100-cps signal merely goesthrough a minimum. This is the result of second-harmonic distortion in the frequency-modulation waveform.2 With a properly phased reference for the synchronous detectorof the loop this 100-cps minimum causes no error, since it is in quadrature with thedesired 100 cps that appears as a result of detuning the excitation frequency from thepeak of the resonance. However, if the second-harmonic distortion is large, the sta-bility of the clock will become a critical function of the phase drifts that might occurbetween the modulating signal and the synchronous detector reference, for any suchdrifts will disturb the quadrature relationship. A 10" shift in the reference phase, forexample, would cause a detuning of several parts in 1012 if the second-harmonic dis-tortion is 1 per cent. Drift might originate in the narrow-band 100-cps amplifier, ormight be the result of changes in the temperature of the hot wire of the beam tube.Because a mechanical synchronous detector was used in the present application, we foundit convenient to mount a "quadrature" brush electrically 90' away from the "in-phase"brush on the commutator. The dc voltage appearing at this quadrature brush is a meas-ure of the magnitude of the quadrature component and hence of the second-harmonic dis-tortion of the modulation. It was therefore used as an error signal in a second servoloop which, by injecting the precise amount of counteracting second harmonic, produceda true 100-cps null at the output of the 100-cps amplifier.As a check on the ability of the quadrature loop to make the phase relationshipbetween modulating signal and synchronous detector reference less critical, a phaseshift of 17. 20 was inserted at the output of the 100-cps amplifier. With the quadratureloop open, a detuning of five parts in 1010 was observed. With the quadrature loopclosed, no effect was visible. With the quadrature loop open, a 10 per cent change inthe current of the hot wire caused a detuning of six parts in 1011. Again, with the quad-rature loop in operation, no effect was visible.The desired stability of ± one part in 1012 has been observed occasionally over inter-vals of 10 minutes, but is generally an order of magnitude below this. Shift of the reso-nance characteristic of the beam tube with power level, an effect that is easily demon-strated, could be the predominant source of remaining drift.R. S. Badessa, V. J. Bates(VI. MOLECULAR BEAMS)References1. C. L. Searle, R. S. Badessa, and V. J. Bates, Quarterly Progress ReportNo. 64, Research Laboratory of Electronics, M. I. T., January 15, 1962, p. 54.2. R. T. Daly, Final Report on Atomic Frequency Standard (Contract Nonr-1707(00),NR375-750), National Company, Inc., Malden, Massachusetts, 15 November 1956, p. 59.B. MEASUREMENT OF FREQUENCY STABILITYThe stability of frequency standards is usually expressed as the mean-square devi-ation in frequency or phase for several averaging times. It now appears that thesespecifications can be quite easily derived from the autocorrelation of the phase deviation.This provides a handy and unifying concept for the understanding of the specificationof stability.We are now trying to determine whether the autocorrelation can be computed withno more difficulty than the previous methods used for specification. Should this proveto be the case, the new method will be a practical means of measurement, as well asan aid to understanding. Part of the difficulty is that the autocorrelation (7T) will beof interest out to a maximum T of approximately 24 hours, and also for differentialshifts AT of 2 seconds when T is small.D. A. Brown, C. L. SearleC. VARACTOR MULTIPLIER STUDIESIn experiments with cesium atomic resonance devices a source of X-band signal isrequired, which must be highly stable in frequency and fairly stable in power. At pre-sent, this signal is obtained by direct multiplication to X-band of a stable crystal oscil-lator signal running at 16.415414 mc. System power requirements set the X-bandsignal level to be at least 20-40 Lw. In the past, several varactor multipliers have beenbuilt which have satisfied the power requirement,1 but they have displayed typicalvaractor problems, such as critical tuning, day-to-day adjustment, sensitivity to shockand vibration, and severe drain on bias batteries.A recently completed varactor multiplier chain of a balanced design (suggested byR. P. Rafuse) for the lower frequency doublers (Fig. VI-1) appears to have solved theseproblems, as well as yielded additional output power at X-band. This balanced designincorporates symmetry to decouple input and output circuits while suppressing the unde-sired third-harmonic signals and, with the use of iron-core coils, lends itself to a rug-ged mechanically stable design. This design also results in more stable electricalcharacteristics. For an input drive of 1. 