Patent Publication Number: US-7915840-B1

Title: RF power recovery feedback circulator

Description:
GOVERNMENT INTEREST 
     The United States Government has rights in this invention pursuant to Contract No, W-31-109-ENG-38, between the U.S. Department of Energy (DOE) and the University of Chicago. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the recovery of reflected RF (radio frequency) energy. 
     BACKGROUND OF THE INVENTION 
     The coupling of RF (radio frequency) circuits often causes signal reflection. This reflected signal is captured and sent to a “dummy load,” a resistive cell. Normally, RF systems, such as those used in communications, are specifically designed to eliminate or at least minimize signal reflectivity. However, in non-communication RF systems, such as those employed in particle accelerators, reflectivity can be a very serious problem. These systems often have a reflectivity greater than about 80% and as high as about 99%, resulting in large RF energy transfer inefficiencies. 
     Particle accelerators such as ATLAS (Argonne Tandem-Linac Accelerator System) at Argonne National Laboratory use superconducting resonators. These superconducting resonators have a very high Q (quality factor), and therefore have a very narrow frequency bandwidth. Therefore, careful attention must be made to ensure the resonator resonance frequency is at the frequency of the RF Energy Source (RF drive frequency) supplied to the superconducting resonator. Careful attention must also be made to ensure that the resonator RF field is in phase with the RF Energy Source. For example, the resonators used in ATLAS at Argonne National Laboratory have a loaded Q on the order of 10 7 , and a resonance frequency of 97 MHz plus or minus only a few hertz. Frequencies outside of the resonance frequency have little to no effect. 
     Unfortunately, the resonance frequency of the superconducting resonators is continuously altered by factors such as cryogenic pressure variations, background mechanical vibrations known as microphonics, and ponderomotive (force from ion movement) detuning. 
     One method of ensuring the resonator can efficiently utilize RF energy from the RF Energy Source is to overcouple the resonator, effectively increasing the bandwidth of the resonator. By increasing the resonator bandwidth, RF phase errors due to RE resonant frequency variations in the resonator can be reduced. 
     Unfortunately, when overcoupled, a significant amount of RF energy from the RF Energy Source is reflected back to the RF Energy Source. As the Reflected Energy can potentially damage the RE Energy Source, this Reflected Energy is normally routed through a circulator to a “dummy load” for dissipation. As resonators used in superconducting particle accelerators can have a reflectivity as high as 99%, a very significant amount of energy is lost due to overcoupling. 
     Another method of ensuring the resonator resonance frequency matches the frequency of the RE Energy Source is to continuously match the resonator frequency using a VCX (voltage controlled reactance) system, VCX systems are generally focused on achieving phase synchronization with the resonance frequency of the resonator. 
     VCX systems such as those used by ATLAS at Argonne National Laboratory are based on PIN diodes used to switch the superconducting resonator between high and a low frequency states chosen to bracket the resonate frequency. In the high-frequency state, the resonator RF phase processes forward relative to the phase of RF Energy Source, and in the low frequency state, backwards. 
     Phase control is achieved with a diode driver which switches the diodes between the two states. Within the switching period, the diodes can be turned on for a controlled time, which generally vary from 5% to 95% of the switching cycle. Modulation of the duty factor provides an effectively continuous control of the direction of phase precession, hence also the mean frequency received by the resonator. Unfortunately, VCX systems add additional complexity to the resonator and are problematic in high RF field applications. This additional complexity increases startup costs, maintenance costs, as well as operating costs. These additional components also absorb energy reducing the efficiency of the system, while generating heat. 
     Therefore, there exists a need for a simple, reliable, low cost, and energy efficient means of recuperating reflected RF energy. More particularly, there exist a need of supplying energy to a superconducting resonator, while minimizing ene loss and system complexity. 
