ISIS OPTIMVS NEVTRONVM SPALLATIONENSIVM FONS MVNDI Introduction to RF at ISIS ISIS Lecture, 16...

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ISIS OPTIMVS NEVTRONVM SPALLATIONENSIVM FONS MVNDI

Introduction to RF at ISIS

ISIS Lecture, 16 February 2006

David FindlayAccelerator DivisionISIS DepartmentRutherford Appleton Laboratory

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?

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)

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

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

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

y = sin (2 f t + )

0 90 180 270 360 450 540 630 720 810 900 990 1080

Degrees

Alternating voltage V(t) = A sin (2 f t + )

= 240° 120° 0°

E.g. three-phase AC mains

50 Hz AC mains in house

4000 miles

200 MHz RF in ISIS linac

5 feet

2½ feet

Positive

Negative

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 beamsDC

RF ns – µs spacing

Sound waves set up inside

milk bottle

Electromagnetic waves set

up inside hollow metal

cylinder

+ – + – + – + – + –

– + – + – + – + – +

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 factor160 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

RF amplifier

Frequency source

~1 W RF ~1 MW RF

Devices that amplify RF

Transistors~100 watts maximum per transistorCouple lots together for kilowatts

Valves / vacuum tubesTriodes, tetrodesLargest can deliver several megawatts (peak)

KlystronsHigh powers, high gainsLimited 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 transistorsMajority 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

Anode power supply

Heater

Cathode

Electrons

Valve-based audio hi-fi amplifiers

Debuncher amplifier: commercial TV transmitter

Linac triode5 MW peak

75 kW mean

Synchrotron tetrode

1000 kW peak350 kW mean

Typical valve parameters at ISIS

TH116 4648Type Triode TetrodeHeater 20 V, 500 A 4 V, 1600 AAnode volts 35 kV 16 kVAnode current 175 A 8 APeak power o/p 2 MW 75 kWMean power o/p 40 kW 40 kWCooling water 100 l/min 200 l/min

Resonant circuits

Parallel LC-circuitImpedance Z “infinite” at f = f0(2f0)² = 1 / LCL C

length l

Shorted lineImpedance Z “infinite” at l = /4, 3/4, 5/4, ...

Only ratio of diameters matters

Output

HT (+ve)

AnodeScreen gridControl gridCathodeHeater

Tetrode

Essence of a tuned RF amplifier — 1

Output

HT (+ve)

AnodeScreen gridControl gridCathodeHeater

Tetrode

Essence of a tuned RF amplifier — 2

ISIS RFQ 200 kW tetrode driver

Input (grid) tuned circuit

Output (anode) tuned circuit

Tetrode

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

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 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

– + – + – + – +

Electric field

Dielectric material

No external electric field

Dielectric material

Dielectric constantCeramic 6Nylon 3Perspex 3½Polystyrene2½Water 80

Loss tangent — leads to dielectric heatingCeramic 0.001Nylon 0.02Perspex 0.01Polystyrene0.0001Water 0.1 — microwave ovens

RF amplifier

Accelerating cavity

BeamVacuu

Air Vacuum

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

V ref. accel. fieldPhase comp.

Volt. comp.

Phase comp.

Motor drive

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

ISIS depends almost entirely on RF

↓ DC 0.004%

35 keV

↓ RF

665 keV

↓ RF 99.996%

70 MeV

↓ RF

800 MeV

Supplementary detail

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

ISIS OPTIMVS NEVTRONVM SPALLATIONENSIVM FONS MVNDI Introduction to RF at ISIS ISIS Lecture, 16...

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