5 watts at 16.4 mc, an output power of0. 150 watt at 263 mc is easily obtained, this 150 mw level being determined by the2-PSI- 117-47BNC2-PSI- 116-22 2- PSI- 115- 10 2pA- MA 4325- EFour-stage varactor multiplier (16.4-263 mc).Fig. VI-1.-- 67.5 VTc(VI. MOLECULAR BEAMS)LINE STRETCHERX-BANDWAVEGUIDESHORTED STUBSFig. VI-2. Varactor multiplier(263-9192 mc).B- 15V 180 MA1.5 WATTSFig. VI-3. Transistor amplifier (16.4 mc).burnout power of the high-frequency varactor in the following stage. Battery bias drainwas sharply reduced by using 1-megohm bias potentiometers. The factor of 35 from263 me to 9192 me was accomplished in a single transmission-line circuit (Fig. VI-2).While development work is continuing on this device, results obtained already show anoutput power in excess of 100 ±w for the 150-mw input signal. Replacement of MicrowaveAssociates 4298 Varactor by a higher cutoff, higher power type should result in moresignal.A complete package was constructed containing a 16.4-mc transistor amplifier(Fig. VI-3) and the four doubler stages in a brass channel 1 1/8" wide, 3/4" high,11 " long.V. J. Bates263 MCIN10 MW(VI. MOLECULAR BEAMS)References1. C. L. Searle, R. S. Badessa, and V. J. Bates, Cesium beam tube investigation,Quarterly Progress Report No. 64, Research Laboratory of Electronics, M.I. T.,January 15, 1962, pp. 54-56.D. SUPERCONDUCTING LEAD CAVITYDuring the past quarterly period, the cryogenic system described in a previousreport1 has been discarded and replaced by a double dewar system made by the SuperiorAir Company. In this design, the cavity holder is now submerged in helium.The cavity was cooled to 4. 2 K and measurements indicated a Q of approximatelyone million. Then the Badessa oscillator 2 was assembled with the superconductingcavity as the feedback element. The maximum stability of the oscillator was three partsin 1010 for a 6-second averaging time.The most likely cause of this instability is mechanical vibration. The cavity is madein two cup-shaped halves that are held together by a spring; thus a movement of-83 X 10-8 cm is enough to cause the instability observed. Therefore, welding or solderingthe cavity into one piece seems to be necessary.Temperature change was also considered, but the thermal masses and thermal timeconstants of the elements that determine the stability rule out this possibility. The tem-perature of the cavity also eliminates Johnson noise in the cavity as a possible causeof the instability.Thus the cavity halves were soldered together with Wood's metalzand a Q measure-ment showed a Q of only 100, 000. Then the cavity halves were joined by fusing themtogether in two places. Again, the Q was low. A second cavity made from a purersample of lead was also tried, and it seemed to have a maximum Q of about 70, 000 -regardless of the surface treatment. Then the surface of the original cavity wasmachined smooth; the cavity was reassembled without soldering, and the Q was 90, 000.After that, the cavity was chemically polished with a solution of 1 part 30 per cent hydro-gen peroxide and 4 parts glacial acetic acid. Again the cavity was held together bysprings only, and the Q was approximately 200, 000. Then, without any further adjust-ments the Q increased to 1.2 million in a period of approximately 30 minutes. No expla-nation of this behavior has been found.J. E. SteelmanReferences1. 1. J.E. Steelman, Superconducting lead cavity, Quarterly Progress Report No. 61,Research Laboratory of Electronics, M. I. T., April 15, 1961, p. 109.2. C. L. Searle et al., Short-term stability of cesium atomic clocks, Quarterly Prog-ress Report No. 60, Research Laboratory of Electronics, M.I.T., January 15,1961, p. 105.