COUPLING OF RF INTO HE1 1 WAVEGUIDE

Field of the Invention

The present invention relates to an RF mode converter and coupler for a gyrotron. In particular, the present invention relates to an apparatus and method for coupling the RF power generated in a gyrotron cavity and traveling as whispering gallery (WG) mode in a cylindrical waveguide to the HE11 mode. In one example, WG mode is coupled from a circular waveguide to a first and second reflector for direct coupling to a corrugated waveguide.

Background of the Invention

Modern high power gyrotrons produce power in high- order TE modes (TE _mn modes with m,n>>l) . These modes cannot be efficiently transported as RF (radio frequency) power in a low loss transmission system. In addition, it is

advantageous to separate the RF transmission from that of the spent electron beam within the gyrotron. Both of these considerations are typically addressed using an internal mode converter and step-cut launcher, which is commonly referred to as a quasi-optical (QO) launcher. The mode converter has small deformations in the waveguide surface to transform the high-order cavity mode into a set of modes whose combined fields have a Gaussian-like profile. The Gaussian-like beam can then be efficiently launched, focused, and guided by mirrors inside the vacuum envelope of the gyrotron. In this way, the RF power is converted to a mode more suitable for low loss transmission, and the RF beam is separated from the electron beam. This allows implementation of a depressed collector with large surfaces for thermal dissipation without affecting the quality of the RF beam. This method has been the primary technique for RF- electron beam separation in high power gyrotrons since the early 1990s. The development of this technique was one of the key technologies enabling the development of mega-watt (MW) level gyrotrons. One drawback of this approach is the internal mirrors must be adjustable for optimum performance to prevent device overheating from internal losses at the high power levels. Additionally, since these large mirrors are external to the gyrotron cavity, the RF power must be coupled out of the gyrotron through a large aperture, which is typically fabricated from expensive materials such as diamond which have the desired low RF loss and high thermal conductivity required. There are several deficiencies in this technique including internal diffraction losses, electron beam potential depression, and mirror alignment issues. It is desired to provide a mode converting device which converts high order WG modes travelling helically in a cylindrical waveguide into HE11 mode for coupling into a corrugated waveguide inside the gyrotron, thereby greatly reducing the deficiencies of the prior art approaches. In addition, substantial cost savings can be realized by eliminating the need for the two to three adjustable mirrors in the gyrotron and the external mirror optical unit used to couple the output Gaussian beam to the

corrugated waveguide transmission line. A final cost savings would be realized by the large reduction in the required diameter of the diamond material in the output window. Objects of the Invention

A first object of this invention is a launcher for a gyrotron having a whispering gallery mode (WGM) waveguide with dimpled surface features which increase in radial depth over an axial extent of the WGM waveguide, the WGM waveguide carrying a primary mode and co-propagating high order modes, the WGM waveguide having a step cut launcher with a launch edge, and coupling RF energy to a first mode converting reflector which generates RF with an elliptical radiation pattern and coupling the RF into a second mode converting reflector generating free space RF radiation pattern with a minimum beam waist at the entrance to a corrugated waveguide, the minimum beam waist also having a near-Gaussian phase distribution at the entrance to the corrugated waveguide, thereby providing efficient coupling of the RF into the corrugated waveguide for propagation as HE11 mode RF in the corrugated waveguide. A second object of this invention is a launcher for a gyrotron having a whispering gallery mode (WGM) waveguide with dimpled surface features which increase in radial depth over an axial extent of the WGM waveguide, the WGM waveguide carrying a primary mode and co-propagating high order modes, the WGM waveguide having a step cut launcher with a launch edge, and coupling RF energy to a single mode converting reflector which generates RF with a minimum beam waist at the entrance to a corrugated waveguide, the minimum beam waist also having a near-Gaussian phase distribution at the entrance to the corrugated waveguide, the near-Gaussian phase distribution provided by dimples of less than a wavelength in depth on the single mode

converting reflector, thereby providing efficient coupling of the RF into the corrugated waveguide for propagation as HE11 mode RF in the corrugated waveguide, the WGM waveguide and mode converting reflector optionally fabricated as a single structure. A third object of this invention is a gyrotron having a Whispering Gallery (WG) mode waveguide with a step-cut launcher, the step-cut launcher having a launch edge and coupling into one or more mode converting reflectors, a first mode converting reflector positioned on the order of a wavelength from the step-cut launcher and launch edge, the one or more mode converting reflectors generating RF with a minimum RF beam waist and also near-Gaussian phase at the entrance to a corrugated waveguide carrying the RF as HE11 mode. Summary of the Invention

