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Synchrotron Talk
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An Introduction to theISIS Synchrotron
Chris Warsop
Outline
• Introduction
• Acceleration and Trapping
• Transverse Focusing and Injection
• Extraction
• The Challenges of a High Intensity Machine
What does the Synchrotron do for ISIS?
• On Target Require (with 10 Hz Variations for Target 2)
2.5x1013, 800 MeV Protons, in < 1 μs pulse @ 50 Hz
3.7x1013, 800 MeV Protons, in < 1 μs pulse @ 50 Hz
• Linac Provides
2.8x1013, 70 MeV H-, in 250 μs pulse @ 50 Hz
4.0x1013, 70 MeV H-, in 250 μs pulse @ 50 Hz
• Synchrotron
Compresses Pulse Length: 250 → 1 μs
Accelerates Beam: 70 → 800 MeV
52 m
Basic Principles• Acceleration
Circular Machine
Repeated Use of Relatively Low RF Acceleration Voltage
ISIS Ring: Max ~140 kV per turn, (163 m), but over 104 turnsISIS Linac: ~ 70 MV in a single pass (50 m)
Synchrotron Principle
Keep Beam on Constant Bend RadiusVary Confining Field and Particle Momentum in Synchronism
• Compression
Multi-Turn Charge-Exchange Injection, Bunching and Acceleration
Motion in a Dipole Magnet - Uniform Field
BveF
v
BF
Be
P
Bevmv
2
• For particle momentum P, charge e, in uniform field B
Trajectory is Circular Arc, Radius
The Synchrotron
teBP
21
2
01
tBe
cm
c
Ltrev
dt
tdBe
dt
dP dt
tdBce
dt
d ][
• Many Parameters Determined by
Main Dipole Magnet Field Variation with Time: B[t]
dt
tdBeLturn
][
0.005 0 0.005 0.01 0.015Times0.2
0.3
0.4
0.5
0.6
0.7
dleiFT
Dipole Field vs Time
Magnetic Field on ISIS ….
]2[ tfCosBBtB magacdc
fmag = 50 Hz
Binj = 0.176 T
Bext = 0.697 T= 7.002 m
• Main Magnet Field is a Biased 50 Hz Sinusoid
• Allows Energy Recovery in Magnet Power Supply
• Main Dipoles and Quads all in series (White Circuit)
Acceleration
RF Acceleration
• Pass particles through sinusoidally excited RF cavity• Gain energy ε from field across accelerating gap
EeF
]2[][ 0 tfSinVtV RF
]2[0 tfSineV RF ][. tVesEesF
E
dt
tdBeLturn
][
Needs to match energy gain requirements
Acceleration in a Ring
tfSineVdt
tdBeL RFturn 2
][0
• Lock RF frequency to a harmonic of revolution frequency
• Define ideal “Synchronous” particle, which follows
• This particle sees the same phase of RF on every turn
- surfs on wave propagating around machine
• Only two points on the wave where particle may receive correct energy
- Synchronous Phase
0 0.2 0.4 0.6 0.8 1Timeus10
5
0
5
10
dleiFVk
Accelerating Field vs Time
Acceleration Parameters on ISIS
• Two bunches on ISIS REVRF ff 2
• Using relations above with ISIS B[t], get
]2[0 tfSineV RF dt
tdBeLturn
][
21
2
012
tBe
cm
L
cfRF
0 0.002 0.004 0.006 0.008 0.01Times
1.5
1.75
2
2.25
2.5
2.75
3
FR
ycneuqerFzHM
RF Frequency Law
0 0.002 0.004 0.006 0.008 0.01Times0
20
40
60
80
stloVnruT
Vk
Minimum Volts per Turn Required
ISIS Ring RF System Details
• Harmonic Number 2 → 2 bunches
• 6 Tuned Resonant Cavities– Sweep 1.3-3.1 MHz
– 0 - 26 kV per cavity
– Supply ~150 kW of beam power
• Ferrite Loaded Coaxial Resonator– Bias current 200-2000 A
• RF Driver– Two 250 kW tetrodes per cavity
• Multiple Control Loops– Frequency
– Voltage
– Beam position, current etc.
• 4 additional cavities for DHRF
Finite Bunch Length, Oscillations in the Bunch
• What about particles not at correct phase?– i.e. non synchronous particles?
• Depends on Revolution Time– variation of velocity with P– variation of path length with P
• On ISIS obeys ‘common sense’– Higher energy particles take less time – Lower energy particles take more time
2 4 6
-0.02
-0.01
0.01
0.02
Graphics
2 4 6
-1
-0.5
0.5
1
RF Phase →
RF
Vol
ts →
P E
rror
→
The ISIS Machine Cycle
0.005 0 0.005 0.01 0.015Times
0.2
0.3
0.4
0.5
0.6
0.7dlei
FT
Dipole Field vs Time
Extraction
Acceleration
Trapping
Injection
Bunch Size : Stable Regions
• Injected beam is effectively unbunched – Fills the machine circumference
• Phase Stable region shrinks as dB[t]/dt increases• Most particles trapped in stable regions
– Not all Trapping Loss!
