Abstract:
An Electron-coupled transformer for generating a high voltage output pulse as an amplified version of an input pulse comprises a cylindrical triode electron tube with a central anode along main axis and a grid and cathode radially spaced from the anode. The anode has a first end connected to ground and a second end insulated with respect to said ground. The cathode and the grid form a traveling wave electron gun that produces a wave of ground potential in the Transverse Electric Mode, which causes electrons stored in the cathode-grid gap to be extracted from said gap. The extracted electrons form a radially symmetric collapsing traveling wave that sweeps as a wave along the anode at the speed of light, causing a voltage output pulse on the second end of the anode whose magnitude is an amplified version of an input pulse applied to the cathode.

Description:
PRIORITY APPLICATION 
       [0001]    This application claims priority from U.S. Provisional Patent Application No. 60/809,453 entitled “Method &amp; Apparatus for Controlled Fusion Reactions” filed May 30, 2006 and (2) U.S. patent application Ser. No. 11/754,928, filed May 29, 2007, for Method and System for Controlled Fusion Reactions by Curtis A. Birnbach. The foregoing applications are incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to electron-coupled transformers for providing a voltage output. 
       BACKGROUND OF THE INVENTION 
       [0003]    Transformers for providing a voltage output are known. For example, high speed transformers such as a Magnetic Linear Adder Transformer include a secondary, for example, a “stalk,” with one end attached to ground and the other end being the high voltage output terminal. A series of Toroidal primaries are stacked on the stalk. Each primary pulse adds to the energy (voltage) in the secondary. 
         [0004]    A disadvantage of the Magnetic Linear Adder Transformer is that Toroidal-shaped primaries will go into saturation and collapse the field if they are driven with too large a pulse. This limits the amount of energy that one can extract from this type of transformer. A further disadvantage is a resulting pulse having a “staircase” leading edge, rather than a smooth leading edge. 
         [0005]    Therefore, it would be desirable to provide a transformer for creating a high voltage pulse that avoids the foregoing problems of saturation and the leading edge of a pulse having a “staircase” pattern. 
       SUMMARY OF THE INVENTION 
       [0006]    Accordingly, the invention provides an electron-coupled transformer for generating a high voltage output pulse as an amplified version of an input pulse, comprising: 
         [0000]    a cylindrical triode electron tube with a central anode along the main axis of the tube and a grid and cathode radially spaced from the anode, the anode having a first end connected to ground and a second end insulated with respect to said ground;
 
the cathode and the grid forming a traveling wave electron gun that produces a wave of ground potential in the Transverse Electric Mode, when the grid is grounded with a pulse, which propagates along the length of the traveling wave electron gun at the speed of light in a direction from the first end of the anode to the second end of the anode; said wave of ground potential causing electrons stored in the cathode-grid gap to be extracted from said gap;
 
said extracted electrons forming a radially symmetric collapsing traveling wave that sweeps as a wave along the anode at the speed of light, causing a voltage output pulse on the second end of the anode whose magnitude is an amplified version of an input pulse applied to the cathode;
 
the energy of the Electron-Coupled Transformer is pumped in a linear fashion by at least the energy contained in distributed interelectrode capacitance of the cathode and grid.
 