The present invention is a launch coupler for a gyrotron having helically propagating energy contained by a cylindrical waveguide which terminates into a step-cut launcher having a launch edge, the RF energy propagating helically in a whispering gallery (WG) mode down the axis of a cylindrical waveguide. RF energy from the launch edge is coupled to a first mode converting reflector which is in close proximity to the launch edge, and thereafter to a second mode converting reflector which directs the

propagating RF onto a path which may be parallel to the central axis, where the first mode converting reflector and second mode converting reflector have surfaces selected such that the RF energy which leaves the second mode converting reflector is substantially coupled into the entrance of a corrugated waveguide, after which the RF energy propagates in HE11 mode and may be subject to a variety of standard HE11 waveguide direction changing reflectors. In one example of the invention, the inner surface of the input cylindrical waveguide has depressions in the direction of wave propagation and also depressions perpendicular to the direction of wave propagation for enhanced generation of high order modes which interact with the first mode converting reflector and second mode

converting reflector to generate a quasi-Gaussian intensity profile at the entrance of the corrugated waveguide. The quasi-Gaussian profile is not a pure first order Gaussian function in intensity distribution, but has the approximate characteristics of a Gaussian intensity distribution which is created through the introduction of high order modes in the waveguide 220 and mode changing reflectors 240 and 250 of figure 2A. In one embodiment of the invention, the first mode changing reflector is located within .25 to 4 wavelengths of the launch edge of the cylindrical

waveguide, such that RF energy reflected from the first mode changing reflector has an amplitude profile with a substantially elliptical radiation pattern, and the shape of the second mode changing reflector is selected to convert the incident elliptical radiation amplitude profile into a circularly symmetric free space wave with a beam waist which is narrow enough to efficiently couple into a corrugated waveguide which is optimized for propagation of an HE 11 mode. Brief Description of the Drawings

Figure 1A shows a cross section view of a prior art gyrotron coupled to a mirror optical unit for generation of HE11. Figure IB shows a cross section view of prior art figure 1A. Figure 1C shows a cross section view of prior art figure 1A. Figure 2A shows a cross section view of a gyrotron launch coupler. Figure 2B shows a cross section view of figure 2A. Figure 2C shows a waveguide of figure 2A cut open and rolled flat. Figure 2D shows a cross section view of figure 2A. Figure 2D-1 shows a cross section view of figure 2A showing a different embodiment for a first mode changing reflector. Figures 2E and 2F show cross section beam profile plots in a plane orthogonal to the RF beam. Figure 2G-1 and 2G-2 show the beam profiles through the y' and x' axis, respectively, of figure 2E. Figure 2H shows the beam amplitude profile for plot 2F. Figure 3A shows a cross section view of a gyrotron with a single beam shaping mirror. Figures 3B and 3C show section views D-D and E-E, respectively, of the beam shaping reflector of figure 3A. Figure 4A is a cross section view of the entrance of waveguide 220 opposite the launch edge. Figure 4B is a cross section view of waveguide 220 near the launch edge. Figure 4C is a cross section projection view of traveling wave energy in the gyrotron launcher. Figure 5 is a chart showing the power levels for co- propagating modes for a primary (m, n) mode.

Detailed Description of the Invention The various figures and views of the invention

identify each structure with a reference numeral which is understood to indicate the same structure in other figures or views. Additionally, certain figures include orthogonal x, y, and z axis indicators to clarify the plane of the particular view. Figure 1A shows a prior art Gyrotron 100. An electron gun assembly 102-1 produces an annular electron beam that propagates about axis 102 through input beam tunnel 104 into a cylindrical cavity 105 where electron beam energy is converted to an RF mode with the RF energy propagating helically along the waveguide. High power gyrotrons use transverse electric modes with high radial and azimuthal mode numbers. A typical mode example is TE2 ₄ ,6, with this high order mode RF propagating helically along the inner surface of the waveguide in a surface wave mode referred to as a whispering gallery (WG) mode. The RF propagates from the cavity 105 into a waveguide of

