2900-239-T Report of Project MICHIGAN X - BAND LADDER-LINE TRAVELING-WAVE MASER G. I. HADDAD J. E.'ROWE February 1961 57titut ~a Scaerce acEd 7ec4d~ THE U N I V E R S I T Y OF M I C H I G A N Ann Arbor, Michigan

NOTICES Sponsorship. The work reported herein was conducted by the Institute of Science and Technology for the U. S. Army Signal Corps under Project MICHIGAN, Contract DA-36-039 SC-78801. Contracts and grants to The University of Michigan for the support of sponsored research by the Institute of Science and Technology are administered through the Office of the VicePresident for Research. Distribution. Initial distribution is indicated at the end of this document. Distribution control of Project MICHIGAN documents has been delegated by the U. S. Army Signal Corps to the office named below. Please address correspondence concerning distribution of reports to: U. S. Army Liaison Group Project MICHIGAN The University of Michigan P. O. Box 618 Ann Arbor, Michigan ASTIA Availability. Qualified requesters may obtain copies of this document from: Armed Services Technical Information Agency Arlington Hall Station Arlington 12, Virginia Final Disposition. After this document has served its purpose, it may be destroyed. Please do not return it to the Institute of Science and Technology. *i

Institute of Science and Technology The University of Michigan PREFACE Project MICHIGAN is a continuing research and development program for advancing the Army's long-range combat-surveillance and target-acquisition capabilities. The program is carried out by a full-time Institute of Science and Technology staff of specialists in the fields of physics, engineering, mathematics, and psychology, by members of the teaching faculty, by graduate students, and by other research groups and laboratories of The University of Michigan. The emphasis of the Project is upon basic and applied research in radar, infrared, information processing and display, navigation and guidance for aerial platforms, and systems concepts. Particular attention is given to all-weather, long-range, high-resolution sensory and location techniques, and to evaluations of systems and equipments both through simulation and by means of laboratory and field tests. Project MICHIGAN was established at The University of Michigan in 1953. It is sponsored by the U. S. Army Combat Surveillance Agency of the U. S. Army Signal Corps. The Project constitutes a major portion of the diversified program of research conducted by the Institute of Science and Technology in order to make available to government and industry the resources of The University of Michigan and to broaden the educational opportunities for students in the scientific and engineering disciplines. Progress and results described in reports are continually reassessed by Project MICHIGAN. Comments and suggestions from readers are invited. Robert L. Hess Technical Director Project MICHIGAN

Institute of Science and Technology The University of Michigan CONTENTS Notices................................. ii Preface................................ iii List of Figures............................. vi Abstract..............................1 1. Introduction............................... 1 2. Traveling-Wave Maser Theory....................... 2 3. Maser Slow-Wave Structures..................... 4 4. Karp-Type Slow-Wave Structure..................... 6 5. Phase Shift vs. Frequency for the Karp Structure............12 6. Traveling-Wave Maser Assembly.................. 14 7. Experimental Results......................... 16 8. Conclusions....................1..........20 References................................21 Distribution List................................22 V

Institute of Science and Technology The University of Michigan FIGURES 1. Ladder-Line RF Structures....................... 6 2. TWM Structure............................ 8 3. Karp Structure Equivalent Circuit................... 8 4. Cutoff Conditions of the TE10 Mode in a Ridged Waveguide........ 10 5. Cutoff Conditions for a Dielectric Loaded Waveguide......... 10 6. Phase Shift per Section, 0, vs. Frequency with CZ as a Parameter... 12 7. Phase Shift per Section, 0, vs. Frequency with d as a Parameter. 1. 13 8. vg/p vs. Frequency for Various Values of CZO............ 14 9. Phase Shift per Section, 0, vs. Frequency for the Double-Ridge Karp Structure...................... 14 10. Assembled Traveling-Wave Maser.................. 15 11. Disassembled Traveling-Wave Maser.1............... 16 12. The Output Power from the Slow-Wave Structure Showing the Effect of the Dark Ruby Isolator................... 17 13. Electronic Gain in the TWM, Case 1............... 19 14. Electronic Gain in the TWM, Case 2................. 19 15. Electronic Gain in the TWM, Case 3................. 19 vi

X-BAND LADDER-LINE TRAVELING-WAVE MASER ABSTRACT The factors to be considered in the design of a broadband traveling-wave solidstate maser are outlined, with particular consideration given to the choice of an rf propagating structure. Desirable structure characteristics for a TWM (travelingwave maser) are low loss, high slowing, ease of coupling between the pump signal and the paramagnetic crystal, simple, rugged construction, and easily accessible regions of different senses of circular polarization of the rf magnetic field. An X-band ruby TWM has been constructed and tested in which the rf structure is a double-ridge ladder line and the signal is coupled into and out of the structure with coaxial lines. The pump power is propagated in a waveguide mode and the device is operated at and below liquid-helium temperatures. A 4-inch electromagnet was used. The TWM was operated at the push-pull point with a pump frequency of 24 kmc and a signal frequency of approximately 9. 65 kmc; the magnetic field was 4. 1 kilogauss. Gross gains of 15 db have been obtained and bandwidths as high as 130 mc were observed. Reduced structure losses and longer sections of ruby promise a squareroot voltage gain times the bandwidth (G /2B) of 1100 mcs (30 db over 35 mc). 1 INTRODUCTION In 1956, Bloembergen proposed an excitation scheme through which continuous microwave amplification could be obtained by utilizing three energy levels of a paramagnetic material, called the three-level solid-state maser (Reference 1). This scheme employs a microwave pump signal to transform the material from an absorptive to an emissive state when such a material is stimulated by radiation at the signal frequency. His prediction that it would exhibit very low noise characteristics generated a great deal of interest in the maser, and since then this principle has been successfully utilized to construct maser amplifiers of both the cavity and traveling-wave types (References 2 through 6). In a cavity maser the microwave radiation is coupled to the paramagnetic salt by making the cavity, containing the paramagnetic salt, resonant at both the signal and pump frequencies. Such masers, however, do not utilize the full line width of the material because of the high degree of regeneration required to obtain a significant amount of gain. This regeneration, in turn, limits the bandwidth of operation. A measure of the performance of these systems is the product of the square-root voltage gain times the bandwidth in megacycles. Representative 1

