Patent Number: 052895092
Section: description

DETAILED DESCRIPTION OF THE INVENTION The following description is of the best mode presently contemplated for carrying out the invention. This description is not to be taken in a limiting sense, but is made merely for the purpose of describing the general principles of the invention. The scope of the invention should be determined with reference to the claims. In order to better understand the primary application of the present invention, i.e., a launcher of magnetosonic waves into a plasma-forming device, it will first be helpful to provide an brief description of a tokamak --the preferred plasma-forming device with which the invention is used. Hence, reference is made to FIG. 1 where there is shown a diagrammatic view of the main elements of a tokamak 20, with a portion thereof cutaway. The design and operation of such a tokamak is well described in the art, see, e.g., Artsimovich and/or Furth, supra, so only a very cursory overview of the tokamak's construction is presented herein. Basically, the tokamak 20 includes a toroidal vacuum vessel 22 that is centered about a major axis 24. A minor axis 25, centrally located within the toroidal vessel 22, encircles the major axis 24. The relationship of the major and minor axes 24 and 25 is shown in FIG. 1A. The vessel 22 is made from a conductive material, such as non-magnetic stainless steel or inconel, and is constructed with sufficiently thick walls to withstand the vacuum pressures that are developed therein. A large number of toroidal field magnetic coils 26 are equally spaced around the vessel 22, each encircling the minor axis 25 and a respective segment of the vessel 22. Eighteen such coils 26 are illustrated in FIG. 1, but this number is only exemplary. When energized with an electrical current, the toroidal coils 26 combine to produce a toroidal magnetic field B.sub.T, represented by the arrow 28, that encircles the major axis 24 within the vacuum vessel 22. A plurality of poloidal field magnetic coils 30 are positioned inside of the toroidal field coils 26, yet still outside of the vacuum vessel 22, so as to encircle the major axis 24. As depicted in FIG. 1, the windings of the poloidal field coils 30 are substantially perpendicular to the windings of the toroidal field coils 26. When energized with an appropriate electrical current, the poloidal field magnetic coils 30 combine to produce a poloidal magnetic field B.sub.P, represented by the arrow 32, that encircles the minor axis 25 of the vacuum vessel 22. Because plasma is an ionized gas, it is also an electrical conductor, with the movement of electrons (negatively charged particles) in one direction and the movement of positively charged ions in the other direction representing the flow of electrical current. An important part of the operation of a tokamak is the creation of axial current flow through the plasma contained within the vessel 22. Such current flow serves to heat the plasma, and is frequently referred to as the "current drive" of the tokamak. The current drive follows the minor axis 25 of the tokamak and is depicted in FIG. 1 by the arrow 36. The current drive may be initiated and maintained by launching a suitable plasma wave into the vacuum vessel 22 that propagates in the direction of the minor axis. The comb-line antenna structure described hereinafter provides one means for launching such a wave. In addition, ohmic heating primary windings 34 may be positioned inside of the toroidal field coils 26, in close contact with the vacuum vessel 22, so as to encircle the primary axis 24, much like the poloidal field coils 30. When energized with an electrical current, the field coils 30 (acting as a transformer primary winding) induce an electrical current, I.sub.P, in the plasma (acting as a transformer secondary winding), which electrical current I.sub.P also contributes to the current drive of the tokamak. Not shown in FIG. 1, but understood to be part of any tokamak or similar plasma-confining structure are conventional means for establishing a desired vacuum pressure within the vessel 22, and means for injecting the appropriate gases into the vessel from which plasma may be formed. In operation, appropriate gases are introduced into the vacuum vessel 22 at the appropriate pressure. These gases, e.g., .sup.2 H and .sup.3 H, are heated to extremely high temperatures in order to form a hot plasma. The toroidal magnetic field B.sub.T confines the plasma to a toroidal volume inside of the vessel 22 that does not touch the walls of the vessel. This occurs because the toroidal magnetic field B.sub.T has lines of magnetic force coincident or parallel with the minor axis 25, and plasma, as a whole, is substantially confined to and follows magnetic lines of force, forming as it were a plasma ring. The poloidal magnetic field B.sub.P is needed to complete the plasma confinement against drifts caused by gradients in B.sub.P. The combined fields form, as it were, a plasma and magnetic vortex. The externally applied component of B.sub.P is also used to