Abstract:
A load for traveling microwave energy has an absorptive volume defined by cylindrical body enclosed by a first end cap and a second end cap. The first end cap has an aperture for the passage of an input waveguide with a rotating part that is coupled to a reflective mirror. The inner surfaces of the absorptive volume consist of a resistive material or are coated with a coating which absorbs a fraction of incident RF energy, and the remainder of the RF energy reflects. The angle of the reflector and end caps is selected such that reflected RF energy dissipates an increasing percentage of the remaining RF energy at each reflection, and the reflected RF energy which returns to the rotating mirror is directed to the back surface of the rotating reflector, and is not coupled to the input waveguide. Additionally, the reflector may have a surface which generates a more uniform power distribution function axially and laterally, to increase the power handling capability of the RF load. The input waveguide may be corrugated for HE 11  mode input energy.

Description:
The present invention was developed under the United States Department of Energy grant DE-SC0001930. The government has certain rights in this invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a load for the termination of high power microwaves traveling through a waveguide or transmitted in a quasi-optical beam. In particular, the invention relates to a microwave load which minimally reflects applied power back to the input waveguide or RF source. 
     BACKGROUND OF THE INVENTION 
     A high power load coupled to an input waveguide must satisfy several operational requirements. One requirement is the uniform dissipation of a large input power which is presented through the input waveguide as a narrow and high energy density microwave beam. A second requirement is the reflection and distribution of input power in a manner which minimizes the formation of standing waves in the load, since standing waves can result in electric field enhancement and plasma arcing, which causes non-sustainable erosion of the load device. A third requirement is the minimization of reflected energy back to the input port. 
     Prior art microwave loads have attempted to trade off some of these requirements against other requirements. A prior art device capable of handling high input power density is described in U.S. Pat. No. 5,949,298 by Ives et al. In the device of Ives, RF power travels from an input waveguide into a cylindrical cavity to a far wall reflector, and the reflected power is subsequently directed against a plurality of dissipation surfaces. One difficulty of this prior art device is that some fraction of the input energy is reflected back to the input port. A computed and observed reflected power coupling of the prior art device of Ives shows 6% or more (−12 dB) of the applied power is reflected back to the input port. Because the input port of this device is exposed to a fraction of the reflected power in the cylindrical dissipation cavity, it is not possible to reduce the reflected input power below this level. A new microwave load device is desired which provides an additional reduction in the level of power reflected back to the input port. Additionally, the device of Ives is input power limited by the power density presented to the first reflection surface from the rotating reflector for certain traveling wave modes. For example, HE 11  mode waves have a radial Gaussian energy profile with a “hot spot” at the center of the microwave beam which impinges on the coated interior wall, and removing heat from this beam profile with an elevated central power density limits the power handling capacity of the entire device, since power density of the central beam hot spot governs the temperature rise of the RF absorbing coating  140 , and an RF absorptive coating such as black rutile is limited in operating temperature to less than 300° C. before damage to the coating occurs. 
     OBJECTS OF THE INVENTION 
     A first object of this invention is a load for high power microwave operation, the load having: 
     a cylindrical body positioned about a z-axis, the cylindrical body having an extent and forming a volume enclosed by a first end cap and a second end cap, the inner surfaces of the enclosed volume having a coating which reflects a fraction of impinging RF (radio frequency) and absorbs the remainder of the RF; 
     an input waveguide located on said z axis, said input waveguide having a stationary part and a rotating part; 
     a rotating reflector located in an extent between the first end cap and the second end cap, the rotating reflector coupled to the rotating part of the input waveguide, the rotating reflector coupling microwave energy from the input waveguide to an inner surface of the cylindrical body; 
     the RF absorbing coating of the cylindrical body inner surface having a comparatively small thickness over an extent of first inner surface reflection, a comparatively greater thickness over an extent of subsequent inner surface reflection over an extent from said small thickness extent to said second end cap, and a comparatively greater thickness over a terminal surface extent from the first end cap to the first inner surface extent. 