increasing diameter 106 and into cylindrical waveguide 107 having entrance 127. Because whispering gallery modes cannot be easily transported in waveguide or efficiently used by downstream devices, the whispering gallery mode is typically converted to a quasi-optical mode inside the gyrotron. This is accomplished by radiating the RF power from a step cut launch edge 123 in cylindrical waveguide 107. The radiated wave energy propagates through free space to focusing mirrors 108a and 108b. Mirrors 108a and 108b modify the phase and amplitude distribution of the RF wave such that the beam passing through vacuum window 112 of window support 111 is a Gaussian-shaped, quasi-optical, free space wave . In one example gyrotron, waveguide 107 inner surface is modified to shape the waveguide whispering gallery mode such that the RF beam radiated from spiral cut 123 has reduced side lobes with increased power in the central lobe of the RF beam directed toward reflectors 108a and 108b. Such shaping is accomplished using surface field integral analysis and coupled with advanced optimization routines. A disadvantage of the device 100 is that additional modifications of the free space output beam 109 are

required to couple the RF power into a waveguide for transport to downstream devices, such as an antenna. This is accomplished with a device commonly referred to as a Mirror Optical Unit (MOU) 170, which is coupled to the output beam 109 of the gyrotron 100. The output beam 109 may travel through one or more diamond vacuum-sealing apertures 112 and to phase shaping mirrors 174 and 176, fabricated from high thermal conductivity and high

electrical conductivity metals such as copper, which are profiled to shape the large cross section beam diameter (also known as beam waist in the art of free space wave propagation) of the free space Gaussian beam profile 172 to minimize reflections as the free space Gaussian wave transitions to HE11 mode at the waveguide entrance, and one of the objectives of the mirrors is to reduce the free space beam waist before delivery to the entrance of

waveguide 186 where the RF beam 178 continues to propagate. Because the gyrotron 100 produces an RF beam with an output beam axis which relies on the angle relationship of many reflective surfaces including launch edge 123, first reflector 108b and second reflector 108a, the axis of the beam output 109 may vary from device to device. To

compensate for these geometric variations, MOU first reflector 174 and MOU second reflector 176 are usually separately adjustable about each mirror's orthogonal mirror axis, which allows adjustment of the beam angle delivered to waveguide 186, and waveguide 186 additionally has a 2- axis translation so that the beam may be centered in the waveguide. The various mirror 174 and 176 angle

adjustments (184 and 182, respectively) and output

waveguide 186 translation adjustment results in significant setup time and cost, and the adjustment settings may change because of the long beam path and wide mirror spacing as a result of factors such as thermal expansion of structures along this path. A further disadvantage of the gyrotron 100 is that the output window 112 which couples energy out of the gyrotron 100 must be relatively large in diameter due to the radial extend of the Gaussian quasi-optical free wave mode which travels through window 112, which is fabricated using a chemically vapor deposited (CVD)

diamond, which has a low RF absorption and high thermal conductivity, which are required for high power (1MW and above) gyrotrons to prevent damage to the window from thermal energy absorbed from the high power beam. The large diameter Gaussian quasi-optical mode which propagates through window 112 results in a large diameter aperture compared to the reduced diameter output waveguide 186 diameter after conversion to HE11. Additionally, the RF leaving the gyrotron is directed through the spent electron beam 158 to collector 103, where undesirable interactions may occur. Also shown are cathode 113, heater power supply 150, modulated anode 114, modulated anode power supply 152, main power supply 154, and solenoidal magnetic field generator 119, all of which are well known in the art.

Figure 1A has cross section views A-A and B-B, shown in figures IB and 1C respectively, which shows section views of the structures previously described, including

cylindrical waveguide 107, for additional clarity. Figure 2A shows an example embodiment of a gyrotron launch coupler 200 of the present invention which may be used to replace the cylindrical waveguide 107, upper mirror 108a and lower mirror 108b over axial extent 156 of figure 1A, and also the mirror optical unit 170 of figure 1A, such that HE11 waves may be directly coupled into a corrugated waveguide such as 186 of figure 1A without the use of MOU 170 of figure 1A. Corrugated waveguides are well known in the art for transmission of HE11 wave energy, and an example corrugated waveguide 260 with axis 254 is shown in figure 21, corresponding to the structures of figure 2A. Gyrotron enclosure 256 supports internal structures enclosed in vacuum chamber 201 isolated from external pressure by diamond window 270. The gyrotron launch coupler 200 shown in figure 2A receives helically