     SUMMARY OF THE INVENTION 
     A device and method for improving the efficiency of RF systems having a Reflective Load. Generally, an RF Energy Source supplies a Supply Energy to a Reflective Load. Some energy is reflected off the Reflective Load producing a Reflected Energy. Reflected Energy from the Reflective Load is reintroduced to the Reflective Load after the phase of the Reflected Energy is properly aligned, in the case of a substantially constant phase shift (torn substantially constant reflections off the load), the phase of the Reflected Energy is adjusted by a constant amount to substantially match the phase of the RF Energy Source. In systems having a variety (variable) of phase shifts (from a substantially variable reflections off the load), a phase feedback loop to account for the various phase shifts is preferable. 
     It is an object of an embodiment of the present invention to increase the efficiency of RF systems by recovering and reusing reflected RF energy. It is another object of an embodiment of the present invention for the efficient and economical energy delivery to a particle accelerator utilizing a superconducting resonator. 
     Still, it is yet another object of an embodiment of the present invention for the efficient and economical energy delivery to a heavy particle accelerator utilizing a superconducting resonator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts an embodiment of an RF Power Recovery Feedback Circulator. 
         FIG. 2  depicts the preferred embodiment of an RF Power Recovery Feedback Circulator. 
         FIG. 3  depicts an embodiment of an RF Power Recovery Feedback Circulator. 
         FIG. 4  depicts a graph of the power gain resulting from the preferred embodiment, depending on the reflectivity of the load (in percent). 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A device and method for improving the efficiency of RF systems having a Reflective Load. Generally, an RF Energy Source supplies a Supply Energy to a Reflective Load. Some energy is reflected off the Reflective Load producing a Reflected Energy. Reflected Energy from the reflective load is reintroduced to the reflective load after the phase of the Reflected Energy is properly aligned. In the case of a substantially constant phase shift (from substantially constant reflections off the load), the phase of the Reflected Energy is adjusted by a constant amount to substantially match the phase of the RF Energy Source. In systems having a variety (variable) of phase shifts (from a substantially variable reflections off the load), a phase feedback loop to account for the various phase shifts is preferable. 
     An embodiment of an RF Power Recovery Feedback Circulator  1 , shown in  FIG. 1 , generally comprises a RF Energy Source  3 , a Combiner  5 , an RF Circulator  7 , a Reflective Load  9 , and a Phase Adjuster  11 . The RF Energy Source  3  is connected to the Combiner  5 , preferably by a wire  13 . The Combiner  5  is connected to the Circulator  7 , preferably by a wire  15 . The Circulator  7  is connected to the Reflective Load  9 , preferably by a wire  17 , and the Phase Adjuster  11 , preferably by a wire  19 . The Phase Adjuster  11  is also connected to the Combiner  5 , preferably by a wire  21 . 
     The RF Energy Source  3  generates a Supply Energy. The RF Energy Source  3  is preferably connected to the Combiner by a wire  13 , preferably a coaxial cable, or a stripline. In the alternative, waveguides may be substituted for the various wires. For example, in particle accelerator applications, the RF Energy Source  3  generates a Supply Energy in order to feed the Reflective Load  9  (resonator), in order to accelerate charged particles. Preferably, the RF Energy Source  3  produces a Supply Energy of about 10 Watts up to 10 Megawatts depending on the output power desired. In the preferred embodiment, the RF Energy Source  3  is a 1-10,000 MHz generator producing about 10 Watts to about 10 Megawatts of Supply Energy, in the another embodiment, the RF Energy Source  3  produces a Supply Energy of about 10 Watts up to about 10,000 Watts at about 40-200 Mhz. In yet another embodiment, the RF Energy Source  3  produces a Supply Energy in excess of 10 Megawatts. 
     The Combiner  5  has a First Input Connection, a Second Input Connection and an Output Connection. The Combiner  5  combines RF signals (RF waves) at the First Input Connection and the Second Input Connection into the Output Connection of the Combiner, while minimizing any reflections and energy loss. The First. Input Connection is connected to the RF Energy Source  3 , by a wire  13 , preferably a coaxial cable, or a stripline. The Second Input Connection is connected to the Phase Adjuster  11 , by a wire  21 , preferably a coaxial cable, or a stripline. The Output Connection of the Combiner  5  is connected by a wire  15 , preferably a coaxial cable, or a stripline, to the Circulator  7 . In the alternative, waveguides may be substituted for the various wires. 
