I n S-/i - _ ---- --- RADC-TDR-63- 509 4915-14-Q = RL-2117 MICROWAVE-PLASMA COMPONENTS STUDY TECHNICAL DOCUMENTARY REPORT NO. RADC-TDR63-509 December 1963 Techniques Branch Rome Air Development Center Research and Technology Division Air Force Systems Command Griffiss Air Force Base, New York Project No.5573, Task No.557301 (Prepared under Contract No. AF30(602)-2605 by Ahdrejs Olte and E. K. Miller, The University of Michigan, College of Engineering, Department of Electrical Engineering, Radiation Laboratory) I -- - —-

Suggested Keywords: Plasma Physics, Microwave Components ABSTRACT An equivalent microwave circuit is proposed to exhibit the main features of the electron cyclotron resonance interaction in a waveguide. The essential part of the circuit consists of a non-reciprocal transmission line and similarly coupled resonant circuit. The experimental effort on the plasma package is discussed and some of the experimental difficulties reviewed. PUBLICATION REVIEW This report has been reviewed and is approved. For further technical information on this project, contact EMATE, Mr. Joseph M. Schenna, X4251 0 C ^/ ^ /^======^ -—. Approved: - / SEP. SCHENNA / Projet Engineer Electron Devices Section Techni ues Branch ( i- f —) C i Approved: J^ C L -- ^p ARTHUR J. FROHLICH, Chief Techniques Branch Surveillance and Control Division

TABLE OF CONTENTS Page Introduction 1 I. Theoretical Derivation of Absorption in the Electron Cyclotron Resonance Isolator 1 II. Experimental Study of Plasma Package Stability 8 InI. Conclusions 14 IV. Future Plans 15 iii

LIST OF FIGURES Figure Page 3 2 Electron Cyclotron Resonance Isolator and its Equivalent Circuit Minimum Power Transmission Coefficient for Non-Uniform Magnetic Field as a Function of f and v with f:f-=3.9Gc p c c Schematic of Vacuum System The Plasma Package 9 3 4 11 12 v

INTRODUCTION This Quarterly Status Report presents work done on Air Force Contract AF 30(602)-2605 for the period 1 July to 30 September 1963+. Section I proposes an equivalent circuit for the electron cyclotron resonance Isolator. Section II presents the experimental work. Section III gives the conclusions, and Section IV discusses future work. I THEORETICAL DERIVATION OF ABSORPTION IN THE ELECTRON CYCLOTRON RESONANCE ISOLATOR This Is a continuation of previous work (Quarterly Status Report 4915-13-Q) in which we have developed a first order theory for the non-reciprocal absorption of the TM 11 mode In a square wavegulde that has a relatively thin transverse plasma slab parallel to tha waveguide wall and spaced about one quarter distance of the guide width from It. The primary approximation in the theory is the neglect of the electrical effect of the glass container of the plasma on the wave propagation, and the further assumption that the fields In the plasma have the same transverse variation as the TM Ad mode fields in the empty guide. These approximations are not severe because the absorption is expected to exhibit a stationary character with respect to a variation In thc fields. The non-reciprocal behavior of the device results from two facts: + NOTE: Although this Is called Quarterly Status Report No. 14, It should be noted that It covers work done in the second quarter of the second year. The first 12 reports were monthly. -1 -

1) lk(H ) = kl(-i o), where Ho is the magnetostatic field, and 2) the magneto-Ionic plasma slab Is placed In an asymmetric position with respect to the magnetic fields of the TM 11 mode. An Implicit assumption In the theory Is that only the TM 11 mode may propagate. But In our case we have a multlmode waveguide; the lower order modes TEl0 and TE01 may propagate as well. However, In the circuit for which we are using the solution we have only a section of the square waveguide which contains the magneto-lonic package. On both ends of this section are the TEMTM 1 mode transducers, as shown in Figure 1. In the same figure we have also presented an equlvalent microwave circuit which represents the main features of Interaction of the waves with this structure. Starting from the left hand side of the circuit we have elements as discussed below. 'The TEM mode of the co-axial line is represented by an Ideal transmission line of characteristic impedance, Zo and the propagation constant j3,o Both constants are real. If the reference planes are selected properly any loss -free four-port network may be replaced by an Ideal transformer, We have chosen to so present the TEM to TM i mode transducer. The section of the square wavegulde between the mode transducer and the magneto-Ionic package is represented by ideal transmission line of characteristic impedance ZI and phase constant j[31, both again real. This representation is with respect to the TM11 mode. The section of the square guide containing the -2 -

