Andrew Hutton

1
THE SCIENCE BEHIND THE CEBAF ACCELERATOR

• Andrew Hutton
• Director of Operations
• Jefferson Lab

2
Outline

• Review of the basic accelerator layout
• Injector
• making electrons
• creating electron bunches
• Linacs
• How does RF acceleration work
• Setting up the RF
• Secrets of the Arcs
• Emittance and Optics notions
• Special features

3
Summary Accelerator Description

• Beam performance objectives
• 5.7 GeV, 200 µA, CW
• 3 simultaneous beams, independent energy and
  current adjustment
• Beam polarization 75
• Parity quality
• Design concept recirculating, superconducting
  CW Linac
• CW for high beam quality
• RF superconductivity for efficiency
• 5-pass beam re-circulation gives multi-energy
  operation
• Isochronous, achromatic beam recirculation arcs

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Continuous Electron Beam Accelerator Facility
2 gain switched diode lasers _at_ 499 MHz 1 G0 laser
_at_ 31 MHz Df 120?
0.6 GeV linac
(20 cryomodules)
1497 MHz
67 MeV injector
(2 1/4 cryomodules)
1497 MHz
RF separators
A
B
C
499 MHz
B
A
B
C
A
Pockels cell
C
Gun
Double sided
septum
6
Making Electrons

• Photo-electric Gun
• Shine laser light on a semiconductor wafer
• Photo-electric effect kicks out an electron
• same effect as solar cells which make electricity
  from the sun
• Because the laser can be pulsed, electron beam is
  also pulsed
• Use three separate lasers, one for each Hall
• provides greater flexibility
• Cathode material is strained Gallium Arsenide
• Laser light of 780nm gives weakly polarized beams
  35
• but high Quantum Efficiency (QE1)
• Laser light of 840 nm gives strongly polarized
  beam 75
• but low QE0.2

7
Polarized photoinjector

• 2 identical horizontal guns installed in 1998

Gun 2 Oct 2000 to Jan 2001
Gun 3 Feb 2001 to Mar 2002
Gun service required once per year. Both guns
provide high polarization (gt70).
8
Source perspective
A 100 keV beam from either gun is deflected 15
by a magnet to a common pre-accelerator beamline.
Two laser tables straddle the beamline and
provide a direct optical path to the cathode.
9
Diode Laser

• Diode lasers like in your CD player
• Requires additional amplifier to obtain 100 mW
  of light
• Highly reliable
• Pulse length d 20 ps
• But
• Amplifier does not fully switch off between
  pulses
• Creates bleed-through
• Have tried to pulse amplifier with some success
  still a problem
• We always probably use diode laser for Hall B
• Presently using diode laser for Hall A

10
Ti-Sapphire Laser

• High power without additional amplifier
• Switches off cleanly unmeasurable bleed-through
• Early laser had noise on the beams
• Led to trips of the safety system
• A Ti-Sapphire laser from TimeBandwidth Inc. in
  Switzerland (Tiger) was ordered for the G0
  experiment at 31.2 MHz
• Uses a different, proprietary method for locking
  to RF frequency
• Has operated flawlessly
• Can create pulses with d from 20ps to 70ps
• We have ordered another Tiger laser for 499MHz
  operation
• Expect good performance
• Will be used for future, high current Hall A
  experiments

11
Dynamic laser configuration
Re-re-re-re-configuration. . . 3 end-stations
makes for a dynamic physics program which
requires that the laser table be configurable for
beam qualities Intensity (power) Polarization
(wavelength) RF (1497, 499, 31.1875) Parity
(Independent)
G0
Future HAPPEx2
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Making Bunches

• Beam from 100 keV gun sent through 499 MHz
  chopper cavity (one third of accelerating
  frequency)
• Rotates beam in circle of 1.5 cm radius
• Slits at 240, 0 and 120 degrees allow bunches
  of electrons to pass
• Slits are individually controlled to regulate
  currents for Halls A, B C
• The three beams are recombined by another 499 MHz
  chopper cavity
• Creates 110 picosecond bunch
• Hall A Hall B

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Bleed-Through

• To reduce bleed-through, Hall B uses about 20
  µA beam current from the cathode and rejects all
  but a few nA at the slits
• Improves rejection of leakage from Hall A and
  Hall C beams
• But
• Makes Hall B current very sensitive to position
  movement at the slit
• Have installed a fast feedback to stabilize
  position
• But
• BPMs read position of the average currents for
  Halls A, B C
• Displaces low current beam if all the beams are
  not exactly coincident
• Cannot stabilize against laser spot movements