Institute of Science and Technology The University of Michigan values of G 1/2B for cavity masers vary from 40 to 100 mcs. Various schemes such as resonant coupling and stagger tuning several cavities have been proposed to increase achievable values of G1/2 B. Square-root gain bandwidth products of 200-300 mcs have been obtained using resonant coupling of maser cavities at X-band. A TWM, on the other hand, employs a slow-wave structure in place of the resonant cavities and should permit larger instantaneous bandwidths. The velocity of energy propagation along the rf structure can be made very low so that effective coupling between the microwave radiation and the spins of the paramagnetic salt can be obtained. Unidirectional gain can be obtained if a slow-wave structure which has separated accessible regions of circular polarization of the rf magnetic field is chosen. A material with circularly polarized signal frequency transitions can then be located in the structure in such a manner that coupling between the rf field and the material is obtained for only one direction of propagation. When the material is located in this way, and a pump signal of the proper frequency is employed, the material exhibits a negative resistance as viewed from the rf structure and an exponential growing wave results in the one direction, while little or no growth is obtained in the reverse direction. Unidirectional loss can also be incorporated in the structure in the same manner as gain. A high degree of isolation can be obtained by using either a highly doped paramagnetic material, whose thermal equilibrium cannot be disturbed by the pump power, or a ferrimagnetic material such as yttrium iron garnet. Because unidirectional gain and loss can be obtained in aTWM, freedom from regenerative instability can be insured with the result that the TWM has a higher degree of gain stability than the cavity maser. This is of great practical importance in many systems applications; e. g., in radiometers and radio telescopes. Since the passband of the slow-wave structure can be made much larger than the material line width as defined by the magnetic susceptibility curve, full utilization of the line width can be achieved so that a larger instantaneous bandwidth than that of the cavity results. Another advantage of the TWM is the possibility of electronic tuning over a wider frequency range. This tuning is accomplished by changing the pump frequency and the d-c magnetic field. 2 TRAVELING-WAVE MASER THEORY The gain expression for a TWM has been derived in the literature (Reference 5). One form is: 27. 3SN 2

Institute of Science and Technology The University of Michigan where S = c- = the electromagnetic wave slowing factor V g N = -= the number of free-space wavelengths along the structure 0 Q = the magnetic Q, determined by the material and the filling factor in the structure c and v = the velocities of light and of energy propagation along the structure, respecg tively It is clear from Equation 1 that in order to obtain high gain, the slowing factor should be as high as possible and the magnetic Q as low as possible for the frequencies of interest. For a typical magnetic Q of 150, a slowing factor of 50, and a structure length of 4 inches, a gain of approximately 30 db at 10 kmc can be obtained. It has also been shown by Degrasse et al. (Reference 5) that, assuming a Lorentzian line shape, the 3-db bandwidth of a TWM is approximately given by B=B 3 (2) m NGdb- 3 where B is the frequency bandwidth over which the imaginary part of the complex permeability is greater than one-half its peak value. For a gain of 30 db the bandwidth is approximately equal to 0. 335 B. B depends on both the material and the point of maser operation that is m m employed. For ruby, which has been found to be a good maser material, a typical value of Bm at 6 kmc and at an angle of 900 between the d-c magnetic field and the ruby c-axis is approximately 60 mc. At the push-pull point and for X-band maser operation, B is of the order of 120 mc. Thus a gain of 30 db over a bandwidth of more than 35 mc would be obtained at this point of operation. The group velocity in a slow-wave structure can be related to the pitch and the width of the passband as follows. For the fundamental mode of a periodic structure operating in the passband, the phase shift per section varies between fL = O and 3L = iT, where L is the pitch and jL is the phase shift per section. Therefore, dw Aw 27 Af v = = =2LAf (3) g do AO AO The slowing factor S of Equation 1 can then be written as v = 2 L —f = 2-L )f ) g

Institute of Science and Technology The University of Michigan It can easily be seen from Equation 4 that the smaller the pitch and the passband the higher the slowing factor. Since the gain in the TWM is directly proportional to the slowing factor, both the pitch and the passband of the slow-wave structure must be made as small as possible. This requirement results in TWM rf structures which have a very small pitch for a given frequency. 3 MASER SLOW-WAVE STRUCTURES A slow-wave structure which is suitable for TWM operation must possess the following characteristics: (1) The structure must be designed so that a high degree of slowing is achieved in the signal energy propagation along the structure; i. e., high slowing factors should be obtainable in such a structure. (2) It must possess different senses of circular polarization of the rf magnetic field in different regions which are readily accessible. This makes unilateral gain and loss possible. (3) The insertion loss due to ohmic losses in the structure must be as small as possible. This, of course, will improve the net gain of the amplifier. (4) The structure must be constructed so that provision can be made to allow effective coupling between the pump signal and the paramagnetic material. This could be in the form of either a broadband resonant mode or a propagating mode. (5) Relatively large size (compared with X0), ease of manufacturing, ruggedness, and easy means of coupling the signal power into and out of the structure conclude the list of features a slow-wave structure must have to be applicable in a TWM scheme. Several structures which have been discussed in the literature (Reference 5) possess the characteristics cited above and a few of them (References 4-6) have been successfully applied to construct TWM amplifiers. These amplifiers have been operated in the frequency range of 6 kmc and lower (References 4-6). A double-ridge Karp structure has been employed at the Electron Physics Laboratory to develop a TWM at X-band frequencies. A detailed discussion of the characteristics of this structure and its utilization in a TWM is presented in the following sections. 4

Institute of Science and Technology The University of Michigan There are several means for achieving significant wave slowing, including dielectric, geometric, resonant, and dispersive slowing. In dielectric slowing, perhaps the most familiar of these methods, the group velocity of the wave is proportional to 1/V-. A slowing factor of 50 would therefore require a material with a dielectric constant of 2500. Since available materials with such a high dielectric constant are generally lossy, unstable, and temperaturesensitive, this method of slowing is not considered very practical. Materials such as titanium dioxide with an e = 100 are particularly useful in TWM's operating above X-band, where many free-space wavelengths can be obtained in a short physical distance. It is also desirable that the Qm be relatively small. If dielectric slowing is used in a rectangular waveguide and if the energy is propagated in the TE10 mode, the size of the waveguide is also reduced by the square root of Er, which can become very tiny at high frequencies and thus very difficult to fabricate. Geometric slowing is best exemplified by the helix, which is a nonresonant periodic rf structure. The wave travels in the direction of the helix wire with the velocity of light, while the axial phase velocity is reduced in proportion to the number of turns per wavelength. High slowing can be obtained in a helix over a large bandwidth. However, since the field configuration is not favorable, a spiraling d-c magnetic field would be required to orient the d-c field perpendicular to the circularly polarized rf field. The various types of ladder structures, e. g., the interdigital line, the comb structure, and the Karp structure, are examples of resonant slowing. In these structures ladder elements extend from the walls into the region between the walls and in some cases are connected to the walls at both ends. Each period of the ladder structure is a resonant element, and the energy is reflected back and forth between the side walls in the form of a propagating TEM wave. These types of periodic structures have a definite passband with associated upper and lower cutoff frequencies. As shown in Equation 4, the narrower the passband in such structures and the smaller the pitch, the higher the slowing. As a result, slowing factors of very high magnitudes can be achieved in these structures. Limits do exist, however, because the passband of the structure must be at least as large as the material line width, and because the pitch must not be so small as to prevent effective interaction between the rf fields and a large number of spins. This lower pitch limit is due to the fact that, as the spacing between fingers becomes smaller, the fields become more tightly bound to the fingers. Dispersive slowing is obtained by operating very close to the cutoff frequencies of a microwave filter, whether of the low-pass, high-pass, or bandpass type. Very high slowing factors can be obtained in such a region of operation, but the Q of the structure must be high to avoid excessive amounts of insertion loss. 5