propagating WG mode guided RF in waveguide 220, which is launched via launch edge 230 into adjacent first mode conversion mirror 240, which produces an elongated or elliptical Gaussian beam 264 propagating in free space (with extents shown as beam plot 280 of figure 2E viewed perpendicular to the local beam axis 281), which is reflected by second mode changing mirror 250, where the free space Gaussian mode wave reduces in beam diameter shown as beam 266 and with a beam extent perpendicular to beam axis 254 shown in figure 2F and having a beam diameter or beam waist 282, and becomes circularly symmetric about the propagation axis (281 of figure 2F and 254 of figure 2A) of the beam 266. This free space Gaussian beam is then suitable for direct coupling to corrugated waveguide 260 and RF mirror 212, which results in a greatly reduced beam diameter (beam waist energy extent) and associated diamond window 270 diameter compared to the beam waist energy extent and associated window 112 diameter of figure 1A. Spent electron extent 214 remains as shown in figure 1A. Figures 2D and 2D-1 show section D-D through figure 2A for two respective embodiments of the edge launcher. In the launch coupler of figure 2A, RF energy conveyed in an electron beam (not shown) is propagated helically as higher order transverse electric (TE whispering gallery) RF mode in cylindrical waveguide 220. Cross section C-C of figure 2A shows cylindrical waveguide 220 in figure 2B including a single "ray tracing" 218 which indicates the individual surface reflections of the quasi-optical helical RF beam 219, as is known in the art of WGM RF propagation. For clarity in understanding the invention, a "split line" 228 is shown in waveguide 220 of figure 2B, and if the cylindrical waveguide 220 were split on this line 228 and laid flat, the traveling whispering gallery mode (WGM) waves which propagate across this surface would travel through launch region 204 of figure 2A as shown in figure 2C, where the continuous helical wave propagation appears as individual linear propagation paths 221, 223, 225, 227 about split line 228. As is clear to those skilled in WGM propagation, a helically propagating wave inside waveguide 220 propagates with a fixed axial velocity, and

accordingly, if waveguide 220 were longitudinally cut and unwrapped as shown in figure 2C, the single path of helical propagation becomes the continuous path shown as segments 221, 223, 225, 227. Accordingly, each of the propagation paths has associated whispering gallery mode radiation intensity contour patterns along the continuous line of propagation of path 221, path 223, path 224, and path 227, with the RF field along path 223 shown as contour 222 extending to contour 224, thereafter continuing along path 225 with contour 226, for a succession of wave features representing the surface RF energy intensity of adjacent RF nodes at an instant of time as the propagation paths 221, 223, 225, 227 lead to helical launch edge 230. A first mode-changing reflector 240 is positioned adjacent to helical launch edge 230, and, as shown in figure 2A, a second mode-changing reflector 250 is positioned in the propagation path centerline 252 axis as the second

reflector 250 reflects energy to corrugated waveguide 260 as HE11 energy along propagation path centerline 254. The positioning of first mode-changing reflector 240 in the range 0.25 wavelengths and 4 wavelengths from helical launch edge 230 is typical, as RF radiated from helical launch edge 230 immediately interacts with first mode changing reflector 240, after which it is directed to second mode changing reflector 250, usually with an

elliptical or elongated radiation pattern with the

radiation pattern long axis (shown as the x' axis in figure 2E) substantially parallel to the propagation paths 221, 223, 225, and 227 and the radiation pattern short axis (shown as y' in figure 2E) which is substantially parallel to the helical launch edge 230. Second mode changing reflector 250 has a surface profile selected to reshape the aspect ratio of the incident RF beam from an elliptical or elongated radiation pattern to precisely match the circular electromagnetic field pattern of HE11 supported by

corrugated waveguide 260 and having a beam waist which optimally couples into the entrance of corrugated waveguide 260. The RF beam can be efficiently propagated through waveguide 260 and redirected as required by one or more miter bends 212 and through RF vacuum window 270, as shown in figure 2A. Figure 2E shows an RF beam profile 280 in an x',y' plane perpendicular to the local beam axis 281 and in the region 264, as shown in figure 2A, between the first mode converting reflector 240 and second mode converting