     A second object of the invention is a load which couples traveling wave energy from an input waveguide having a stationary part to a rotating part of the input waveguide, the rotating part passing through an aperture in a first end cap and coupling power to a rotating reflector and thereafter onto the inner surface of a cylindrical body, the opposite end of the cylindrical body closed by a second end cap, the cylindrical body inner surface having a terminal reflection extent which begins at the first end cap, a secondary reflection extent which begins at the second end cap, and a primary reflection extent between the terminal extent and secondary extent; 
     where power from the rotating reflector is directed to the primary reflection extent of the inner surface of the cylinder, the primary reflection extent having an inner dissipation surface coated with a microwave energy absorbing material, the reflected energy thereafter being directed to the secondary reflection extent of the cylindrical body, the reflected energy thereafter directed to the terminal extent and back surface of the rotating reflector which prevents coupling to the input waveguide. 
     SUMMARY OF THE INVENTION 
     The present invention is a load device for radio frequency (RF) traveling in a waveguide, the load having a first end cap, a second end cap, and a cylindrical body interposed therebetween. The inner surfaces of the end caps and cylindrical body have a surface coating which reflects part of the impinging RF and absorbs the remaining impinging RF, and the resulting thermal energy is removed with water passages located in the cylindrical body and end caps. The first end cap has an aperture for an input waveguide having a stationary part with an input port coupled to the source of microwave energy, and a rotating part of the input waveguide which is coupled through the first end cap to a rotating reflector. The rotating reflector redistributes the power density profile of the input RF beam and also re-directs microwave energy to the interior surfaces of the cylindrical body and end caps in a manner which dissipates the energy, minimizes the formation of standing waves, and has a reflection geometry which minimizes the reflected energy travelling back to the input port, such as by including a baffle on the opposite side of the rotating reflector which and selecting a reflector and inner surface geometry such that multi-path reflection impinge on the back surface of the reflector and are thereby prevented from entering the input waveguide. For a single mode input wave with a Gaussian profile, the rotating reflector may have a reflection surface which generates a uniform power density at a first reflection surface of the cylindrical body. The rotating reflector may also have an axial profile for spreading the input energy across an axial extent of the cylindrical body, and a different azimuthal profile for spreading the input energy circumferentially across the cylindrical inner surface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a diagram of a lateral section view in the YZ plane for a microwave load according to the present invention. 
         FIGS. 2A ,  2 B,  2 C are axial projection views in the XY plane showing the lateral spreading of power over multiple reflections in the load of  FIG. 1 . 
         FIG. 3  is an axial section view in the XY plane showing a reflector and inner wall. 
         FIG. 4A  is the plot for a power distribution function at the reflector. 
         FIG. 4B  is a section view of an example reflector. 
         FIG. 4C  is a plot of the reflected power density at the inner surface of the cylindrical body. 
         FIG. 5A  is a plot of the power distribution function applied to a reflector. 
         FIG. 5B  is a section view of an example reflector profile for a peak-flattened reflector. 
         FIG. 5C  is a plot of the reflected power density for an example peak-flattened reflector at the inner surface of the cylindrical body. 
         FIG. 6  is a diagram of a lateral section view in the YZ plane of a reflector and an inner wall. 
         FIG. 7A  is a plot of the power distribution function for an input waveguide. 
         FIG. 7B  is a diagram for an example linear reflector profile such as the one shown in  FIG. 6 . 
         FIG. 7C  is a plot of the power distribution function at a reflector dissipation surface. 
         FIG. 8  is the diagram for a section view in the YZ plane of a reflector an inner wall. 
         FIG. 9A  is a plot of the power distribution function for the power in a waveguide applied to a peak flattening reflector. 
         FIG. 9B  shows a peak flattening reflector profile. 
         FIG. 9C  is the plot of a power distribution function of a reflection from a peak-flattening reflector. 
