RF Sources and Combiners

1. RF Sources and Combiners Emma Wooldridge

2. Outline • Very quick RF power overview • RF power sources • Triodes • Tetrodes • Klystrons • IOTs (Inductive Output Tubes) • RF power combining • Waveguides • Combining cavity Joint Accelerator Workshop

3. Coax or Waveguide E Field B Field Joint Accelerator Workshop

4. Standing and travelling waves Joint Accelerator Workshop

5. Triodes • Invented in the early 1900s • Differs from a diode due to a grid added between the cathode and the plate • By applying an RF voltage across the grid it is possible to obtain bunches of electrons instead of a continuous stream of electrons Joint Accelerator Workshop

6. Tetrodes • This is the same as a triode but with an added screen grid • This grid protects the anode and control grid from being linked due to capacitance. • The screen grid is at ground so most electrons go past and continue to hit the anode Joint Accelerator Workshop

7. IOTs (Inductive Output Tubes) • First designed by Haeff in the late 1930s • Electrons are bunched as in a tetrode • The power is obtained from the electron beam as it passes the cavity Joint Accelerator Workshop

8. Klystrons • Invented in the late 1930s by the Varian brothers • Extract energy in a similar way to an IOT • Produces electron bunches by velocity modulation Joint Accelerator Workshop

9. Klystrons Expensive Long lead time High power Higher gain IOTs Comparatively cheap Shorter lead time Lower power Lower gain RF Power combining – Why? • It is not always possible to get the perfect power source for each job Joint Accelerator Workshop

10. Magic Tee Large Structure. Failure of 1 input results in loss of symmetry and failure of the system. Combining Cavity Small/Compact. Failure of 1 or more components gives graceful degradation of power. Methods of Combining Joint Accelerator Workshop

11. Magic Tees Joint Accelerator Workshop

12. Combining Cavity • Magic Tees are the standard for RF power combining • H. Bohlen of CPI has modelled a combining cavity in 1D using 6 inputs • Thales have built a working model cavity using 5 inputs Joint Accelerator Workshop

13. Modelling the cavity • Modelled in CST Microwave Studio. • With either: 2,3 or 4 IOTs. • At 1.3 GHz. • 1st step - modelling the cavity at the required frequency. • 2nd step – Optimising the IOT inputs into the cavity by varying their width and length within the cavity. Joint Accelerator Workshop

14. Results • Efficiency of all models between 65%-99%. • Efficiency of 65% only seen in models with 3 inputs • Due to the analysis not converging to a single result • Reflection minimised to less 1% for most models Joint Accelerator Workshop

15. 4 IOT plot • For the 2 and 4 IOT models the output for each IOT is identical. Output Power 396% Reflectedpower 1% Joint Accelerator Workshop

16. 3 IOT plot • Due to constraints on the way the ports are defined the asymmetry of the 3 IOT model led to it not being optimised fully. Reflectedpower 12% Output Power 208% Reflectedpower 52% Joint Accelerator Workshop

17. Joint Accelerator Workshop

18. Technical Demonstrator Joint Accelerator Workshop

19. Specifications • Main body is made of aluminium • Couplers are made from brass • Inner cavity radius 88.5mm • Outer cavity radius to be 150mm • Both input and output couplers can move in and out of the cavity by ±10mm Joint Accelerator Workshop

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24. Measurements • Symmetric placing of couplers to compare against Microwave Studio results • Asymmetric positioning of couplers • Rotate the couplers • Alter the length of coupler inside the cavity Joint Accelerator Workshop

25. Conclusions • Power from RF sources can be combined efficiently • The combining cavity could be a new way of doing this • Modelling suggests that an efficient IOT combining cavity can be created • Prototype will be ready this week • Measurements to be taken in the next few weeks Joint Accelerator Workshop

26. Any Questions?