shape the cross sectional area of the plasma ring within the toroidal plasma volume to a desired shape. For example, at some points within the vessel, or at some times when the plasma is within the vessel, the cross sectional area of the plasma cloud may be "squeezed", thereby compressing the plasma into a smaller volume, and further increasing its temperature. At other points within the vessel, or at other times, the cross sectional shape of the plasma cloud may be expanded, with some of the plasma particles being diverted away from the main plasma body. Such control of the cross-sectional shape of the plasma cloud is, as indicated, controlled by the poloidal field coils 30. For this reason, such coils are sometimes referred to as the "shaping field coils" or "shaping field windings". Referring next to FIG. 2, the manner in which a comb-line antenna structure 40 made in accordance with the present invention may be used to launch unidirectional plasma waves into the plasma of a tokamak is diagrammatically illustrated. The toroidal vacuum vessel 22 of a tokamak is depicted in FIG. 2 from a view along the major axis 24. Once formed, the plasma is confined within the vessel 22 so as not to touch the walls of the vessel 22, i.e., to reside within the area bounded by the dotted lines 38, thereby forming a plasma ring 39. One or more comb-line antenna structures 40, described more fully below, are mounted to the inside of an outer wall of the vessel 22 so as to front or face the toroidal plasma ring 39. Two such structures 40 are shown in FIG. 2, but such is only exemplary. Typically, the comb-line structures 40 will be recessed within the outer wall of the vessel 22. However, as will be evident from the description that follows, the comb-line structures are sufficiently shallow to enable their mounting on the inside of the outer wall of the vessel 22 without being recessed and still not encroach on the area to which the plasma ring 39 is confined. Still referring to FIG. 2, an rf generator 44 generates rf input power, represented by the arrow 45, that is applied to an input port 48 of the comb-line structure 40 through a conventional waveguide 46. As described more fully below, the rf input power 45 is directly coupled to a first current strap within the comb-line structure 40, and inductively coupled to other current straps within the comb-line structure by means of a traveling wave propagating along the structure. In the vacuum region immediately surrounding the comb-line structure, the electromagnetic field of this traveling wave is evanescent in the radially inward direction. Such wave traveling along the comb-line is diagrammatically illustrated in FIG. 2 by the arrows 42. If the distance to the plasma 39 is small, such an evanescent field couples to a magnetosonic wave suitable for providing current drive and heating in the plasma mass, while the power flow along the comb-line structure is damped by the power transfer to the plasma. The comb-line structure 40 also includes an rf output port 50. Any of the rf input power 45 that is coupled to a last current strap within the comb-line structure 40 without being converted to the traveling wave 42, or without otherwise being dissipated within the comb-line structure 40, is received at the output port 50. This rf output power may either be applied through a conventional waveguide 54 (or other suitable transmission line) to a load 58, or to a recirculation system 52. If applied to a recirculation system, the recovered rf power is combined with the input power generated from the rf generator 44 in order to reapply it to the input port 48, thereby improving the efficiency of the comb-line structure 40. A representative recirculation system 52 is described below in conjunction with the description of FIG. 5. Referring next to FIGS. 3A and 3B, one embodiment of a comb-line antenna structure 60 is shown. FIG. 3A shows a "top" view, similar to the view orientation of FIG. 2, and FIG. 3B shows a "side" view, showing the comb-line structure as viewed from the plasma. As with the generic comb-line structure 40 of FIG. 2, the comb-line structure 60 includes a single input port 62 and a single output port 64. The structure 60 resembles, electrically as well as mechanically, a comb-line bandpass filter. The structure 60 includes a plurality of parallel current straps 66 that are supported in a plane above a conductive surface 68. Each strap 66 has an approximate length l, width W, and thickness t. As best seen in FIG. 3B, discrete capacitors 72 are placed at one end of each of the straps 66. What is shown as item 72 in FIG. 3B is one plate of the capacitor, with the conductive wall 70 functioning as the other plate. Each current strap 66 is separated a distance S from an adjacent strap. The plane formed by the straps 66 thus has the approximate dimensions of l by N(W+S), where N is the number of current straps that are used. The plane of the straps 66 is separated from the conductive surface 68 by a standoff distance d.sub.1. Such plane is spaced adjacent to the plasma mass 39 a