Lecture 11 Radiofrequency Cavities III

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Lecture 11 - Radiofrequency Cavities III

• Emmanuel Tsesmelis (CERN/Oxford)
• John Adams Institute for Accelerator Science
• 19 November 2009

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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)

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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

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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

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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

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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.
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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 ? of
• driving wave.

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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.

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Klystrons

• Klystron output power
• U0 klystron supply voltage (e.g. 45 kV)
• Ibeam beam current (e.g. 12.5 A)
• ? klystron efficiency (45 - 65)

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Large Hadron Collider (LHC)
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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.

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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 25-30 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.

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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.

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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 ?
• 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.

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Large Hadron Collider
The Main Beam and RF Parameters
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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
  Earths B-field

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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.

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Linear Colliders
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Linear Collider Baseline
Physics motivation "Physics at the CLIC
Multi-TeV Linear Collider Report of the CLIC
Physics Working Group, CERN Report 2004-5

• LEP 209 GeV
• next Electron-Positron Collider
• Centre-of-mass-energy
• 0.5 - 3 TeV
• Luminosity gt21034

Storage Ring not possible, energy loss DE E4 ?
two linacs, experiment at centre

• 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
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Basic Differences ILC-CLIC
ILC Superconducting RF 500 GeV
CLIC normal conducting copper RF 3 TeV
Accelerating gradient 31.5 MV/m
100 MV/m
(35 MV/m target) RF Peak power 0.37 MW/m ,
1.6 ms, 5 Hz 275 MW/m, 240 ns, 50 Hz RF
average power 2.9 kW/m
3.7 kW/m
Total length 31 km
48.4 km Site power 230
MW 392 MW
Particles per bunch 20 109
3.7 109
2625 bunches / pulse of 0.96 ms
312 bunches / pulse of 156 ns
Bunch spacing 369 ns
0.5 ns
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International Linear Collider(ILC)
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The ILC Baseline Design
Schematic lay-out of the ILC complex for 500 GeV
CM
Basic ILC design parameters (Values at 500 GeV CM)
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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 RD
• Quality factor Q relates to the cryogenic cooling
  power

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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 17 000 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

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

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Technical Systems

• Multi-Beam Klystrons (MBK)
• 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

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Multi-Beam Klystrons
Toshiba E3736
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Compact Linear Collider(CLIC)
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CLIC Acceleration System
CLIC Compact Linear Collider
(length lt 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 15000 m Pulse length 240 ns, 50 Hz
RF out
RF in
Beam
Efficient RF power production !!!!!
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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 - 240 MeV)
• high current (100A)

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The CLIC Two Beam scheme
Bunch charge 8.4 nC, Current in train 100 A
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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)
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The Full CLIC scheme
Not to scale!
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Why 100 MV/m and 12 GHz ?

• 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 gt 60 106 structures 100 MV/m 12 GHz
chosen, previously 150 MV/m, 30 GHz
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CLIC Accelerating Module
Main Beam
Transfer lines
Drive Beam
Main Beam
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Accelerating Structures

• Objective
• Withstand of 100 MV/m without damage
• breakdown rate lt 10-7
• Strong damping of HOMs

Technologies Brazed disks - milled quadrants
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Power Extraction PETS
8 Sectors damped on-off possibility
Status CTF3 up to 45 MW peak (3 A
beam, recirculation) SLAC 125 MW _at_ 266 ns
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CLIC Main Parameters