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.
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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
...
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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
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Wavelengths and frequencies?
c=lf
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)
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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
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Relative size
matters
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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
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Phase
y = sin V(t)
(2 f t=
+ )
Alternating voltage
A sin (2p f t + f)
1.0
0.8
0.6
Amplitude
0.4
0.2
0.0
0
90
180
270
360
450
540
630
720
810
900
990
1080
-0.2
-0.4
-0.6
-0.8
-1.0
Degrees
f = 240°
120°
0°
E.g. three-phase AC mains
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50 Hz AC mains in house
House
4000 miles
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200 MHz RF in ISIS linac
Positive
2½ feet
Negative
5 feet
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Why is RF used at all in accelerators?
Cathode ray tube in TV set doesn’t need RF
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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
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Air
Sound waves
set up inside
milk bottle
RF
Electromagnetic
waves set up
inside hollow
metal cylinder
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RF
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RF
+
– +
–
+
–
+
–
+
–
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23
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24
–
+ –
+
–
+
–
+
–
+
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Interior of linac tank
How much RF power? All beam power from RF
ISIS mean current 200 µA
Linac 70 MeV
Synchrotron 800 MeV
70 MeV × 200 µA = 14 kW
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
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Two commercial
0.5 MW short
wave radio
transmitters
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RF powers
Big radio and TV transmitters 0.5 MW
Mobile phone transmitters
30 W
Mobile phones
1W
Sensitivity of mobile phones 10–10 W
ISIS linac
3 × 2 MW + 1 × 1 MW
ISIS synchrotron
6 × 150 kW + 4 × 75 kW
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Where does RF power come from?
Big amplifiers
Usually purpose built
The basics:
Accelerator
Frequency
source
RF
amplifier
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~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
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Transistors usually junction transistors (NPN, PNP)
Essentially minority carrier device
But RF transistors usually field effect transistors
Majority carrier device
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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
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
Type
Heater
Anode volts
Anode current
Peak power o/p
Mean power o/p
Cooling water
TH116
Triode
20 V, 500 A
35 kV
175 A
2 MW
40 kW
100 l/min
4648
Tetrode
4 V, 1600 A
16 kV
8A
75 kW
40 kW
200 l/min
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Resonant circuits
L
C
Parallel LC-circuit
Impedance Z “infinite” at f = f0
(2pf0)² = 1 / LC
Shorted line
Impedance Z “infinite” at
l = l/4, 3l/4, 5l/4, ...
length l
Only ratio of diameters
matters
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HT (+ve)
Output
Tetrode
Input
Anode
Screen grid
Control grid
Cathode
Heater
Essence of a tuned RF amplifier — 1
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HT (+ve)
Output
Tetrode
Input
Anode
Screen grid
Control grid
Cathode
Heater
Essence of a tuned RF amplifier — 2
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Input (grid)
tuned circuit
Tetrode
Output (anode)
tuned circuit
ISIS RFQ 200 kW tetrode driver
Klystron gain ~50 dB
(× 105 power gain)
IOT gain ~25 dB
(× 300 power gain)
E.g. 10 W in, 1 MW out
E.g. 200 W in, 60 kW out
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5 metres, 3 tons
Toshiba E3740A 3 MW 324 MHz klystron
Skin depth
RF currents flow in surface of conductor only
Skin depth d  1   (frequency) (exponential)
In copper, d = 7 /  (frequency) (cm)
50 Hz
1 cm
1 MHz
70 µm
200 MHz 5 µm
In sea water
50 Hz
~100 feet
10 kHz
~10 feet
ELF / submarines
VLF / submarines
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ISIS RFQ — vessel copper-plated stainless steel
Different currents
on different
surfaces of same
piece of metal
Linac high power
RF amplifier
Dielectric material
No external electric field
Atoms
–
+
–
+
–
+
–
+
Electric field
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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
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Accelerating cavity
Beam
Vacuum
Air
Air
RF
amplifier
Vacuum
RF
Window
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RF feed to linac tank
Window and aperture
Good and failed RF windows
Cavity n

Low
level
RF
Phase
comp.
Volt.
comp.
RF amp. chain
V ref. accel. field
Phase
comp.
Motor
drive
Tuner
beam
Servo systems on amplitude, phase and cavity tuning
Linac RF block diagram
Three amplifiers in
previous slide
Synchrotron high power RF systems
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
↓
35 keV
↓
DC
0.004%
RF
665 keV
↓
RF
70 MeV
99.996%
↓
RF
800 MeV
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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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