Tiara Workshop on RF power generation for accelerators, Uppsala 2013 Prospects for Phase Locked Magnetrons, Magnicons and Gyro-Klystrons as RF Power Sources for Accelerators Amos Dexter (speaker), Graeme Burt and Chris Lingwood efficient inefficient RF Output RF Input B field into page RF Input RF Output Input Cavity Output Cavity B field along axis Cathode

Tiara Workshop on RF power generation for accelerators, Uppsala 2013 Bunching Magnetrons, Magnicons and Gyro-Klystrons all have electron orbits that circulate around the magnetic fields lines. Bunching is in the orbit around the magnetic field. The Gyro-Klystron uses a magnetron injection gun (MIG) so the transverse momentum is created close to the gun. Bunching is partly ballastic but also depends on the relativistic cyclotron maser instability. The Magnicon starts with a linear beam and then employs synchronous rotating modes in deflection cavities to spin and bunch the beam. Output for the Gyro-klystron and the Magnicon is a bremsstrahlung process in a “fast wave” output cavity. In a Magnetron electrons have no motion in the direction of the magnetic field. Bunching occurs by the interaction of circulating electrons and the slow wave structure of the anode. As electrons are collected on the capacitive gaps of the slow wave structure hence output can be viewed simply as the synchronous charging of a circuit. As electrons are retarded by the slow wave output it is also a Cherenkov process.

Tiara Workshop on RF power generation for accelerators, Uppsala 2013 Groups with recent work on/relevant to cross field devices for accelerators Phase locked magnetrons Varian Associates (MA) (1991)Treado, Hansen, Jenkins (Short pulse) Univ. Mitchigan (-2013)Gilgenbach et al.(Relativistic Magnetrons) Univ. Lancaster (2003 – 2010)Dexter, Tahir, Carter, Burt(CW Cooker type) J-Lab (2006 – 2013)Wang(CW Cooker type) Muon Inc., Fermi-Lab & (2007 – 2013)Kazakevich, Yakovlev(Power combining) Efficient L Band Magnetrons SLAC, CTL, RaytheonTantawi et al. (2004)(CW Coaxial? 300kW) Diado Instit. Tech. Japan (1991)Shibata (1991)(CW Coaxial 600kW Gyro Klystrons IAP Nizhny NovgoradLebedev Univ. Maryland Lawson Calabazas Creek Magnicons Budker Inst. NonosibirskKozyrev, Makarov, Omega-P Inc, & YaleNezhevenko, LaPointe, Shchelkunov, Yakovlev, Hirshfield Gyro TWT (Univ. Strathclyde, MIT,IAP Nizhny Novgorad, Univ. Maryland, NRL Washington, Univ. Mitchigan)

Tiara Workshop on RF power generation for accelerators, Uppsala 2013 Magnetrons for Accelerators Single magnetrons GHz, 5 MW, 3  s pulse, 200 Hz repetition are used to power linacs for medical and security applications. Multiple magnetrons have been considered for high energy normal conducting linacs but the injection power needed for an unstabilised magnetron made it uncompetitive with a Klystron. Courtesy of e2v Overett, T.; Bowles, E.; Remsen, D. B.; Smith, R. E., III; Thomas, G. E. “ Phase Locked Magnetrons as Accelerator RF Sources” PAC 1987 Benford J., Sze H., Woo W., Smith R., and Harteneck B., “Phase locking of relativistic magnetrons” Phys. Rev.Lett., vol. 62, no. 4, pp. 969, Treado T. A., Hansen T. A., and Jenkins D.J. “Power-combining and injection locking magnetrons for accelerator applications,” Proc IEEE Particle Accelerator Conf., San Francisco, CA Chen, S. C.; Bekefi, G.; Temkin, R. J. “ Injection Locking of a Long-Pulse Relativistic Magnetron” PAC 1991 Treado, T. A.; Brown, P. D.; Hansen, T. A.; Aiguier, D. J. “ Phase locking of two long- pulse, high-power magnetrons”, IEEE Trans. Plasma Science, vol 22, p , 1994 Treado, Todd A.; Brown, Paul D., Aiguier, Darrell “New experimental results at long pulse and high repetition rate, from Varian's phase-locked magnetron array program” Proceedings Intense Microwave Pulses, SPIE vol. 1872, July 1993

Tiara Workshop on RF power generation for accelerators, Uppsala 2013 Amplifier Selection MagnetronGyro-KlystronMagniconKlystron FrequencyAbove ~ 200MHzabove a few GHzAbove a few GHzAbove ~ 350MHz Peak PowerLowerVery high High Average powerLowerHigh GainLowerHigh Tuneable rangeLargeSmall Instantaneous bandwidthVery smallSmall Slew rateVery smallSmall Noise figureHigherLower Best Efficiency L band~ 90%ILC ~ 69% Best Efficiency X band~ 50%50%60%XL5 = 40% Pushing figureSignificant Pulling figureSignificant Amplifier costLowhigh Modulator & magnet costLowervery highVery highhigh