         [0007]    One embodiment of an electron-coupled transformer includes a cylindrical triode electron tube with a solid central anode along the main axis of the tube and a grid and cathode radially spaced from the anode. 
         [0008]    The anode includes a first end connected to ground and a second end insulated with respect to said ground. The cathode and the grid form a traveling wave electron gun that produces a radially symmetrical collapsing traveling electron wave that travels at the speed of light along the linear axis of said electron tube in a direction from a first end of the anode to the second end of the anode, is in the Transverse Electric Mode, and is of sufficient energy such that electrons penetrate a surface of the anode and cause a zone of Bremsstrahlung and electrons. 
         [0009]    The zone sweeps as a wavefront along the anode, causing a voltage to build along the length of the anode as a function of said swept zone so as to result in a voltage on the second end of the anode ramping upwardly in absolute value from ground. In addition, the Electron-Coupled Transformer is pumped in a linear fashion by at least the energy contained in distributed interelectrode capacitance of the cathode and grid. 
         [0010]    The foregoing electron-coupled transformer beneficially avoids the saturation and ‘staircase’ leading pulse edge problems associated with Magnetic Linear Adder Transformers mentioned above. 
         [0011]    Other advantages and features of the invention will become apparent from reading the detailed description in conjunction with the drawing figures. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0012]      FIGS. 1A and 18  are simplified end and side cross-sections, respectively, of a basic Stimulated X-ray Emitter (SXE) Energy Driver, from which an electron-coupled transformer is derived. 
           [0013]      FIG. 2A  is a perspective view of Grid and Phase Matching Network used with the SXE of  FIGS. 1A and 1B . 
           [0014]      FIG. 2B  is a view of the Grid and Phase Matching Network of  FIG. 2A  showing the cross section indicated as “FIG.  2 A- 2 B” in that figure. 
           [0015]      FIG. 2C  is a schematic diagram of the Phase Matching Network of  FIG. 2A . 
           [0016]      FIG. 3  is a perspective view of a capacitor-enhanced version of the SXE of  FIG. 1 . 
           [0017]      FIG. 4  is a sectional view along the length of an Electron Coupled Transformer. 
           [0018]      FIG. 5  shows the typical waveforms of the Electron Coupled Transformer of  FIG. 4 . 
           [0019]      FIG. 6  is a sectional view along the length of a combined SXE-Vircator driver. 
           [0020]      FIG. 7  is a partly sectional view along the length of the Vircator RF head of  FIG. 6 . 
           [0021]      FIG. 8  is a sectional view along the length of a combined SXE-MILO driver. 
           [0022]      FIG. 9  is a partly sectional view along the length of the MILO RF head of  FIG. 8 . 
           [0023]      FIG. 14A  is a sectional view along the length of a Drift. Tube used in the MILO RF head of  FIG. 8 , and  FIG. 108  is an enlarged view of the circled region in  FIG. 10A  entitled “FIG.  10 B”. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0024]    A list of drawing reference numbers, their associated parts and preferred materials for the parts can be found near the end of this description of the preferred embodiments. 
         [0025]      FIGS. 1A and 1B  are cross-sections of a basic SXE Energy Driver, as first described by the inventor of this current invention in U.S. Pat. No. 4,723,263.  FIG. 1A  is an end view and  FIG. 1B  is a side view. Visible in these views are an anode  64 , a grid  66 , and a cathode  68 . The SXE is a triode electron tube. It has a novel electron gun structure, comprised of the cathode  68  and the grid  66 . The physical juxtaposition of these elements is such that it forms a circular waveguide. A circular waveguide supports the Transverse Electric Mode (TEM) which always propagates at the Speed of Light (“c”) in a vacuum. This property is critical to the function of the SXE in that it ensures that the electron beam, as shown by arrows, sweeps the anode at the speed of light and thus the phase velocity and the group velocity of the wavefront are matched. This is known as the Collapsing Traveling Wave. This is important in the x-ray formation process in that it ensures that the x-rays formed by the beam are always traveling in a highly ionized zone and are therefore not absorbed by self-absorption processes. 