(VI. MOLECULAR BEAMS)E. MEASUREMENT OF THE VELOCITY OF LIGHTThis report summarizes the progress that has been made since the last report inpreparation for actual measurement and evaluation of possible sources of error.1. Pneumatic Measurement of Cavity PerfectionThe pneumatic system previously described in Quarterly Progress Report No. 63(pages 109-112) has permitted us to obtain the cavity topology shown in Fig. VI-4. Thestraightness of motion of the moving piston (on which the nozzles are mounted) was inde-pendently ascertained with the help of the alignment interferometer previously described.-5It accounts for approximately 1 fringe (10-5 inch) of the curves in Fig. VI-4. The6.346 INCHES -6 (NOT TO SCALE)SRUN I5- + RUN 2o RUN 3-LJT4 -H /<C /lL -a_ LLJ, Allw2 O0 -I -2 -3 3 2FRINGES (I FRINGE - 10- 5 INCHES)Fig. VI-4.I 0 -i -2Fused-quartz cavity topology with pneumatic detection. Curves showdepartures from straightness in 10-5-inch units along two diametricallyopposed sections of the cavity. No correction was made for departurefrom straightness of travel of the moving piston. Independent inter-ferometric measurements indicate that these errors could account forone-half of the departure from straightness.(VI. MOLECULAR BEAMS)result is that the straightness of the cavity is more than satisfactory for the measure--6ment of c with an accuracy better than 10 without any corrections for dimensionalperturbation, as we had hoped. This is particularly true also for the measurement oftwo diameters 900 from each other which were found to differ only by approximatelytwo and one-half fringes. The diameter measurement was carried out with a pair ofnozzles, with the nozzles at opposite ends of a diameter. A more extensive series ofmeasurements of cavity diameters will be undertaken with the help of an inner pistonthat has been finished for this purpose, rather than with the aluminum piston previouslyused, which required rather delicate temperature control.2. Iris Dimensions for Cavity Excitation and Resonance DetectionIdeally, one would want to excite the cavity, as used in transmission, through iriseswithout any losses in the cavity. Noise considerations, however, require a decouplingof no more than ~40 db. Figure VI-5 shows a number of experimental energy-drop-60 -IRIS TYPE STRAIGHT HOLEIRIS TYPE - 450IRIS TYPE - 900TAPERED HOLE-40 h1 4 INCH--.-4501/4D TT-0.125 0.150 0.175 0.200 0.225CAVITY-SIDE APERTURE D0.250 0.275 0.300Microwave energy transmission of irises used to excite the cavity anddetect resonance. Graphs are shown for various iris shapes.-20 L0.100Fig. VI-5.INCHES-50(VI. MOLECULAR BEAMS)2_1200 o16011Fig. VI-6. Final iris shape transmitting 40 db at 9192 mc.curves obtained by transmission through irises of various shapes. Figure VI-6 showsthe final iris shape that was obtained. It has the smallest possible opening diameterat the cavity side for the desired 40-db decoupling. Two such irises are used, 900 fromeach other: one for cavity excitation. and one for resonance detection as previouslydescribed. This iris shape will be used in the optical plates that will be manufactured.A cavity of dimensions comparable to the actual cavity was used with a completeexcitation and detection plate to verify the fact that the total transmission through thecavity was indeed the desired 80 db.3. High-Speed Interferometer Length MeasurementIn the past, it was considered that optical fringe counting and fractional measure-ment were restricted to the range from 2 to 10 fringes per second (2-3 X 10- 5 inchper second), and would rarely exceed 20 per second, because of imperfections in themechanical apparatus. This is the case for grating ruling engines, for example. Forour velocity-of-light apparatus, it appeared essential to strive for fringe rates in excessof 100 fringes per second, and possibly to work in the vicinity of from 500 to 1000 fringesper second, that is, up to the order of 1/100 inch per second. A resonance measure-ment could thus be completed in only a matter of minutes,, with our apparatus, ratherthan in several days, as we thought when measurement on the M. I. T. ruling machinewas first considered (see below).We have indeed made considerable advances in measurement of high-speed opticalfringe distance. Figure VI-7 shows an excellent example bf a fringe signal obtained onour apparatus at 90 fringes per second. This was first recorded from a photoelectricmultiplier tube on magnetic tape and then photographed on an oscilloscope screen. Fig-ure VI-8 shows a fringe signal similarly obtained at 900 fringes per second. Theseremarkable results are due not only to the fine adjustment of the optical and electronicInterferometric fringe signal obtained at 90 fringes persecond with velocity-of-light apparatus. (1 fringe = 10-5inch with green line of Hg 1 9 8 source used.) The photo-electric fringe signal shown here was recorded on mag-netic tape and then displayed on the long-persistencescreen of an oscilloscope. The superposition of fringeswith very little relative shift is a measure of very gooduniformity of the piston motion in our apparatus.Fig. VI-8. High-speed interferometric fringesignal obtained for 900 fringes persecond (~0. 