Stabilised Magnetrons Presentation to mark Professor Richard Carters
Stabilised Magnetrons Presentation to mark Professor Richard Carter’s contributions to Vacuum Electronics Delivered by Amos Dexter with thanks for contributions from Dr Imran Tahir and of course Richard Carter. Fest 14 th July 2010
Rough Investigation Extract magnetron Saw open Look inside Operation now plain to see ? Carter. Fest 14 th July 2010
The Carter Video Lectures Carter. Fest 14 th July 2010
Magnetron Operation Magnetic Field into Page Anode forms slow wave structure cathode (negative volts) sub synchronous zone spoke vane anode (earth) Carter. Fest 14 th July 2010
Trajectories and Charge Density Litton 4 J 50 - rigid rotating field model ANODE VANE SPOKE MAGNETIC FIELD INTO PAGE Electrons which gain energy from the RF field return to the cathode, those which lose move to the anode ELECTRIC FIELD LINES RF + DC SUBSYNCHRONOUS ZONE CATHODE Carter. Fest 14 th July 2010
Carter the Educator Carter. Fest 14 th July 2010
Magnetron Instabilities Moding • Large frequency jump (several MHz at S band) • Low output, reduced efficiency, increased voltage. Multipactor • Reduction in efficiency • Arc precursor Gauss Discontinuities • Anode currents where the magnetron does not operate • Depend on magnetic field and heater power Twinning • Small frequency jump ( < 1 MHz at S band) • Efficiency and output often acceptable at both frequencies • No good for Radar or Accelerators • Depends on magnetic field, heater power, cathode condition Carter. Fest 14 th July 2010
Twinning Carter. Fest 14 th July 2010
Gauss Line Discontinuities Carter. Fest 14 th July 2010
Frequency Stabilisation with Phase Lock Loop (PPL) Pushing Curve Water Load 3 Stub Tuner Loop Coupler Low Pass Filter 8 k. Hz cut-off Compare frequency to reference and adjust anode current with PI controller (loop filter) to prevent frequency drifting. Frequency Divider / N Divider /R Phase - Freq Detector & Charge Pump ADF 4113 Micro. Controller 10 MHz TCXO 1 ppm High Voltage Transformer Power supply 325 V DC with 5% 100 Hz ripple Loop Filter 40 k. Hz Chopper Pulse Width Modulator SG 2525 1. 5 k. W Power Supply Carter. Fest 14 th July 2010
Spectral Improvement National (Panasonic) M 137, 1. 2 k. W CW “cooker” magnetron, full heater power, 5% ripple at 100 Hz on dc supply As left but 4. 2 W heater Bandwidth ~ 200 k. Hz (depends on comparison frequency and loop filter) With frequency stabilisation Carter. Fest 14 th July 2010
Heater Power Dependence of Magnetron Pushing Curves At heater powers of about 18 W to 21 W two frequencies are possible at the same anode current. National (Panasonic) M 137, 1. 2 k. W CW “cooker” magnetron, 800 k. Hz Cooker operation Pushing curves can only be measured in this range with a stabilised magnetron hence we had a world first. Twining Carter. Fest 14 th July 2010
Twinning National (Panasonic) M 137, 1. 2 k. W CW “cooker” magnetron, At one particular cathode temperature there are two possible frequencies for the same anode current. We believe the lower frequency corresponds to a state where the subsynchronous zone is not space charge limited. Heater Power If a pulsed magnetron is operated in the lower frequency state (having less associated noise) then if too many electrons are released from the cathode during the pulse then the magnetron twins. Ball and Carter only studied pulsed magnetrons driven from modulators They observed that the anode current for “ twinned” pulses, start identically but diverges early for low currents and later for high currents. They observed dependence on anode current, cathode coating, heater power and magnetic field. Direct comparison is difficult between the CW magnetron and the pulsed magnetron as the modulator current and voltage change when twinning occurs. Carter. Fest 14 th July 2010
The Magnetron Reflection Amplifier • Linacs require accurate phase control • Phase control requires an amplifier Cavity Magnetron • Magnetrons can be operated as reflection amplifiers Load Circulator Compared to Klystrons, in general Magnetrons Injection Source - are smaller - more efficient - can use permanent magnets - utilise lower d. c. voltage but higher current - are easier to manufacture Consequently they are much cheaper to purchase and operate Carter. Fest 14 th July 2010