         [0026]    The interrelation between an SXE and the claimed Electron-coupled Transformer (ECT) is as follows. The ECT is a pulse amplification device, with the pulse being an amplified version of an input pulse. An ECT is a novel electron tube derivative of the foregoing-mentioned SXE. A high voltage pulse can be generated if one end of the SXE is grounded and the other end is highly insulated. When an input pulse is injected into the cathode, an output pulse appears at the anode output. When being used for generating stimulated X-ray emission, the anode of the SXE is hollow and filled with a lasing material. If stimulated X-ray emission is not desired, the anode can be solid. However, the SXE can simultaneously exhibit stimulated X-ray emission and exhibit the ECT principle of generating a high voltage pulse, by grounding one end of the SXE and highly insulating the other end. 
         [0027]    The grid structure (discussed in detail in  FIG. 2 ) is highly symmetrical. This ensures that the collapse of the wave towards the anode is perfectly symmetrical. 
         [0028]    Referring now to  FIGS. 2A ,  2 B and  2 C, these figures show details of the Grid and Phase Matching Network of the SXE of  FIGS. 1A and 18 .  FIG. 2A  shows the entire Grid—Phase Matching Network and details of the Grid Insulation. These two elements are actually part of a single structure.  FIG. 2B  shows details of a preferred Grid Tensioning and insulating means.  FIG. 2C  shows the electrical schematic of the Phase Matching Network. Common to all tubes with Traveling Wave Electron Guns (TWEG) are the design requirements for the grid. No matter what scale or power level the tube is designed for, the following characteristics are common and must be present in order for the Traveling Wave Electron Gun to operate. 
         [0029]    The Traveling Wave Electron Gun (TWEG) is a unique structure in that it uses the close juxtaposition of the grid  66  and the cathode  68  to produce a Circular Waveguide structure that supports a Transverse Electric Mode (TEM). The Transverse Electric Mode in a Circular waveguide always travels at the speed of light (“c”) in a vacuum. This aspect of the TWEG accounts for its extremely fast risetime (one nanosecond for each foot or 30.48 cm of gun length). 
         [0030]    The grid is also used to both produce the electric field necessary to extract electrons from the cathode and to control the flow of such electrons. This is accomplished by selectively biasing the grid relative to the cathode. Both switching and modulation functions can be attained by appropriate biasing of the tube. 
         [0031]    There are several critical conditions that must be met when designing a grid for a TWEG structure. They are:
       (1) The grid-cathode spacing must be constant across the length of the grid. This is usually accomplished by placing the grid under high tension or building it with a rigid structure   (2) The number of elements in the grid must be high enough to ensure a constant and uniform electric field in the grid-cathode region.   (3) There must be no sharp edges or burs anywhere on the grid structure. Individual elements can be round, flat or high aspect-ratio elliptical shapes. All edges must be fully radiused. In this context, fully radiused means that the edge in question has a radius equal to half the thickness of the material; an example of fully radiused appears at  125  in  FIG. 10 .       
 