distance d.sub.2. In operation, the edge of the plasma mass 39, as well as the density of the plasma at its edge, will vary. Thus, it is understood that the distance d.sub.2 will also vary. While the plane of the straps 66 is depicted in FIG. 3A as being straight, it is also to be understood that there will typically be some curvature associated with such plane as it fits against or within the outer wall of the toroidal vacuum vessel 22 (FIG. 2). Still referring to FIGS. 3A and 3B, side walls 70 are attached to the conductive surface 68, thereby forming a housing or "box", within which the straps 66 are supported. The straps 66 may be mechanically supported from one section of such wall 70. A first conductive strap 66', on one edge of the plane formed by the straps 66, is electrically connected to the rf input port 62. A last conductive strap 66", on an opposing edge of the current strap plane, is electrically connected to the rf output port 64. In operation, the comb-line structure 60 is inherently a traveling wave device, in the sense that power applied to its input port 62 launches a wave traveling toward the output port 64, with power being inductively coupled between adjacent straps. The structure 60 shown in FIGS. 3A and 3B, as well as the preferred structure shown below in FIGS. 4A and 4B, offers the significant advantage that far fewer feedthroughs and tuning elements are required to launch the same total power than are used in prior art devices that individually drive each current strap. Such feedthroughs (input and output ports) and tuning elements are costly and occupy valuable space near the tokamak. Of course, approximately the same current per strap is required to launch a given power per strap regardless of whether the straps are fed directly or inductively. However, the power per feedthrough can be much higher for the comb-line structure shown in FIGS. 3A and 3B (as well as in FIGS. 4A and 4B) because the input port 62 sees a matched load. That is, with individually fed straps, the standing wave ratio at each feedthrough is necessarily high because the resistive component of the strap impedance is typically only a few ohms. In contrast, the input impedance of the comb-line structure 60 reflects the accumulated loading of all the straps. The resonant elements are internal to the comb-line structure 60, so the standing wave ratio is low at the feedthrough (input port 62). The concept of using a comb-line structure to launch fast waves into a plasma is described in Chiu, et al., Nuclear Fusion, Vol. 24, p. 717 (1984), cited previously. An analysis of a more practical form of the comb-line structure 60 is provided in Moeller et al., "A Comb Line Structure For Launching Unidirectional Fast Waves", Europhysics Conference Abstracts on Radiofrequency Heating and Current Drive of Fusion Devices, Vol. 16E, pp 53-56, Brussels (Jul. 7-10, 1992). These two references--the Chiu et al. and Moeller et al. references--are incorporated herein by reference. A preferred comb-line structure 80, made in accordance with the present invention, is depicted in FIGS. 4A and 4B. Such structure 80 resembles the structure 60 described in connection with FIGS. 3A and 3B above except for two major differences: (1) there are no discrete capacitors used with the structure 80; and (2) the structure 80 includes a multiplicity of wickets (or hoops) 92 that enclose each current strap. FIG. 4A shows a "top" view, similar to the view orientation of FIG. 2, and FIG. 4B shows a "side" view, showing the comb-line structure 80 as viewed from the plasma 39. As with the generic comb-line structure 40 of FIG. 2, the comb-line structure 80 includes a single input port 82 and a single output port 84. The structure 80 includes a plurality of parallel current straps 86 that are supported in a plane above a conductive surface 88. Each strap 86 has an approximate length l, width W, and thickness t. Each current strap 86 is separated a distance S from an adjacent strap. The plane formed by the straps 86 thus has the approximate dimensions of l by N(W+ S), where N is the number of current straps that are used. The plane of the straps 86 is separated from the conductive surface 88 by a standoff distance d.sub.1. Such plane is further spaced adjacent to the plasma mass 39 a distance d.sub.2. In operation, the edge of the plasma mass 39, as well as the density of the plasma at its edge, will vary. Thus, it is understood that the distance d.sub.2 will also vary. Like the description above in connection with FIGS. 3A and 3B, it is noted that while the plane of the straps 86 is depicted in FIG. 4A as being straight, it is to be understood that there will typically be some curvature associated with such plane as it fits against or within the outer wall of the toroidal vacuum vessel 22 (FIG. 2). Still referring to FIGS. 4A and 4B, side walls 90 are attached to the conductive surface 88, thereby forming a housing or "box", within which the straps 86 are supported. The straps 86 may be mechanically supported from one section of such wall 90. A first conductive strap 