A magnetron solution for proton drivers Amos Dexter Achieving precise phase control of magnetrons opens the possibility of their application to drive accelerators where multiple sources are required. It has been well known for 50 years that the phase of a magnetron can be locked and hence controlled by an injection signal. In the early 1990’s extensive studies of short pulse, high power, injection locked, relativistic magnetrons were pursued by Varian as potential RF sources for particle accelerators. [1]  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, 1989. [2] 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 1991. The ultimate aim of our project is to investigate the overall phase performance that might be achieved when injection locked magnetrons, with state of the art control electronics are used to drive high Q cavities for long pulse and CW accelerator applications. Simulation Using Tech-X’s VORPAL e.m. code

Collaborations Lancaster Richard Carter, Graeme Burt, Ben Hall, Chris Lingwood JLab Haipeng Wang, Robert Rimmer CEERI Shivendra Maurya, VVP Singh, Vishnu Srivastava TechX Jonathan Smith CERN ? ESS ?

The 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 (at 704 MHz) - utilise lower d.c. voltage but higher current - are easier to manufacture Consequently they are much cheaper to purchase and operate The most convenient way to set up a magnetron as a reflection amplifier is with a circulator. 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

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. Double Balance Mixer Phase shifter Spectrum Analyzer 1 2.45 GHz Panasonic 2M137 1.2 kW Magnetron Oscilloscope Spectrum Analyzer 2 Load 3 Stub Tuner 2 Loop Coupler Circulator 3 Stub Tuner 1 Loop Coupler Phase shifter 1W Amplifier Circulator 2 Load 2 Cathode heater control Load LP Filter 8 kHz cut-off IQ Modulator (Amplitude & phase shifter) DAC ADC Oscilloscope DAC Digital Phase Detector HMC439 Digital Signal Processor High Voltage Transformer ÷ 2 ÷ 2 42 kHz Chopper 300 V DC +5% 120 Hz ripple Pulse Width Modulator SG 2525 Agilent E4428 signal generator providing 2.45 GHz Control voltage 1.2 kW Power Supply

SCRF cavity powered with magnetron Injection but magnetron off Injection + magnetron on + control Injection + magnetron on

Next Steps Development of a 704MHz Magnetron (440kW – 880kW ) Collaboration with CEERI, Pilani, India Procure standard modulator Hope to use klystron modulator with different pulse transformer however rate of voltage rise is tightly defined. Need to deal with impedance change on start up. The CI have a suitable 3 MW magnetron modulator for short pulses up to 5 micro-seconds and could be used for characterisation Establish test station with Television IOT as the drive amplifier Could be used for conditioning SPL and ESS components Understand locking characteristics of new magnetron Commission advanced modulator with in-pulse current control Establish minimum locking power Establish two magnetron test stand Develop LLRF for simultaneous phase and amplitude control

Layout using one magnetron per cavity Permits fast phase control but only slow, full range amplitude control Cavity Standard Modulator 880 kW Magnetron Load Pulse to pulse amplitude can be varied 4 Port Circulator Slow tuner ~ -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 LLRF 60 kW IOT Could fill cavity with IOT then pulse magnetron when beam arrives

Layout using two magnetrons per cavity Permits fast full range phase and amplitude control Phasor diagram output of magnetron 1 output of magnetron 2 Cavity combiner / magic tee Advanced Modulator Advanced Modulator 440 kW Magnetron 440 kW Magnetron Load Fast magnetron tune by varying output current Fast magnetron tune by varying output current 440 W 440 W ~ -30 dB needed for locking LLRF

Reflection Amplifier Controllability Phase of output follows the phase of the input signal Phase shift through magnetron depends on difference between input frequency and the magnetrons natural frequency Output power has minimal dependence on input signal power Phase shift through magnetron depends on input signal power There is a time constant associated with the output phase following the input phase Magnetron frequency and output vary together as a consequence of Varying the magnetic field Varying the anode current (pushing) Varying the reflected power (pulling) 915MHz Anode Voltage 916MHz 10kW 20kW 30kW 40kW 12.0 kV 3.00A 11.5 kV 2.92A 2 3 4 6 900 W 800 W 700 W towards magnetron VSWR +5MHz +2.5MHz -2.5MHz -5MHz Moding +0MHz Arcing 0o 270o 180o 90o Power supply load line 11.0 kV 2.85A 2.78A 10.5 kV 2.70A Magnetic field coil current 10.0 kV 1 2 3 4 5 Anode Current Amps

S-band, 3.1 MW Pulse Tunable Magnetron for Accelerator CEERI Collaboration Dr Shivendra Maurya of the Microwave Tube Division, CEERI, PILANI, India visited Lancaster University from 1st August to 31st November to start work on the design of a suitable magnetron. This visit has been funded by the Royal Academy of Engineering. If there is sufficient interest CEERI will seek funding to manufacture the magnetron. CEERI already manufacture a range of tubes mainly for use in India. 5 MW (pk), 5kW(avg) S-band Klystron as RF amplifier for injector microtron in Synchrotron Radiation Source at RRCAT, Indore S-band, 3.1 MW Pulse Tunable Magnetron for Accelerator

Specification of initial device Frequency 704 MHz Power 200 kW to 1 MW Pulse length 5ms to 5 ms (for max power) Max average power 100 kW Efficiency > 90% above 500 kW Magnet NyFeB (< 0.5 T) External Q ~ 50 (for ease of locking) Mechanical Tunability ~ 5 MHz Cathode heating indirect and controllable

Approximate Calculations 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

Expected operating range Threshold for moding Short circuit regime VORPAL simulations

VORPAL Predictions at 30 kV Take B = 0.3 T, Va = 32 kV, Ic = 60 A Predict Ianode = 19 A, Efficiency = 92%, Power = 560 kW Z = 1684 W Amps Anode current (A) time (s) Volts Cathode voltage (V) time (s) Watts Output power (W) time (s)

Moding Issues Excitation in the mode at 1060 MHz might be a problem. Voltage in magnetron Excitation in the mode at 1060 MHz might be a problem. We think the coarse mesh or other issues with the simulation might exacerbate the issue. Volts time (s) Volts time (s) FFT (dB) time (s)

MWS modes p mode at 702 MHz p-1 mode at 1060 MHz p+1 mode at 1063 MHz

Efficient Orbits An efficient orbit should have no loop

Magnetron Size dg dm Magnets hm air cooling for cathode 704 MHz dg ~ 360 mm dm ~ 165 mm hm ~ 650 mm cost £8000 water cooling for anode dg dm Magnets hm If magnetron design is similar to industrial design with similar tolerances and can be made on same production line then cost may not be much more air cooling input for dome

High Efficiency Klystrons Design of high efficiency klystrons for ESS in collaboration with CLIC Similar Klystrons (704.4 MHz, 1.5 MW, 70% efficiency) allow synergetic activities with CLIC. Focus on understanding of bunching process and space charge in the output cavity. Using evolutionary algorithms to improve optimisation New design concepts to achieve optimum beam modulation Single and Multiple beams investigated Images courtesy of Thales Electron Devices