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Available RF power sources for reliable and robust medical accelerators

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Available RF power sources for reliable and robust medical accelerators

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  1. Available RF power sources for reliable and robust medical accelerators Ivan Konoplev University of Oxford

  2. Acknowledgment • Graeme Burt (CI, Lancaster University) • David A. Constable (CI, Lancaster University) • Robert Apsimon (CI, Lancaster University) STFC-ICEC-CERN, Gaborone, Botswana

  3. Outline • RF power drivers: • Introduction (klystrons, magnetrons) • Technical requirements (TR) • Vacuum tubes operation - overview • Vacuum tubes – availability to satisfy the TR • Suggestions and possibilities • Discussion STFC-ICEC-CERN, Gaborone, Botswana

  4. RF Power Technologies • Linacscan potentially be driven by Magnetron, Klystron or solid state devices • Choice is defined by: frequency, power, device stability and reliability, complexity of the power systems and users capabilities to operate and maintain the RF power system • The RF power supplies which are the most commonly used: • Klystrons and Magnetrons We will discuss these two RF power sources but will briefly look at others as well STFC-ICEC-CERN, Gaborone, Botswana

  5. Klystroninertial bunching mechanism of RF generation • Efficiency - up to 85% (research) and 60% (commercial) –running cost savings • Commercial availability in broad frequency range operating range from 0.1GHz up to 15GHz • Versatile, reliable they are operating at 100s (A) beam current and 10s (kV) • Can be high average power up to 10MW • However they are more expensive Basic schematic of two cavity klystron Technical drawings of a multi-cavity cavity klystron

  6. Magnetronelectrons are losing “potential” energy • It is an Oscillator • Not expensive (1/3rd of klystron price) but not as reliable, life time (typically 3 years) is an issue for high-power devices • Power up to 3MW (peak power) good for low energy LINCAS (up to 4MeV) • Operating frequency range 0.1GHz to high 10s GHz • Versatile but not stable and operating frequency may walk around. Complex stabilisation circuitry is required. The cathode in a magnetron is harder to cool than in a klystron and gets additional heating from back bombardment so the maximum output is far less than a klystron

  7. Magnetron Schematic view of the magnetron and its operation Cathode is heated to generate the electrons but it is also heated by the electrons coming back – erosion of the cathode Magnetrons are cheap to manufacture but it cannot be controlled to the level of the klystron Heated cathode

  8. RF Power Technologies: outline of technical requirements • Challenge to use: • State-of-the-art unique systems; • Complex system consisting of many parts which require unique specialisation to maintain • Systems which require complex facilities such as specialised cooling and clean room • Bulky and heavy systems with large footprints • Systems which can easily be compromised

  9. RF Power Technologies: general outline of the technical requirements • Consider to use: • Standard, well developed, high performance technologies (may not be cheap); • Modulus, with well defined life-time of each module (maintenance simplification); • Robust and reliable; • Compact and relatively light (up to 150kg); • External environment and handling safe

  10. Choice of Frequency CAS, Zürich, March 3rd , 2018 Steffen Döbert, BE-RF The dimensions (i.e. footprint) of an accelerating structure and RF power source are scaling with the operating frequency. High frequency means smaller structures and smaller RF power needed LEP-Cavity 350 MHz Original CLIC-Cavity 30 GHz

  11. Alternative RF: Solid State Amplifire CAS, Zürich, March 3rd , 2018 Steffen Döbert, BE-RF Soleil/ESRF BoosterSSPA, 150 kW,352 MHz • Initiallydeveloped by SOLEIL •Transfer of technology to ELTA / AREVA Pair ofpush-pull transistors x 128 x 2 150 kW, 352.2 MHz Solid State Amplifiers for the ESRF booster (7 in operation) 650 W RF module 75 kWCoaxial combiner tree with l/4transformers 6thgeneration LDMOSFET(BLF Efficiency: > 57 % at nominal power 578 / NXP),Vds = 50 V Efficiency: 68 to 70 %

  12. Required RF Power for a Linac • To allow comparison of RF source performance: Assuming ~6MeV system 1.45m 0.6m

  13. Peak Output Power Comparison • Comparison of single devices: • Larger RF power required for lower frequency. • Magnetrons & klystrons offer largest peak power across all frequencies. • Few single devices meet required specification. X-band klystrons

