Lecture 6 LLRF and Distribution

1. Lecture 6 LLRF and Distribution Dr G Burt Lancaster University Engineering

2. Waveguide High power RF is transported in waveguide between the RF source and the cavity. The TE10 mode is almost always used. The waveguides can be in air or vacuum depending on the power levels. For exceptionally high powers pressurised SF6 is used to suppress plasma discharges.

3. Directional Couplers and Splitters Directional couplers take a small sample of the wave propagating in one direction. They mostly come in 3, 10, 20 ,30, and 50 dB varieties. They work over a wide frequency range and are used for diagnostic measurements. H-plane 3dB splitter A 3dB coupler is known as a splitter as it splits the power 50-50 in both directions. This is used to feed two cavities from one source.

4. Hybrid- Magic Tee A magic Tee is similar to a 3dB splitter but has a special property when dealing with two inputs. If the inputs are in phase at both ports 2 and 3, the power is transmitted back to port 1. However if the inputs are 180 degrees out of phase all the power goes to port 4. 4 2 3 This is useful as if we place the cavities correctly we can prevent power being reflected back to the RF Source. 1

5. Impedance Matching – Reactance Impedance is a combination of both capacitance and inductance If a system shows poor transmission, it is possible to improve the match by adding reactance. To add capacitance to the system you could use Capacitive Shutters If the system requires more inductance, Inductive shutters can be added. Occasionally you may be required to add both, in the form of an iris.

6. Stub Tuners • If there is an impedance mismatch between two sections in the transmission line we get reflections. • If we place reflective elements in the correct locations we can cancel out reflections from other elements. • A three stub tuner is a device that has a variable reflection so that all reflections can be cancelled and all power is transmitted. It works by varying the position of three capacitive stubs in the waveguide hence creating impedance mismatches.

7. Coax-waveguide adapter • Most LLRF is distributed in coaxial cable. • Almost all high power RF is produced and distributed in waveguide. • We often require waveguide-to-coaxial adapters to convert between the two.

8. Attenuators Attenuators are lossy sections of transmission line. The attenuation is always a set percentage of the input power independent of input power or frequency (over the operating range). They should ideally have zero reflections. A rotary vane attenuator is a type of variable attenuator. Only electric field components parallel to the vane are attenuated. By rotating the vane we are able to alter the attenuation.

9. Loads • RF loads are similar to attenuators, except that they attenuate all the power delivered to the input. • Coaxial loads have a fixed impedance so that if the correct transmission line is used there are no reflection. • Waveguide loads are usually lossy dielectrics tapered so that the impedance varies slowly and hence the reflections are low over a large frequency range.

10. Circulators and Isolators Circulator (Ferrite) • Ferrites allow the design of waveguide devices with non isotropic properties. • The two main uses for this is in isolators and circulators. • In isolators RF power can only flow in one direction and is used for protecting amplifiers from reflections. • Circulators only allow power to flow in a circle in a specific direction. Port 1 Port 2 Port 3

11. Phase Shifters • A phase shifter is a device which can alter the phase difference between the input and the output without creating reflections. • This can be done by using ferrites or mechanical phase shifters.

12. Mixers • A mixer is a non-linear that mixes two RF signals together and filters out the high frequency components. • VIF=V0cos[(wRF-wLO)t+(fRF-fLO)] • The LO (local oscillator) is usually a reference signal. • The downconverted IF (intermediate frequency) signal allows us to make low frequency measurements of the high frequency RF signal. • The mixer allows phase and frequency measurements to be made in a simple manner.

13. Vacuum Windows The windows allow RF to pass with little or no reflections while still holding vacuum. They are often made from ceramics (either Alumina or Berylia). For most frequencies the waveguide is too large to make the window in one piece so often they are placed in slots or iris. Matching posts are added to reduce reflections.

14. Combiner Cavities If we have zero reflections the power in from each coupler must equal power difference in steady state, (Pout=N Pin), so the required Q’s must be, Qin=N Qout A combiner cavity takes several input signals and couples them into a low loss cavity with no reflections. All the incident power is then transferred to a single high Q coupler.

15. SCRF SCRF SCRF Phase shift 1MW Klystron Load CIRC RF 500MHz RF system- 1MW Klystron • Only requires one klystron • Requires high power components (phase shifters and circulators) • Don’t need to worry about Klystron transients • If the klystron fails, all 3 cavities are out of action.

16. SCRF SCRF SCRF Circ Circ Circ Load 300KW Klystron Phase Shift 500MHz 300KW Klystron • Requires three klystrons (expensive) • Phase shifting is performed at low power • Klystron transients may cause phase errors • If a klystron fails, only one cavity is out of action.

17. SCRF SCRF SCRF 75KW 4 IOT’s 500MHz 75KW Inductive Output Tubes • Requires twelve IoTs and three combiner cavities (or 9 magic Tee’s) • Phase shifting is performed at low power • Use IoTs which have higher efficiency, but require a pre-amplifier • If an IoT fails, there is only a small effect and IoTs are faster to replace

18. Power Compressed Pulse Klystron Pulse time Pulse Compression For pulse linacs it is often cheaper and easier to produce longer RF pulses and compress them to produce higher peak powers. This is performed by storing the RF in a cavity and switching the external Q of the cavity (or otherwise increasing the output power).

19. Low Level RF Feeder System Klystron Cavity DC Power Supply Typical RF System • Low Level RF (LLRF) Control Tasks • Provide low noise RF sources for all acceleration points in the machine • Maintain phase and amplitude of the RF system to accelerate the beam • Combat beam induced instabilities etc • Provide diagnostics and be flexible • Minimise environmental effects on the machine • Temperature • Vibration

20. Phase Control Loop • The input phase is adjusted so that the output phase (and hence cavity phase) is always equal to a preset value. • The mechanical phase shifter is used so that the phase detector will work in the correct range. • Phase control loop can control the phase to less than 0.25 degree Master Oscillator Electronic Phase Shifter 3dB Output Phase Shifter Amplifier Phase Detector Feeder Waveguide Attenuator

21. Frequency Control Loop Change in cavity resonant frequency causes a change in phase difference between the cavity input and the cavity output, S21. input • Cavity is tuned by squashing the shape using a stepper motor and a pivot. • The motor torque is varied to keep the phase constant. cavity phase detector tuner motor motor control

22. Amplitude Control Loop gap voltage from cavity 1 gap voltage from cavity 2 • Voltage controlled attenuator controls input power to the source • Loop keeps gap voltages constant (~1%) counteracting beam loading. Attenuator Attenuator detector detector amplitude loop board Gap voltage setting voltage controlled attenuator RF drive in RF drive out

23. Basic LLRF system • 25dB range • 360 Deg. • 1% Max full power • 1Deg. error

24. Digital LLRF • For digital systems we use inphase and quadrature (real and imaginary) instead of phase and amplitude. • This is because phase and amplitude are coupled to each other, where I and Q are independent. • This can be used to calculate an instantaneous phase and amplitude.