         [0035]    The actual implementation of these design rules is determined by the size of the grid being built. The grid can be made from a single piece or, more commonly, a series of individual elements constrained by mounting rings on either end  130 ,  132 , provided with suitable electrical insulators  136 ,  140  to prevent arcing, and a means of maintaining tension on the grid structure. In the preferred embodiment shown, each grid element is provided with a tensioning means in the form of a heavy spring  146 , washer  148 , and nut  150 . The nuts of the various grid elements are tightened with a torque wrench to ensure uniform tension on all elements. 
         [0036]    The electrical connection to the grid is made by means of a phase matching network  134 ,  136  that is connected to the input end of the grid. The phase matching network consists of a series of wires  134  of exactly equal length, with a typical tolerance of +/−0.0005″ (+/−12 microns). Each wire of the phase matching network is connected to the lower grid support ring  132  at a paint equidistant from the two adjacent grid elements. There is a plurality of phase matching network wires symmetrically disposed around the grid support ring. 
         [0037]    The other ends of the phase matching network wires are connected to a common connector element  136 . This has a number of holes on one end equal to the number of phase matching network wires, and a single hole on the opposite end. A wire is attached to this hole and runs to the grid vacuum feedthrough. The wires are silver soldered or welded by the Tungsten-Inert Gas method (TIG) as appropriate, TIG welding is preferred but not always possible. 
         [0038]    The purpose of this phase matching network is to ensure that the entire base of the grid responds to the control signal at the same moment with an accuracy that is preferably in the picosecond range. This results in a highly symmetrical wave propagating in the TWEG structure. This is a wave of ground potential and it has the effect of allowing energy stored in the grid-cathode gap and also energy available to the grid to propagate to the anode resulting in conduction of this signal. When the grid is grounded through the phase matching network, a radially symmetrical collapsing traveling wave of electrons is formed and propagates along the length of the TWEG structure. 
       Energy Storage Enhancement of SXE 
       [0039]      FIG. 3  shows a projected view of the SXE driver enhanced with an energy storage capacitor  70  integrated directly into its structure. 
         [0040]    The entire inner surface of the capacitor is bonded in intimate electrical contact to the cathode. The capacitor is then wound around the SXE driver until it has a suitable diameter to provide the required capacitance to store the energy necessary for the reaction. 
         [0041]    It should be noted that the cathode-grid interelectrode space is a capacitor by itself and stores a considerable amount of energy. A three-inch (75 mm) diameter structure stores approximately 200 picofarads per foot (30.48 cm). A two foot (61 cm) diameter device would store 1.6 nanofarads per foot (30.48 cm) if operated at 500.000 Volts and would store approximately 4 Kilojoules in the cathode-grid interelectrode space. 
       Electron Coupled Transformer 
       [0042]      FIG. 4  shows a cross-section of an Electron-coupled Transformer. The Electron Coupled Transformer (ECT) is a novel electron tube derivative of the SXE. The ECT is a pulse amplification device. It utilizes the same style electron gun as the SXE (i.e., cathode  68  and grid  66 ). The difference lies in the design and installation of the anode  64 . 
         [0043]    In the SXE, the anode is always hollow and filled with a lasing material. The input end (left, lower,  FIG. 4 ) is always connected to ground. The inventor of the current invention realized that the basic SXE structure was very similar to a class of high speed transformers known as “Linear Adder Transformers”. In these devices, the secondary is a “stalk” with one end attached to ground and the other end as the high voltage output terminal. A series of Toroidal secondaries are stacked on the stalk. These are pulsed in sequence, such that the time between pulses is equal to the propagation time of the pulse up the stalk. Each secondary pulse adds to the energy (voltage) in the secondary. 
         [0044]    The disadvantage of the Magnetic Linear Adder Transformer is that the Toroidal primaries will go into saturation and collapse the field if they are driven with too large a pulse. This limits the amount of energy that one can extract from this type of transformer. 
         [0045]    The inventor of the current invention realized that there was a strong similarity between the Linear Adder Transformer and the SXE. The both incorporated a “Stalk. Both used a sequential drive mechanism, but the SXE had a much larger current-handling capacity due to the large current-handling capacity of its cold cathode. In early SXE experiments, both ends of the anode were grounded so no high voltage was observed. An experiment was conducted in late 2006 where a version of the SXE was constructed that had only one end grounded and the other end highly insulated. A solid anode  64  ( FIG. 4 ) was used in this test. A pulse was injected into the cathode and the anode output was measured. A final distinction between the Linear Adder Transformer and the ECT is that in the Linear Adder Transformer, the primaries are separate distinct entities. The pulse that results has a “staircase” leading edge as a result. The ECT, in its preferred embodiment, has a continuous primary (cathode) and thus has a smooth leading edge to its pulse. The ECT is much lighter than a Linear Adder due to the lack of magnetic cores. A 100 KV 100 KA ECT weighs less than 200 pounds (90.7 kilograms). 
         [0046]      FIG. 5  shows the results of the foregoing test. The input pulse  86  and output pulse  84  were measured with identical voltage dividers on a high speed oscilloscope. The output pulse was several times larger than the input pulse, thus verifying the operational concept of the ECT. 