Electron Cyclotron Resonance Isolator and its Equivalent Circuit. TEMI to TMT to TM11 Square Waveguide with the Co-a -- Mode Transducer- [ Magneto-ionic Package TM to TEM Mode - W- Transducer ~-C.,o-ax. - I: iA Z, +j/ 0 0t 4 - - -Agk — - 0 F igure 1

magneto-Ionic package is represented again with respect to the TM 11 mode by a non-reciprocal transmission line. When the propagation is from left to right, both the characteristic Impedance, Z3, and propagation constant 5', are complex and the wave suffers attenuation. For propagation from right to left however, the magneto-lonic package Is invisible to the wave, and hence the transmission line parameters are the same as for the empty square guide. I the magnetostatic field was reversed, then the non-reciprocal transmission line would also reverse. Since the structure is symmetric with respect to a transverse plane passing through the center of the plasma package, the equlvalent microwave circuit to the right of the plasma package consists of an ideal transformer connecting two loss-free transmission lines. This is the same type of circuit as to the left of the magneto-Ionic package which already has been discussed. One really should Include a coupling network on the ends of the non- reciprocal transmission line. In a general case the network should be a "T" network. However, In some moreelementary,but similar situations where the problem has been worked out+, the effect of this network on the wave propagation Is of practically Insignificant Importance. Therefore, In our case it Is reasonable to neglect It as well. + Collln, Rowert E., F4eld Tor of Guided Waves, McGraw-Hill Book Company, Inc., New York,pp 244-247 (1960). -4 -

This would be the end of the story of tile equivalent microwave circuit of the device If the square waveguide were a single mode guide. On the contrary, It may also propagate the TE10 and TE01 modes in addition to the 'TM mode. However, these modes are axially asymmetric and therefore the TM l to TEM mode transducer will not convert them Into a TEM mode. But this Is the only mode which is allowed to propagate on the co-axlal line. Thus we have a situation In which the mode transducers form shorts on both ends of the square waveguide section as far as th9 TE01 and TE 0 modes are concerned., In effect, we have a cavity In which may exist TE and TEn resonatnt modes; Oln 10n n in our case being an appropriate integer. These modes may exist only for certain microwave frequencies. At these resonant frequencies the resonant modes will be coupled by the plasma paclukge to the 11TM 1 modeO That means that energy will be taken from the travelling mode and dissipated primarily in the wall losses of the resonant mode fields. The couplhin will occur only for the direction of travel In which the plasma section of the gulde appears lossy to the TM mode. To take into account the powrr loss out of the TM, mode due to these coupled cavity resonances we have included a series resonant circuit which Is coupled by an Ideal transformer to the center of the lossy transmission line. It Is doubtful that this part of the circuit may be justified rigorously, but the 5 -

Inclusion of the present model appears not only reasonablk, but also necessary. Because the cavity walls and the plasma package Itself disslpate energy out of the resonan t cavity mode we have Included a resistor In the series resonant circuit. The Inductance, capacitance, and resistance is determined by the character of the resonant mode fields and the electrical energy dissipation in the cavity. The coupling transformer, with turns ratio n1, we have chosen to interpret in such a way as to Introduce the nature of directivity In the resonant mode excitation. When the TM u mode is travelling from left to right, then n1 Is of some appropriate finite value; for the opposite direction of TM, mode travel we have no coupling and hence 1 = oo. When the frequency of the TM 11 mode is not within line-width of any coupled cavity resonance, then the series resonant circuit presents a large impedance to the transmission line and hence its loading effect on the transmission line is negligible. For frequency Intervals where this is true the operation of the device may be explained on the basis of the non —reciprocal transmission line alone. We revert to this discussion and continue the work started in Quarterly Status Report No. 13. Equation (1.47) of QSR No. 13 predicts the power transmission coefficient of the device when the wave travels from left to right,. e. when the line is attenuating. For convenience

we reproduce the equation: -54.5b 6+ 01a (jna ( PTC = 0.136+0.197 -+0.1731) 31a2 0 2 where "a"t is the size of the square guide, 'b" 1s thei length of the plasma package, 311 Is the phase constant for the unperturbed TMI mode. Note that in Figure 1 we have used 3, but that Is the same as3 ll. We write out the last factor 6 as = 6 VP2 1 (2) + 0 (Y-1)2+V2 where Pc H V = - Y = -- P= - LA) J U v, WH w and w being the collision frequency, electron cyclotron resonance 'c ' p frequency, plasma frequency, and the microwave frequency, respectively. It Is of some Interest to examine the conditions under which maximum absorption occurs. Maximum absorption means minmuum power transmission coefficient, and from (1) It Is clear that this requires + to be at a minimum. As (2) Implies this demands that we set Y = 1, and also taking f - 3,9 Gc we reduce (1) to PTC = -4.5 (3) V

The above formula is expressed on the db scale. The dimensions of the plasma package and the device are given In quarterly Status Report No. 13. The fact that the magnetostatic field in our devlce Is non-uniform Increases the PTC by approximately one-half on the db scale. We introduce this factor In (3) to keep our discussion as close as possible to our experimental device. The equation (3) then becomes, when written explicitly In terms of f and ^, both P e measured In Gc. f2 PTC = -3.6.f (4) C The results of this formula have been plotted In Figure 2, as lines of constant PTC; P Is the ordinate axis and f the absclssa axis. We see that Increasing C P the plasma frequency Increasos the attenuation, but Increasing collision frequency, decreases It, From Figure 2 one may select desirable plasma properties for specified absorption In the device. II EXPERIMENTAL STUDY OF PLASMA PACKAGE STABILITY At the beginning of the second quarter of this work, the Varlon Ion-pump mentioned In Quarterly Status Report No. 13, could no longer be pumped down beyond Its Ionization phase. According to the local Varlan representative, the trouble was either contamination of the pump elements, or deterioration of the electrodes. Since in either cast, considerable expense and timne would have -8 -