     The Combiner  5  may utilize various designs such as quadrature or hybrid structures. In quadrature structures, the Output Connection of the Combiner  5  is the combination of the First Input Connection and Second Input Connection of the Combiner  5  having a 90 degree phase difference between them. Preferably, the Combiner  5  is selected considering the desired phase shift, frequency range, insertion loss, isolation, and RF connector type. Examples of suitable Combiners include the Combiners described in U.S. Pat. Nos. 6,377,133; 5,892,414; and 5,455,546, hereby fully incorporated by reference. 
     The RF Circulator  7  comprises an Input Connection (port), and Output Connection (port) and a Reflection Connection (port). The Input Connection connected to the Output Connection of the Combiner  5 , by a wire  15 , preferably a coaxial cable, or a stripline. The Output Connection (port) is connected to the Reflective Load  9 , by a wire  17 , preferably a coaxial cable, or a stripline. The Reflection Connection (port) is connected to the Phase Adjuster  11 , by a wire  19 , preferably a coaxial cable, or a stripline. In the alternative, waveguides may be substituted for the various wires. The RF Circulator  7  transfers energy inputted at the Input Connection to the Output Connection, while minimizing reflections and energy loss. The RE Circulator  7  also transfers energy inputted at the Output Connection to the Reflection Connection, while minimizing reflections and energy loss. 
     The RF Circulator  7  is preferably a passive microwave ferrite device with three (P 1 , P 2 , P 3 ) or four (P 1 , P 2 , P 3 , P 4 ) ports in which the ports can be accessed in such an order that when a signal is fed into any port it is transferred to the next port, in the direction P 1 -P 2 -P 3 -P 1  or P 1 -P 2 -P 3 -P 4 -P 1 , while minimizing any RF reflections and energy loss. In one embodiment, the RF Circulator  7  is a four port waveguide circulator based on Faraday rotation of propagating waves in a magnetized material, or a three port “turnstile” or “Y-junction” circulator based on cancellation of waves propagating over two different paths near a magnetized material. In the preferred embodiment, the RF Circulator  7  is a three port Y-junction coaxial RF Circulator Examples of suitable RF Circulators includes the RF Circulators described in U.S. Pat. Nos. 3,573,666; 5,384,556; and 5,963,108, hereby fully incorporated by reference. 
     The Reflective Load  9  is a device capable of absorbing the Supply Energy produced by the RF Energy Source  3  while at least partially reflecting the Supply Energy creating a Reflected Energy.  FIG. 4  shows a graph of the power gain resulting from the preferred embodiment, depending on the percentage of reflectivity of the Reflective Load  9 . The Reflective Load  9  preferably has a reflection coefficient greater than 41.4%, the unity power gain. In the preferred embodiment, the Reflective Load  9  is a superconducting resonator of a particle accelerator having a Q (quality factor) greater than about 10 6 , more preferably about 10 8 , and even more preferably 10 11 . In another embodiment, the Reflective Load  9  is an antenna, or any another RE load known in the art. 
     The Phase Adjuster  11  has an input Connection and an Output Connection, and aligns the phase of the RF signal at the Input Connection to the phase of the RF Energy Source  3  at the Combiner  5 . The Input Connection of the Phase Adjuster  11  is connected to the Reflection Connection of the Circulator  7  by a wire  19 , preferably a coaxial cable, or a stripline. The Output Connection of the Phase Adjuster  11  is connected to the Second input Connection of the Combiner  5 , by a wire  21 , preferably a coaxial cable, or a stripline. In the alternative, waveguides may be substituted for the various wires. 
     Depending on the Combiner  5 , the Phase Adjuster  11  may need to provide a determined phase relationship between the Supply Energy from the RF Energy Source  3  and the Reflected Energy from the Reflection Connection of the Circulator  7  to compensate for any phase modification from the Combiner  5 . The Phase Adjuster  11  is preferably a low loss passive device. In systems where the Reflective Load  9  produces Reflected Energy with a constant phase shift, the Phase Adjuster  11  is designed to shift the phase of the Reflected Energy from the Reflected Load  9  (passed through the Circulator  7 ) by a constant amount. In the case where the Reflective Load  9  produces Reflected Energy having a variety of phase shifts, the Phase Adjuster  11 , should be designed to adapt to phase of the Reflected Energy to that of the RE Energy Source  3 . 
