1. Andrew Hutton Director of Operations Jefferson Lab
THE SCIENCE BEHIND THE CEBAF ACCELERATOR
Andrew Hutton
Director of Operations
Jefferson Lab

2. Outline Review of the basic accelerator layout Injector Linacs
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

5. Continuous Electron Beam Accelerator Facility
2 gain switched
diode 499 MHz
1 G0 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
Pockels cell A 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 (QE~1%)
Laser light of 840 nm gives strongly polarized beam ~75%
but low QE~0.2%

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 (>70%).

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 δ ~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 Time–Bandwidth 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 δ 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, )
Parity (Independent) G0
Future
HAPPEx2

12. 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

13. 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
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

17. Wave acceleration

18. 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

20. 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 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

21. 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

22. Linac Cryomodules
CEBAF has 42¼ cryomodules with a total active length of 169 meters

23. 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

25. 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)

26. Improving RF arc trip rate
Installed >50 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

27. FSD Trip Rate Versus Energy October 99 – June 01

28. 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

29. 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

30. 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

31. 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

32. 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%)

33. Global Procedures Krest (intrusive) Momod (parasitic)
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
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’, δE, δL
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 ARCS
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

39. Principle of Isochronicity
Recombiner
Linac
Spreader
Arc
Arc
Spreader
Linac
Recombiner
Isochronous if path length difference in Arcs equals
path length difference in spreaders and recombiners

40. 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 Louiville’s 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

44. 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 Master’s)
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
That’s my job!