Abstract:
An apparatus for modulating the density of an electron beam as it is emitted from a cathode, comprised of connecting a source of pulsed input power to the input end of a nonlinear transmission line and connecting the output end directly to the cathode of an electron beam diode by a direct electrical connection.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional application of U.S. patent application Ser. No. 13/245,250 filed on Sep. 26, 2011, and claims the benefit of the foregoing filing date. 
    
    
     STATEMENT OF GOVERNMENT INTEREST 
     The conditions under which this invention was made are such as to entitle the Government of the United States under paragraph I(a) of Executive Order 10096, as represented by the Secretary of the Air Force, to the entire right, title and interest therein, including foreign rights. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention is generally related to a method for modulating the density of an electron beam as it is released from a cathode, and in particular relates to coupling a cathode to a nonlinear transmission line to modulate an electron beam emitted by the cathode. 
     In many electron beam-related applications, it is highly desirable or necessary to be able to modulate the density of an electron beam as it is released from the cathode. In grid-controlled microwave tubes, such as inductive output tubes and planar triodes, this is done by applying a dc voltage between the cathode and anode of a vacuum diode and then using a control grid with a time varying voltage bias a very short distance (as little as ˜0.1 mm) from the cathode. The control grid bias determines the amount of current that is released from the cathode. The highest frequency of these tubes is limited by the electron transit time in the cathode to grid region. The requirement for a cathode control grid increases expense and complexity as well as introducing additional failure methods (such as inadvertent shorting of the cathode to the grid due to contaminates or warping of the grid or cathode). 
     In many accelerators, a modulated electron beam is created using laser light pulses to eject electrons from a photocathode. The laser system and associated focusing optics add considerable cost and complexity to accelerator cathodes. 
     This invention provides a novel and efficient way to modulate the current density of an electron beam emitted from a cathode without the need for complicated control grids or laser-based photoemission techniques used in current microwave tubes and accelerators. 
     SUMMARY 
     The present invention provides a novel and efficient way to modulate the current density of an electron beam emitted from a cathode without the need for complicated control grids or laser-based photoemission techniques currently in use. The current density is modulated by coupling a vacuum diode to a nonlinear transmission line (NLTL). This connection may be made from the NLTL to the cathode or from the NLTL to the anode of the electron beam diode. 
     A dispersive NLTL can be used to convert a pulsed voltage input into a modulated output at microwave frequencies. A non-dispersive NLTL, or shockline, can be coupled to the cathode to produce an electron beam with a very sharp density gradient on the leading edge of the beam. Because the NLTL can be incorporated into the power system, this invention enables one to directly modulate the input voltage pulse to the cathode in a controllable and repeatable manner at high frequencies (&gt;500 MHz) and provides an apparatus that is simpler, less expensive, and more robust than current devices. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a conceptual drawing which describes the coupling of a nonlinear transmission line to the cathode or anode of an electron beam diode in order to allow for the generation of a modulated electron beam. 
         FIGS. 2A, 2B and 2C  comprise conceptual drawings which respectively describe the coupling of a nonlinear transmission line to the cathode or anode of an electron beam diode via an impedance transformer, a capacitive connection, and an inductive connection, to provide for the generation of a modulated electron beam. 
         FIG. 3  is a plot of the input signal for a hypothetical non-dispersive nonlinear transmission line “shock line.” 
         FIG. 4  is a plot of the output signal for a hypothetical non-dispersive nonlinear transmission line “shock line.” The long rise time input pulse of  FIG. 3  is converted to a very short rise time voltage pulse by the shock line. 
         FIG. 5  is plot of the predicted cathode current as a function of time for a cathode with an emission threshold of Vt 0  in an electron beam diode across which the voltage waveform of  FIG. 4  is applied. 
         FIG. 6  is a plot of the input and output voltage signals for a dispersive nonlinear transmission line. The input signal is converted to a modulated output signal by the nonlinear transmission line. 
         FIG. 7  is a plot of the output voltage signal of  FIG. 6  applied as applied across an electron beam diode with voltage thresholds Vt 1  and Vt 2  shown. 
         FIG. 8  is a plot of the expected current output of a cathode which is driven by the output of the nonlinear transmission line associated with the traces depicted in  FIG. 6  and which has the emission threshold voltage Vt 1 . 