         FIG. 10  shows a cross section view of a rear driven low reflection RF load with a rear rotary seal. 
         FIG. 11  shows a cross section view of a rear driven low reflection RF load without rotary seals. 
         FIGS. 12A and 12B  show cross section views of  FIG. 11 . 
         FIG. 13  shows an example cross section view of a rear driven low reflection RF load without rotary seals. 
         FIGS. 13A and 13B  show cross section views of  FIG. 13 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows an example embodiment of the load device  100 . Microwave traveling wave energy enters through input waveguide  130  through stationary waveguide  102  and rotating waveguide  104  which are electrically coupled and mechanically isolated through rotary waveguide joint  106  including optional seals and bearings  107  as is known in the prior art. The stationary and rotary waveguides  102 ,  104 , and  106  may be of any construction suitable for the mode of the waves propagated. For the propagation of HE 11  waves, a corrugated waveguide is commonly used, and other waveguide types are also possible which are commonly configured to preferentially propagate RF energy which travels in a particular wave mode. In the present example embodiment, the input waveguide is circularly symmetric about a central axis  131 . RF energy which travels in the direction of an applied input power wave is referred to as “co-propagating”, whereas reflected RF energy which follows the same trajectory but travels in the opposite direction is referred to as “counter-propagating”, and this counter-propagating RF energy which traces a reflection path back to the input port becomes undesired reflected power, which is characterized by the scattering parameter S 11 . The input characteristic for power applied to the input port is characterized by the scattering parameter S 21 . 
       FIG. 1  shows the trajectory  134  (with center of beam shown as a dashed line) of input RF propagation path  134  which includes the first reflection from the surface of rotating reflector  114 , second reflection  116  from cylindrical body  108 , third reflection  118 , fourth reflection  120 , fifth reflection  122 , etc. Rotating reflector  114  has a surface profile and angle selected such that the complete trajectory of the reflections ultimately reflect reflected energy  137  to the back surface  136  of the rotating reflector  114 , which minimizes the amount of counter-propagating energy directed to the input waveguide  130 , thereby minimizing reflected power. The power absorbed in the first reflection from the rotating reflector is minimized to reduce power dissipation in the rotating reflector, since the power density at the reflector is the highest in the device, and the power dissipation and reflected energy for the subsequent reflections from the cylindrical body are provided with a variable absorptive coating thickness which is selected to match the local power dissipation capability of the inner surface. The peak localized power dissipation for a particular region is related to the areal power density of the RF beam size. In one embodiment of the invention which uses absorptive coatings with thicknesses related to the objective of reducing power dissipated in the initial reflections, the inner surface of cylindrical body  108  is divided into three dissipation regions, a primary reflection region  150  where the incident power level is the highest and the incident beam size is the narrowest, resulting in the highest areal power density, a secondary reflection region  152  of subsequent reflection energy (a region which includes the inner surface of the second end cap  156 ), and a terminal reflection region  154  where the beam power is largely exhausted after dissipation in reflections in primary reflection extent  150  and secondary reflection extent  152 . A coating of absorptive material such as black rutile or carbon is applied with a coating thickness selected according to the areal power density in each region. The method of application of the absorptive material may include sputter coating, plasma deposition or other means which results in a thermal bond to the underlying end caps  158 ,  156  or cylindrical body  108 . The primary reflection region  150  has the highest areal power density, and a corresponding thinnest coating thickness, such as a coating thickness T 1   151  which absorbs on the order of 30% (1.5 dB) of the incident RF power. The secondary reflection region  152  has a reduced areal power density compared to the primary region, and the coating thickness T 2   153  is on the order of 50% (3 db) of the incident RF power, and the terminal reflection region  154  (which includes first end cap  158 ) has the highest RF power absorption of 80% (7 dB), and accordingly has the greatest coating thickness T 3   155 . Transition zones  160  and  162  taper the absorptive coating thickness between attenuation extents  154  to  150  and  150  to  152 , respectively, to avoid enhancement of internal electric fields from discontinuous steps, and these transition zones should span many hundreds of wavelengths, but because of the process used to sputter coat the absorptive coating  140 , the transition zone has an extent on the order of a hundred millimeters. The first end cap  158  and second end cap  156  may include regions of angled wall intersecting body  108  which tend to reflect the beam power away from the reflector  114  for minimization of reflected power to the input waveguide  130 . Rotary waveguide  106  is typically coupled via gear or belt drive to a motor which causes rotation of the rotating waveguide  104  and attached reflector  114  at a rate on the order of 28 RPM, or any rate sufficient to prevent the excess buildup of thermal energy in absorptive coating  140  or cylindrical body  108 . 