86', near one edge of the plane formed by the straps 86, is electrically connected to the rf input port 82. A last conductive strap 86", on an opposing edge of the current strap plane, is electrically connected to the rf output port 84. A multiplicity of current wickets 92 (or conductive hoops) enclose each of the current straps 86. These wickets are oriented so as to lie substantially orthogonal to the current straps. Such wickets are grounded to the conductive surface 88. In some instances, it may be desirable to mechanically support the wickets 92 to standoff bars 94 (only a couple of which are shown in FIG. 4A) that are attached to, or form an integral part of, the conductive surface 88. Such standoff bars 94 not only facilitate the mechanical connection of the wickets to the surface 88 (which surface 88 may be referred to as a conductive "backplane"), but also help to stiffen the backplane in the region where the attachment of the wickets is made. The wickets 92 are made from a suitable conductive wire, such as inconel, molybdenum, titanium, or other high temperature conductor. Such wire is formed in the general shape of a hoop, and mounted to the backplane 88 so as to loop around or enclose the current strap, as best seen in FIG. 4A. It is important that the wicket 92 be kept close to the current strap 86, but not touch the current strap 86. Keeping the wicket close to the current strap increases the capacitance between the strap and shield, which is advantageous because it reduces the required length of the strap. In general, the separation distance between a given wicket 92 and the respective current strap that it encloses may be on the order of 0.5 to 1.0 cm with the U-shaped conductive wire being more or less equally spaced from the three sides of the current strap about which it loops. At an input frequency in the range of 100 to 200 MHz, the number N of current straps within the comb-line structure should be at least 10, with the length l of each strap being approximately 15 to 30 cm, the separation distance S between adjacent current straps being approximately 2.5 to 5.0 cm, and the width W of the current straps also being about 2.5 to 5.0 cm. In lieu of wickets 92, some applications of the invention may utilize a row of posts that separate the current straps, which posts may be substantially the same as the legs of the wickets. Such posts would extend up from the backplane 88 just past the current straps, and would provide electrostatic shielding between the straps. In most instances, only a single row of posts would be required between current straps, although a double row could be used to increase the capacitance, if needed. Further, it is noted that while each current strap 86 will generally have the same number of wickets enclosing it as do the other current straps, the wickets may be arranged in alternate rows so as to overlap, as shown in FIG. 4C, which shows a partial view of the antenna from the same vantage point as FIG. 4B. An advantage achieved by the comb-line structure 80 shown in FIGS. 4A and 4B over the structure 60 shown in FIGS. 3A and 3B is the avoidance of discrete capacitors. At a typical operating frequency of 120 MHz, such discrete capacitors, when used, need only have a capacitance value of from 16 to 20 pf. However the peak voltage seen by such capacitors is on the order of tens of kilovolts. Given the type of hostile environment associated with plasma, this means that a spacing of at least 1 cm is needed between the plates of the capacitor in order to avoid breakdown if solid dielectrics are not used. Solid dielectrics do not survive well in or near a plasma environment. A spacing of 1 cm, in turn, translates to approximately a 200 cm.sup.2 area that is required to order to achieve the necessary capacitance. Such area is not readily available. Hence, being able to avoid the use of discrete capacitors, as is accomplished with the comb-line antenna structure 80 shown in FIGS. 4A and 4B, offers an important advantage. Another advantage achieved by the comb-line structure 80 (FIGS. 4A and 4B) over the structure 60 (FIGS. 3A and 3B) is the electrostatic shielding provided by the wickets 92. There is no such shielding provided by the structure 60. That is, the wickets 92 (FIGS. 4A and 4B) function as a Faraday shield, shielding the plasma 39 from the electrostatic fields that are present at the current straps. The desired plasma wave is excited inductively by the currents flowing in the current straps 86, and is referred to as a "magnetosonic" wave. Unfortunately, an electrostatic field is also produced by such straps that can excite undesired plasma waves. Such undesired waves not only represent wasted power, but also lead to acceleration of ions near the walls of the vessel 22 (FIG. 2). Advantageously, the Faraday shield provided by the wickets allows the divergence free, inductive field to readily pass therethrough, thereby allowing the desired inductive coupling between adjacent current bars to take place, but also blocks the curl free, electrostatic field. That is, because the