14
Making Bunches (cont)

• Beam then goes through buncher section
• Decelerates front of bunch, accelerates back of
  bunch
• After a drift space, bunch is compressed
  (shorter) 5 picosecond
• but energy spread in the bunch is increased
• Beam then goes through capture which continues
  process
• Final bunch length 1 picosecond (3/10 of a
  millimeter)
• Phases of Injector Rf must be stable to within 1
  ps
• Can achieve this for short periods of time
• Best in summer and winter
• Injector very unstable in spring and fall
  sun-up and sun-down

15
Measuring Bunch Length

• Measure the time of arrival of the bunch at a
  high frequency cavity (5.988 GHz)downstream of
  the buncher
• Move the phases of the chopper cavities and
  re-measure the time of arrival
• Plot the time of arrival as a function of the
  phase
• Each point is the center of gravity of the beam
• By moving the center of gravity, the result
  mimics the behavior of particles distributed
  along the bunch
• This measurement is accurate to about 15
  femtoseconds

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Wave acceleration
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How does RF acceleration work?

• Imagine a surfer riding a wave
• Get on the wave at right time and right speed -
  acceleration
• Get on the wave at wrong time or wrong speed,
  deceleration, wipe-out
• A good surfer will speed up to catch the wave and
  will then start to move across the wave to avoid
  overtaking the wave and wiping out
• Matching the phase velocity and the group
  velocity
• Now imagine an electron traveling in a
  radio-frequency (RF) wave
• If it arrives at exactly the right time it will
  gain energy
• If it arrives at the wrong time it will gain less
  energy
• But If it is traveling near the speed of light,
  it will not speed up or slow down it will gain
  or lose mass as it gains energy
• No wipe -out, but not the right energy at the end

19
How does RF acceleration work? cont.

• RF acceleration comes from multiple, independent
  klystrons, each feeding one 5-cell cavity
• The trick is to ensure that as a bunch arrives at
  each cavity, the fields are correct to continue
  acceleration
• The waves (fields) in each cavity have to be in
  phase
• Phases have to be set up correctly
• Phases have to stay correct
• Both of these are a challenge
• Each klystron is fed from the same phase
  reference, but . . . . . .
• must be accurate to a few picoseconds
• klystrons are up to half a mile away from the
  source
• Thermal problems, stability problems in repeaters
  . . . .

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Superconducting Cavities

• Use superconducting niobium cavities to create
  the RF fields for acceleration
• Very low losses on the cavity walls
• cavity resonance very sharp (Qext 106)
• sensitive to vibration (microphonics)
• Cooled by superfluid liquid Helium _at_ 2 K so heat
  transfer is easy
• no bubbles to vibrate the cavity
• Helium temperature (pressure) changes the tuning
• keeping all of the cavities in tune requires
  automated software
• CEBAF Acceleration System
• 330 cavities in 41 1/4 cryomodules, installed and
  functional
• Gradient limit and Q twice as good as
  specification

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CEBAF SRF Cavities
CEBAF 5-cell cavities operate at 1497 MHz with an
active length of 50 cm each There are eight
cavities per cryomodule
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Linac Cryomodules
CEBAF has 42¼ cryomodules with a total active
length of 169 meters
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Superconducting Cavity Treatment

• Gradient specification of CEBAF cavities was 5
  MV/m
• Average gradient of superconducting cavities as
  installed was 7.3 MV/m
• Two approaches to improve performance of
  cavities,
• Helium processing and waveguide vacuum
  processing
• Carried out in situ
• Aim to reduce field emission
• Average gradient of superconducting cavities is
  now 7.76 MV/m
• Limit is established with CW beam under standard
  operating conditions

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FSD Trips

• Fast Shut Down (FSD) trips are triggered by RF
  arcs and protect the SRF cavities from arc damage
• Caused by charging up of ceramic widows?
• Caused by three dimensional gas discharges?
• Strongly dependent on accelerating gradient
• Weakly dependent on total linac beam current
• Pushing energy to
• 5.7 GeV ? 10 hit in availability (just
  acceptable)
• 6 GeV ? 20 hit in availability (unacceptable)

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Improving RF arc trip rate

• Installed gt50 stub tuners to improve match
• Reduces klystron power for same beam power
• Helium processed all cryomodules (some twice)
• Reduces electron emission in cavities
• Improved algorithm for calculating set points
• Optimize for gradient, current, cryogenic load,
  etc
• Problem is inherent, getting close to limit
• This year, will try to reduce time to reset trip