     The cylindrical body  108 , first end cap  158  and second end cap  156  may be fabricated from any material with high thermal conductivity such as aluminum, and the rotating reflector  114  may be fabricated from any material with minimum reflective loss and high thermal conductivity, such as oxygen free copper. The rotating reflector  114 , cylindrical body  108 , first end cap  158  and second end cap  156  are all water cooled (not shown), and the rotary joint waveguide  106  may be a vacuum-tight joint such that the inner volume  132  can be evacuated of any gas which could interact with the high RF fields to form a plasma which may etch or erode the inner dissipation surface coatings. Optical viewing ports (not shown) may also be present for the detection of internal arcing, which is commonly used in conjunction with an interlock system which disables the microwave source. 
       FIG. 2A  shows an axial projection of the example load of  FIG. 1  for an example reflector  202  with a second reflection  206  and third reflection  210  (over different Z axis extents) and best understood in combination with  FIG. 1 . Incoming axial RF energy which reflects from the reflector  202  surface  201  such as reflector  114  of  FIG. 1  is shown in ray-optic form with extent boundaries  204 , where the power is reflected from the reflector  201  to primary reflection inner surface  206  through focal point  212  to a secondary reflection surface  210 .  FIG. 2B  shows the subsequent reflected energy  220  from  208  which reflects from surface  222  with extents  224  and to reflection surface  226 , and  FIG. 2C  shows the ray-optic incident extent  224  which is reflected from surface  232  to surface  234 . One of the issues for a high power load is the tradeoff between removing a maximum amount of power at each reflection surface, in particular the first reflection surface  206  where the beam width is the narrowest and power density highest, while limiting the temperature increase of the dissipation surface  206 .  FIG. 3  shows one solution for reducing the power density at the cylindrical body first reflection  308 , where the reflector surface  302  has a convex profile which spreads the input energy over a greater angular surface.  FIG. 4A  shows a plot of the power density function  402  of the RF applied to the input waveguide and directed to the reflector  302 . An example convex reflector profile  404  (in cross section) is shown in  FIG. 4B , and the resulting second reflection power density at the inner surface of the cylindrical body is shown as power density plot  406  with a peak areal power density at the center point  408 . It is possible to form the reflector surface such that the peak  408  is reduced and the power uniformly spread over the same extent, such as by using the reflector profile shown in  FIG. 5B  with region  505  modified to redistribute the peak level over the same extent. The application of power of  FIG. 5A  plot  502  results in a more uniform power density, as shown in plot  506  of  FIG. 5C  with reduced peak  508 . 
       FIG. 6  shows the geometry for the reflector in the YZ plane. Incoming RF power  602  is directed to flat reflector  604 , which is shown in detail view  608  in  FIG. 7B . Incident power distribution function plot  704  from the input waveguide is shown with peak power  702 . Reflector  706  angle  712  is selected to ensure that minimal RF energy which reflects from first reflection surface  610  is directed back to reflector  604  and returning to the input waveguide, and this requirement for non-reflection to the reflector provides the constraint that angle  712  be less then 45 degrees, or less considering the beamwidth after first reflection. Conversely, angle  712  should be large enough to result in at least 3 reflections, preferably at least one reflection in each of the primary reflection extent, secondary reflection extent, and terminal reflection extent, which results in angle  712  being at least larger than 5-10 degrees. In the present example shown in the figures, angle  712  is 30 degrees and is selected to maximize the number of attenuative inner surface reflections and minimize the reflected power reflected to the input port, preferably instead directing reflected power to the baffled rear reflector surface which is isolated from coupling this power to the input waveguide. 