reflector 250, the beam profile 264 of figure 2A shown closer to the second mode converting reflector 250. The beam profile 280 tends to be elongated or elliptical, and with an aspect ratio on the order of 5:1. Figure 2G-1 shows the amplitude profile 284 of the RF beam 280 across the y' axis, and Figure 2G-2 shows the amplitude profile 285 of the RF beam 280 (shown in figure 2E) across the x' axis, each of which tend to be a quasi-Gaussian function across their respective axis. The dependent axis of each of figures 2G-1, 2G-2 and 2H are labeled |A| to indicate absolute value of amplitude for clarity in understanding the invention. Figure 2F shows the RF beam profile in the plane x' ',y' ' perpendicular to the RF beam axis at the output of the second mode converting reflector. The second beam reflector 250 corrects for the incoming elliptical beam profile shown in figure 2E, and generates a substantially circularly symmetric radiation pattern 282 with a beam profile 286 as shown in figure 2H. The RF beam profile which exits second mode converting reflector 250 tends to have a beam profile 282, or beam waist W, which has a minimum waist diameter, and the location of the beam waist minimum is the preferred location for the entry of the beam into corrugated waveguide 260. Because of the reduced radial extent of the RF beam within the HEll waveguide, RF window 270 shown in figure 2A can have a significantly smaller diameter than would be required for a free space quasi-optical Gaussian mode beam 109 of figure 1A. Moving the RF window to a region near the HEll waveguide allows the diameter of the RF window to reduce to the diameter of the MOU output waveguide 186. Additionally, since the gyrotron of figure 2A has greatly reduced path lengths between reflective surfaces and the structures are closely associated compared to the gyrotron of figure 1A, it is not necessary to perform the beam alignment associated with adjustable mirrors, as the HEll beam can be directly coupled into output corrugated waveguide 260. This results in significant cost reduction through the reduced number of structures, reduced exit window 270 diameter, and elimination of the MOU 170

alignment requirements compared to the device of figure 1A. In one example of the invention shown in figure 2D-1, the cylindrical waveguide 220, launch edge 230, first mode converting reflector 240, and second mode converting reflector 250 of figure 2A are formed from a single

heterogeneous material such as copper, so there are no mechanical interfaces or joints to change the alignment. In one example of the invention, the device operates at a frequency of 110 GHz, waveguide 220 has a radius 232 (of figure 2D) of 20.5mm, and the first reflector 240 has a circular cross section with a radius 242 less than 20.5mm, and an axial extent approximately equal to the axial extent of the launch edge 230, which is computed from the wave number of the propagating RF in WG mode. The included angle of the first reflector 240 about its center of radius is approximately 90 degrees, or one quarter of the circular waveguide 220, although this can range from 30 degrees to 120 degrees. Second reflector 250 has an angle with respect to the axis 202 which is selected to re-direct the RF propagating on axis 254 to be parallel to the axis 202 of figure 2A, although this angle can be selected based on the preferred exit angle for RF coupling into the

corrugated output waveguide 260. Many example embodiments are possible for the surface shape of waveguide surface 220, first mode changing

reflector 240, and second mode changing reflector 250. In one embodiment of the invention, the cylindrical waveguide 220, first mode changing reflector 240, and second mode changing reflector 250 have surface shapes and profiles which are optimized by using surface integral field

analysis, including finite element analysis software coupled with advanced electro-magnetic field optimization software. In another embodiment of the invention shown in figure 2D-1, the first reflector 240 is shown with respect to launch edge 230, and the first reflector 240 is integral with cylindrical waveguide (shown as dashed outline 241) and includes a discontinuous region 243 where first

reflector 240 has a surface which is generally radial and perpendicular in region 243 and also adjacent to launch edge 230. The first reflector 240 has a region 241-1 which is optionally tangent to the projected diameter of input waveguide 241 (shown as dashed line) , and in one embodiment of the invention, the first reflector 240 includes active surfaces which are adjacent to launch edge 230 and which are within a quarter wavelength to 4 wavelengths of the WG RF propagating within input waveguide 241. Internal to cylindrical waveguide 220 are a series of deformations that convert the mode incident from the gyrotron to a Gaussian like beam. In one example

embodiment of the invention, cylindrical waveguide 220 has surface deformations which generate enhanced currents which provide a semi-Gaussian beam which is not circularly symmetric in radiation pattern, but one which has an intensity profile with an elliptical intensity cross section as previously described, and with an initially long axis parallel to the arc formed by a radial line which is perpendicular to the center axis 202 and swept along helical path 221, 223, 224, 227, shaped principally by reflector 240 of figures 2A, 2C, and 2D. The long axis x' (parallel to path 223, 225, 227 of figure 2C) of the radiation pattern is focused by reflector 240 of figures 2A, 2C, 2D, and 2D-1 such that the long axis x' extent reduces along path 252 of figure 2A and reaches a minimum extent at the entrance to corrugated waveguide 260, optionally also shaped and focused for x' extent along the propagation path 252 by second reflector 250. Second reflector 250 may also provide surface shaping to reduce the beam extent in the short axis y' of the radiation pattern (parallel to launch edge 230) until it similarly reaches a minimum extent at the entrance to corrugated waveguide, with the radiation at the entrance to corrugated waveguide 260 preferably achieving a substantially circular cross section radiation pattern. The profiles of first reflector 240 of figures 2A, 2C, 2D, and 2D-1 and second reflector 250 of figure 2A are selected to provide maximum coupling efficiency for the free space quasi-Gaussian RF into the waveguide 160. The elliptical quasi-Gaussian output beam containing high order modes is thereby focused and shaped into a substantially circular cross section suitable for free-space coupling into the circular