101 101 ~20d O.51.0 2.0 fp In Go 3.0 Figure 2.- Mintmuin Pow,,er Transmissi[on Coeff icient for Nonunif orm Magnetic Field As a Functio2 of f and vc with f==. Ge. p -.9-.

been required to put the unit back into operation, we decided to build a new vacuum station which would also be more suitable for the present application. The vacuum station which was assembled In this past quarter consists of a Welch Fore Pump, Model 1402 and a Vacuum Instrument Research Company 2? Diffusion Pump, with a 1 inch copper manifold and various connecting fittings for the plasma chamber, gas Inlet, pressure gauge ports, etc. A 1 Inch stainless steel valve with teflon seat Is used between the manifold and the diffusion pump. A schematic of the system is shown in Figure 3 This system has been In operation since the middle of September. Pressures as low as 3x107mm Hg have been obtained after baking the manifold for four days at 90 - 100~ C. After valving off the manifold from the diffusion pumnp, the pressure has remained below Zx10-6 mm over a period of six hours, and due to the light pumping action of the [on pressure gauge, the pressure was actually decreasing at the end of this time. A plasma envelope considerably more elaborate than the previous one has been assembled. A photograph of the plasma tube is shown in Figure 4. The tube has maximum outside dimensions of approximately4.3/4 x 3 x 1-1/2 inches. The cathode is a direct heated Barlum-Nlckelate type supplied to us as engineering samnples by the General Electric Company. It Is the cathode used In thu GE Type 5544 Inert gas filled Thyratron, and has a temperature -10 -

I" Stainless Steel Valve I-A 1 N2. c, diameter copper line SYMBOLS: L ) Glass Stopcock c6FDr Glass Taper ManI;cvac Guage Maesures Forepump Pressure SCHEMATIC OF VACUUM SYSTEM Figure 3

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limited emission of 3.2 amperes. The cathode Is partially shielded by a nicekel heat shield, 0.005 in thick. The anode is of platinum foil 0.002 in thick. Two barium getters are also mounted at the cathode end of the tube. The entire cathode assembly Is mounted on a standard elght-pin tube base that may be removed from the chamber and replaced. The nickel parts of the tube were hydrogen fired before assembly. Nickel could not be used in areas of the tube where the temperature would be less than 360~C (the Curie temperature of nickel) since the tube will be placed In a strong magnetic field. The tungsten and plat Inum were not fired; outgassing from these metals is not a serious problem. One should state at this point the essential operations which have to be successfully completed before we have a useful plasma package. The first operation Is to construct a slab-like diode with ample cathode area. The diode has to be well baked out to remove all the impurities after which the cathode must be developed. The slab-like form is structurally weak and thus we are limited to rather low bake-out temperatures. Second, we have to obtain a pue noble gas. The oxygen and water impurity should not exceed one part per million. Third, we have to transfer the gas via the pumping manifold to the diode without Increasing the impurity levels. Lastly, we have to seal the tube from the manifold by "pulling" a glass seal. -13 -

Oxide cathodes, once they are developed, cannot be exposed to atmosphere. The coating absorbs atmospheric water and is destiroyed. Thus the first step is Irreversible, and we have to be sure that we can make the second and third steps before we attempt the first. For this reason a simple diode was constructed, consisting of a 4 1/2 In long cathode of two strands of 0. 010 in diameter thorlated tungsten wire and an anode of 0.10 in diameter tungsten rod mounted In a glass envelope 5 In long by 1 1/4 In outside diameter. The cathodc was apparently developing normally, when one of the feed-through beads developed a leak. This was repaired, and the tube put back on the system. A subsequent gas fill gave a high rate of poisoning. The problem is under Investigation as to whether the gas had higher contaminant levels than expected, or if It was contaminated In the manifold. III CONCLUSIONS The principal interaction of the electromagnetic fields with the magnetoIonic plasma In a multlmode guide can be represented by an equivalent circuit that consists of a non-reciprocal transmission line and coupled resonant circuits. Our main difficulty is now in the experimental area. TIhe problem consists of three stages: 1) constructing a large cathode diode that is well baked out, 2) obtaining a very pure usable gas, and 3) transferring it without contamination to the diode, -14 -

IV FUTURE PLANS In addition to the present manifold we want also to assemble a complete metal manifold system that may be baked out to higher temperatures than the present one. In addition we want to construct a plasma package with a thoriated tungsten filament. This filament may be exposed to the atmosphere when it is cold without any ll effects. The moisture of the atmosphere destroys oxide coated cathodes, On the theoretical side, further computations will be carried out on the cyclotron resonance Interaction, -15 -