Institute of Science and Technology The University of Michigan 4 KARP-TYPE SLOW-WAVE STRUCTURE The Karp-type slow-wave structure consists of a planar array of conductors which are situated in a waveguide in such a manner that they are electrically connected to the side walls. The waveguide can be either double- (Figure lb) or single-ridge (Figure lc). When the ridges are not present the structure becomes nonpropagating, because the electric and magnetic couplings between adjacent rods cancel. Since this structure, known as the Easitron structure (Figure la), has a zero passband, it must be loaded in some manner to obtain a finite passband. The double-ridge Karp structure has been employed here in developing a TWM at Xband frequencies. The characteristics of the double-ridge structure are very similar to those of the single-ridge structure and will be discussed in detail in Section 6. (a) (b) (c) FIGURE 1. LADDER-LINE RF STRUCTURES. (a) Easitron nonpropagating structure. (b) Double-ridge Karp propagating structure. (c) Singleridge Karp propagating structure. The arrows indicate the sense of circular polarization. The Karp-type structure possesses all of the characteristics essential to a TWM application. Very high slowing factors can be obtained by adjusting the dimensions and by properly loading the structure with a dielectric which in this case can be the active maser material itself. The magnetic field has different senses of circular polarization above and below the plane of the fingers. Therefore, unilateral gain and attenuation can be obtained by placing the active 6

Institute of Science and Technology The University of Michigan maser material and the isolation material below and above the fingers, respectively. The magnetic field intensity is maximum at the shorted ends of the fingers and zero in the middle. The electric field is maximum in the middle of the fingers and zero at the shorted ends. Thus, by placing a dielectric material such as ruby next to the side walls, effective magnetic coupling can be achieved without producing high dielectric loading, which might otherwise affect the structure characteristics considerably. The microwave pump power, which is necessary to invert the spin population in the paramagnetic crystal, can be propagated in the ridged waveguide which encloses the ladder line. If this power propagates through the waveguide in a TEl0 mode, and if the ladder line is placed in the middle of the waveguide (Figure lb), a minimum amount of coupling between the waveguide mode and the ladder will result. The waveguide can be shorted at one end and a coupling iris placed at the other to enhance the coupling of the pumping field to the maser crystal. The group velocity, which is needed to compute the gain, and the tunable bandwidth of the maser can be computed from the frequency vs. phase-shift-per-section characteristics of the slow-wave structure. Since these characteristics are essentially the same in the double- and single-ridge Karp structures, the phase vs. frequency characteristics of the single-ridge structure have been analyzed in detail. The effect of dielectric materials placed in different regions of the structure, namely, in the middle and at the sides, has also been considered. Figure 2 shows a single-ridge Karp structure. The material on the sides has a relative dielectric constant, erl, which in this case is that of the active maser material, ruby, whose e = 9. The material between the ridge and the ladder has a relative dielectric constant er2. This material can be either rutile (E; 100) or air (er = 1). A useful equivalent circuit for this structure is shown in Figure 3. The fingers are considered as two shorted transmission lines with a characteristic impedance of ZO. L and C include the effects of the ridge and the dielectric material. For the equivalent circuit of Figure 3, the lower frequency cutoff occurs when the admittance of the shunt arm is equal to zero. This can be expressed as 1 Yshunt j27rf L +j27TfC =0 (5) c where f is the lower cutoff frequency. L is then expressed as L= 2 (6) c C

Institute of Science and Technology The University of Michigan Ladder Ridge t Er d _ _ r 1 r - FIGURE 2. TWM STRUCTURE a/2 FIGURE 3. KARP STRUCTURE EQUIVALENT CIRCUIT

Institute of Science and Technology The University of Michigan where o = 2rf. The susceptance of the shunt arm is conveniently written as C C shunt =jOC - If the series arm in Figure 3 is considered as two shorted transmission lines in parallel, the susceptance of this arm, B series' becomes Bseries =-j2YO cot 2 (8) where a = the finger length Y0 = the characteristic admittance of the fingers considered as a transmission line 3 = phase shift in radians per unit length If fl is taken as the frequency at which the slot length a is one half a free-space wavelength, then Equation 8 becomes series = j2Y cot (2 f) (9) Filter theory is used to relate the phase shift per section, 0, to the admittances of the series and the shunt arms through the expression Bshunt cos 0 = 1 +s (10) 2B series Substituting the values of B and B from Equations 7 and 9 into 10 yields shunt series cos =1- 2Tf (4) -(f )tan (2) (11 Thus for a certain fc' fl' and CZ0, a phase shift vs. frequency curve can be obtained. The lower cutoff frequency is determined by the waveguide dimensions, by the kind of dielectric material that is employed, and by the location of this material in the waveguide. The cutoff conditions in a ridged waveguide have been presented in the literature (References 7 and 8) and are plotted in Figure 4 for convenience. The cutoff conditions for a dielectric loaded waveguide, which are of interest here, have also been plotted in Figure 5. These graphs enable one to obtain the lower cutoff frequency of the structure and also the cutoff frequency of the TE20 mode, which is of interest when the pump power is propagated through the structure in a waveguide mode. 9

Institute of Science and Technology The University of Michigan d/b - 0. 1 0 0.1 0.2 0.3 0. 4 0.5 0.6 c/a FIGURE 4. CUTOFF CONDITIONS OF THE TE10 MODE IN A RIDGED WAVEGUIDE b/a = 0. 25. 24 4 5 br r2 r 1~~ ~~2 ~~ 10 0 0.2 0.4 0.6 0.8 1.0 0 0.2 0.4 0.6 0.8 1.0 (a) c/a (b) c/a FIGURE 5. CUTOFF CONDITIONS FOR A DIELECTRIC LOADED WAVEGUIDE. (a) e = 9, e = 100. (b) E = 9, E = 1. 1 2 10