Tiara Workshop on RF power generation for accelerators, Uppsala 2013 Gyro-Klystron Experimental results InstitutionFreq. (GHz) Output Mode Power (MW) Eff.Gain (dB) Band- width IAP, Nizhny Novgorod30.0TE (5,3)1540%300.17% TE (3,2)1230%380.17% Univ. Maryland8.57TE (0,1)7532%300.2% 9.87TE (0,1)2732%360.2%max pow 9.87TE (0,1)1637%330.2%max eff 9.87TE (0,1)2028%350.2%max gain 19.76TE (0,3)3229%270.1% Calabazas Creek %55 Novosibirsk Magnicon7.0TM(2,1,0)4649%55 XL5 Klystron %500.4% For a review of klystrons verses other source for the NLC see

Tiara Workshop on RF power generation for accelerators, Uppsala 2013 Calabazas Creek Gyro-Klystron

Tiara Workshop on RF power generation for accelerators, Uppsala 2013 Slide taken from CLIC test stand study by Erk Jensen CPI study: Gyroklystron 30 GHz, 50 MW made a design study for CERN in A 200 MW power station would consist of –four 50 MW Gyroklystrons, 1.2 μs, 100 Hz with fundamental TE 011 coaxial cavities, –four SC 2 T solenoids + power supplies, (≈ 300 k$ each) –two 15 kW drivers + power supplies, (≈ 370 k$ each) –one modulator 500 kV, 1.2 kA, (≈ 1000 k$ ) –two power combiners 50 MW + 50 MW (≈ 20 k$ each) –ancillary systems. Lead time ≈3 years (15 months for first 50 MW amplifier) This example would sum up to 7.5 M$

Tiara Workshop on RF power generation for accelerators, Uppsala 2013 Opportunities for Magnetrons Lancaster started worked on phase locked magnetrons in 2003 The conceptual application was for intense proton beams as would be required for a neutrino factory or future neutron spallation sources. Magnetrons can become an option for intense proton beams where they give significantly greater efficiency than other devices and bring down the lifetime cost of the machine without sacrificing performance and reliability. The easiest applications are where beam quality is not a key issue. One would consider using multiple magnetrons to drive an accelerator unless cost and efficiency are potentially show stopping considerations.

Tiara Workshop on RF power generation for accelerators, Uppsala 2013 Magnetron Size at 704 MHz Magnets dgdg dmdm hmhm 704 MHz dgdg ~ 360 mm dmdm ~ 165 mm hmhm ~ 650 mm cost£8000 air cooling input for dome water cooling for anode air cooling for cathode If an accelerator magnetron design is similar to industrial heating magnetron design, has similar tolerances and can be made on same production line then cost may not be much more The magnetron easily fits in the accelerator tunnel and this might offer huge unexpected savings.

Tiara Workshop on RF power generation for accelerators, Uppsala 2013 The Reflection Amplifier J. Kline “The magnetron as a negative-resistance amplifier,” IRE Transactions on Electron Devices, vol. ED-8, Nov 1961 H.L. Thal and R.G. Lock, “Locking of magnetrons by an injected r.f. signal”, IEEE Trans. MTT, vol. 13, 1965 Linacs require accurate phase control Phase control requires an amplifier Magnetrons can be operated as reflection amplifiers They run in saturation which is a problem Cavity Injection Source Magnetron Circulator Load Compared to Klystrons, in general Magnetrons - are smaller -can be more efficient at L band - can use permanent magnets (at 704 MHz) - utilise lower d.c. voltage but higher current - are easier to manufacture Consequently they are expected to be much cheaper to purchase and operate

Tiara Workshop on RF power generation for accelerators, Uppsala 2013 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 (amplifier in saturation) 2.Phase shift through magnetron depends on difference between input frequency and the magnetrons natural frequency (same as driving a cavity with a source) 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

Tiara Workshop on RF power generation for accelerators, Uppsala 2013 Adler’s Equation for Injection Locking J.C. Slater “The Phasing of Magnetrons” MIT Technical Report 35, 1947 Shien Chi Chen “Growth and frequency Pushing effects in Relativistic Magnetron Phase – Locking”, IEEE Trans. on Plasma Science Vol. 18 No 3. June To get Adler’s equation set The basic circuit model for the phased locked magnetron is the same as for a cavity to give Load impedance includes pulling effects. Negative impedance to represent magnetron spokes excitation of the anode. Includes static pushing effects. LR C ZwZw -Z S Injection

Tiara Workshop on RF power generation for accelerators, Uppsala 2013 Solution of Adler’s Equation   magnetron ang. oscillation frequency without injection  in j injection angular frequency  phase shift between injection input and magnetron 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. High frequency phase jitter will be filtered by a superconducting the cavity 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.

