Innovative Approach to Magnetron Phase Locking for RF Applications

This document explores a novel method for phase locking magnetrons, essential for various high-power RF applications. Traditional methods involve injecting RF signals through additional components, leading to complexity and inefficiency. The proposed method utilizes varactor diodes and feedback loops to tune and synchronize magnetron oscillation with an RF reference, aiming to improve phase noise characteristics. This innovative approach could enhance the performance and practicality of using magnetrons as accelerator RF sources.

• Magnetron
• RF applications
• Phase locking method
• Varactor diode
• Innovative approach

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1. A Novel Magnetron Phase Locking Method Lili Ma 3/23/2020

2. Introduction - Some Unique Features of Magnetron Magnetron is Cross field RF oscillator High power (kW~ MW), high frequency(1GHz ~10GHz, or higher), high efficiency (~80%). Compact size, simple in design and construction. Cheap RF source, $7000 for NLM915 L-band 915MHz industrial magnetron, 100kW CW output power. Application areas: RF heating, medical linac, RF power transmission, radar, communication systems. Difficult to be phase locked. Can we use magnetron as accelerator RF source? We need to synchronize magnetron oscillation with an RF reference. Even radar and communication systems require some control over frequency, phase and noise emitted by the source.

3. Current Status of Magnetron Phase Locking There are a few publications on magnetron phase locking. Authors includes researchers from Fermi Lab, Jefferson Lab, Lancaster university, etc. One popular way is to inject RF signal into magnetron through its RF output port to lock the magnetron oscillation to a reference. The method involves another RF tube, e.g., a TWT, or a solid state amplifier to lock the magnetron; The locking signal generator must be protected from the magnetron output power by a circulator. The ability of the magnetron to follow sudden changes in injected frequency or phase is proportional to the injected power level. The total range of frequency locking is also dependent on the injected power. The overall system is complicated, expensive and less efficient. Fermi Lab and Calabaza Creek Research developed a phase locked, 100kW peak, 10 kW average 1.3 GHz magnetron-based RF system for driving accelerators.

4. Calabaza Creek Research & Fermi Lab 1.3GHz Magnetron RF System [Ref]: M. Read, B. Chase, A100kW 1.3GHz Phase Locked Magnetron for Accelerators , 2018 IEEE International Vacuum Electronics Conference (IVEC)

5. Calabaza Creek Research & Fermi Lab 1.3GHz Magnetron RF System Need 300Watts to drive 100kW! Fig. Spectrum of the magnetron with a drive power -24.8dB below the magnetron power at the natural frequency of the magnetron [Ref]: M. Read, B. Chase, A100kW 1.3GHz Phase Locked Magnetron for Accelerators , 2018 IEEE International Vacuum Electronics Conference (IVEC)

6. A novel Magnetron Phase Locking Method Use varactor diode to tune magnetron oscillation frequency. Feed back loop to lock magnetron oscillation to RF reference. Want to know magnetron phase noise measured from 10Hz to MHz. didn t find any from existing publications.

7. Algorithm of Magnetron Phase locking with a Varactor Diode RF RF0 Reference controller - PI Low Pass varactor Magnetron RF1 Lock magnetron oscillation to RF reference by phase lock loop Tune magnetron oscillation by varactor This method is based on two established technologies. Tuning of the magnetron frequency by introducing a capacitance from a varactor; A laser-RF synchronization control system developed by SLAC.

8. Lock Magnetron Frequency/Phase with a Varactor Diode In laser phase control system, we use Piezo actuator to tune laser cavity length therefore tune the laser pulse rep rate. Can we do the same to magnetron as we did for laser? Insert reactance component into magnetron resonance structure, turn magnetron into a voltage controlled oscillator. Varactor diode has a much faster tuning speed than Piezo actuators, therefore magnetron phase lock loop should achieve a much larger bandwidth and lower phase noise. Compared to injection locking method. Find varactors that can stand high RF frequency, high RF power. Investigate tuning configuration that provides the control required within the power loading capability of the diodes. Simple overall hardware system, no RF power is required to lock magnetron oscillation, no circulator required. Dramatic reduction in size, weight and power efficiency. Use existing ATCA based laser locker control system.

9. Publication on X Band Pulsed Magnetron Frequency Tuning with Varactor Diode [Ref]. H. Obata, Electronic-Frequency-Tuning Magnetron , IEEE Transactions on Electron Devices, Vol. 59, No. 11, November 2012.