       FIG. 7A  shows the power distribution function at the input waveguide and  FIG. 7C  shows the power distribution function at the second reflection surface  610  after reflection from surface  706 . 
       FIG. 8  shows an alternate embodiment for the axial profile of the reflector surface  808 , shown over extent  804  as modified reflector surface  906  in  FIG. 9B . As was described for the axial section shown in  FIG. 5B  for reducing the peak power and generating a more uniform profile, the reflector surface  906  is shown as modified to create a more uniform power level over the axial extent  808  of the reflection surface. Waveguide power density is shown in plot  904  with a peak power level  902 , and reflector surface  906  redistributes the power into the profile shown in plot  910  of  FIG. 9C , with reduced peak  908  and a more uniform power distribution across the same extent. 
     The internal dissipation of RF energy across the inner surface of the load may be accomplished many different ways. In one example shown in  FIG. 1 , the dissipation surfaces have an absorptive coating which is applied such as by sputtering vapor of the RF absorptive coating material onto the inner surface which is chosen to be highly reflective for RF such as aluminum, copper, or a metallic plating which has high conductivity and a thickness which exceeds the skin depth of the impinging RF. Alternatively, in another embodiment, the cylindrical body and end caps may be fabricated from a material which exhibits a uniform dissipation upon reflection of the impinging wave, such as stainless steel. In this example embodiment, each inner reflection results in a constant dB loss with a skin depth on the order of hundreds of microns for frequencies in the hundreds of Ghz, and the cylindrical body axial length (and also optionally diameter) is increased to generate additional reflections over an extended axial extent, resulting in the same multipath attenuation before the returning energy strikes the back surface  136  of the reflector  114 , compared to the increased attenuation and shorter multiple reflection path of absorptive coating  140  with thickness variations T 1 , T 2 , and T 3  shown in  FIG. 1 . 
     The RF load is suitable for any modes or frequency of applied electromagnetic radiation which exhibits quasi-optical behavior, including the domains of traveling RF waves in space or in waveguides, and high power optical sources including lasers and the like. 
     Water cooling of the heat developed in the inner absorptive surfaces of the load device  100  of  FIG. 1  may be achieved in any standard method by introducing water jackets into the structures of the cylindrical body  108 , end caps  156 ,  158 , and rotating reflector  114 . 
     Another embodiment of the load  1000  is shown in  FIG. 10 , which has certain cost advantages when the input waveguide and load is evacuated to reduce plasma arcing from high electric fields inside the waveguide and load. 
     Vacuum isolation of the load  100  of  FIG. 1  requires rotary seals  107 , which are present in the form of two separate seals on the two opposing surfaces of the rotary waveguide  106 . As rotary vacuum seals are expensive and potential failure points, a variation of the load of figure  1  is shown in  FIG. 10  with a drive shaft  1006  at the rear of the load through second end plate  1018  with a single rotary vacuum seal  1004  and rotary bearing  1002  which are both located in a region of the device with optionally lower standing RF energy compared to their location at the RF input in  FIG. 1 . The use of a single vacuum seal  1004  as shown in  FIG. 10  results in a reflection geometry which produces internal reflections of the RF from the drive shaft  1006 , which shaft  1006  can be designed with an RF reflective surface, or including a water cooled jacket (not shown) which is contiguous to reflector  1008 , and the shaft  1006  may have a partially RF absorptive surface. The other structures of the load  1000  are generally similar to the previously described  FIG. 1  load, with the drive shaft  1006  coupled axially to reflector  1008 , where reflector  1008  has attached a co-rotating input waveguide  1014  formed from the reflector  1008  and separated from the RF  130  of stationary waveguide  102  which input waveguide  102  may be formed from the first end cap  1016 . As before, reflector  1008  has a back surface  1010  and a reflection geometry selected where multi-path reflections are directed to prevent coupling of reflected RF energy back to the input waveguide  102  of the device. 