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FSD Trip Rate Versus Energy October 99 June 01
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Ponderomotive Force

• Ponderomotive force of RF fields tends to lower
  cavity resonant frequency
• RF fields have energy, tends to push outwards,
  makes cavity bigger
• Cavity tuner changes to maintain correct
  operating frequency
• In a few cavities (so far)
• If cavity trips off at high gradient, resulting
  frequency shift is so large
• Cavity goes out of resonance
• Cannot be switched back on
• This is the most serious problem for RF control
  of the new cryomodules for 12 GeV

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Linac RF Operation

• Operation of the linacs requires sophisticated
  high-level software
• Automated Cavity Tuning
• Make it resonate at exactly the right frequency
• Automated Cavity phasing
• Make all the cavities resonate in phase
• Linac Energy Management
• Set up exactly the right energy
• KREST
• Set up exactly the right phase for each cavity
• MOMOD
• Maintain the right phase for each linac

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Automated Cavity Tuning

• Sweep Mode (Exact tuning)
• The cavity frequency is modulated over a range of
  200 Hz in steps of 5 Hz.
• Find the typical response of a resonant system to
  harmonic excitation
• The measured de-tuning angle ? as a function of
  modulating frequency is
• compared to the predicted curve to determine the
  resonance frequency
• as well as the phase offset ???
• Autotrack (locks cavity on frequency)
• Uses the phase offset ???determined above to keep
  the cavity tuned to the operating frequency

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Linac Auto-Phasing

• Purpose Precise cavity phasing is achieved by
  maximizing the Linac energy.
• Arc is used to measure energy changes
• The higher the energy, the heavier it is, the
  harder it is to bend so it travels on the outside
  of the bend
• Phase of an individual cavity is changed by 30.
• Initial beam position and beam position changes
  are recorded.
• Crest phase is found from
• ?0 initial phase setting, y0 beam position at
  ?0, y beam position at ? ?0 ??
• Reproducibility better than 1, Phasing time 2
  min/cavity

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Linac Energy Management (LEM)

• For a given energy at the end of North or South
  Linac and available cavities
• Optimizes the cavity gradient distribution using
  individual cavity characteristics
• Calculates energy profile along the linacs
• Calculates Linac quad values consistent with
  calculated energy profile
• Downloads and sets RF, Quads (including
  hysteresis) and skew quads
• Required input
• Maximum gradient permitted for each cavity
• List of available cavities
• Integrated into the maintenance log
• Most useful number energy overhead available
  (usually 6 10)

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Global Procedures

• Krest (intrusive)
• Modulate the phase of each cavity
• Observe the change in energy at a BPM in the Arc
  where there is dispersion (particles with
  different energies have different orbits)
• Move the cavity phase until energy does not
  change with modulation
• Repeated for each cavity
• Each cryomodule is done by hand at start-up
• Momod (parasitic)
• Modulate the phase of the RF phase reference for
  each Linac using two different frequencies 800
  Hz during accelerator operation
• Move the cavity phase until energy does not
  change with modulation
• Linac is now on crest

34
What is Emittance?

• The beam size in an accelerator can be separated
  into two parts
• The emittance or phase space which is a property
  of the beam
• The beta functions or R matrix elements which
  describe the focusing
• In an accelerator, the emittance is defined by
  the gun characteristics and cannot be improved
  later but it can be degraded
• For CEBAF, emittance is the extent of the beam in
  six dimensions
• X, X, Y, Y, dE, dL
• As the beam is accelerated, the transverse
  emittance should be adiabatically damped (reduced
  proportionally to the energy)
• As the beam is accelerated the transverse energy
  of the beam stays constant but the longitudinal
  energy increases. It is like pulling on a rubber
  band it gets thinner

35
Basic Accelerator Optics

• DIPOLES
• magnets that bend the beam (usually horizontally,
  sometimes vertically and sometimes at an angle)
  They define the shape of the machine
• Higher energy electrons will be bent less (like a
  prism in optics)
• The edges of the beam have fields that are not
  uniform, so they focus as well (like aberrations
  due to non-flat prism faces)
• QUADRUPOLES
• magnets that focus the beam (like a lens in
  optics)
• main difference with an optical lens is that if
  the lens focuses in the horizontal, it will
  defocus in the vertical and vice versa
• Strong focusing if the quadrupoles are
  sufficiently strong (and close together) overall
  they will focus in horizontal and vertical
• the result is rather like a periscope in optics
  and is stable

