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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 CarterFest 14th July 2010 Rough Investigation Extract magnetron Saw open Look inside Operation now plain to see ? CarterFest 14th July 2010 The Carter Video Lectures CarterFest 14th July 2010 Magnetron Operation Magnetic Field into Page Anode forms slow wave structure cathode (negative volts) sub synchronous zone spoke vane anode (earth) CarterFest 14th July 2010 Trajectories and Charge Density Litton 4J50 - 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 CarterFest 14th July 2010 Carter the Educator CarterFest 14th 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 CarterFest 14th July 2010 Twinning CarterFest 14th July 2010 Gauss Line Discontinuities CarterFest 14th July 2010 Frequency Stabilisation with Phase Lock Loop (PPL) Frequency (GHz) 2.452 2.450 2.448 2.446 2.444 2.442 Pushing Curve 2.440 Water Load 2.438 2.436 100 150 200 250 300 350 400 3 Stub Tuner Loop Coupler Anode Current (mA) Compare frequency to reference and adjust anode current with PI controller (loop filter) to prevent frequency drifting. Low Pass Filter 8 kHz cut-off Frequency Divider / N Divider /R Phase - Freq Detector & Charge Pump ADF 4113 MicroController 10 MHz TCXO 1ppm High Voltage Transformer Power supply 325 V DC with 5% 100 Hz ripple Loop Filter 40kHz Chopper Pulse Width Modulator SG 2525 1.5 kW Power Supply CarterFest 14th July 2010 Spectral Improvement 0 Amplitude(dBm) -20 -40 -60 -80 2.430 -20 -40 -60 -80 2.440 2.450 2.460 2.470 2.43 2.44 2.45 2.46 Frequency(GHz) Frequency (GHz) National (Panasonic) M137, 1.2kW CW “cooker” magnetron, full heater power, 5% ripple at 100Hz on dc supply As left but 4.2W heater 0 Amplitude (dBm) Amplitude (dbm) 0 -20 Bandwidth ~ 200 kHz (depends on comparison frequency and loop filter) -40 -60 -80 2.43 2.44 2.45 2.46 2.47 Frequency (GHz) With frequency stabilisation CarterFest 14th July 2010 2.47 Heater Power Dependence of Magnetron Pushing Curves At heater powers of about 18W to 21W two frequencies are possible at the same anode current. 2.4525 Frequency (GHz) 2.4520 Pushing curves can only be measured in this range with a stabilised magnetron hence we had a world first. National (Panasonic) M137, 1.2kW CW “cooker” magnetron, 2.4530 800 kHz 2.4515 Cooker operation 2.4510 fc(36W) fc(33W) fc(30W) fc(27W) fc(25W) fc(21W) fc(18W) fc(16W) 2.4505 2.4500 2.4495 2.4490 160 Twining 200 240 280 320 Anode Current CarterFest 14th July 2010 360 400 Twinning National (Panasonic) M137, 1.2kW CW “cooker” magnetron, We believe the lower frequency corresponds to a state where the subsynchronous zone is not space charge limited. 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. 2.4525 Frequency (GHz) At one particular cathode temperature there are two possible frequencies for the same anode current. 2.4530 2.4520 2.4515 Heater Power 2.4510 21 W 2.4505 18 W 2.4500 2.4495 2.4490 160 200 240 280 320 Anode Current 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. CarterFest 14th July 2010 360 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 CarterFest 14th 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 10kW 915MH z 30kW 20kW 916MHz 40kW 12.0 kV 3.00A 11.5 kV 2.92A 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) 0o Arcing Power supply 11.0 kV load line towards magnetron Moding 2.85A 900 W 800 W 700 W 2.78A 10.5 kV 2 270o VSWR 3 4 6 90o 2.70A 10.0 kV 1 2 3 Anode Current Amps 4 5 Magnetic field coil current +5MHz +2.5MHz -5MHz -2.5MHz +0MHz 180o CarterFest 14th July 2010 Solution of Adler’s Equation The phase of injection locked oscillators is determined by Like Vinj o d sin o i dt VRF 2 Q L o inj Vinj /RF d A const dt for small hence phase stabilises to a constant offset = 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 o = i then → 0 if o close to i then → a fixed value (i.e. when sin < 1 then locking occurs) if o far from i then → no locking unless Vinj is large Steady state sin 2 Q L PRF o i Pinj o If the natural frequency of the magnetron is fluctuating then the phase 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. CarterFest 14th July 2010 Power Needed for Injection Locking Adler steady state solution sin 2 Q L PRF o i Pinj o i o Minimum power Pinj 4 PRF Q 2L requirement for locking o Minimum power given when sin = 1 2 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 Frequency (GHz) 2.452 2.450 2.448 2.446 2.444 2.442 2.438 2.436 100 Plock 4 Q2 Poutput 2 Panasonic Pushing Curve 2.440 2 f i f o 2 2.455 2.450 4 100 0.166 f 2 . 450 o 150 200 250 300 350 400 Anode Current (mA) This is big hence must reduce fi – fo ( can do this dynamically using the pushing curve) CarterFest 14th 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, pp2096-2130 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, pp1721-1729 0 dBm RBW = 100Hz Span = 100 kHz Centre = 2.44998488 GHz Locked spectral output -50 dBm Lancaster has successfully demonstrated the injection locking of a cooker magnetron with as little as -40 dB injection power by fine control the anode current to compensate shifts in the natural frequency of the magnetron. -100 dBm -50 kHz +50 kHz CarterFest 14th July 2010 Frequency Shift Keying the Magnetron Input to pin diode Output from double balanced mixer after mixing with 3rd frequency CarterFest 14th 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 Advanced Modulator 440 kW Magnetron Fast magnetron tune by varying output current 440 kW Load ~ -30 dB needed for 440 W locking Magnetron 440 W LLRF 440 kW Magnetron design is less demanding than 880 kW design reducing cost per kW, and increasing lifetime and reliability. CarterFest 14th July 2010 Fast magnetron tune by varying output current Magnetrons for Proton Drivers The Carter solution for IFMIF “Conceptual Design of a 1 MW 175MHz CW Magnetron”, IVEC 2008 Diacrode Magnetron Anode Voltage 14 kV 60 kV Anode Current 103 A 20 A Efficiency 71% 90% Gain 13 dB ~ 30 dB Drive Power 50 kW ~ 1 kW Cooling Anode Anode and Cathode Electromagnet No Yes ( ~ 1.5 kW) CarterFest 14th July 2010