001 in./sec) advance ofmoving piston with hydraulic con-trol in the velocity-of-light appa-ratus.Fig. VI-7.tFig. VI-9. View of velocity-of-light apparatus used in our experimental work.i B,ii~s(VI. MOLECULAR BEAMS)equipment but also, in particular, to the smoothness of motion and absence of any "stick-slip" in our hydraulic and mechanical piston advance mechanism. We recall that themoving piston that carries the moving plate of the interferometer is mounted on a cylin-der, 20 inches long, sliding between brass ways. The entire piston and cylinderassembly rides on a hypodermic syringe assembly that governs the rate of piston motion.Considerable simplification has been achieved in the return motion of the cylinder. Weare now in a position to raise the piston hydraulically in a matter of seconds, over thefull course of 2 or 3 inches used in these experiments without introducing any air bubblesinto the system. A maximum traverse of 6 inches can be made with the present syringe.4. Thesis Research on Silver Conductivity and Cavity Diameter MeasurementsTwo theses have been completed in connection with this experimental work. ArthurT. Funkhouser has completed a Bachelor's thesis entitled "A Method for Measuring theDistance between Two Fabry-Perot Interferometric Plates," and J. F. Brenner has com-pleted a Bachelor's thesis entitled "On Measuring the Electrical Conductivity of Thin,Deposited Metallic Films."The results of this work will be used in our research and will be more fully describedelsewhere.5. Dry-run of Our ExperimentWe have now chemically silver-plated our fused-quartz cavity and are proceedingwith a dry-run of the entire experiment, including the mechanics, optics, microwaveelectronics, and recording of optical and resonance signals on magnetic tape.A view of the apparatus used in these experiments is shown in Fig. VI-9.6. Remark on This ExperimentThis experiment was started in 1955 by Professor J. R. Zacharias, Dean G. R.Harrison, G. W. Stroke, Professor S. J. Mason and C. L. Searle. l This measure-ment was proposed when there was a possibility of combining precise interfer-ometric length measurements in terms of light waves (developed at the Spec-troscopy Laboratory, M. I. T., by Dean Harrison, G. W. Stroke, and associates) withthe precise microwave frequency standard of the Cs 1 3 3 atomic clock, developedby Professor Zacharias and associates, for the purpose of attempting a measure-ment of the velocity of light in terms of primary standards, in a microwavecavity resonance experiment.The experiment was actively pursued by Zacharias, Searle, and G. W. Stroke untilthe end of 1958, by which time a precise fused-quartz cavity had been manufactured tooptical precision by the Optical Shop at Boston University, and the mechanical parts ofthe apparatus had been built by the R. L. E. Shop. Microwave equipment for the cavity(VI. MOLECULAR BEAMS)excitation and resonance detection had also been assembled, and a dry-run with anexperimental Pyrex cavity carried out.Work on the experiment was resumed in July 1961 by G. W. Stroke (QuarterlyProgress Report No. 63, p. 105) and was devoted to assembly of optical parts and testingof the apparatus in preparation for an actual measurement. Dr. Stroke has accepteda position as Professor of Electrical Engineering at the University of Michigan and willcomplete the experimental work there.G. W. Stroke, M. A. Yaffee, A. FunkhouserReferences1. J. R. Zacharias et al., Proposed measurement of the velocity of light in termsof two primary standards, Cesium (Cs 3 3 ) and Mercury (Hg 98), Quarterly ProgessReport, Research Laboratory of Electronics, M. I. T., January 15, 1956, pp. 59-68.

Molecular Beams

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Molecular Beams

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