Institute of Science and Technology The University of Michigan The frequency fl is fixed by a choice of the length of the fingers. This choice and the amount of capacitive loading that is introduced by the ridge and the dielectric determine the upper cutoff frequency of the slow-wave structure. The amount of capacitive loading is measured by C, the capacitance of the shunt arm, and can be approximately expressed in terms of the dimensions and the dielectric constant through the expression C E Ec(p - x) (12) d where p = the pitch of the ladder line d = the distance between the ridge and the ladder c = the width of the ridge x = the width of the slot in the ladder E = the dielectric constant of the material between the ridge and the ladder The above equation gives an approximate expression for C. The major contribution to C arises from loading at the middle because the electric field is maximum there and becomes zero at the side walls. This value will be increased, however, when a dielectric material is placed on both sides of the ridge as is done in a TWM. The increase depends on the distance the material extends from the side walls to the ridge and also on the distance from the ladder line to the bottom plate. From transmission line theory, the characteristic impedance Z0 of Equation 11 can be approximately expressed as x Z 120 {n- (13) 0 t where t = the thickness of the ladder line. Thus, CZ0, which appears in Equation 11, can be written as 120~cp 1 - CZ0 = 2dP(1 ) n - (14) Thus for a certain set of dimensions of the structure, fc' f1, and CZ0 can be calculated so that Equation 11 can be used to calculate a phase shift vs. frequency curve. From this plot the group velocity can be calculated at any frequency in the passband. Several of these curves have been plotted and are presented in Figures 6-9. 11

Institute of Science and Technology The University of Michigan 5 PHASE SHIFT vs. FREQUENCY for the KARP STRUCTURE Figure 6 illustrates the effect of CZ0 on the characteristics of the structure. It can be seen from these curves that for a certain lower cutoff frequency, fc, and a certain f1, the upper cutoff frequency is decreased when CZ0 is increased. This decreases the structure passband and higher slowing factors result. CZ0 is related to the structure dimensions and to the dielectric constant of the material through Equation 14. 180 160 - 140 - t -:120 i t 8.12 0 9.5 10.0.10 /, 80 60 40 00 8 8. 5 9. 0 9. 5 10. 0 10. 5 11. 0 11. 5 FREQUENCY (kmc) FIGURE 6. PHASE SHIFT PER SECTION 0, vs. FREQUENCY WITH CZO AS A PARAMETER. fc = 8 kmc, fl = 11 kmc. It is obvious from this relation that CZ0 can be increased in several ways. One way is to decrease d, the distance between the ladder and the ridge. This, however, has a slightly negative effect in that the lower cutoff frequency is decreased when d is decreased while the other dimensions are kept the same. The net effect is favorable, however, because the upper cutoff frequency decreases more rapidly than the lower one, which results in the passband becoming narrower. This effect is illustrated in Figure 7. Another way to increase CZ0 is to increase the dielectric constant of the material between the ridge and the ladder. This has the same effect as decreasing d. 12

Institute of Science and Technology The University of Michigan 180 - 140 — / (D, ~ -ii- d/b= 0.08 (D100 60 2 3 4 5 6 7 8 9 10 11 FREQUENCY (kmc) FIGURE 7. PHASE SHIFT PER SECTION, 0, vs. FREQUENCY WITH d AS A PARAMETER The group velocity, v, which can be easily derived from Equation 11, is expressed as follows: v (7rf8 - 27Tf [l- ) tan(2 (15) [ I - )] an(f) 2 + Tf P - 1/2 f / andI Tf cfi Graphs of vp vs. frequency for different values of CZ and specified values of f and f are Graphs of v-p vs. frequency for different values of CZgand specified values of fcand f1are shown in Figure 8. These plots show that v can be made as small as desirable, either by decreasing p or by increasing CZ0, while keeping f fixed. There are limits, however, on how small the pitch can be made. These were mentioned in Section 3. CZ0 can be increased without affecting the lower cutoff frequency by increasing the ratio x/t. It has been found that the passband can be narrowed and that higher slowing factors can be obtained by properly loading the structure with a dielectric material. This is illustrated in Figure 9, where the characteristic of an unloaded structure is compared with that of a rubyloaded structure. For the unloaded structure the lower and upper cutoff frequencies were measured and found to be 8. 5 and 12. 5 kmc respectively. The loaded structure showed a lower cutoff frequency of 7 kmc and an upper cutoff frequency of 10 kmc. This structure was employed in the TWM experiments. Detailed discussions of the TWM assembly, the double-ridge Karp structure, and the experimental results are given in the following sections. 13

Institute of Science and Technology The University of Michigan 20 18 16.. 180'~~~~~1~ o~~ ~160 Ruby-Loaded,-2t 140 Structure (b) 10, O,. X 120 lo120 100 6 8 — I 40 ---- 0 0 8.0 9.0 10.0 11.0 7.0 7.5 8.0 8.5 9.0 9.5 10.010.010.5 FREQUENCY (kmc) FREQUENCY (kmc) FIGURE 8. vg/p VS. FREQUENCY FOR VARIOUS FIGURE 9. PHASE SHIFT PER SECTION, 0, VALUES OF CZ0. p is the pitch. fc = 8 kmc, fl = vs. FREQUENCY FOR THE DOUBLE-RIDGE 11 kmc. KARP STRUCTURE 6 TRAVELING-WAVE MASER ASSEMBLY A double-ridge Karp-type slow-wave structure was employed in the TWM experiments. The ladder line of this structure has an over-all length of 4 inches, but the effective length of the interaction region is approximately 3.5 inches because the diameter of the pole pieces of the magnet is only 4 inches. The ladder line is photo-etched and made out of copper. It has a pitch of 0. 040 inch, a separation between fingers of 0. 020 inch, and a finger length of 0. 44 inch. The phase shift vs. frequency characteristics of this structure are given in Figure 9. The active material is light ruby, which has approximately 0. 05% Cr+ concentration. Both dark ruby, which has approximately 1% Cr concentration, and polycrystalline yttrium iron garnet have been used for isolation. An assembly view of the TWM is shown in Figure 10. The two coaxial lines provide the input and output coupling of the signal power to the structure. A sliding short which is adjustable by a screw movement, as well as the K-band waveguide through which the pump power is coupled into the structure, can also be seen in this figure. A provision for lowering the liquid 14

Institute of Science and Technology The University of Michigan FIGURE 10. ASSEMBLED TRAVEELING-WAVE MASER helium temperature by using a large forepump in a closed cryogenic system has also been included in the assembly. However, no provision has been made to measure the absolute value of the bath temperature. A closeup of the interaction region is presented in the disassembled views of Figure 11, and the coupling of the signal power into and out of the structure is clearly shown. The outer conductors of the coaxial lines are terminated at one side wall of the structure, while the center conductors extend to the other side wall. These portions of the center conductors are flattened out to the desired thickness and located in the plane of the ladder. Figure 11(b) shows the location of the flattened portions relative to the ladder line. The location of the ruby crystals is also shown in the above figures. Two slabs (each 2 inches long) of light ruby, which is the active material, are placed on each side of the ridge of the bottom plate. These slabs extend all the way along the 4-inch ladder line and are located so that they touch the fingers. The four slabs of dark ruby which provide the isolation are similarly located in the top plate and do not touch the fingers but are very close to them. In recent experiments, however, the dark ruby slabs have been replaced by light ruby, and a slab of polycrystalline yttrium iron garnet was placed beside the ruby pieces in the top plate helium~~~ ~~ teprtr yuigalrefrp na lsdcygncsse a lobe n cluded in the~ ~ ~~~~~i. assembl.Hoevr,:o:roiso ha.en.aet.maueth.bout au of~~~~~~~~~~.'the:':th tem:raure A lsupo heitrato rgo i rsntdi tedsasmle:'wso igr 1 an h opigo h ina oe noadoto hesrcuei lal hw...eoue conductors of...... coxa line are terminated.....................tre whlethecete 15