       FIG. 11  shows a cross section view of another embodiment of the invention which requires no rotary seals as were required at the input waveguide and first end cap  158  of  FIG. 1  or the second end cap  1018  of  FIG. 10 . As for the previous embodiments, the inner volume  132  is formed from cylindrical body  108 , first end cap  1016 , and second end cap  1118 , which is sealed to a cylindrical support  1106  which is attached to a movable bellows  1102 , the opposite end of which is sealed to an undulating reflector  1110 , which is shown as conical, but may be any shape which distributes input energy from waveguide  139  onto the dissipation surface  140  as was described previously. The end cap  1118 , cylindrical support  1106 , bellows  1102 , and undulating reflector  1110  provide a vacuum-tight separation from the outer environment surrounding inner volume  132  without the use of rotary seals  1107  of  FIG. 1  or  1004  of  FIG. 10 . In one embodiment of the invention, an inner rotating shaft  1112  is centered about axis  131  and is coupled to a drive mechanism  1114  such as a gear or pulley, and the opposite end of the rotating shaft  1112  enters bellows  1102  where it has an off-axis bend  1104  and is coupled to undulating reflector  1110  with a rotational bearing  1108 . The undulating reflector  1110  moves in a manner which describes a circular trajectory about central axis  131 , but without axial rotation of the reflector  1110  or bellows  1102 .  FIG. 12A  shows the cross section view A-A of  FIG. 11 , and shows an off-axis  131  section  1204  of tip  1110  which has movement path  1202  which defines a circle about central axis  131 .  FIG. 12B  shows a cross section view B-B of  FIG. 11 , through the stationary outer shaft  1106  and rotating inner shaft  1112 . The other references and structures of  FIG. 11  are as previously described for  FIGS. 1 and 10 , and are included for reference to these structures. 
       FIG. 13  shows another embodiment of a low-reflectance RF load similar to  FIG. 11 , but with a ball joint  1308  coupled to the shaft  1306  and  1304 , where the reflector  1302  travels in a circular motion shown in  FIG. 13A  trajectory  1301  about central axis  131 , and the shaft  1306  may be driven as shown in  FIG. 13B  about central axis  131  in trajectory  1310 . In this example embodiment, a flat vacuum bellows  1310  seals the shaft  1304  to the second end cap  1318  to provide a non-rotary sealing surface. Reflector  1302  may be conical, or any surface pattern which directs reflections away from the input waveguide on subsequent multi-path reflections, as was discussed previously. Rear surface  1312  may provide the baffling function to reflect subsequent reflections away from the input waveguide, thereby providing a low-input reflectance RF load. 
     Many other embodiments of the load are possible, and the example given is for illustration only to understand a few variations of the invention, and the examples are not intended to limit the scope of the invention as set forth in the claims. The low reflectance load is suitable for a wide range of frequencies, including those in the range 70 Ghz to 200 Ghz, a frequency range known as millimeter-wave RF. In one example embodiment of the invention tested by the inventors and shown in  FIG. 1 , the length of the cylindrical extent encompassing the final reflection extent, primary reflection extent, and secondary reflection extent is on the order of 100 cm, and the diameter of the cylindrical body is on the order of 50 cm, the input waveguide is 10 cm carrying 2MW continuous wave HE 11  mode microwave energy at 170 Ghz. 
     In another example embodiment of the invention, the rotational waveguide joint  106  of  FIG. 1  includes a vacuum seal  107  attached to first end cap  158  and stationary input waveguide  102  which provides a stationary coupling surface for the input waveguide associated with the microwave source.