         [0047]    Referring back to  FIG. 4 , we see that the ECT is nearly identical in form to the SXE ( FIG. 1 ) but the dimensions, anode, and output are different. In the preferred embodiment, the ECT is encased in a Glass Vacuum Envelope  76 . There is a robust high voltage insulator  80  at the output, which provides a constant impedance electrical connection to the outside world. The Grid and cathode signals are fed in through feedthroughs  74  and  72 , respectively. The entire device is encased in a Lead radiation shield  78  to contain the transverse radiation field that forms. The thickness of shield  78  is a function of the cathode voltage and is calculated by conventional means for determining a radiation safety shield. 
         [0048]    We note that it is both possible and practical to utilize the coaxial capacitor energy enhancement scheme described above in the “Energy Storage Enhancement of SXE” with the ECT. This would be a convenient method of making additional energy available to the ECT for extremely high power applications. 
       Combined SXE &amp; RF Energy Drivers 
       [0049]      FIG. 6  shows an SXE combined with an RF generating means, and  FIG. 7  shows the RF generating means. Specifically, a separate tube known generically as a Virtual Cathode Oscillator (Vircator) is mounted onto the output (right-shown) end of the SXE in  FIG. 7 . In this configuration, we take advantage of the Electron-Coupled Transformer (ECT) principle to use the high voltage pulse that is created by the SXE process and apply it directly to the cathode  90  of the Vircator. The Vircator body forms a resonant cavity  98  which oscillates when the cathode fires. A grid  92  controls the firing of the Vircator. The control signal is obtained from the output terminal  142  of the grid of the SXE, which is located at the opposite end from the phase matching network. The trigger pulse is applied to the Vircator sequentially as a result of the Traveling Wave action of the SXE Grid. The Cathode and grid contain an aperture in their center that the x-ray pulse propagates through. 
         [0050]    The novelty of the foregoing system is that it combines two techniques known by themselves, i.e., Direct x-ray drive and RF Heating, so as to realize increased system efficiency. This concept is practical because the SXE is going to generate a high voltage DC pulse whether it is used or not. However, if the RF heater is not employed, then the SXE output is grounded and no high voltage DC pulse occurs. The electrical energy then leaves the system in the form of a current pulse in the ground return. But, because the HVDC pulse is available, it makes sense to use it, particularly since using it does not affect the x-ray output. 
         [0051]      FIG. 7  shows a cross-section of the Vircator RF head. The principle components are the cathode  90 , the grid  92 , a mesh anode  94 , a resonant cavity  98 , and an output window  96 . The drive pulse comes directly from the anode of the SXE  12 , which is attached directly to the Vircator cathode via the cathode feedthrough  102 . The Vircator is triggered by the output signal from the SXE grid  142 . When the Vircator is triggered, a burst of RF energy is formed by oscillation in the resonant cavity  98 . This energy has a spectral distribution that is determined by the dimensions of the cavity  98 . Typically, this energy is between 200 MHz and 2.5 GHz. The energy exits the Vircator and enters the Target Chamber  10  by the output window  96 . The Vircator is one type of RF source that can be integrated to the SXE  12  to increase system operating performance. The Vircator cathode  90  has an aperture  93  in its center through which the x-ray pulse from the SXE passes into the target chamber  10 . 
         [0052]      FIG. 8  shows a cross-section of an SXE combined with a Magnetically Insulated Linear Oscillator (MILO) at the output (right-shown) end of the SXE. The MILO is another well known, high power RF source, similar to the Vircator. The significant difference is that it can produce much higher frequencies than the Vircator. Structurally, the major difference is the incorporation of a drift tube  122  of  FIG. 10A  and use of a Traveling Wave Electron Gun (TWEG) instead of the planar cathode  90  and grid  92  of the Vircator. There is a resonant cavity  98  and its dimensions in conjunction with the dimensions of the drift tube  122  ( FIG. 10A ) determine the output range. Conventional MILO devices have outputs between 300 MHz and 3.5 GHz. The inventor of the present invention has experimentally verified that by placing a grating surface on the inner face of the drift tube  122  ( FIG. 10A ), as shown  FIG. 10B , it is possible to generate RF at much higher frequencies than those available from a smooth bore drift tube  122 . The source of this RF is due to the Smith-Purcell effect which describes the interaction of a relativistic electron beam with a grating surface  123 . Outputs in the THz range are possible. The grating surface can be formed by many methods. The spacing, face angle and grating geometry all are determinants in the frequency achieved ( FIG. 10B ). It has been determined that the preferred embodiment of the drift tube grating is an internal thread as shown in  FIGS. 10A and 108 . By altering the thread parameters, the output frequency is changed. The ends of the Drift Tube  125  are radiused to minimize formation of undesirable electric field perturbations inside the Resonant Cavity  98 . 
         [0053]    The balance of the SXE-MILO driver is the same as the SXE-Vircator. In fact, the RF heads—Vircator and MILO—can be interchanged. As in the case of the SXE-Vircator, the TWEG of the MILO has a hollow center through which the x-rays pass. The electron output from the TWEG is compressed by the drift tube  122  and oscillates in the resonant cavity  98 . 
       DRAWING REFERENCE NUMBERS 
       [0054]    The following list of drawing reference numbers has three columns. The first column includes drawing reference numbers; the second column specifies the parts associated with the reference numbers; and the third column mentions a preferred material (if applicable) for the parts. 
         [0000]    
       