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Institute of Science end Technology The University of Michigan FIGURE 1.2. T'l1 OTPU PWE FROMAN TH1E SLOX'-WAX'I ST1t"UC-~ TUllE SIIOWNG Et TlEVEFCTO TIlEJ'Al)ARK ItIHI ISOLATOR1 of approximately'24 kmc(,, the coyrre spondi ng simal frequency is a round 9. 65 kmic and the ma"netic field is netar 4. 1 kilog,.auss. TV insertion loss inl the neiAghborhood of the siga f reque ncy is a11-)proximatelIy 1I- 13 db11 at room temperaturc. At liquidhlu tempcratures this loss diecrecased to 8-9 (lb. Thiis loss inchudes the input and output coaixial. hues, which heay -anl attenuation of approximately 0. 4 db1 per foot at rJoom temperature. A total of app~roxima"tely 7 feet of coaxial line was used.'The insertion loss can be:made lower by using wa-vef..uid.s instea<-d of co.a-xi.aI caIles to couple the signal power into a-nd out of the structure, but this would mncAke the masur assembly anid (Iewar quite bulky. A better w~ay to-. improve the loss in the.st'ucture would be to impr'ove the a-ssemibly. Since the magnetic fields; a-re str'ongest at the side wal-:-ls where the top and~ bottom plate.s are ftastened toge'ther~, a very good contact at these points will minimize the losses. The screws, lone might not provide a good contact. Whenfou slb.' f drk uby(two da-bs placed onl each side of the ridlgt~ of the to-p pl:Ate as shown in Figu re- 11(b) were employed fo- isolation, an a,bsorption of 16..17 db1 was obtained with he dc maneti feld- in the- reer ietoantnasrtof prxmtl 2d

Institute of Science and Technology The University of Michigan at a bath temperature of 4. 2~K, and the results are shown in Figure 12. Even though the dark ruby material provides good isolation it was found difficult to align the c-axes of the several ruby crystals in order to obtain amplification from the light ruby and absorption from the dark ruby at the same value of magnetic field. The dark ruby also absorbs an appreciable amount of pump power, which renders its application as the isolating material less attractive. In recent experiments on the TWM, the dark ruby pieces in the top plate were replaced by light ruby pieces. A slab of yttrium iron garnet, cut to the proper size for ferromagnetic resonance and placed beside the light ruby pieces on one side of the ridge in the top plate, was employed for isolation. A forepump was employed to pump the liquid helium in order to reduce its temperature. It was observed that the electronic gain was 2. 5 times higher with the forepump than without it. As mentioned previously the dewar design did not include a provision for measuring the absolute value of the temperature. A high degree of nonreciprocity was achieved by using the garnet slab. When the structure was operated simply as an isolator the ratio of the absorption in the backward direction to that in the forward direction was approximately 15. The addition of the garnet slab greatly improved the passband of the structure. When the maser was operated with no isolation, a high degree of regeneration and very large amounts of regenerative gain and oscillations were observed. The bandwidth, on the other hand, decreased sharply. When sufficient isolation was incorporated in the structure, gross gains of 15 db over a bandwidth of 75 mc were obtained. The square-root gain bandwidth was 420 mcs. This amount of gain was obtained, however, at two slightly different signal frequencies by changing the d-c magnetic field while holding the pump frequency fixed. Also, for a certain signal frequency, gross gains of approximately 15 db over a 75-mc bandwidth were obtained at two slightly different values of d-c magnetic field and at two corresponding values of pump frequency. This is attributed to the misalignment of the c-axes of the light ruby slabs. Figures 13 and 14 clearly illustrate the results mentioned above. It can be deduced from the above results that if the c-axes of the ruby slabs were well aligned, gross gains of approximately 30 db over a bandwidth of possibly 35-40 mc could have been obtained. This corresponds to a G 1/2B of greater than 1100. Attempts to align the c-axes of the available ruby slabs have failed so far, but alignment is certainly possible. In one of the experiments, the c-axes of the ruby slabs were close enough together to result in gross gains of approximately 13 db over 130 mc bandwidth (G1 /2B = 585). By slightly 18

Institute of Science and Technology The University of Michigan changing the magnetic field the gain was increased to 16 db over 70 mc. The total loss in the structure, including the isolator, was approximately 8 db, which left a net gain of 5 db over 130 mc in one case and a net gain of 8 db over 70 mc in the other. This is illustrated in Figure 15. The amplification in the TWM can be tuned over a frequency range of 9.45 to 9.85 kmc (400 mc) by adjusting the pump frequency and the magnetic field.?~~~~~~~ ( a) _ _ ( FIGURE 13. ELECTRONIC GAIN FIGURE 14. ELECTRONIC GAIN IN THE FIGURE 15. ELECTRONIC IN THE TWM, CASE 1. (a) The TWM, CASE 1. (a) The gain at a certain GAIN IN THE TWM, CASE 3. gain at a certain signal frequency signal frequency and at corresponding val- (a) The output power with and at correspondingvalues of d-c ues of d-c magnetic field and pump fre- zero d-c magnetic field. (b) magnetic field and pump frequency. quency. (b) The gain at the same signal The output with d-c field ad(b) The gain at a slightly higher frequency but at slightly different values justed to resonance and pump signal frequency and at a slightly of d-c magnetic field and pump frequency. power on. (c) The output at different value of d-c magnetic (c) The output of the TWM with zero pump a slightly different value of field but at the same pump power but with the d-c magnetic field ad- magnetic field. frequency. justed to resonance. (d) The output with zero field applied. 19