36
The R Matrix

• The R Matrix relates the output angles and
  positions from a beamline to the input angles and
  positions.
• Xout R11 R12 R13 R14 Xin
• X'out R21 R22 R23 R24 X'in
• Yout R31 R32 R33 R34 Yin
• Y'out R41 R42 R43 R44 Y'in
• The matrix for an uncoupled beamline would have
  the cross-terms zero
• If the beam is kicked horizontally, it will
  oscillate in the horizontal plane but
• should never show a vertical oscillation

37
CEBAF Optics

• LINACS
• A Linac (LINear ACcelerator) is straight no
  bends
• In CEBAF, electrons of very different energies
  travel together
• There is a regular array of equi-spaced
  quadrupoles
• They focus strongly for the lowest energy pass,
  more weakly for the highest pass but still
  strong focusing
• ARCS
• At the end of the Linacs the beams of different
  energies are separated by the spreaders
• There is then a regular circular Arc section
  (actually five of them)
• At the end of the Arcs the beams of different
  energies are brought together by the recombiner

38
Achromatic Isochronous

• Bunches must be transported around the arc and
  all of the electrons must arrive at the other end
  at the same place and time
• Achromatic electrons of different energies
  arrive at the same place
• From the Greek a without, khroma color
• Isochronous electrons of different energies
  take the same time
• From the Greek isos equal, chronos time
• The Arc was designed to be both achromatic and
  Isochronous

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Principle of Isochronicity
Recombiner
Linac
Spreader
Arc
Arc
Recombiner
Linac
Spreader
Isochronous if path length difference in Arcs
equals path length difference in spreaders and
recombiners
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Types of Emittance Increase

• Filamentation
• In the longitudinal plane, RF focusing in
  circular machines causes particles of different
  energies to rotate around the RF bucket at
  different speeds. This can cause an effect like
  an egg beater resulting in real emittance
  dilution. This effect is not relevant for CEBAF
• Longitudinal variation of Emittance
• If the front of the beam is not in the same
  position as the back of the beam, the projection
  of the beam on a screen is apparently enlarged
  (think of the beam as a banana the projection
  is thicker than the cross-section of the banana).
  This effect is usually not a problem at CEBAF.
• Emittance Projection
• If the beam is strongly X-Y coupled, the
  projections of the phase space onto the visible
  axes X, X, Y, Y can all be increased this has
  been a problem at CEBAF.

41
Coupled Beams

• Beam coupling refers to a coupling between
  horizontal and vertical oscillations in the beam
• Under normal (uncoupled) conditions, the
  horizontal and vertical motions of the beam are
  independent
• Note that a quadrupole has the effect of focusing
  in one plane and defocusing in the other plane so
  there is a correspondence between the two planes
• When the beams are coupled, an oscillation in one
  plane couples into the other plane, like two
  coupled oscillators
• This can lead to an apparent increase in the beam
  emittance
• Note that Louivilles theorem says that phase
  space (emittance) is conserved, so the increase
  is only apparent but very real!

42
Uncoupled Emittances
X
X
In the absence of coupling, the product of the
projections of the phase space area on the X and
X axes is a constant
43
Coupled Emittances
X
X
In the presence of coupling, the product of the
projections of the phase space area on the X and
X axes is a never a constant and is usually
much larger than when uncoupled
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Causes of Coupling

• Point coupling in the Injector
• Most likely candidates are the counter-wound
  solenoids, the cryounit and the cryomodules
• Uniform Coupling in the Linacs
• Produced by skew fields in the SRF cavities due
  to asymmetry in the HOM couplers
• Corrected by the skew quadrupoles
• Point Coupling in the Arcs
• Produced by mis-steered beams going through
  fringe fields in dipoles
• Most serious in the spreaders and recombiners
• Goal is to reduce coupling as much as possible

45
Summary

• Keeping the accelerator tuned up is a demanding,
  delicate task
• The operators usually have a Batchelor degree in
  Physics (some are studying for their Masters)
• They are trained on the job for six months to
  become an Operator
• It takes another 2-3 years to become Crew Chief
• They try incredibly hard to deliver quality beam
  to Users
• There are new capabilities all of the time
• We are all learning as we go
• Please give the Operators a break and complain
  to me if you are unhappy
• Thats my job!