wickets are electrically grounded, the electric field lines associated with such electrostatic fields terminate at the wickets, thereby confining the electrostatic fields to the immediate region surrounding the wickets. However, the wickets 92, by virtue of being more or less orthogonal to the direction of current flow in the current straps, allow the magnetic fields associated with the current flow in the current straps to readily pass therethrough. The passage of the magnetic fields through the wickets provides for the desired inductive coupling between adjacent current straps, as well as permits the appropriate plasma wave to be launched in the desired direction within the plasma 39. The magnetosonic wave is launched from the current straps into the plasma through an evanescent layer, which concept is well understood by those of skill in the art. An advantage of the comb-line structures 60 or 80 is that the axial wave number of the traveling wave in the structure, usually referred to as n.sub..parallel., can be made to vary by adjusting the input frequency within the passband of the structure. Referring next to FIG. 5, there is shown a schematic diagram of a recirculation system 52 that may be used to maintain a traveling wave in a comb-line structure, such as the comb-line structure 80 of FIGS. 4A and 4B. Such recirculation system 52 is used whenever it is not convenient, especially in existing tokamaks, to use a comb-line structure of sufficient length (i.e., having a sufficient number of current straps, N) to give complete damping under the conditions of poorest coupling. In such instances, a unidirectional wave is maintained by making the comb-line structure 80 part of a ring resonator, as shown schematically in FIG. 5. As seen in FIG. 5, a group of three 3 dB couplers, 102, 104 and 106, in combination with a conventional stub tuner pair 108, form a variable directional coupler 110. An additional 3 dB hybrid coupler 112 and stub tuner 114 form an adjustable phase shifter 116. The phase shifter 116 is the only critical tuning element, in the sense that the resonance condition only requires an integral number of wavelengths around the ring. If the variable coupler 110 is not optimally adjusted, some fraction of the input power, generated by the rf amplifier 44, will go to a dummy load 118, but the ring will remain unidirectional and the generator 44 will not see a reflection, as long as the ring remains resonant. The phase shifter 116 need not have rapid adjustment capability. Rather, any rapid fluctuations in the electrical length of the antenna structure 80 due to plasma movement may be compensated for by small adjustments in the frequency. Also, for reasonable loading of the antenna structure, the circulating power and hence the Q of the ring are low. Specifically, if T is the fraction of power incident at the antenna structure 80 that arrives at the output port 84, and C is the power coupling factor of the adjustable coupler 110, the ratio G of circulating power to generator power is G=C/[1-(1-C).sup.1/2 T.sup.1/2 ].sup.2, which is maximized when C=1-T and G=1/(1-T) and all the power is damped in the antenna. For example, if T=1/e, then G=1.58, which represents a very low Q. This means that resonance is not critical under such conditions. To demonstrate the advantage of using the Faraday shield formed using the wickets 92, the electrical behavior of the comb-line structure 80 with and without an ideal Faraday shield will be compared. An ideal Faraday shield is considered as a shield that is transparent to magnetic fields, but appears as a perfect conductor to curl fee electric fields. For purposes of the comparison, it is assumed that the number of current straps is infinite, so that there are no end effects. Also, to simplify the comparison, the plasma is assumed to be a conducting wall. Using this simplified approach, it is also possible to calculate the plasma loading that determines the actual plasma impedance presented to the antenna structure as a perturbation. Such calculation is carried out in Appendix A for the discrete capacitor embodiment (FIGS. 3A and 3B). It is noted that such calculation is not related to the shielded verses non-shielded comparison presented below. The coordinate system used for the comparison that follows is shown in FIG. 6. Note, in FIG. 6 the Y coordinate is out of the paper. Considering first the unshielded case, and designating I.sub.r (y) and V.sub.r (y) as the current and voltage, respectively, along the r.sup.th strap, the array of straps can be regarded as a multi-conductor TEM mode transmission line. Such transmission line is governed by the equations: ##EQU1## where C.sub.rs is the mutual capacitance per unit length between the r and s strap, with the other straps (other than the r.sup.th strap) grounded, and L.sub.rs is the mutual inductance per unit length. (Note that Eqs. (1)-(3) are in Appendix A). If it is assumed that all of the straps are identical, and that there are no end effects, then it is only necessary to consider a typical element, r=0, and determine L.sub.oS and C.sub.0S. The currents and voltages in the straps may be expressed as: ##EQU2## The dependence of the current and voltage on y is a consequence of the TEM nature of the mode, and the assumption that the straps are all shorted to ground at y=0. The dependence on e.sup.