Reflection Amplifier Controllability 1. Phase of output follows the phase of the input signal 2. 3. Phase shift through magnetron depends on difference between input frequency and the magnetrons natural frequency Output power has minimal dependence on input signal power 4. Phase shift through magnetron depends on input signal power 5. There is a time constant associated with the output phase following the input phase Anode Voltage 10 k. W 915 MH z 30 k. W 20 k. W 916 MHz 40 k. W 12. 0 k. V 3. 00 A 11. 5 k. V 2. 92 A Magnetron frequency and output vary together as a consequence of 1. Varying the magnetic field 2. Varying the anode current (pushing) 3. Varying the reflected power (pulling) 0 o towards magnetron Moding Arcing Power supply 11. 0 k. V load line 2. 85 A 900 W 800 W 700 W 2. 78 A 10. 5 k. V 2 270 o 2. 70 A 10. 0 k. V 1 2 3 Anode Current Amps 4 5 VSWR 3 4 6 90 o Magnetic field coil current +5 MHz +2. 5 MHz -2. 5 MHz 180 o +0 MHz Carter. Fest 14 th July 2010
Solution of Adler’s Equation The phase of injection locked oscillators is determined by Like for small y hence phase stabilises to a constant offset wo winj y Vinj /RF = oscillation angular frequency without injection = injection angular frequency = phase shift between injection input and oscillator output = equivalent circuit voltage for injection signal / RF output Adler’s equation predicts that : if wo = wi then y → 0 if wo close to wi then y → a fixed value (i. e. when sin y < 1 then locking occurs) if wo far from wi then y → no locking unless Vinj is large Steady state If the natural frequency of the magnetron is fluctuating then the phase y will be fluctuating. Advancing or retarding the injection signal allows low frequency jitter to be cancelled and the magnetron phase or the cavity phase to be maintained with respect to a reference signal. Carter. Fest 14 th July 2010
Power Needed for Injection Locking Adler steady state solution Minimum power given when sin y = 1 Minimum power requirement for locking PRF is output power QL refers to the loaded magnetron. For 2. 45 GHz cooker magnetron (fi –fo) due to ripple ~ 2 MHz (fi –fo) due to temperature fluctuation > 5 MHz Panasonic Pushing Curve This is big hence must reduce fi – fo ( can do this dynamically using the pushing curve) Carter. Fest 14 th July 2010
Experiments at Lancaster Tahir I. , Dexter A. C and Carter R. G. “Noise Performance of Frequency and Phase Locked CW Magnetrons operated as current controlled oscillators”, IEEE Trans. Elec. Dev, vol 52, no 9, 2005, pp 2096 -2130 Phase shift keying the magnetron Tahir I. , Dexter A. C and Carter R. G. , “Frequency and Phase Modulation Performance on an Injection-Locked CW Magnetron”, IEEE Trans. Elec. Dev, vol. 53, no 7, 2006, pp 1721 -1729 0 d. Bm RBW = 100 Hz Span = 100 k. Hz Centre = 2. 44998488 GHz Locked spectral output -50 d. Bm Lancaster has successfully demonstrated the injection locking of a cooker magnetron with as little as -40 d. B injection power by fine control the anode current to compensate shifts in the natural frequency of the magnetron. -100 d. Bm -50 k. Hz +50 k. Hz Carter. Fest 14 th July 2010
Frequency Shift Keying the Magnetron Input to pin diode Output from double balanced mixer after mixing with 3 rd frequency Carter. Fest 14 th July 2010
Long pulse proton driver solution for SPL? Phasor diagram output of magnetron 1 Permits fast full range phase and amplitude control output of magnetron 2 Cavity combiner / magic tee Advanced Modulator Fast magnetron tune by varying output current 440 k. W Magnetron 440 k. W Load ~ -30 d. B needed for 440 W locking Magnetron 440 W LLRF 440 k. W Magnetron design is less demanding than 880 k. W design reducing cost per k. W, and increasing lifetime and reliability. Carter. Fest 14 th July 2010 Advanced Modulator Fast magnetron tune by varying output current
Magnetrons for Proton Drivers The Carter solution for IFMIF “Conceptual Design of a 1 MW 175 MHz CW Magnetron”, IVEC 2008 Diacrode Magnetron Anode Voltage 14 k. V 60 k. V Anode Current 103 A 20 A Efficiency 71% 90% Gain 13 d. B ~ 30 d. B Drive Power 50 k. W ~ 1 k. W Cooling Anode and Cathode Electromagnet No Yes ( ~ 1. 5 k. W) Carter. Fest 14 th July 2010
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