1. Lecture 11 - Radiofrequency Cavities III
Emmanuel Tsesmelis (CERN/Oxford)
John Adams Institute for Accelerator Science
19 November 2009

2. Table of Contents III Power Generators for Accelerators
Triode Amplifier, Tetrode Amplifier, Klystron
Large Hadron Collider (LHC)
International Linear Collider (ILC)
Compact Linear Collider (CLIC)

3. Power Generators for Accelerators
The sinusoidal power needed to drive the accelerating structures ranges between a few kW to a few MW.
RF power amplifiers
Triodes & tetrodes: few MHz to few hundred MHz
Klystrons: above a few hundred MHz
Proven to be the most effective power generator for accelerator applications

4. Triode Amplifier Three active electrodes
Cathode (filament)
Grid
Anode (plate)
Anode current obeys Langmuir-Child Law
k = perveance of tube
μ = amplification factor
Va = anode voltage
Vg = grid voltage

5. Tetrode Amplifier Four active electrodes
Cathode (filament)
Control Grid
Screen Grid – reduce space charge between cathode and Control Grid
Anode (plate)
Anode current obeys Langmuir-Child Law
k = perveance of tube
μa = anode amplification factor
μs = screen grid amplification factor
Va = anode voltage
Vcg = control grid voltage
Vsg= screen grid voltage

6. Klystrons Principle of operation
Electrons emitted from round cathode with large surface area.
Accelerated by voltage of a few tens of kV.
Yields a round beam with a current of between a few amperes and tens of amperes.
Electrodes close to the cathode focus the beam and solenoid along the tube ensure good beam collimation.
Outgoing particles from cathode have a well-defined velocity and pass through cavities operated in TM011 mode.
Wave excited in this resonator by external pre-amplifier.
Klystrons are similar to a small
linear accelerator.

7. Klystrons Depending on phase, will modulate velocity
with resonant frequency of particles
(accelerate, decelerate, or have no influence).
In subsequent zero-field drift, faster particles
move ahead, while slower ones lag behind.
Changes hitherto uniform particle density
distribution and bunches of particles are
formed with separation given by λ of
driving wave.

8. Klystrons
Continuous current from cathode becomes pulsed current with frequency of coupled pulsed current.
A second cavity mounted at this location is resonantly excited by pulsed current and the RF wave generated in this second cavity is then coupled out.
A better coupling of beam to output cavity achieved by inserting additional cavity resonators, each tuned to frequencies close to operating frequency.

9. Klystrons Klystron output power
U0 = klystron supply voltage (e.g. 45 kV)
Ibeam = beam current (e.g A)
η = klystron efficiency (45% - 65%)

10. Large Hadron Collider (LHC)

11. Superconducting Cavities (SC)
The use of superconducting material (Nb) at low temperature (2-4 K) reduces considerably the ohmic losses and almost all the RF power from the source is made available to the beam (i.e. ~100% efficiency).
In contrast to normal conducting cavities, SC cavities favour the use of lower frequencies.
Offers a larger opening to the beam.
Reduces the interaction of the beam with the cavity that is responsible for beam instability.

12. Superconducting Cavities
Characteristics
Q0 as high as 109 – 1010 are achievable.
Leads to much longer filling times.
Higher electric field gradients are reached for acceleration – MV/m.
Reduces number of cavities or a higher energy can be reached with a given number of cavities.
Single-cell or multi-cell.
Used for both lepton and hadron machines.

13. LHC RF Parameters
Injected beam will be captured, accelerated and stored using the 400 MHz SC cavity system.
A 200 MHz capture system may be installed if the injected beam emittance increases when intensities much higher than nominal are reached.

14. Parameter Specification
Two independent RF systems.
One per each beam cooled with 4.5 K saturated He gas
Each RF system has eight single-cell cavities
Each cavitiy has 2 MV accelerating voltage, corresponding to a field strength of 5.5 MV/m
R/Q = 45 Ω
RF Power System
Each cavity is driven by individual RF system with a single klystron, circulator and load.
Maximum of 4800 kW of RF power will be generated by the 16 (300 kW) 400 MHz klystrons.
Each klystron will feed via a Y-junction circulator and a WR2300 waveguide line, a single-cell SC cavity.
High Voltage Interface
Each of the 4 main 100 kV power converters, re-used from LEP, will power 4 klystrons.