Institute of Science and Technology The University of Michigan 8 CONCLUSIONS It can be concluded that operation of a TWM at X-band utilizing a Karp-type slow-wave structure is feasible. Gross gains of 30 db over a 35-mc (G 1/2B; 1100) bandwidth can be obtained at the push-pull point of operation. Even broader bandwidths can be achieved by staggertuning either the magnetic field or the ruby crystals. The major difficulty has been the alignment of the c-axes of the several ruby slabs, a difficulty which can certainly be minimized by a more careful procedure in cutting the pieces. The use of either dark ruby or polycrystalline yttrium iron garnet as the isolating material has been demonstrated and both were found feasible. Gross rather than net gain has been emphasized in the measurements because this gives a better measure of the effectiveness of the slow-wave structure as a wave-slowing device. The circuit losses can be improved by fabrication and assembly techniques. The application of a Karp-type slow-wave structure as a TWM circuit can be easily extended into the millimeter wave range. In this range, a very small length of the structure would be required to obtain adequate gain. REF E R E N C E S 1. N. Bloembergen, "Proposal for a New-Type Solid State Maser," Phys. Rev., October 1956, Vol. 104, p. 324. 2. H. E. D. Scovil, G. Feher, and H. Seidel, "Operation of a Solid State Maser," Phys. Rev., January 1957, Vol. 105, p. 762. 3. H. E. D. Scovil, "The Three-Level Solid State Maser," IRE Trans. on Microwave Theory and Tech., January 1958, Vol. MTT-6, No. 1, p. 29. 4. W. S. C. Chang, J. Cromack, and A. E. Siegman, "Cavity and Traveling-Wave Masers Using Ruby at S-Band," 1959 IRE WESCON Convention Record, Part 1, p. 142. 5. R. W. Degrasse, E. O. Schulz-Dubois, and H. E. D. Scovil, "The Three-Level Solid State Traveling-Wave Maser," Bell Telephone System, Tech. Publs. Monographs, March 1959, Vol. 38, No. 3213. 20

Institute of Science and Technology T he University of Mic h i ga n 6. H. D. Tenney, R. Roberts, and P. H. Vartanian, "An S-Band TravelingWave Maser,"1 1959 IRE WESCON Convention Record, Part 1, P. 151. 7. N. Marcuvitz, Waveguide Handbook, MIT Radiation Laboratory Series, McGrawHill, New York, N. Y., 1951, Vol. 10, p. 400. 8. 5. Hopfer, "The Design of Ridged Waveguides, " IRE Trans. on Microwave Theory and Tech., October 1955, Vol. MTT-3, No. 5, pp. 20-29. 9. W. S. Chang and A. E. Siegman, Characteristics of Ruby for Maser Applications, Technical Report Number 156-2, Electron Devices Laboratory, Stanford University, Stanford, Calif., September 1958 (UNCLASSIFIED). 10. C. Kikuchi, J. Lambe, G. Makhov, and R. W. Terhune, "Ruby as a Maser Material," J. Appl. Phys., July 1959, Vol. 30, p. 1061.

Institute of Science and Technology The University of Michigan PROJECT MICHIGAN DISTRIBUTION LIST 5 1 February 1961-Effective Date Copy No. Addressee Copy No. Addressee 1 Army Research Office, ORCD, DA 47 -48 Commander, Army Rocket & Guided Missile Agency Washingtoo 25, D. C. Redstone Arsenal, Alabama ATTN: Research Support Divisioo ATTN: Technical Library, ORDXR-OTL 2 Office, Assistant Chief of Staff for Intelligence 49 Commanding Off icer Department of the Army, Washingtoo 25, D. C. U. S. Army Transportatioo Research Command ATTN: Chief, Research & Development Braoch Fort Eustis, Virginia ATTN: Research Reference Center 3 Commanding General, U. S. Continental Army Command Fort Monroe, Virginia 50 Commanding General ATTN: ATSWD-G Ordnance Tank-Automotive Command, Detroit Arsenal 28251 Van Dyke Avenue 4-5 Commanding General Centerline, Michigan U. S. Army Combat Surveillance Agency ATTN: Chief, ORDMC-RRS 1124 N. Highland Street Arlington 1, Virginia 51 Commanding Officer, Ordnance Weapons Command Rock Island, Illinois 6-8 Office of the Chief Signal Officer AT:ODWG Department of the Army, Washington 25, D. C. AT:ODWG (6) ATTN: Chief, Combat Development 52 Commanding Officer Branch, Rsearch & evelopmen DivisionU. S. Army Diamond Ordnance Fuze Laboratories Branch, Reseach & Developent DivisionWashingtop 25, D. C. (7-8) ATTN: Chief, Signal Research OfficeATNOR L30 Research & Development DivisionATN OR L-0 53-55 Director, U. S. Army Engineer 9-37 Commanding Off icer Research & Development Laboratories U. S. Army Signal Research & Development Laboratory Fort Belvoir, Virginia Fort Monmouth, New Jersey (53) ATTN: Chief, Topographic, Engineer Department ATTN: SIGRA/SL-ADT (54) ATTN: Chief, Electrical Engineering Department 31-39 Commanding General (55) ATTN: Technical Documents Center U. S. Army Electronic Proving Ground 56 Director, Human Engineering Laboratory Fort Huachuca, Arizona Aberdeen Proving Ground, Aberdeen, Maryland ATTN: Technical Library 57 Commandant, U. S. Army Command & General Staff College 40 Director, Weapons Systems Evaluation Group Fort Leavenworth, Kansas Room 1E880, The Pentagon ATTN: Archives Washington 25, 0. C. 58 Commandant, U. S. Army Infantry School 41 Chief of Engineers Fort Henning, Georgia Department of the Army, Washington 25, D. C. ATTN: Combat Developments Office ATTN: Research & Development Division 59-60 Assistant Commandant U. S. Army Artillery & Missile School 42 Chief, Chief of Ordnance, Research &Development Division Fort Sill, Oklahoma Departent o the Amy, Wshingtn 25,D. C.61 Assistant Commandant, U. S. Army Air Def ense School ATTN: ORDTB, Research & Special Projects Fort Bliss, Texas 43 Commanding Officer, Army Map Service, Corps of Engineers 62 Commandant, U. S. Army Engineer School U. S. Army, Washington 25, 0. C. Fort Belvoir, Virginia ATTN: Document Library ATTN: ESSV-L 63 President, U. S. Army Infantry Board 44 Commanding General Fort Henning, Georgia Quartermaster Research & Engineering Command 6 rsdnU.Am rilr or U. S. Army, Natick, Massachusetts 64ore Siden, U.kS.aromya rilr or 45-46 Chief, U. S. Army Security Agency 65 President, U. S. Army Air Defense Board Arlington Hall Station, Arlington 12, Virginia Fort Bliss, Texas