corrugated waveguide 260 which supports HE11 mode, thereby minimizing coupling losses at the free-space wave to corrugated waveguide interface. For the purposes of this invention, "substantially circular" may be defined to be a shape which has a short axis dimension which is within 20% of a long axis dimension. For example, if the long axis of radiation pattern 282 of figure 2F is 20mm and the short axis of this radiation pattern is in the range 16mm to 20mm, this radiation pattern may be considered

"substantially circular". In another example embodiment, the first reflector and mode converter 240 are integrated into the circular

waveguide 220 launcher 230 to directly generate a circular RF beam cross section from the launcher 230 onto

propagation path 252. Second mode converting reflector 250 may be placed within the inner circumference of the tube envelope 256 to match the beam waist radiated from the launcher to the HE11 mode in the corrugated guide. This reflector 250 can also be used to tilt the output beam angle to be parallel to the tube axis 202. In one embodiment of the invention, the cylindrical waveguide 220 has internal depressions on the inner

waveguide surface which maximize the generation of quasi- Gaussian mode free space waves. The internal depressions on the inner waveguide cause the generation of "high order TE modes", which is defined in the present invention as any TE mode with an azimuthal mode greater than 15, such that for TEmn, m>15. In another embodiment of the invention, the first reflector such as 240 provides a surface with an azimuthal radius of curvature which is less than the radius of curvature of the central waveguide 220 to reduce the transverse extent of the coupled RF energy from launcher 230. Figure 3A, which may be viewed in combination with section D-D shown in figure 3B and section E-E shown in figure 3C, shows an embodiment 300 of the invention having a single reflector 316 where the cylindrical waveguide 306 and launch edge 314 provide RF energy to a reflector 316 which is similarly spaced (as in the structure of figure 2A) between a quarter wavelength and four wavelengths from launch edge 314, and which provides beam focusing and mode conversion to generate a circularly symmetric radiation pattern 320 on the RF beam propagation axis 318 and at the entrance to the corrugated waveguide 310. Figure 3A also shows the spent electron beam 322 which, as in figure 2A, is minimally interacting with the free space RF (in

contrast with figure 1A where the RF traverses through the spent RF beam 158 multiple times) , enclosure 308 with evacuated chamber 302, central axis 304, launch region 312, and aperture window 324 for preserving the vacuum of the gyrotron 300. Figure 3A section C-C is identical to the previously described section C-C of figure 2B, and figure 3A section D-D is shown in figure 3B, where the waveguide 306 is formed into a launch edge 314 which surfaces are separated by gap 344 to nearby single dual-purpose

reflector 316, which performs the corrections described for reflectors 240 and 250 of figure 2A, which results in a symmetric minimum waist beam of the free space RF which is provided at the entrance corrugated waveguide 310, which efficiently accepts the free space RF energy and transports HEll mode through the corrugated waveguides and through RF transparent vacuum seal window 324. The additional axial focusing of dual purpose reflector 316 may be seen with the edge relationship to reflector 306 in figure 3C showing section E-E of figure 3A. Additionally, radius 342 and reference circle 340 of figure 3B identify analogous respective elements as figure 2D-1 radius 232 and with reference circle 241 which indicates in dashed line

reference the extent of input waveguide 220. The coupling efficiencies of the free space quasi- gaussian RF coupling into the entrance of the corrugated waveguide, as shown in figures 2A and 3A, provides for very efficient coupling and minimal reflection loss. The

coupling efficiency into the corrugated waveguide for the devices of figure 2A and 3A exceeds 95%, and is typically 98% or more. Because of the close proximity of the components of the invention, as in figure 2A, any of the structures of figure 3A may be formed as a single unit, including any subset or set of: waveguide 306, launch edge 314, reflector 316, and a support (not shown) for the corrugated waveguide 310. The fabrication of these components from a

homogeneous slab of material such as copper can eliminate the need for mechanical adjustments of the prior art, and can also include corrective structures which minimize or eliminate mechanical deformations caused by thermal

