Subcritical-voltage magnetron RF power source

Abstract

A system and method of operating a magnetron power source can achieve a broad range of output power control by operating a magnetron with its cathode voltage lower than that needed for free running oscillations (e.g., below the Kapitsa critical voltage or equivalently below the Hartree voltage) A sufficiently strong injection-locking signal enables the output power to be coherently generated and to be controlled over a broad power range by small changes in the cathode voltage. In one embodiment, the present system and method is used for a practical, single, frequency-locked 2-magnetron system design.

Claims

1. A system for power generation, comprising: a magnetron configured to receive a magnetron input signal and a cathode voltage and to produce a magnetron output signal having an output power; and a voltage supply system coupled to the magnetron and configured to control the output power by controlling the cathode voltage within a subcritical feeding voltage range that is below a critical voltage needed for self-excitation of the magnetron.

2. The system of claim 1, further comprising a power source configured to provide the magnetron with the magnetron input signal, wherein the magnetron input signal has characteristics enabling the output power to be coherently generated and controlled over a first power range by controlling the cathode voltage, the first power range broader than a second power range over which the output power is controllable with the cathode voltage above the critical voltage.

3. The system of claim 2, wherein the power source is configured to produce the magnetron input signal to enable the output power to be coherently generated and controlled over a power range of up to 10 dB by controlling the cathode voltage.

4. The system of claim 2, wherein the power source comprises a solid state amplifier.

5. The system of claim 2, wherein the power source comprises a further magnetron.

6. The system of claim 4, wherein the magnetron is configured to be phase-locked and frequency-locked by the magnetron input signal.

7. The system of claim 6, further comprising an accelerating cavity configured to receive the magnetron output signal, and wherein the voltage supply system comprises: a probe configured to measure an accelerating voltage signal in the accelerating cavity; and a power feedback loop configured to control the cathode voltage using the measured accelerating voltage signal.

8. The system of claim 6, wherein the power source is configured to receive a power source input signal and to be phase-locked by phase modulation of the power source input signal using a phase feedback loop comparing a phase of the power source input signal to a phase of the measured accelerating voltage signal.

9. The system of claim 8, wherein the magnetron is configured to operate in a continuous wave (CW) mode.

10. The system of claim 8, wherein the magnetron is configured to operate in a pulsed mode.

11. A method for power generation, comprising: operating a magnetron by receiving a magnetron input signal and a cathode voltage to produce a magnetron output signal having an output power; providing the magnetron with a subcritical feeding voltage as the cathode voltage, the subcritical feeding voltage below a critical voltage needed for self-excitation of the magnetron; and controlling the power of the magnetron output signal by controlling the cathode voltage.

12. The method of claim 11, further comprising producing the magnetron input signal to have characteristics enabling the output power to be coherently generated and controlled over a first power range by controlling the cathode voltage, the first power range broader than a second power range over which the output power is controllable with the cathode voltage above the critical voltage.

13. The method of claim 12, wherein producing the magnetron input signal comprises producing the magnetron input signal to enable the output power to be coherently generated and controlled over a power range of up to 10 dB by controlling the cathode voltage.

14. The method of claim 12, wherein producing the magnetron input signal comprises operating a power source by receiving a power source input signal, producing a power source output signal to be received by the magnetron as the magnetron input signal, and phase-locking the power source using the power source input signal.

15. The method of claim 14, wherein phase-locking the power source using the power source input signal comprises: comparing a phase of the power source input signal to a phase of the measured accelerating voltage signal; and phase-modulating the power source input signal using an outcome of the comparison.

16. The method of claim 15, further comprising phase-locking and frequency-locking the magnetron using the magnetic input signal.