15. Large Hadron Collider
The Main Beam and RF Parameters

16. Cavity Material
As frequency of 400 MHz is close to that of LEP (352 MHz), the same proven LEP technology of Nb sputtered cavities is applied to the LHC.
Nb Sputtering on Cu
Advantage over solid Nb in that susceptibility to quenching is very much reduced.
Local heat generated by small surface defects or impurities is quickly conducted away by the Cu.
Nb-sputtered cavities are insensitive to the Earth´s B-field

17. Large Hadron Collider Design of a four-cavity cryomodule
A four-cavity module during assembly
Four cavities, each equipped with their He tank and power coupler, are grouped
together in a single cryomodule.
Reduces overall static thermal losses and requires less total space for installation
than a single cavity configuration.

18. Linear Colliders

19. Linear Collider Baseline
LEP: 209 GeV
next Electron-Positron Collider
Centre-of-mass-energy:
TeV
Luminosity: >2*1034
Physics motivation:
"Physics at the CLIC Multi-TeV Linear Collider: Report of the CLIC Physics Working Group,“
CERN Report
Storage Ring not possible, energy loss DE ~ E4
 two linacs, experiment at centre
Linac I
Linac II
total energy gain in one pass: high acceleration gradient
beam can only be used once: small beam dimensions at crossing point
Boundary conditions: site length
Power consumption

20. Basic Differences ILC-CLIC
ILC: Superconducting RF
500 GeV
CLIC: normal conducting copper RF
3 TeV
Accelerating gradient: MV/m MV/m
(35 MV/m target)
RF Peak power: MW/m , 1.6 ms, 5 Hz MW/m, 240 ns, 50 Hz
RF average power: kW/m kW/m
Total length: km km
Site power : MW MW
Particles per bunch: * * 109
2625 bunches / pulse of 0.96 ms
312 bunches / pulse of 156 ns
Bunch spacing ns ns

21. International Linear Collider (ILC)

22. The ILC Baseline Design
Schematic lay-out of the ILC complex for
500 GeV CM
Basic ILC design parameters
(Values at 500 GeV CM)

23. ILC Design Criteria
Choice of gradient is key cost and performance parameter
Dictates length of linacs
Baseline average operational accelerating gradient of 31.5 MV/m represents the primary challenge to the global ILC R&D
Quality factor Q relates to the cryogenic cooling power

24. Cavities
Basic element of the superconducting RF is a nine-cell 1.3 GHz niobium cavity
Each cavity is about 1 m. long
Operated at 2K
Nine cavities are mounted together in a string and assembled in a common low-temperature cryostat (cryomodule)
About cavities are needed for the ILC
Key to high-gradient performance is ultra-clean and defect-free inner surface of cavity consisting of Nb material and electron beam welds
Use of electropolishing in clean-room environment

25. Cavity Design Parameters
ILC 9-cell superconducting cavity design parameters

26. Superconducting RF Structures
A TESLA nine-cell 1.3 GHz
superconducting niobium
cavity.
ILC prototype cryomodules.
Clean room environments are
mandatory for the cavity preparation
and assembly.

27. RF Distribution Units RF unit diagramme
The linacs are composed of RF units, each of which is formed by 3 contiguous
superconducting RF cryomodules containing a total of 26 nine-cell cavities.
Each RF unit has a stand-alone RF source:
Conventional pulse-transformer type high-voltage modulator (120 kV)
A 10 MW multi-beam klystron
Waveguide system that distributes RF power to the cavities
To operate the cavities at 2 K, they are immersed in a saturated He II bath

28. Technical Systems Multi-Beam Klystrons (MBK) Waveguide
10 MW L-band source
High efficiency by using multiple low space charge (low perveance) beams
Waveguide
Distribution system consists of Al WR650 waveguide (6.50” x 3.25”)
Losses can be reduced by plating the inner walls of waveguide with Cu