Tiara Workshop on RF power generation for accelerators, Uppsala 2013 Power Needed for Injection Locking The minimum locking power is given when sin  = 1 P RF is output power Q L refers to the loaded magnetron. Pushing For our 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 Time response ~ ~ 400 ns ~ 1000 RF cycles

Tiara Workshop on RF power generation for accelerators, Uppsala 2013 Experiments at Lancaster D/A 1W Amplifie r ÷ M Micro- Controller GHz PLL Oscillator ADF VCO 10 MHz TCXO 1ppm A/D D/A Frequency Divider / N Phase - Freq Detector & Charge Pump Water Load High Voltage Transformer 40kHz Chopper Pulse Width Modulator SG 2525 Loop Coupler 3 Stub Tuner 1 Circulator 1 Circulator 2 Double Balance Mixer LP Filter 8 kHz cut-off 1.5 kW Power Supply Loop Filter Divider / R IQ Modulator (Amplitude & phase shifter) ADF V DC + 5% 100 Hz ripple ÷ M 10 Vane Magnetron D/A Loop Coupler 2 Stub Tuner 2 Oscilloscope Load C3 Oscilloscope DSP Digital Phase Detector 1.3GHz Power supply ripple Magnetron phase no LLRF Magnetron phase with LLRF pk-pk 1.2 o pk-pk 26 o

Tiara Workshop on RF power generation for accelerators, Uppsala 2013 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.

Tiara Workshop on RF power generation for accelerators, Uppsala 2013 Proof of principle Demonstration of CW 2.45 GHz magnetron driving a specially manufactured superconducting cavity in a VTF at JLab and the control of phase in the presence of microphonics was successful.

Tiara Workshop on RF power generation for accelerators, Uppsala 2013 SCRF cavity powered with magnetron Injection but magnetron off Injection + magnetron on Injection + magnetron on + control

Tiara Workshop on RF power generation for accelerators, Uppsala 2013 Magnetron study for CERN’s SPL? Would like to phase each cavity individually Cavity power requirements increase to ~ 900 kW Klystrons more cost effective at the 3-10 MW scale IOTs not available to get to 900 kW Long pulse L magnetrons should be able to reach 1 MW Hence use IOTs along the linac up to that point power is insufficient After that point use IOTs to injection lock magnetrons Obtain high efficiency where most power is required Obtain best control at front of linac

Tiara Workshop on RF power generation for accelerators, Uppsala 2013 Layout using one magnetron per cavity Permits fast phase control but only slow, full range amplitude control LLRF 880 kW Magnetron 4 Port Circulator Load Slow tuner 60 kW IOT Standard Modulator Pulse to pulse amplitude can be varied Cavity ~ -13 dB to -17 dB needed for locking i.e. between 18 kW and 44kW hence between 42 kW and 16 kW available for fast amplitude control Could fill cavity with IOT then pulse magnetron when beam arrives

Tiara Workshop on RF power generation for accelerators, Uppsala 2013 How do we get high efficiency? Using standard theory one can estimate Magnetic field, anode and cathode radii from requirement data (frequency 704 MHz, efficiency >90% and power Should be able to use same block for efficient generation at both the 500 KW and 1 MW level

Tiara Workshop on RF power generation for accelerators, Uppsala 2013 Expected operating range VORPAL simulations Short circuit regime Threshold for moding

Tiara Workshop on RF power generation for accelerators, Uppsala 2013 PIC code simulations Voltage in magnetron time (s) Volts FFT (dB) The literature suggests that PIC codes give the correct predictions for inefficient magnetrons with solid cathodes. Our simulations have never accurately reproduced performance curves of efficient cook magnetrons with spiral cathodes.

Tiara Workshop on RF power generation for accelerators, Uppsala 2013 Efficient Orbits An efficient orbit should have no loop Orbits associated with the design on the previous slides

Tiara Workshop on RF power generation for accelerators, Uppsala 2013 Cost Calculation 20 mA SPL beam

Tiara Workshop on RF power generation for accelerators, Uppsala 2013 Notes for cost calculation

Tiara Workshop on RF power generation for accelerators, Uppsala 2013 Prospects Intense beams in future user facilities need to be generated efficiently. Developing a new HPRF source is expensive and comparison to available sources is difficult before development is mature. One would not use a magnetron for a superconducting linac if you could afford a klystron or IOT. One would not fund the development a new gyro-klystron with the same specification as a klystron which already exists. Universities will continue to explore new concepts. Need accelerator labs to explore new devices at accelerator test stands to have any chance of new devices becoming feasible alternatives. Future accelerators constrained on cost so research on efficient low cost sources is worthwhile.