17. The method of claim 16, wherein operating the power source comprises operating a solid state amplifier.

18. The method of claim 16, wherein operating the power source comprises operating another magnetron.

19. The method of claim 11, wherein operating the magnetron comprises operating the magnetron in a pulsed mode.

20. The method of claim 11, further comprising transmitting the magnetron output signal to an accelerating cavity, and wherein controlling the cathode voltage comprises: measuring an accelerating voltage signal in the accelerating cavity; and controlling the cathode voltage using the measured accelerating voltage.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is the measured dependence of power of a 2.45 GHz, 1 kW. CW magnetron on the cathode voltage with 53.9 W injected resonant (injection-locking) signal (squares), and in the free running mode with no injection locking signal (circles). Solid lines are linear fits showing the range of coherent magnetron operation for the indicated conditions.

(2) FIG. 2 is a conceptual drawing of the 2-stage magnetron embodiment for wide-band phase and mid-frequency power control over a 10 dB range for an RF cavity. By numbers are depicted: 1input driving resonant RF signal, 2low-power magnetron driven by the resonant injected signal, 3high-power injection-locked magnetron controlled by cathode voltage, 4power loads, 5ferrite circulators, 6an accelerating cavity driven by the high-power magnetron, 7RF couplers, 8low-power HV power supply controlled within a feedback loop, 9main HV power supply, 10current/voltage controller within the Low Level RF system (LLRF) for the low-power power supply. 11RF probe. 12phase controller within the LLRF. The heavy lines with arrows indicate the direction of information or power flow.

(3) FIG. 3 is the absolute efficiency and the range of power control of the 1.2 kW, CW magnetron driven by a resonant (injection-locking) signal for different values of the locking power. Solid lines show the range of power control vs. P.sub.Lock. The magnetron filament power consumption was not included in the efficiency determination.

(4) FIG. 4 is the carrier frequency offset at various powers of the magnetron and the locking signal. The trace P.sub.Mag=0.0 W, P.sub.Lock=30 W shows the offset of the injection-locking signal when the magnetron high voltage power supply is OFF.

(5) FIG. 5 is the modulation of the magnetron power by modulation of the magnetron current, obtained by controlling the SM445G HV switching power supply within a current feedback loop.

(6) FIG. 6 is shows the V-I curves for different values of locking power of the magnetron tube described in this patent application. The Hartree voltage or Kapitza critical voltage is shown as the threshold voltage to create oscillations and produce power without an injection locking signal.

DETAILED DESCRIPTION OF THE INVENTION

(7) The embodiment of a high-power RF source powering a superconducting RF cavity is based on a single, frequency-locked 2-cascade magnetron system. It is controlled in phase over a wide band by a phase-modulated injection-locking signal and controlled in power in the range up to 10 dB by a HV power supply within a fast feedback loop comparing the amplitude of the accelerating voltage in the cavity measured by an RF probe with the required accelerating voltage. The cost-effective 2-cascade magnetron system is shown schematically in FIG. 2. In this embodiment, the required RF signal 1 enters the first low power magnetron 2 to phase lock it via the first circulator 5. The injection-locked output of the low power magnetron is then directed via circulators 5 to enter the high power magnetron 3 to injection-lock its frequency. The output of the high power magnetron is sent to the RF cavity 6 via the first RF coupler 7. The first low-power magnetron operates in a nominal regime. It is frequency-locked by a driving system 12 and controlled in phase comparing phase of the driving signal 1 with phase of the signal measured via the RF probe 11 within a phase feedback loop. This provides the required injection-locking signal for the wideband phase control of the second, high-power magnetron. The phase control is realized by phase modulation of the input injection-locking signal, as it was demonstrated in Ref. [1]. The high-power magnetron 3 is driven by a sufficient injection-locking signal to operate stably with a feeding voltage less than the critical voltage in free run [2]. This allows control of the power via the LLRF 10 of the high-power magnetron over an extended range by varying the current-controlled HV power supply 8 that biases the main HV supply 9 within the feedback loop. Thus one can achieve the quite fast power control with highest efficiency in the range of 10 dB avoiding redistribution of the RF power into a dummy load unlike Refs. [1,3]; the only purpose for the loads 4 in this embodiment is to protect circuitry from unintended reflected signals.

