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

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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

2 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 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 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 5 Wavelengths and frequencies? c = f Velocity = wavelength × frequency Velocity of light= 3×10 8 metres/second = 186,000 miles/second = 670,000,000 miles/hour = 300 m/µs (300 m  twice around the synchrotron)

6 6 FrequenciesWavelengths 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 7 Relative size matters

BBC Droitwich transmitter — Long wave Radio 4

Marconi’s transmitter, 1902 — Nova Scotia

Marconi’s spark transmitter, 1910

Steam engine and alternator

Two of four 5 kV DC generators

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

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 (2  f t +  )  = 240° 120° 0° E.g. three-phase AC mains Phase

17 50 Hz AC mains in house 4000 miles House

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

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

22 RF

23 + – + – + – + – + – RF

24

25 – + – + – + – + – +

26

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 MeV800 MeV × 200 µA = 160 kW So need >14 kW RF for linac, >160 kW RF for synchrotron Linac pulsed, 2% duty factor 14 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 transmitters0.5 MW Mobile phone transmitters30 W Mobile phones1 W Sensitivity of mobile phones10 – 10 W ISIS linac 3 × 2 MW + 1 × 1 MW ISIS synchrotron6 × 150 kW + 4 × 75 kW

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

~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

Field effect transistor

Typical RF MOSFET

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

3 kW max. solid state amplifier mounted in rack

1 kW solid state driver RF amplifier for synchrotron

Valves / vacuum tube made in 1915

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

Valve-based audio hi-fi amplifiers

Debuncher amplifier: commercial TV transmitter

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

45 Typical valve parameters at ISIS TH1164648 TypeTriodeTetrode Heater20 V, 500 A4 V, 1600 A Anode volts35 kV16 kV Anode current175 A8 A Peak power o/p2 MW75 kW Mean power o/p40 kW40 kW Cooling water100 l/min200 l/min

46 Resonant circuits Parallel LC-circuit Impedance Z “infinite” at f = f 0 (2  f 0 )² = 1 / LC LC length l Shorted line Impedance Z “infinite” at l = /4, 3 /4, 5 /4,... Only ratio of diameters matters

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

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

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

50 Klystron gain ~50 dB (× 10 5 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

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

52 Skin depth RF currents flow in surface of conductor only Skin depth   1  (frequency) (exponential) In copper,  = 7 /  (frequency) (cm) 50 Hz 1 cm 1 MHz70 µm 200 MHz 5 µm In sea water 50 Hz ~100 feetELF / submarines 10 kHz~10 feetVLF / submarines

ISIS RFQ — vessel copper-plated stainless steel

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

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

56 Dielectric material Dielectric constant Ceramic6 Nylon3 Perspex3½ Polystyrene2½ Water80 Loss tangent — leads to dielectric heating Ceramic0.001 Nylon0.02 Perspex0.01 Polystyrene0.0001 Water0.1— microwave ovens

57 RF amplifier Accelerating cavity Beam Vacuum Air Vacuum RF Window

RF feed to linac tank

Window and aperture

Good and failed RF windows

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

Three amplifiers in previous slide

Synchrotron high power RF systems

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

Driver amplifier

Cavity and high power RF driver

High power RF drive

68 ISIS depends almost entirely on RF Earth ↓ DC 0.004% 35 keV ↓ RF 665 keV ↓ RF 99.996% 70 MeV ↓ RF 800 MeV

69

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

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