-ir.theta. comes from Floquest's theorem. In this case, .beta.=.omega./c, where .omega. is the angular frequency, and c is the speed of light. The propagation constant along the structure, k.sub.z, is just k.sub.z =.theta./P, where P is the period. Using Eqs. (5a) and (5b) in (4b), it is seen that, looking at the typical r=0 strap, ##EQU3## Although L(.theta.) is defined as a fourier series having mutual inductances as coefficients, it is noted that L(.theta.) simply represents the self inductance per unit length of a strap when the phase shift from strap to strap is .theta.. Combining Eqs. (2a) and (2b), it is also seen that ##EQU4## where l is the length of the straps. If there is an admittance Y.sub.0 at the end of each strap, then ##EQU5## Combining Eqs. (3a) and (4), a dispersion relation is obtained as ##EQU6## where it is assumed that Y.sub.0 =i.omega.C.sub.e, where C.sub.e is the discrete capacitance at the end of each strap, if any. Since .omega./c=.beta., and k.sub.z =.theta./P, a relationship between .omega. and k.sub.z is thus obtained. The phase velocity along the structure is just .omega./k.sub.z, while the group velocity, related to the energy flow, becomes d.omega./dk.sub.z. As the end capacitance, C.sub.e, approaches zero, however, .beta.tan(.beta.l) approaches .infin., which means than .beta.l approaches .pi./2. Thus, .beta., and therefore .omega., are then fixed, independent of .theta., and hence independent of k.sub.z. The group velocity then approaches 0, so that there is no energy flow, and the antenna structure is cut off. In contrast, when a Faraday shield is used, the equations for the shielded multi-conductor transmission line are: ##EQU7## For an ideal Faraday shield, L.sub.rS and L(.theta.) are unchanged from the above unshielded case. C.sub.0 is the capacitance per unit length between a given strap and its shield. From Eqs. (9a), (5a) and (5b), it is seen that Eq. (8a) is again obtained. However, using Eqs. (9b), (5a) and (5b) it is seen that ##EQU8## Eq. (10) thus shows that .beta. is no longer equal to .omega./c. Equivalently, Eq. (7) demonstrates that the velocity of light along the strap, for the shielded case, becomes ##EQU9## and is dependent upon .theta.. The consequence of this is that even if C.sub.e =0, by making .beta.l=.pi./2, the antenna structure is no longer cut off. In fact, when the quantity .beta.=.pi./2l is substituted into Eq. (10), it is seen that ##EQU10## Since C.sub.0 in the shielded case is always greater than C(.theta.) in the unshielded case, .beta. will always be greater than .omega./c, and the larger C.sub.0 becomes, the greater will be .beta.. The advantage of having a large C.sub.0 and large .beta. is that the strap length, l=.pi./2.beta., can be shorter, which is normally desirable. In general, if C.sub.e .noteq.0, Eqs. (8) and (10) show that the dispersion relation ##EQU11## takes the form ##EQU12## where U=l.omega.[L(.theta.)C.sub.0 ].sup.1/2. This means that ##EQU13## where F is some complicated function of l and C.sub.0 /C.sub.e, which reduces to .pi./2 when C.sub.e .fwdarw.0. The form of the dispersion relation remains unchanged from the case where C.sub.e =0, except for a scaling factor. That is, the ratio of the upper to lower cut off frequency, the "pass band ratio", ##EQU14## is independent of l, C.sub.0 and C.sub.e, as is the functional dependance of .omega. on .theta., except for a scaling factor in the latter case. Adding C.sub.e allows l to be smaller, but does not effect the pass band ratio. As described above, it is thus seen that the present invention provides a comb-line antenna structure that launches magnetosonic waves into an adjacent plasma mass. This it does, in the preferred embodiment (shown in FIGS. 4A and 4B) without the need for a discrete capacitor at the end of each current strap. In other embodiments (FIGS. 3A and 3B or variations thereof), it may be desirable to add a small end capacitance. Moreover, as also seen from the above, the described comb-line structure effectively shields the plasma and adjacent current straps from electrostatic fields, yet still retains the requisite inductive coupling needed for the operation of a comb-line device. As further seen from the above, the comb-line launching structure described further provides significant mechanical advantages over prior art launching devices in that the wickets used to enclose each current strap are flexible, yet strong, and are thus able to react to thermal stress, without significant distortion or breakage. While the invention herein disclosed has been described by means of specific embodiments and applications thereof, numerous modifications and variations could be made thereto by those skilled in the art without departing from the scope of the invention set forth in the claims. ##SPC1##