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

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.

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 ...

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

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)

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

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

Marconi’s 1920 valve transmitter

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

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

50 Hz AC mains in house House 4000 miles

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

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

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

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

RF

RF + – + – + – + – + –

– + – + – + – + – +

Interior of linac tank

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 factor 14 kW ÷ 0.02 = 0.7 MW Synchrotron pulsed, 50% duty factor 160 kW ÷ 0.50 = 0.3 MW

Two commercial 0.5 MW short wave radio transmitters

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

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

~1 W RF ~1 MW RF

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

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 Electrons Grid Cathode Heater Basic triode circuit

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

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

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

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

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

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

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

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

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

ISIS RFQ — vessel copper-plated stainless steel

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

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

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 0.0001 Water 0.1 — microwave ovens

Accelerating cavity Beam Vacuum Air Air Vacuum RF amplifier RF Window

RF feed to linac tank

Window and aperture

Good and failed RF windows

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

Three amplifiers in previous slide

Synchrotron high power RF systems

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

Driver amplifier

Cavity and high power RF driver

High power RF drive

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

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