10. Publication on X Band Pulsed Magnetron Frequency Tuning with Varactor Diode - continued Frequency Shifts 21MHz during 4us pulse length; Peak output power 12kW, duty cycle 0.001. [Ref]. H. Obata, Electronic-Frequency-Tuning Magnetron , IEEE Transactions on Electron Devices, Vol. 59, No. 11, November 2012.

11. SLAC Femtosecond Laser Locker System 10GHz photo diode 10GHz photo diode Low Noise Piezo Amplifier Mode locked laser 0~100V 68MHz Phase Sweep & set point 68MHz 5V IQ PI Low pass BandPass ADC CORDIC DAC Amplifier demodul ation control 2856MHz CW LO BandPass Amplifier 2771MHz Low Noise RF Front End Unlock detect and recover IQ ADC demodu lation CORDIC RF reference 2856MHz PI Low pass DAC Phase Noise Analyzer control ATCA ATCA based Femtosecond laser timing synchronization algorithm

12. Phase Noise Measurements of UED Laser Oscillator Red: free running laser phase noise; blue: closed loop laser phase noise; black: phase noise of RF reference.

13. LLRF Algorithm of Magnetron Driven RF System Hybrid of Analog and Digital Loop RF Reference Splitter RF0 Digital loop: real time parameter tuning within several hundreds kHz bandwidth. Analog loop: low latency, fast feedback, broad band noise control. RF cavity Splitter Directional coupler ADC ADC ADC Mixer FPGA Low Pass DAC Digital controller Analog controller Analog summing amplifier Phase Control to Varactor

14. LLRF Algorithm of Magnetron Driven RF System Hybrid of Analog and Digital Loop ??????? ??? ???? ????? Set point ??1 ? + ??1) ?????= (??1+?? ?) ( 1/2 ? + 1/2 ??=?? + ????= ? ??2 ? + ??2) ?????= ??2 ( ?????? ??? fast response ????= (?????+?????) ?? ????

15. Current Status Have first applied for LDRD funding last year. Now submitted proposal to Navy STTR 2020 together with Calabaza Creek Research.

16. References [1]. H. Wang, I. Tahir, Use of an injection locked magnetron to drive a superconducting RF cavity , IPAC2010, Kyoto, Japan. (Jefferson Lab and Lancaster University) [2]. B. Chase, R. Pasquinelli, E. Cullerton, and P. Varghese, Precision vector control of a superconducting RF cavity driven by an injection locked magnetron, J. Instrum., vol. 10, Mar. 2015, Art. no. P03007. [3]. M. Read, R. Lawrence Ives, T. Bui, G. Collins, D Marsden, B. Chase, J. Reid, C. Walker, and J. Conant, A 100-kW 1300-MHz Magnetron With Amplitude and Phase Control for Accelerators , IEEE Transactions On Plasma Science, Vol. 47, No. 9, pp 4268-4273, September 2019. [4]. A. Dexter, Phase Locked Magnetrons for accelerators , LINAC2014. (Lancaster University) [5]. G. Kazakevich, R. Johnson, Phase and Power Control in the RF Magnetron Power Stations of Superconducting Accelerators , 2017. [6]. B. Yang, T. Mitani, Experimental Study on a 5.8 GHz Power-Variable Phase-Controlled Magnetron , 2017. https://www.researchgate.net/publication/320139095_Experimental_Study_on_a_58_GHz_Power-Variable_Phase- Controlled_Magnetron [7]. W. C. Brown, Satellite Power System (SPS) MagnetronTube Assessment Study , https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19810009965.pdf [8]. H. Obata, Electronic-Frequency-Tuning Magnetron , IEEE Transactions on Electron Devices, Vol. 59, No. 11, November 2012. [9] L. Ma, X. Shen, J. Frisch, etc., SLAC UED LLRF System Upgrade , Proceedings of LLRF workshop 2019, Chicago, USA. 10] J. Frisch, R. Claus, M. D Ewart, etc., A FPGA based common platform for LCLS2 beam diagnostics and controls , Proceedings of IBIC2016, Barcelona, Spain. [11] D. Van Winkle, M. D Ewart, J. Frisch, THE SLAC LINAC LLRF controls upgrade , Proceedings of IBIC2016, Barcelona, Spain.