CarterFest 14 th July 2010 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 14 th July 2010 Rough Investigation Extract magnetron Saw open Look inside Operation now plain to see ?

CarterFest 14 th July 2010 The Carter Video Lectures

CarterFest 14 th July 2010 Magnetron Operation cathode ( negative volts ) sub synchronous zone spoke vane anode ( earth ) Magnetic Field into Page Anode forms slow wave structure

CarterFest 14 th July 2010 Trajectories and Charge Density Litton 4J50 - rigid rotating field model Electrons which gain energy from the RF field return to the cathode, those which lose move to the anode CATHODE ELECTRIC FIELD LINES RF + DC SPOKE SUB- SYNCHRONOUS ZONE MAGNETIC FIELD INTO PAGE ANODE VANE

CarterFest 14 th July 2010 Carter the Educator

CarterFest 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

CarterFest 14 th July 2010 Twinning

CarterFest 14 th July 2010 Gauss Line Discontinuities

CarterFest 14 th July 2010 Frequency Stabilisation with Phase Lock Loop (PPL) Compare frequency to reference and adjust anode current with PI controller (loop filter) to prevent frequency drifting. Pushing Curve Frequency Divider / N Phase - Freq Detector & Charge Pump 10 MHz TCXO 1ppm Water Load Loop Filter High Voltage Transformer 40kHz Chopper Pulse Width Modulator SG Stub Tuner Low Pass Filter 8 kHz cut-off Loop Coupler 1.5 kW Power Supply Micro- Controller Divider / R ADF 4113 Power supply 325 V DC with 5% 100 Hz ripple

CarterFest 14 th July 2010 Spectral Improvement National (Panasonic) M137, 1.2kW CW “cooker” magnetron, full heater power, 5% ripple at 100Hz on dc supply As left but 4.2W heater With frequency stabilisation Bandwidth ~ 200 kHz (depends on comparison frequency and loop filter)

CarterFest 14 th July 2010 Heater Power Dependence of Magnetron Pushing Curves At heater powers of about 18W to 21W two frequencies are possible at the same anode current. Pushing curves can only be measured in this range with a stabilised magnetron hence we had a world first. Twining 800 kHz Cooker operation National (Panasonic) M137, 1.2kW CW “cooker” magnetron,

CarterFest 14 th July 2010 Twinning 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 sub- synchronous 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. 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. National (Panasonic) M137, 1.2kW CW “cooker” magnetron, Heater Power

CarterFest 14 th July 2010 The Magnetron Reflection Amplifier Linacs require accurate phase control Phase control requires an amplifier Magnetrons can be operated as reflection amplifiers Cavity Injection Source Magnetron Circulator Load Compared to Klystrons, in general Magnetrons - 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 14 th July 2010 Reflection Amplifier Controllability 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) 1.Phase of output follows the phase of the input signal 2.Phase shift through magnetron depends on difference between input frequency and the magnetrons natural frequency 3.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 Current Amps 10.0 kV 10.5 kV 11.0 kV 11.5 kV 12.0 kV Anode Voltage Power supply load line 916MHz 915MH z 10kW20kW30kW40kW 2.70A 3.00A 2.85A 2.92A 2.78A Magnetic field coil current W 800 W 700 W towards magnetron VSWR +5MHz +2.5MHz -2.5MHz -5MHz Moding +0MHz Arcing 0o0o 270 o 180 o 90 o

CarterFest 14 th July 2010 Solution of Adler’s Equation   = oscillation angular frequency without injection  in j = injection angular frequency  = phase shift between injection input and oscillator output V inj /RF = equivalent circuit voltage for injection signal / RF output Adler’s equation predicts that :- if   =  i  then  → 0 if   close to  i  then  → a fixed value (i.e. when sin  < 1 then locking occurs) if   far from  i  then  → no locking unless V inj is large Like for small  hence phase stabilises to a constant offset Steady state 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. The phase of injection locked oscillators is determined by

CarterFest 14 th July 2010 Power Needed for Injection Locking P RF is output power Q L refers to the loaded magnetron. Panasonic Pushing Curve For 2.45 GHz cooker magnetron This is big hence must reduce f i – f o ( can do this dynamically using the pushing curve) (f i –f o ) due to ripple ~ 2 MHz (f i –f o ) due to temperature fluctuation > 5 MHz Minimum power given when sin  = 1 Minimum power requirement for locking Adler steady state solution

CarterFest 14 th July 2010 Experiments at Lancaster RBW = 100Hz Span = 100 kHz Centre = GHz -100 dBm -50 dBm 0 dBm -50 kHz +50 kHz Phase shift keying the magnetron Locked spectral output 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 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 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.

CarterFest 14 th July 2010 Frequency Shift Keying the Magnetron Input to pin diode Output from double balanced mixer after mixing with 3 rd frequency

CarterFest 14 th July 2010 Long pulse proton driver solution for SPL? Permits fast full range phase and amplitude control Cavity ~ -30 dB needed for locking 440 kW Magnetron design is less demanding than 880 kW design reducing cost per kW, and increasing lifetime and reliability. Load 440 W Advanced Modulator Fast magnetron tune by varying output current 440 W 440 kW Magnetron Advanced Modulator Fast magnetron tune by varying output current LLRF output of magnetron 1 output of magnetron 2 Phasor diagram combiner / magic tee

CarterFest 14 th July 2010 Magnetrons for Proton Drivers DiacrodeMagnetron Anode Voltage14 kV60 kV Anode Current103 A20 A Efficiency71%90% Gain13 dB~ 30 dB Drive Power50 kW~ 1 kW CoolingAnodeAnode and Cathode ElectromagnetNoYes ( ~ 1.5 kW) The Carter solution for IFMIF “Conceptual Design of a 1 MW 175MHz CW Magnetron”, IVEC 2008