1. ISIS OPTIMVS NEVTRONVM SPALLATIONENSIVM FONS MVNDI
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 :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 :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 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 – MHz

5. Wavelengths and frequencies? c = l f
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 – 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
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. Alternating voltage V(t) = A sin (2p f t + f)
Phase
Alternating voltage V(t) = A sin (2p f t + f)
f = 240° ° °
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. Sound waves set up inside milk bottle
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 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 factor kW ÷ 0.02 = 0.7 MW
Synchrotron pulsed, 50% duty factor 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. + – Basic triode circuit 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 ISISTH
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. Essence of a tuned RF amplifier — 1
HT (+ve)
Output
Tetrode
Anode
Screen grid
Control grid
Cathode
Heater
Input
Essence of a tuned RF amplifier — 1

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

49. ISIS RFQ 200 kW tetrode driver
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

51. Toshiba E3740A 3 MW 324 MHz klystron
5 metres, 3 tons
Toshiba E3740A 3 MW 324 MHz klystron

52. Skin depth
RF currents flow in surface of conductor only
Skin depth d µ 1 / Ö (frequency) (exponential)
In copper, d = 7 / Ö (frequency) (cm)
50 Hz 1 cm
1 MHz 70 µm
200 MHz 5 µm
In sea water
50 Hz ~100 feet ELF / submarines
10 kHz ~10 feet VLF / submarines

53. ISIS RFQ — vessel copper-plated stainless steel

54. Different currents on different surfaces of same piece of metal
Linac high power RF amplifier

55. No external electric field
Dielectric material
No external electric field
Atoms
– – – –
Electric field

56. Dielectric material
Dielectric constant
Ceramic 6
Nylon 3
Perspex 3½
Polystyrene 2½
Water 80
Loss tangent — leads to dielectric heating
Ceramic 0.001
Nylon 0.02
Perspex 0.01
Polystyrene
Water — microwave ovens

57. Accelerating cavity
Beam
Vacuum
Air
Air
Vacuum
RF amplifier RF
Window

58. RF feed to linac tank

59. Window and aperture

60. Good and failed RF windows

61. Servo systems on amplitude, phase and cavity tuning
Cavity n 
RF amp. chain
Phase comp.
Motor drive
Volt. comp.
Phase comp.
Tuner
V ref. accel. field
Low level RF
beam
Servo systems on amplitude, phase and cavity tuning
Linac RF block diagram

62. Three amplifiers in previous slide

63. Synchrotron high power RF systems

64. Synchrotron low-level RF systems block diagram
Frequency sweeper
Beam compensation loop
Voltage loop
Cavity tuning
Phase loop
Synchrotron low-level RF systems block diagram

65. Driver amplifier

66. Cavity and high power RF driver

67. High power RF drive

68. ISIS depends almost entirely on RF
Earth
↓ DC %
35 keV
↓ RF
665 keV
↓ RF %
70 MeV
800 MeV

70. Supplementary detail
RF transistors — hand-waving
Electron and hole mobilities in Si ~1000 (cm/s)/(V/cm)
Breakdown field strength in Si is ~300 kV/cm
So maximum speed of electron or hole in Si is ~3×10^8 cm/s = 0.01 c
In big transistor say characteristic size = 1 cm
So electron or hole would take ~3 ns to travel across/through transistor
RF period must be >> 3 ns, say 10 ns, thereby limiting RF frequency to 100 MHz
If make transistor bigger to dissipate more heat, then more and more limited in frequency