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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
22
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.
33
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
...
44
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
55
Wavelengths and frequencies?
c = 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)
66
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
77
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
1515
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
1616
y = sin (2 f t + )
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
0 90 180 270 360 450 540 630 720 810 900 990 1080
Degrees
Am
plit
ud
e
Alternating voltage V(t) = A sin (2 f t + )
= 240° 120° 0°
E.g. three-phase AC mains
Phase
1717
50 Hz AC mains in house
4000 miles
House
1818
200 MHz RF in ISIS linac
5 feet
2½ feet
Positive
Negative
1919
Why is RF used at all in accelerators?
Cathode ray tube in TV set doesn’t need RF
2020
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
2121
Air
Sound waves set up inside
milk bottle
RF
Electromagnetic waves set
up inside hollow metal
cylinder
2222
RF
2323
+ – + – + – + – + –
RF
2424
2525
– + – + – + – + – +
2626
Interior of linac tank
2828
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
2929
Two commercial 0.5 MW short wave
radio transmitters
3030
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
3131
Where does RF power come from?
Big amplifiers
Usually purpose built
The basics:
Accelerator
RF amplifier
Frequency source
~1 W RF ~1 MW RF
3333
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
3434
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
Load
Anode power supply
+
–
Anode
Heater
Cathode
Grid
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
4545
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
4646
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
4747
Output
Input
HT (+ve)
AnodeScreen gridControl gridCathodeHeater
Tetrode
Essence of a tuned RF amplifier — 1
4848
Output
Input
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
5050
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
5252
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
5555
– + – + – + – +
Electric field
Dielectric material
No external electric field
Atoms
5656
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
5757
RF amplifier
Accelerating cavity
BeamVacuu
m
Air
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. fieldPhase 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
6868
ISIS depends almost entirely on RF
Earth
↓ DC 0.004%
35 keV
↓ RF
665 keV
↓ RF 99.996%
70 MeV
↓ RF
800 MeV
6969
7070
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