         FIG. 9  is a plot of the expected current output of a cathode which is driven by the output of the nonlinear transmission line associated with the traces depicted in  FIG. 6  and which has the emission threshold voltage Vt 2 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  is a conceptual drawing of one embodiment of the present invention in which a nonlinear transmission line  1  (NLTL) is coupled to an electron beam diode of an electron beam device  2 . A first terminal  3  of the electron beam diode is connected to the output of the nonlinear transmission line (NLTL)  1  via a connection  4  which can represent either a direct connection between terminal  3  and the NLTL or a connection via a length of transmission line. In this drawing, a second terminal  5  is connected to ground  6 . In the case where the modulated potential applied to the first terminal  3  is negative with respect to the grounded terminal  5 , the first terminal  3  will be the cathode and the modulated electron beam  7  will travel from the cathode toward the grounded terminal or anode  5 . In the case where the modulated potential applied to the first terminal  3  is positive with respect to the grounded terminal  5 , the grounded terminal will be the cathode and the modulated electron beam  7  will travel from the cathode  5  toward the anode  3 . The input pulser  8  provides pulsed input power to the NLTL. The NLTL may be coupled to the anode or cathode of an electron beam diode by either a direct electrical connection or via a capacitive or inductive coupling connection. The specific nature of the connection will change depending on the type of NLTL or cathode/anode used as would be apparent to one skilled in the art. 
     The nonlinearity of the electromagnetic response of the nonlinear transmission line may be due nonlinear dielectric materials, nonlinear magnetic materials, or a combination of nonlinear dielectric and nonlinear magnetic materials. Additionally, this nonlinear transmission line may be dispersive or a shock line. 
       FIG. 2A  depicts a NLTL coupled to an electron beam diode  2  via an impedance transformer  9 . This type of configuration would prove to be advantageous in cases where the electron beam diode impedance differs substantially from the output impedance of the NLTL. Alternatively, the impedance transformer  9  of  FIG. 2A  may simply consist of the capacitive coupling  28  shown in  FIG. 2B  or the inductive coupling  29  shown in  FIG. 2C . 
     The electron beam diodes depicted in  FIG. 1  and  FIG. 2  are greatly simplified to allow for ease of understanding of the present invention. Additionally, although the grounded terminal  5  of  FIG. 1  and  FIG. 2  is shown to be tied to ground for the sake of simplicity, both the cathode and anode could, in principle, be separately biased with respect to ground such that the effective voltage across the diode would be the difference of the dc biases on the cathode and anode plus the modulated voltage output of the NLTL. 
       FIG. 3  is a plot of an input signal of a simulated nonlinear transmission line shock line. The long rise time input voltage pulse  10  is sharpened to a much shorter rise time voltage pulse  11  during its transit down the shock line as seen in  FIG. 4 . The voltage scales and the time scales in both plots are normalized. The voltage threshold Vt 0  is chosen as an example emission threshold for a hypothetical cathode. 
       FIG. 5  is a plot of the predicted cathode current  16  as a function of time for a cathode with an emission threshold of Vt 0  in a electron beam diode, across which the voltage waveform  11  of  FIG. 4  is applied. For the purposes of this illustration, it was assumed that the cathode is an idealized space-charge-limited emission cathode in which the electron emission scales as a function of voltage to the 3/2 power, V 3/2 . In actual practice, the emission properties and type of each individual cathode must be taken into account when calculating predicted current yields. The cathode current scale in this plot is normalized for simplicity. The time scale is the same as that used in  FIG. 4 . 
       FIG. 6  is a plot of the input and output voltage signals from a simulated dispersive nonlinear transmission line. The NLTL converts the video pulse-like input signal  18  into an RF output signal or output signal consisting of a series of electromagnetic soliton-like pulses  19 . A normalized voltage scale and time scale were used in this plot. The output signal  19  of the NLTL data in  FIG. 6  is again shown in  FIG. 7  as it is applied across an electron beam diode. The voltage thresholds Vt 1  and Vt 2  are also shown. These voltage thresholds represent electron emission voltage thresholds for two different hypothetical cathodes. The voltage scale and time scale are the same as those used in  FIG. 6 . As will be evident from the next two figures, the choice of emission threshold allows a degree of control of the modulation amplitude imposed on the electron beam. 
       FIG. 8  is a plot of the predicted cathode current  24  as a function of time for a cathode with emission threshold Vt 1  in an electron beam diode, across which the voltage waveform  19  of  FIG. 7  is applied. For the purposes of this illustration, it was assumed that the cathode is an idealized space-charge-limited emission cathode in which the electron emission scales as a function of voltage to the 3/2 power, V 3/2 . As is evident from the plot, the cathode would emit an electron beam which is modulated at the frequency of the output of the NLTL. The cathode current scale is normalized for simplicity. The time scale is the same as that used in  FIG. 6 . 
       FIG. 9  is a plot of the predicted cathode current  24  as a function of time for a cathode with emission threshold Vt 2  in an electron beam diode, across which the voltage waveform  19  of  FIG. 7  is applied. For the purposes of this illustration, it was assumed that the cathode is an idealized space-charge-limited emission cathode in which the electron emission scales as a function of voltage to the 3/2 power. In this case, the choice of electron emission of the cathode results in stronger relative modulation of the electron beam in that discrete electron bunches being emitted from the cathode at the frequency of the output of the NLTL. The cathode current scale is normalized for simplicity. The time scale is the same as that used in  FIG. 6 .