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AD Div. 25/6 UNCLASSIFIED AD Div. 25/6 UNCLASFE Institute of Science and Technology, U. of Michigan, Ann Arbor I. Title: Project MICHIGAN Institute of Science and Technology, U. of Michigan, Aon Arbor I. Title: ProetMCIA X-BAND LADDER-LINE TRAVELING-WAVE MASER by G. I. II. Haddad, G. I., Rowe, J. E. X-BAND LADDER-LINE TRAVELING-WAVE MASER by G. I. II. Haddad, G.IRwe.E Haddad and J. E. Rowe. Rept. of Proj. MICHIGAN. Feb 61. III. U. S. Army Signal Corps Haddad and J. E. Rowe. Rept. of Proj. MICHIGAN. Feb 61. III. U. S. ArmySganop 21 p. inc. illus., 10 refs. IV. Contract DA-36-039 21 p. ind. illos., 10 refs. IV. Contract DA3I -3 (Rept. no. 2900-239-T( SC-78801 (Rept. no. 2900-239-T( SC-78110 (Contract DA-36-039 SC-78801( Unclassified report (Contract DA-36 -039 SC-78801) Unclassified report The factors to be considered in the design of a broadband travel- The factors to be considered in the design of a broadband traveling-wave solid-state maser are outlined, with particular consid- ing-wave solid-state maser are ootlined, with particular consideration given to the choice of an rf propagating structure. De- eration giveo to the choice of an rf propagating strnctore. Desirable structure characteristics for a TWVM (traveling-wave sirable stroctore characteristics for a TWVM (traveling-wave maser) are low loss, high slowing, ease of coupling between the maser( are low loss, high slowing, ease of coupling between the pomp signal and the paramagnetic crystal, simple, rugged con- pomp signal and the paramagnetic crystal, simple, rugged construction, and easily accessible regions of different senses of struction, and easily accessible regions of different senses of circular polarization of the rf magnetic field. circular polarization of the rf magnetic field. An X-band ruby TWM has been constructed and tested in which An X-band ruby TWM has been constructed and tested in which the rf structure is a double-ridge ladder line and the signal is Armed Services the rf structure is a double-ridge ladder line and the signal isAre couped ito ad ou ofthe trucure ithcoaxal lnes.TheTechnical Intormation Agency coupled into and out of the structure with coaxial lines. The Technical InforainAec (over( UNCLASSIFIED (over) UNCLASIFE AID Div. 25/f UNCLASSIFIED AD Div. 25/6 UNCLASIFE Institute of Science and Technology, U. of Michigan, Ann Arbor I. Title: Project MICHIGAN Institute of Science and Technology, U. of Michigan, Ann Arbor I. Title: Projc, ICIA X-BANTI LADDER-LINE TRAVELING-WAVE MASER by G. 1. II. Haddad, G. I., Rowe, J. E. X-BAND LADDER-LINE TRAVELING-WAVE MASER by G. I. II. Haddad, G.IRweC.E Haddad and J. E. Rowe. Rept. of Proj. MICHIGAN. Feb 61. III. U. S. Army Signal Corps Haddad and J. E. Rowe. Rept. of Proj. MICHIGAN. Feb 61. III. U. S. ArmySganop 21 p. incI. illus., 10 refs. IV. Contract DA-36-039 21 p. inc. illus., 10 refs. IV. Contract DAI -3 (Rept. no. 2900-239-T( SC-78801 (Rept. no. 2900-239-T( SC-78801 (Contract DA-36-039 SC-78801( Unclassified report (Contract DA-36-039 SC-78801( Unclassified report The factors to be considered in the design of a broadband travel- The factors to be considered in the design of a broadband traveling-wave solid-state maser are outlined, with particular consid- ing-wave solid-state maser are outlined, with particular consideration given to the choice of an rf propagating structure. De- eration given to the choice of an rf propagating structure. Desirable structure characteristics for a TWM (traveling-wave sirable structure characteristics for a TWM (traveling-wave maser) are low loss, high slowing, ease of coupling between the maser( are low loss, high slowing, ease of coupling between the pump signal and the paramagnetic crystal, simple, rugged con- pump signal and the paramagnetic crystal, simple, rugged construction, and easily accessible regions of different senses of struction, and easily accessible regions of different senses of circular polarization of the rf magnetic field. circular polarization of the rf magnetic field. An X-band ruby TWM has been constructed and tested in which An X-band ruby TWM has been constructed and tested in which the rf structure is a double-ridge ladder line and the signal is Armed Services the rf structure is a double-ridge ladder line and the signal is Armed Srie coupled into and out of the structure with coaxial lines. The Technical Information Agency coupled into and out of the structure with coaxial lines. The Technical InforainAec (over( UNCLASSIFIED (over( UNCLASFE

AD UNCLASSIFIED AD UNCLASSIFIED pump power is propagated in a waveguide mode, and the device DESCRIPTORS pump power is propagated in a waveguide mode, and the device DESCRIPTORS is operated at and below liquid-helium temperatures. A 4-inch Paramagnetic crystals is operated at and below liquid-helium temperatures. A 4-inch Paramagne electromagnet was used. The TWM was operated at the push- electromagnet was used. The TWM was operated at the push- Radiofreque pull point with a pump frequency of 24 kmc and a signal frequen- R requeny ampiers pull point with a pump frequency of 24 kmc and a signal frequen-Ruby Ruby Ruby cy of approximately 9. 65 kmc; the magnetic field was 4. 1 kilo- X-band cy of approximately 9. 65 kmc; the magnetic field was 4. 1 kilo- X-band gauss. gauss. Gross gains of 15 db have been obtained, and bandwidths as high Gross gains of 15 db have been obtained, and bandwidths as high as 130 mc were observed. Reduced structure losses and longer as 130 mc were observed. Reduced structure losses and longer sections of ruby promise a G1/2B of 1100 mcs (30 db over 35mc). sections of ruby promise a G1/2B of 1100 mcs (30 db over 35mc). UNCLASSIFIED UNCLASSIFIED AD UNCLASSIFIED AD UNCLASSIFIED pump power is propagated in a waveguide mode, and the device DESCRIPTORS pump power is propagated in a waveguide mode, and the device DESCRIPTORS is operated at and below liquid-helium temperatures. A 4-inch Paramagnetic crystals is operated at and below liquid-helium temperatures. A 4-inch Paramagnetic crytal electromagnet was used. The TWM was operated at the push- electromagnet was used. The TWM was operated at the push-reqency amp pull point with a pump frequency of 24 kimc and a signal frequen- Rapull point with a pump frequency of 24 kmc and a signal frequen- Ruby Ruby Ruby cy of approximately 9. 65 kmc; the magnetic field was 4. 1 kilo- X-band cy of approximately 9. 65 kmc; the magnetic field was 4. 1 kilo- X-band gauss. gauss. Gross gains of 15 db have been obtained, and bandwidths as high Gross gains of 15 db have been obtained, and bandwidths as high as 130 mc were observed. Reduced structure losses and longer as 130 mc were observed. Reduced structure losses and longer sections of ruby promise a G1/2B of 1100 mcs (30 db over 35 mc). sections of ruby promise a G1/2B of 1100 mcs (30 db over 35mc). UNCLASSIFIED UNCLASSIFIED