  14. RF Power Technologies: conventional systems Length available for accelerator ~1.5m Length available for accelerator <0.6m (a) (b) beam optics to cathode RF feed Feedback from accl. Conventional positions of the RF power supplies in the systems Stationary at the back for the high energy beams On the rotating platform. It moves together with the accelerator

  15. Services/Maintenance of RF power sources It requires several levels of the service and support: • Local technical personal to maintain and fix system locally. It involves a “low level access” to the system checking electrical, mechanical connections, system parameters; changing oil, filters and water. • Mechanical and electrical workshops to replace small broken parts and make new minor corrections to installations without challenging the service agreement • Facilities to support the operation of the RF power supply: water, clean room, and climate control system • System servicing facilities and conditions to allow significant changes/repair of the system by an external specialist locally without sending the equipment away These brings an additional cost to run the machine as well as additional stop time.

  16. Services/Maintenance of RF power sources for robust and reliable operation • Local technical personal to maintain and fix system locally. It involves a low level access to the system checking electrical, mechanical connections, system parameters; changing oil, filters and water. • Minimum external local facilities to support the operation of the RF power supply • Remote and advanced diagnostics and warning of the equipment failure. Long time warranty (extended life time of RF power supply) and replacing the RF power source under the warranty.

  17. Klystron vs Magnetron A klystron more expensive and heavy but it has vert good (cost*complexity)/(life-time*reliability*robustness*stability) ratio

  18. What to expect from vendors • The tube price is about £100k and can be negotiated with up to 15% reduction if one buys 10 of them. • Spec. for life time is 5000 hours (sure can run > 10 000 but not specified as such in official offers). • Tube can be repaired (new cathode) for 30% of initial price. • New tube as compared to existing tubes with similar peak power should save >30% of modulator cost (low voltage, no oil tank) and it is very compact (1/3 of standard modulator in volume). Discussion

  19. Medical LINACs: X-ray bulb concept I. V. Konoplev, H. Zhang, S. Dey, M.Topp-Mugglestone, J. Adelinia (JAI, Department of Physics, University of Oxford) Beam focusing permanent magnet Schematic of SW accelerating structure. Gridded gun Beam focusing permanent magnet Gridded gun Schematic of TW accelerating structure.

  20. Medical LINACs: X-ray bulb concept I. V. Konoplev, H. Zhang, S. Dey, M.Topp-Mugglestone, J. Adelinia (JAI, Department of Physics, University of Oxford) • General description: • Compact in a range of 30cm -40cm • Light up to 50 kg (to manage by two people) • Can be replaced by two (medical/technical) personnel: for example medical doctor and support person • Self contained: • Vacuum sealed • Permanent focusing magnets • Electron beam gun incorporated • X-ray target incorporated • Minimum input power ports: power for gun and RF for accelerator • Minimum output ports: X-ray; diagnostics X-ray bulb will be used as a quick solution to replace the source of X-ray (Co60) in old machines while keeping most of the infrastructure intact i.e. minimum invasion in the operation of the machines

  21. Schematic of old machine with X-ray bulb Length available for accelerator ~1.5m X-ray bulb to cathode RF feed Feedback from accl.

  22. Initial parameters and design

  23. Preliminary design 8.5 8 Cell length 7 12.2 6 0 10 30 20 GHz 11.8 Cell number 11.4 180 60 0 120 Operating frequency as a function of the phase shift per cell

  24. Results of preliminary studies

  25. Results of preliminary studies Normalised electric field amplitude

  26. Medical LINAC: first design Schematic diagram of the acceleration section including the RF couplers and accelerating cavities.

  27. Medical LINAC • Objectives: • Design and build TW 12GHz vacuum sealed, cathode included accelerating structure enabling acceleration of the electron beam from 50keV to 8MeV (x-ray bulb) • Minimise the construction and run cost • Compatible with permanent magnets • Requirements: • Stability (no need to retune or service – “light bulb” approach) • Compactness • Modularity (same composition as for vacuum tubes used in aviation and industrial applications) Technical drawing of the prototype Prototype of the TW accelerating structure