(8) Proof-of-Principle of the Invention

(9) Proof-of-Principle of the invented technique was demonstrated with 2.45 GHz, 1 kW magnetrons operating in pulsed and CW modes, [2].

(10) The experiments in CW mode demonstrated the capability of power control over the range of 10 dB, with the efficiency remaining quite high over all the range of power control, given sufficient power of the locking signal as seen in FIG. 3. The average efficiency of power control drops less than by 20% for power variation over a 10 dB range. Although the maximum efficiency of the tested magnetron was 70%, modern high power magnetrons operate with efficiencies closer to 88%.

(11) Being injection locked, the magnetron provides precise frequency stability of the output signal to control of the output power over a 10 dB range as shown in FIG. 4. With sufficient injection-locking signal power, the magnetron produces no more than 50 dBc noise over 10 dB range of output power.

(12) The 2.45 GHz transmitter model demonstrates capability for dynamic power control that is necessary for superconducting accelerators as shown in FIG. 5.

(13) Normally, the bandwidth of the invented power control technique is limited by the bandwidth of magnetron current regulation. Presently, one estimates this bandwidth up to 10 kHz without compromise of the transmitter efficiency. Due to the bandwidth of the phase control in the MHz range, [1], the phase pushing arising from the current control of the high-power magnetron can be compensated to a level of about 60 dB or better. It is suitable for various superconducting accelerators.

(14) The realization of the invented technique of power control of frequency-locked cascaded magnetron transmitters is very promising for high-power RF sources for superconducting and normal-conducting high-power accelerators.

(15) Model of Operation

(16) Detailed consideration of the kinetic simplified model describing the radial and azimuthal drift of the charge in a conventional magnetron driven by a resonant (injection-locking) signal is presented in Ref. [4]. Below we present a simple estimate of the power control range vs. the power of the locking signal.

(17) As it follows from performance charts of typical magnetrons, [6], the minimum magnetron power. P.sub.min is usually of the nominal power, P.sub.nom, and corresponds to the threshold voltage of self-excitation. For commercial magnetrons, as it is noted in Ref. [7], the energy of the RF field in the interaction space is approximately 3 times less than the energy stored in the magnetron RF system including the interaction space and cavities; half of the energy in the interaction space is associated with the RF field in the synchronous wave. Thus, an injection of the resonant driving wave with power P.sub.Lock in the magnetron RF system, as it follows from the law of energy conservation increases the energy stored in the RF system. From the point of view of the magnetron start up, it is equivalent to the decrease of the critical magnetron voltage, U.sub.C, by
U.sub.C.Math.(1(P.sub.Smin/(P.sub.Smin+P.sub.D/6)).sup.1/2).(2)
With the magnetron dynamic impedance Z.sub.D, the minimum current I.sub.minD can be estimated to be:
I.sub.minDI.sub.minU/Z.sub.D,(3)
where I.sub.min is the minimum current in free run. The power range, R.sub.D, allowable for regulation in the driven
magnetron one estimates at the nominal magnetron current, I.sub.nom, as:
R.sub.D=P.sub.nom/P.sub.minD(I.sub.nom/I.sub.minD).(4)
For the 1 kW magnetron model considered in Ref. [4] at P.sub.Lock10 dB of the nominal power one obtains: R.sub.D10 dB. As it is shown in FIG. 3, the estimations correspond to measured results.

(18) Many people familiar with the operation of magnetron power sources understand their operation based on V-I plots. FIG. 6 shows the V-I plots for the operation of our subcritical magnetron that show strait-line fits for each value of the locking power. Although these measurements correspond to a new subcritical operating mode, the straight line behavior indicates that the loss of coherence in the magnetron is insignificant and should relatively low level noise.