++ AD Div. 25/6 UNCLASSIFIED AD Div. 25/6 UNCLASSIFIED Institute of Science and Technology, U. of Michigan, Ann Arbor I. Title: Project MICHIGAN Institute of Science and Technology, U. of Michigan, Ann Arbor I. Title: Project MICHIGAN X-BAND LADDER-LINE TRAVELING-WAVE MASER by G. I. II. Haddad, G. I., Rowe, J. E. X-BAND LADDER-LINE TRAVELING-WAVE MASER by G. I. II. Haddad, G.I., Rowe, J. E. Haddad and J. E. Rowe. Rept. of Proj. MICHIGAN. Feb 61. III. U. S. Army Signal Corps Haddad and J. E. Rowe. Rept. of Proj. MICHIGAN. Feb 61. III. U. S. Army Signal Corps 21 p. incl. illus., 10 refs. IV. Contract DA-36-039 21 p. incl. illus., 10 refs. IV. Contract DA-36-039 (Rept. no. 2900-239-T) SC-78801 (Rept. no. 2900-239-T) SC-78801 (Contract DA-36-039 SC-78801) Unclassified report (Contract DA-36-039 SC-78801) Unclassified report The factors to be considered in the design of a broadband travel- The factors to be considered in the design of a broadband traveling-wave solid-state maser are outlined, with particular consid- ing-wave solid-state maser are outlined, with particular consideration given to the choice of an rf propagating structure. De- eration given to the choice of an rf propagating structure. Desirable structure characteristics for a TWM (traveling-wave sirable structure characteristics for a TWM (traveling-wave maser) are low loss, high slowing, ease of coupling between the maser) are low loss, high slowing, ease of coupling between the pump signal and the paramagnetic crystal, simple, rugged con- pump signal and the paramagnetic crystal, simple, rugged construction. and easily accessible regions of different senses of struction, and easily accessible regions of different senses of circular polarization of the rf magnetic field. circular polarization of the rf magnetic field. An X-band ruby TWM has been constructed and tested in which An X-band ruby TWM has been constructed and tested in which the rf structure is a double-ridge ladder line and the signal is Armed Services the rf structure is a double-ridge ladder line and the signal is Armed Services coupled into and out of the structure with coaxial lines. The Technical Information Agency coupled into and out of the structure with coaxial lines. The Technical Infoi g y (over) UNCLASSIFIED (over) UNCLASSIFIED ++ AD Div. 25/6 UNCLASSIFIED AD Div. 25/6 UNCLASSIFIED Institute of Science and Technology, U. of Michigan, Ann Arbor I. Title: Project MICHIGAN Institute of Science and Technology, U. of Michigan, Ann Arbor I. Title: Project MICHIGAN X-BAND LADDER-LINE TRAVELING-WAVE MASER by G. I. II. Haddad, G. I., Rowe, J. E. X-BAND LADDER-LINE TRAVELING-WAVE MASER by G. I. II. Haddad, G.I., Rowe, J. E. Haddad and J. E. Rowe. Rept. of Proj. MICHIGAN. Feb 61. III. U. S. Army Signal Corps Haddad and J. E. Rowe. Rept. of Proj. MICHIGAN. Feb 61. III. U. S. Army Signal Corps 21 p. incl. illus., 10 refs. IV. Contract DA-36-039 21 p. incl. illus., 10 refs. IV. Contract DA-36-039 (Rept. no. 2900-239-T) SC-78801 (Rept. no. 2900-239-T) SC-78801 (Contract DA-36-039 SC-78801) Unclassified report (Contract DA-36-039 SC-78801) Unclassified report The factors to be considered in the design of a broadband travel- The factors to be considered in the design of a broadband traveling-wave solid-state maser are outlined, with particular consid- ing-wave solid-state maser are outlined, with particular consideration given to the choice of an rf propagating structure. De- eration given to the choice of an rf propagating structure. Desirable structure characteristics for a TWM (traveling-wave sirable structure characteristics for a TWM (traveling-wave maser) are low loss, high slowing, ease of coupling between the maser) are low loss, high slowing, ease of coupling between the pump signal and the paramagnetic crystal, simple, rugged con- pump signal and the paramagnetic crystal, simple, rugged construction, and easily accessible regions of different senses of struction, and easily accessible regions of different senses of circular polarization of the rf magnetic field. circular polarization of the rf magnetic field. An X-band ruby TWM has been constructed and tested in which An X-band ruby TWM has been constructed and tested in which the rf structure is a double-ridge ladder line and the signal is Armed Services the rf structure is a double-ridge ladder line and the signal is Armed coupled into and out of the structure with coaxial lines. The Technical Information Agency coupled into and out of the structure with coaxial lines. The Technical Infori e (over() UNCLASSIFIED (over) UNCLASSIFIED ++

AD UNCLASSIFIED AD UNCLASSIFIED pump power is propagated in a waveguide mode, and the device DESCRIPTORS pump power is propagated in a waveguide mode, and the device DESCRIPTORS is operated at and below liquid-helium temperatures. A 4-inch Pcrystals is operated at and below liquid-helium temperatures. A 4-inch Paramaget tl electromagnet was used. The TWM was operated at the push- ar unc a l electromagnet was used. The TWM was operated at the push- ar ea ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~Radiofreqoeocy amplifiers pull point with a pump frequency of 24 kmc and a signal frequen- Radiofrequency amplifiers pull point with a pump frequency of 24 kmc and a signal frequen- Ruby poiot magnetic ~~~~~~~~Ruby Xubyo cy of approximately 9.65 kmc; the magnetic field was 4.1 kilo X-band cy of approximately 9.65 kmc; the magnetic field was 4. 1 kilo- Xband gauss. gauss. Gross gains of 15 db have been obtained, and bandwidths as high Gross gains of 15 db have been obtained, and bandwidths as high as 130 mc were observed. Reduced structure losses and longer as 130 mc were observed. Reduced structure losses and longer sections of ruby promise a G1/2B of 1100 mcs (30 db over 35mc). sections of ruby promise a G1/2B of 1100 mcs (30 db over 35mc). C;___ z UNCLASSIFIED UNCLASSIFIED O - AD UNCLASSIFIED AD UNCLASSIFIED pump power is propagated in a waveguide mode, and the device DESCRIPTORS pump power is propagated in a waveguide mode, and the device DESCRIPTORS is operated at and below liquid-helium temperatures. A 4-inch Paraagnetic crystals is operated at and below liquid-helium temperatures. A 4-inch Paramagnetc c tl electromagnet was used. The TWM was operated at the push- ar uncya s electromagnet was used. The TWM was operated at the push- ar el pull point with a pump frequency of 24 kmc and a signal frequen- Radofrequ ecy aplifirs pull point with a pump frequency of 24 kmc and a signal frequen- adiofrequ ency amplifiers Ruby Ruby cy of approximately 9. 65 kmc; the magnetic field was 4.1 kilo- X-band cy of approximately 9. 65 kmc; the magnetic field was 4.1 kilo- X-band gauss. gauss. Gross gains of 15 db have been obtained, and bandwidths as high Gross gains of 15 db have been obtained, and bandwidths as high as 130 mc were observed. Reduced structure losses and longer as 130 mc were observed. Reduced structure losses and longer sections of ruby promise a G1/2B of 1100 mcs (30 db over 35mc). sections of ruby promise a G1/2B of 1100 mcs (30 db over 35mc). UNCLASSIFIED UNCLASSIFIED