29. Multi-Beam Klystrons
Toshiba E3736

30. Compact Linear Collider (CLIC)

31. CLIC Acceleration System
CLIC = Compact Linear Collider
(length < 50 km)
Acceleration in travelling wave structures:
CLIC parameters:
Accelerating gradient: 100 MV/m
RF frequency: 12 GHz
64 MW RF power / accelerating structure
of 0.233m active length
 275 MW/m
total active length for 1.5 TeV: 15’000 m
Pulse length 240 ns, 50 Hz
RF out
RF in
Beam
Efficient RF power production !!!!!

32. The CLIC Two Beam Scheme
Individual RF power sources ?
 Not for the 1.5 TeV linacs
Two Beam Scheme:
Drive Beam supplies RF power
12 GHz bunch structure
low energy (2.4 GeV MeV)
high current (100A)

33. The CLIC Two Beam scheme
decelerator, 24 sectors of 876 m
Main Beam
Drive Beam
240 ns
5.8ms
2904 bunches
83 ps (12 GHz)
139ms, 24 trains
Time structure of Drive Beam
Bunch charge: 8.4 nC, Current in train: 100 A

34. CLIC Drive Beam Generation
Accelerate long bunch train with low bunch rep rate (500 MHz)
with low frequency RF (1 GHz) (klystrons)
interleave bunches between each other
to generate short (280 ns) trains with
high bunch rep rate (12 GHz)
EPS 2009 G.Geschonke, CERN

35. The Full CLIC scheme
Not to scale!

36. Why 100 MV/m and 12 GHz ? After > 60 * 106 structures:
Structure limits:
RF breakdown – scaling
RF pulse heating
Beam dynamics:
emittance preservation – wake fields
Luminosity, bunch population, bunch spacing
efficiency – total power
Figure of merit:
Luminosity per linac input power
Take into account cost model
After > 60 * 106 structures:
100 MV/m 12 GHz chosen,
previously 150 MV/m, 30 GHz

37. CLIC Accelerating Module
Main Beam
Drive Beam
Transfer lines
Drive Beam
Main Beam

38. Accelerating Structures
Objective:
Withstand of 100 MV/m without damage
breakdown rate < 10-7
Strong damping of HOMs
Technologies:
Brazed disks - milled quadrants

39. Power Extraction : PETS
Travelling wave structures
Small R/Q : 2.2 kW/m
(accelerating structure: kW/m)
100 A beam current
136 MW 240 ns per PETS
(2 accelerating structures)
0.21 m active length
total number : 35’703 per linac
8 Sectors
damped
on-off possibility
Status:
CTF3: up to 45 MW peak (3 A beam,
recirculation)
SLAC: ns

40. Center-of-mass energy
CLIC Main Parameters
Center-of-mass energy
CLIC 500 GeV
CLIC 3 TeV
Beam parameters
Conservative
Nominal
Total (Peak 1%) luminosity
0.9(0.6)·1034
2.3(1.4)·1034
1.5(0.73)·1034
5.9(2.0)·1034
Repetition rate (Hz) 50
Loaded accel. gradient MV/m 80
100
Main linac RF frequency GHz 12
Bunch charge109
6.8
3.72
Bunch separation (ns)
0.5
Beam pulse duration (ns)
177
156
Beam power/beam (MWatts)
4.9 14
Hor./vert. norm. emitt (10-6/10-9)
3/40
2.4/25
2.4/20
0.66/20
Hor/Vert FF focusing (mm)
10/0.4
8 / / 0.3
4 / 0.07
Hor./vert. IP beam size (nm)
248 / 5.7
202 / 2.3
83 / 2.0
40 / 1.0
Hadronic events/crossing at IP
0.07
0.19
0.57
2.7
Coherent pairs at IP 10
5 107
BDS length (km)
1.87
2.75
Total site length km
13.0
48.3
Wall plug to beam transfert eff
7.5%
6.8%
Total power consumption MW
129.4
415