gradients in the gyrotron coupling structures. Two important figures of merit for the gyrotron launcher of figures 2A and 3A are coupling efficiency, measured by the fraction of RF energy injected into

waveguide section 220 which is coupled into the HE11 waveguide 260 of figure 2A and 310 of figure 3A, and mode purity, which is the fraction of desired mode power

compared to sum of the power in all modes being propagated. In prior art gyrotrons such as the example shown in figure 1A, typical mode purity is less than 90% and the coupling efficiency is less than 90% at the output waveguide section 178. For the construction of figures 2A or 3A, the

coupling efficiency approaches 97-98% and the mode purity also approaches 97-98%. With regard to coupling efficiency on a 1MW gyrotron, a change in coupling efficiency from 90% to 98% corresponds to a change from 100KW power dissipation to 20KW power dissipation, respectively, or a factor of 5 reduction in dissipated power.

Without modification of the interior surface of the whispering gallery mode waveguide 220, a wide range of high order RF modes will naturally propagate in waveguide 220. In the present invention, a series of dimples and/or grooves are provided which provide preferential coupling for selected particular modes. This is done by taking advantage of the "beat wavelength" which results from one high order mode mixing with another. The beat wavelength is determined by the pattern of constructive and

destructive interference, which are many wavelengths long in the direction of propagation. The beat wavelengths are the result of the constituent RF waves which form the beat wavelength of the conveyed mode each propagating with a different phase velocity.

In the selection of the dimples and/or grooves on the inner surface of waveguide 220, many different high level modes can be selected for propagation. For a particular selected propagating mode, the table of figure 5 identifies the particular modes which are the subject of the surface features of the WG waveguide 220, first mirror 240, and second mirror 250 of figure 2A (or alternatively the single dual-purpose mirror 316 of figure 3A) . The selection of surface waveguide features for these structures is done in combination to optimize for a Gaussian-like phase front TEMoo mode at the entrance of the HE11 waveguide 260 of figure 2A or 310 of figure 3. This Gaussian-like beam couples efficiently to the HE11 mode in the waveguide 260. Figure 5 lists the co-propagating modes and power levels for a particular (m,n) primary mode. For example, for the earlier example of TE2 ₄ ,6 optimization, the beat wavelengths listed in Figure 5 would be TE ₂₁ , ₇ , TE ₂₇ , ₅ , TE ₂₅ , ₆ , TE ₂₃ , ₆ , TE ₂₂ ,7, TE ₂ o,7 _/ TE ₂ 8,5 _/ and ΤΕ ₂ 6,5·

The inner surface profile of waveguide 220 changes over the axial extent of the waveguide to provide a

boundary condition which encourages the formation of particular modes based on the beat wavelengths for a particular primary mode, as shown in figure 5 which shows the modes which co-propagate with a particular (m,n) primary mode, where m is the azimuthal mode and n is the axial mode. The whispering gallery mode waveguide 220 inner surface is initially substantially cylindrical as shown in figure 4A. Along an axial 202 extent of the waveguide 220, the inner surface profile changes gradually from a cylindrical inner surface to an inner surface defined by:

where:

Ro is the nominal radial distance from the axis to the inner surface of the waveguide, as shown in figure 4A.

R((|),z) is the swept inner radius of the waveguide 220; a(z) is the dimple amplitude of the azimuthal

variation of the inner surface of waveguide 220 with respect to the swept angle φ;

N is the number of variations in a rotation through φ (N=5 in figure 4B) ;

m(z) is a linear term varying with z which provides an "azumithal twist" to the dimple pattern shown in cross section 4B, to preserve the sinusoidal variation and period experienced by a helically traveling wave through the extent of waveguide 220 as shown in the cross section figure 4B, showing a simplified view of the inner surface of the waveguide. The primary and co-propagating high order modes propagate in a series of reflections helically down the whispering gallery mode waveguide 220, as

previously described, and the pitch of the succession of dimples 404a, 404b, and 404c is chosen with a length of the beat wavelength of the mixture of primary and co- propagating high order modes. The depth of the dimples may vary axially down the waveguide 220 as the reinforcement of selected modes occurs. In addition to the sinusoidal azimuthal variation described in the equation above, an axial sinusoidal variation in the surface profile is also present, which generates a dimpled surface with local regions of maxima and minima, rather than continuous axial grooves. This complex waveguide inner surface contour provides the waveguide boundary necessary to couple the co- propagating high order modes of the table of figure 5 with the initial particular (m,n) primary mode. Accordingly, the cross section of figure 4B is not intended to indicate that the dimples 404a, 404b, 404c, and others are of fixed rotational position, or of fixed depth. A series of cross section views such as 4A would typically show a rotation clockwise or counterclockwise of the dimple features in successive axial positions along axis 202 (representing the m(z) term in the equation above, and the depth of the dimple or groove features 404a, 404b, 404c with respect to the center axis 202 may increase or decrease, since the wave propagation is helical and the spacing from dimple to dimple is based on the beat wavelength. Figure 4C shows the effect of the superposition of a plurality of propagating waves in a waveguide where N=5.