     For systems having reflectivity with a variety (variable) of phase shifts, the embodiment of an RF Power Recovery Feedback Circulator  27 , shown in  FIG. 2 , is preferable.  FIG. 2  shows a Phase Adjuster  11  having a Phase Detector  23 , a Feedback Controller  29 , and a Phase Shifter  12 . The Phase Detector  23  has a First Input Connection, a Second Input Connection and an Output. Connection. The Feedback Controller  29  has an Input Connection and an Output Connection. The Phase Shifter  12  has an Input Connection, Output Connection, and a Control Input Connection. 
     The Phase Detector  23  and the Feedback Controller  29  form a phase feedback loop controlling the Phase Shifter  12 . The First Input of the Phase Detector  23  is connected to the RF Energy Source  3  by a wire  13 . The Second Input Connection of the Phase Detector  23  is connected to the Output Connection of the Phase Shifter  12  by a wire  21 . The Output Connection of the Phase Detector  23  is connected to the input Connection of the Feedback Controller  29  by a wire  25 . The Output. Connection of the Feedback Controller  29  is connected to the Control Input Connection of the Phase Shifter  12  by a wire  31 . The input Connection of the Phase Shifter  12  is connected to the Reflection Connection of the Circulator  7 . The Output Connection of the Phase Shifter  12  is connected to the Second Input Connection of the Combiner  5  and the Second Input of the Phase Detector  23   
     The Phase Detector  23  detects the phase difference between the Supply Energy from the RF Energy Source  3  and the phase-adjusted energy from the Output Connection of the Phase Shifter  12  and outputs a signal thereof. In the preferred embodiment, the Phase Detector  23  outputs a DC voltage related to the phase difference of the Supply Energy from the RF Energy Source  3  and the phase-adjusted energy from the Output Connection of the Phase Shifter  12 . 
     The feedback controller  29  stabilizes the phase relationship between the Supply Energy from the RF Energy Source  3  and phase-adjusted energy from the Output Connection of the Phase Shifter  12 . Preferably, the feedback controller  29  is a PID (Proportional-Integral-Derivative) controller, or a Digital controller. In the preferred embodiment, the feedback controller  29  amplifies the Phase Detector  23  output to achieve high feedback loop gain and while also stabilizing the feedback loop. 
     In the case where the Reflective Load  9  has a constant reflectivity, whereby energy reflected off the Reflective Load  9  has the same phase adjustment, the Phase Detector  23  and Feedback Controller  29 , may be omitted as shown in  FIG. 3 , in this embodiment  14 , the Phase Shifter  16  is selected and configured to adjust the phase of the Reflected Energy from the Reflective Load  9  by the predetermined phase difference between the Reflected Energy of the Reflective Load  9  and the Supply Energy of the RF Energy Source  3 . 
     It is to be understood that the above-described arrangements are only illustrative of the application of the principles of the present invention. Numerous modifications and alternative arrangements may be devised by those skilled in the art without departing from the spirit and scope of the present invention and the appended claims are intended to cover such modifications and arrangements. For example, Reflected Energy in a communication system may be similarly rerouted to the RF Energy Source of the communications system, reintroduced into the transmission stage of the communication system, or routed directly to the Reflective Load. In yet another embodiment, reflections in a microwave system may be similarly rerouted to the RF Energy Source of the microwave system, reintroduced into the transmission stage of the microwave system or routed directly to the Reflective Load. 
     All publications and patent documents cited in this application are incorporated by reference in their entirety for all purposes to the same extent as if each individual publication or patent document were so individually denoted. 
     Any element in a claim that does not explicitly state “means for” performing a specified function, or “step for” performing a specific function, is not to be interpreted as a “means” or “step” clause as specified in 35 U.S.C §112, ¶6. In particular, the use of “step of” in the claims herein is not intended to invoke the provisions of 35 U.S.C §112, ¶6.