         
               
               
             
               
               
               
             
           
               
                   
               
               
                 REFERENCE NUMBER LIST 
                 PREFERRED MATERIAL 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 62 
                 Collapsing Traveling Wave 
                 Electrons 
               
               
                 64 
                 Anode 
                 Refractory Metal; Hi-Z 
               
               
                 66 
                 Grid 
                 Refractory Metal 
               
               
                 68 
                 Cathode 
                 Graphite (Preferred 
               
               
                   
                   
                 Embodiment) 
               
               
                 70 
                 Coaxial Capacitor 
                 Dielectric/Metal Layers 
               
               
                 72 
                 Cathode Feedthrough 
                 Ceramic &amp; Metal 
               
               
                 74 
                 Grid Feedthrough 
                 Ceramic &amp; Metal 
               
               
                 76 
                 Glass Vacuum Envelope 
                 Glass (Ceramic, Stainless 
               
               
                   
                   
                 Steel) 
               
               
                 78 
                 Radiation Shield 
                 Lead 
               
               
                 80 
                 Anode Output Insulator 
                 Ceramic 
               
               
                 84 
                 ECT Input Waveform 
                 n/a 
               
               
                 86 
                 ECT Output Waveform 
                 n/a 
               
               
                 90 
                 Vircator Cathode 
                 Graphite 
               
               
                 92 
                 Vircator Grid 
                 Refractory Metal 
               
               
                 93 
                 Aperture in Vircator Cathode 
                 n/a 
               
               
                 94 
                 Anode Mesh 
                 Refractory Metal 
               
               
                 96 
                 Output Window 
                 RF Transparent Low-Z 
               
               
                   
                   
                 Ceramic 
               
               
                 98 
                 Resonant Circular Cavity 
                 Stainless Steel or Copper 
               
               
                 100 
                 Mounting Flange 
                 Stainless Steel 
               
               
                 102 
                 Cathode Feedthrough 
                 Ceramic &amp; Metal 
               
               
                 104 
                 Cathode Support 
                 Refractory Metal 
               
               
                 106 
                 Grid Feedthrough 
                 Ceramic &amp; Metal 
               
               
                 108 
                 Grid Support 
                 Refractory Metal 
               
               
                 110 
                 Getter Pump 
                 n/a 
               
               
                 112 
                 Getter Pump Feedthrough 
                 Ceramic &amp; Metal 
               
               
                 114 
                 MILO Cathode 
                 Graphite 
               
               
                 116 
                 MILO Cathode Support 
                 Refractory Metal 
               
               
                 118 
                 MILO Grid 
                 Refractory Metal 
               
               
                 120 
                 MILO Grid support 
                 refractory Metal 
               
               
                 122 
                 Drift Tube 
                 Refractory Metal 
               
               
                 124 
                 Drift Tube Support 
                 Ceramic 
               
               
                 125 
                 Radiused end of Drift Tube 
                 Refractory Material 
               
               
                 126 
                 Internal Anode Insulator 
                 Ceramic 
               
               
                 128 
                 Grid Insulator 
                 Ceramic 
               
               
                 130 
                 Upper Grid Support Ring 
                 Stainless Steel 
               
               
                 132 
                 Lower Grid Support Ring 
                 Stainless Steel 
               
               
                 134 
                 Phase Matching Network Wire 
                 Stainless Steel 
               
               
                 136 
                 Phase Matching Network Connector 
                 Stainless Steel 
               
               
                 138 
                 Internal Anode Insulator 
                 Ceramic 
               
               
                 140 
                 Phase Matching Network End 
                 Ceramic 
               
               
                   
                 Insulator 
               
               
                 142 
                 Grid Output Terminal 
                 Refractory Metal 
               
               
                 144 
                 Wire to Grid Feedthrough 
                 Ceramic &amp; Metal 
               
               
                 146 
                 Grid Tension Spring 
                 Stainless Steel 
               
               
                 148 
                 Washer 
                 Stainless Steel 
               
               
                 150 
                 Hex Nut 
                 Stainless Steel 
               
               
                 152 
                 Phase Matching Network Insulator 
                 Ceramic 
               
               
                   
               
             
          
         
       
     
         [0055]    In accordance with the invention, the foregoing describes an electron-coupled transformer that avoids the problems, inherent in Magnetic Linear Adder Transformers, of saturation and the leading edge of a pulse having a “staircase” pattern. 
         [0056]    While the invention has been described with respect to specific embodiments by way of illustration, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true scope and spirit of the invention.