Figure 4B also shows the dimple features 404a, 404b, 404c of the whispering gallery mode waveguide 220 providing a focusing and guiding function. In the case where the beam is propagating helically in a clockwise direction in the view of figure 4B, two reflected segments are shown with the preceding and successive segments not shown for clarity. As a propagating helical wave, it is understood that RF beam segments 410, and 416, and associated dimple features 404a, 404b, and 404c are at different axial positions, but are shown in the simplified projection view of figure 4B for clarity. Each interaction of the RF beam with features 404a, 404b, and 404c serves to focus and guide the beam to the elongate radiation pattern previously described for figure 2E. Additionally, the surface dimple shapes and positions are selected to provide a repeated focusing of the beam, as is see with beam 410 reducing to a narrow diameter 412 and subsequently expanding as shown in region 414. This successive focusing and guiding occurs at beam 416 after reflection at dimple 404b, and in the beam reflected by dimple 404c. This focus progression continues until the RF is launched as quasi-optical RF energy at launch edge 230 of figure 2A. The mirrors after launch (240 and 250 of figure 2A and 316 of figure 3A) have a first order focusing which reshapes the beam profile as was described for figures 2E, 2F, 2G-1, 2G-2 and 2H. This first order correction provides a minimum beam waist diameter at the entrance 266 of corrugated waveguide 260 of figure 2A or entrance 320 of corrugated waveguide 310 of figure 3A. The surface of the mirrors after launch also have a second order phase correction through the use of minor surface deformations (which are less than a

wavelength in depth) which provides a second order phase correction to provide a modified quasi-Gaussian phase front TEMoo at the corrugated waveguide entrance to maximize coupling to the HE11 mode in the waveguide 260.

In this manner, a wide variety of whispering gallery mode waveguide surface profiles and mirrors can provide very efficient coupling and high mode purity in a gyrotron using three criteria in combination, as follows. The first criteria is selection of a primary mode and a particular set of high order co-propagating modes (such as from figure 5) for propagation through the whispering gallery mode waveguide 220, which is provided by the surface dimples and features of the waveguide 220. On one embodiment of the invention, the surface dimples have a beat period separation which is greater than a wavelength in the direction of RF propagation.

The second criteria is the generation of a minimum RF beam diameter with a Gaussian-like profile at the entrance of the corrugated waveguide, which is provided by the geometric shape of the focusing mirrors the free space quasi-optical RF beam encounters after the launch edge of the WG waveguide.

The third criteria is generation of substantially uniform phase at the Gaussian beam phase front occurring at substantially the same extent in the beam axis as the minimum beam diameter at the corrugated waveguide entrance of the second criteria, and this third criteria is met through the minor surface deformations on the focusing mirrors (240 and 250 of figure 2A, or 316 of figure 3A) which perform these phase corrections, the minor surface deformations being less than a wavelength of the propagating RF.

The novel result of coupling directly into a

corrugated waveguide inside the gyrotron cavity 201 or 302 is based on the application of the above three criteria.

The gyrotron launcher of the present invention thereby comprising a whispering gallery mode waveguide with a step- cut launcher and one or more mirrors, where the whispering gallery mode waveguide provides the formation of a primary (m, n) mode and the co-propagating modes of figure 5 are used to form beat wavelengths which are used to select the waveguide inner surface profile, the beat wavelength relationships between the primary (m,n) mode and co- propagating modes of figure 5 determining the inner surface profile on the whispering gallery mode waveguide, the one or more mirrors having minor phase corrections of less than a wavelength in depth to provide a Gaussian-like beam with minimum waist diameter at the entrance of the corrugated waveguide, which entrance is preferably located inside the gyrotron evacuated chamber 201 or 302.

Accordingly, many different variations of the

illustrated embodiments which rely on different high order modes may be used which operate according to the three criteria described above. The invention is best understood not by the particular examples given for understanding of the invention, but by the claims which follow.