Introduction to Radio Frequency at ISIS

1. Introduction to RF at ISIS ISIS Lecture, 16 February 2006 David Findlay Accelerator Division ISIS Department Rutherford Appleton Laboratory ISIS OPTIMVS NEVTRONVM SPALLATIONENSIVM FONS MVNDI

2. From ISIS MCR Beam News 3-NOV-2005 00:04 A burnt out valve base has been found on system 4 RF. We are in the process of changing it. Further update at 03:00 Hrs. 17-NOV-2005 13:30 The beam tripped due to Modulator 3 tripping off. Whilst attempting to bring RF back on a large breakdown was heard in the feedline / 116 Valve area. We have investigated the problem and found a significant water leak. Experts are in attendance to rectify the problem. Update at 14.30 Hours.

3. What is RF? • RF = Radio frequency • Used as shorthand for • Alternating voltages at radio frequencies • Alternating currents at radio frequencies • Electromagnetic waves at radio frequencies • Power carried in electromagnetic waves • Apparatus generating RF power • ...

4. What are radio frequencies? • Long waves ~200 kHz • Medium waves ~1 MHz • Short waves ~3–30 MHz • VHF radio ~100 MHz • TV ~500 MHz • Mobile phones ~1000–2000 MHz • Satellite TV ~10000 MHz • Accelerators ~1 MHz – 10000 MHz • http://www.ofcom.org.uk/static/archive/ra/publication/ra_info/ra365.htm#table

5. Wavelengths and frequencies? • c = lf • Velocity = wavelength × frequency • Velocity of light = 3×108 metres/second • = 186,000 miles/second • = 670,000,000 miles/hour • = 300 m/µs • (300 m  twice around the synchrotron)

6. Frequencies Wavelengths • Long waves ~200 kHz ~1500 m • Medium waves ~1 MHz ~300 m • Short waves ~3–30 MHz ~10–100 m • VHF radio ~100 MHz ~3 m • TV ~500 MHz ~2 feet • Mobile phones ~1000–2000 MHz ~6–12 inches • Satellite TV ~10000 MHz ~1 inch • Accelerators ~1 MHz – 10000 MHz • 240 VAC mains 50 Hz ~4000 miles

7. Relative size matters

8. BBC Droitwich transmitter — Long wave Radio 4

9. Marconi’s transmitter, 1902 — Nova Scotia

10. Marconi’s spark transmitter, 1910

11. Steam engine and alternator

12. Two of four 5 kV DC generators

13. 12 kV stand-by battery (6000 cells! 2 GJ stored energy!)(cf. RAL SC3: 5 J)

14. Marconi’s 1920 valve transmitter

15. Alternating voltages, currents, electric fields, magnetic fields, ... • Need to describe by three quantities • Frequency, amplitude and phase • E.g. three-phase AC mains: • All phases “240 V” • But different phases are very different! • Phase varies along a wire carrying alternating current • How much phase changes depends on wavelength and hence on frequency

16. Phase Alternating voltage V(t) = A sin (2p f t + f) f = 240° 120° 0° E.g. three-phase AC mains

17. 50 Hz AC mains in house House 4000 miles

18. 200 MHz RF in ISIS linac Positive 2½ feet Negative 5 feet

19. Why is RF used at all in accelerators? Cathode ray tube in TV set doesn’t need RF

20. Particles accelerated using electric field For 100 keV can use 100 kV DC power supply unit. Even 665 kV for old Cockcroft-Walton But 800,000,000 V DC power supply unit for accelerating protons in ISIS not possible Instead, for high energies, use RF fields, and pass particles repeatedly through these fields RF fields produce bunched beams DC RF ns – µs spacing

21. Air RF Sound waves set up inside milk bottle Electromagnetic waves set up inside hollow metal cylinder

22. RF

23. RF +–+–+–+–+–

25. –+–+–+–+–+

27. Interior of linac tank

28. How much RF power? All beam power from RF • ISIS mean current 200 µA • Linac 70 MeV 70 MeV × 200 µA = 14 kW • Synchrotron 800 MeV 800 MeV × 200 µA = 160 kW • So need >14 kW RF for linac, >160 kW RF for synchrotron • Linac pulsed, 2% duty factor14 kW ÷ 0.02 = 0.7 MW • Synchrotron pulsed, 50% duty factor 160 kW ÷ 0.50 = 0.3 MW

29. Two commercial 0.5 MW short wave radio transmitters

30. RF powers • Big radio and TV transmitters 0.5 MW • Mobile phone transmitters 30 W • Mobile phones 1 W • Sensitivity of mobile phones 10–10 W • ISIS linac 3 × 2 MW + 1 × 1 MW • ISIS synchrotron 6 × 150 kW + 4 × 75 kW

31. Where does RF power come from? • Big amplifiers • Usually purpose built • The basics: Accelerator Frequency source RF amplifier

32. ~1 W RF ~1 MW RF

33. Devices that amplify RF • Transistors • ~100 watts maximum per transistor • Couple lots together for kilowatts • Valves / vacuum tubes • Triodes, tetrodes • Largest can deliver several megawatts (peak) • Klystrons • High powers, high gains • Limited to frequencies >300 MHz • IOTs (inductive output tubes) • Often used in TV transmitters (esp. digital TV) • Output limited to ~50 kW

34. Transistors usually junction transistors (NPN, PNP) • Essentially minority carrier device • But RF transistors usually field effect transistors • Majority carrier device

35. Field effect transistor

36. Typical RF MOSFET

37. Solid state RF amplifier: few watts in, 3 kW max out

38. 3 kW max. solid state amplifier mounted in rack

39. 1 kW solid state driver RF amplifier for synchrotron

40. Valves / vacuum tube made in 1915

41. Load + Anode power supply Anode Electrons – Grid Cathode Heater Basic triode circuit

42. Valve-based audio hi-fi amplifiers

43. Debuncher amplifier: commercial TV transmitter

44. Linac triode 5 MW peak 75 kW mean Synchrotron tetrode 1000 kW peak 350 kW mean

45. Typical valve parameters at ISIS TH116 4648 Type Triode Tetrode Heater 20 V, 500 A 4 V, 1600 A Anode volts 35 kV 16 kV Anode current 175 A 8 A Peak power o/p 2 MW 75 kW Mean power o/p 40 kW 40 kW Cooling water 100 l/min 200 l/min

46. Resonant circuits Parallel LC-circuit Impedance Z “infinite” at f = f0 (2f0)² = 1 / LC L C Shorted line Impedance Z “infinite” at l = /4, 3/4,5/4, ... Only ratio of diameters matters length l

47. HT (+ve) Output Tetrode Anode Screen grid Control grid Cathode Heater Input Essence of a tuned RF amplifier — 1

48. HT (+ve) Output Tetrode Anode Screen grid Control grid Cathode Heater Input Essence of a tuned RF amplifier — 2

49. Input (grid) tuned circuit Tetrode Output (anode) tuned circuit ISIS RFQ 200 kW tetrode driver

50. Klystron gain ~50 dB (× 105 power gain) E.g. 10 W in, 1 MW out IOT gain ~25 dB (× 300 power gain) E.g. 200 W in, 60 kW out