(19) Application to Superconducting RF Particle Accelerators

(20) Modern superconducting accelerators require highly efficient megawatt-class RF sources with dynamic phase and power control to compensate changes of the accelerating field in Superconducting RF (SRF) cavities due to microphonics. Lorenz force detuning, etc. That is, the small changes in cavity dimensions due to sonic vibrations and to forces from normal operation induce time varying frequency variations that, due to the high quality factor of superconducting cavities (typical loaded quality factors are in the range 10.sup.7-10.sup.8) can destroy the beam quality. This can cause beam losses that can destroy accelerator components or make the beam unusable for many applications. It is known that these effects can be mitigated if the power sources feeding the cavities can react to the changes in the cavities by controlling their phase and power with the proper low level RF system.

(21) Magnetron power sources have long been considered inappropriate for many high-power SRF applications because they are essentially oscillators that operate over a small range of output power that is insufficient and too slow to control microphonics. Some recent studies that use vector power control to provide power control with sufficient bandwidth look promising, but they are effectively diverting some of the power of the magnetrons into dummy loads that heat water, representing an intrinsic inefficiency of these methods.

(22) Unlike these vector power control techniques that manage the accelerating voltage in the SRF cavity by redistribution of magnetron power between the cavity and a dummy load (the magnetron in this method always operates at nominal power), our innovative technique directly controls the output power of the magnetron, without a dummy load, to achieve the highest possible operating efficiency. This is realized by a wide range of power control made possible by operating with subcritical cathode voltage. i.e. below the level needed for self-excitation of the tube, where the injection-locking signal creates the conditions to excite the magnetron and is thereby extremely effective to control its phase, frequency, and output power. This innovative technique has been demonstrated in experiments that show a wide range of power control (up to 10 dB) with low phase noise and precise frequency stability.

(23) Other Applications

(24) Applications include marine radar, microwave assisted sintering of ceramics and other materials, and microwave assisted processes in the chemical industry. A likely profitable usage is RF transmitters for TV and high frequency radio, where the efficiency and cost of magnetrons are significantly better than for other choices.

(25) Previous Patent References

(26) Previous patent references related to magnetron cathode voltage operation below or near the critical or Hartree voltage are listed below. We were unable to find any previous patents where the magnetron operation was significantly below the critical voltage and we found none that used the power from the injection-locking signal to enable operation with the cathode below the critical value.

REFERENCES CITED

U.S. Patent Documents

(27) U.S. Pat. No. 2,576,108 Amplitude Modulation of Magnetrons U.S. Pat. No. 2,620,467 Amplitude Modulation of Magnetrons, RCA

Other Patent Documents

(28) GB541120A Improvements relating to the modulation of magnetron oscillators GB2235775A Magnetron RF generation for electron paramagnetic resonance

Other Publications

(29) [1] G. Kazakcvich, EIC 2014, TUDF1132_Talk, http://appora.fnal.gov/pls/eic14/agenda.full [2] G. Kazakevich. R. P. Johnson, G. Flanagan. F. Marhauser, V. Yakovlev, B. Chase, V. Lebedev. S. Nagaitsev, R. Pasquinelli N. Solyak, K. Quinn, D. Wolff, V. Pavlov, NIM A 760 (2014) 19-27. [3] B. Chase, R. Pasquinelli, E. Cullerton, P. Varghese, JINST, 10, P03007, 2015. [4] G. Kazakevich, V. Lebedev, V. Yakovlev, V. Pavlov, NIM A 839 (2016) 43-51. [5] G. Kazakevich, R. Johnson, M. Neubauer. V. Lebedev. W. Schappert, V. Yakovlev, TUA2CO03, NAPAC 2016 Conference, Chicago, Ill., USA. [6] G. B. Collins, from Microwave Magnetrons, New York, McGraw-Hill Book Co., 1948. [7] P. L. Kapitza, HIGH POWER ELECTRONICS, Sov. Phys. Uspekhi, V 5, #5, 777-826, 1963.