• Have simplified processes– Complicated Trajectories– Space Charge 2 4 6
-0.02
-0.01
0.01
0.02
2 4 6
-0.02
-0.01
0.01
0.02
2 4 6
-0.02
-0.01
0.01
0.02
2 4 6
-0.02
-0.01
0.01
0.02
2 4 6
-0.02
-0.01
0.01
0.02
2 4 6
-0.02
-0.01
0.01
0.02
RF Phase →
P E
rror
→
Dual Harmonic RF Upgrade
• Add More RF Cavities– Run at 2 x frequency– Half the volts– Carefully Phased
• Enlarge Stable Region– Optimise Trapping– Reduce Loss -3 -2 -1 0 1 2 3
-2
-1
0
1
2
-3 -2 -1 1 2 3
-60000
-40000
-20000
20000
40000
60000
-3 -2 -1 0 1 2 3
-2
-1
0
1
2
-3 -2 -1 1 2 3
-40000
-20000
20000
40000
P E
rror
→
RF Phase →
Single Harmonic
RF Phase →
Dual Harmonic
New DHRF Cavities
6 h=2 cavities
2 h=4 cavities
2 h=4 cavities
Some Measurements of Longitudinal Motion
Injected Chopped Beam (100 ns, 1/15 of circumference)
RF Off RF On
Transverse Motion• Beam consists of many particles, not all aligned with reference orbit
– small angular spread about beam direction
• Need to focus beam to prevent it diverging– hitting vacuum vessel
• Quadrupole Magnet provides the focusing required
Beam with angular spread
Focusing Elements
Focused Beam
Unfocused Beam
Motion in a Quadrupole – Linear Focusing Field
BveF
v
BF
yxgveF
xygB
,
,
• Linear force with transverse displacement– Focusing in one plane– Defocusing in the other
• Arrange Alternating Channel– Overall Focusing in both planes
Form a Stable Focusing Channel
0 2.5 5 7.5 10 12.5 15
0
0.02
0.04
0.06
0.08
• Design repeating pattern of magnets for optimal stability (lattice)• "Modified" Simple Harmonic Motion
– distorted sinusoidal oscillation about equilibrium orbit
• Beam larger in focusing elements → overall focusing• ISIS Synchrotron Lattice
– Horizontal → [QD QF QD BF] x 10 (plus trim quads)– Vertical → [QF QD QF QD] x 10
Bea
m W
idth
→
Distance along beam axis →
Horizontal
Vertical
Lattice and Main Magnets
• 10 super-periods– 16.3 m long
• Main Dipole– 4.4 m long– 0.16 – 0.69 T
• Main Quads– Doublet & Singlet– 0.7 m long– 0.8 – 3.0 T/m
• Half Apertures– ~ 60 mm x 80 mm
• IAC = 400 A, IDC = 660 A• Power 1.8 MW• Also 20 Trim Quads
– Programmable
Particle Motion and Formation of a Beam
0 25 50 75 100 125 150Distance Around Machinem-0.15
-0.1
-0.05
0
0.05
0.1
0.15
esrevsnarT
tnemecalpsiD
mParticleTrajectories
0 25 50 75 100 125 150Distance Around Machinem-0.15
-0.1
-0.05
0
0.05
0.1
0.15
esrevsnarT
tnemecalpsiD
mParticle Trajectories
0 25 50 75 100 125 150Distance Around Machinem-0.15
-0.1
-0.05
0
0.05
0.1
0.15
esrevsnarT
tnemecalpsiD
mParticle Trajectories
0 25 50 75 100 125 150Distance Around Machinem-0.15
-0.1
-0.05
0
0.05
0.1
0.15
esrevsnarT
tnemecalpsiD
mParticle Trajectories
0 25 50 75 100 125 150Distance Around Machinem-0.15
-0.1
-0.05
0
0.05
0.1
0.15
esrevsnarT
tnemecalpsiD
mParticle Trajectories
-0.01 -0.005 0 0.005 0.01Transverse Displacement
-0.002
-0.001
0
0.001
0.002
elgnA
Oscillationin Phase Space
• Single Particle TrajectoryOscillation about equilibrium orbit
• Number of Oscillations per turn– Q Value– On ISIS QH=4.31, QV=3.83
• Avoid Integer Q– Stability
• Beam formed by– many particles (~1013)– incoherent oscillations
• Maximum Extent of beam– Beam Envelope
• Oscillation in (x,x') space
Tra
nsve
rse
Ang
le
Transverse Position
Beam Self-Field for Uniform Charge Distribution
0 0.05 0.1 0.15 0.2rm0
2000
4000
6000
8000
10000
rEVm
0 0.05 0.1 0.15 0.2rm0
210-6
410-6
610-6
810-6
0.00001
0.000012
BT
BvEeF
ca
reB
202
22
0
2
12
a
reBeveEF zrr
• High Intensity Beam – must allow for beam's own field
• Simplest case– long uniform cylinder of charge– moving at velocity βc
c
2
0
1
2 a
reEr
ar
• Gauss/Ampere Law• Defocusing Radial Force
– Strong energy dependence
• At low energy affects stability– Charge distribution is important– Take great care to control it
Relationship BetweenTransverse Particle Motion and Charge Distribution
12
34
56
-1-0.75
-0.5
-0.25
0.250.5
0.75
1
12
34
56
-1
-0.50.51
• Transverse Motion– Modified SHM
• Average Charge Density– Depends on amplitude distribution– Avoid small amplitudes
• Optimise Amplitude Distribution– Approach Uniform
• Injection Process Crucial– Determines initial distribution
particles with large amplitudes
-0.5 0 0.5 1
1
2
3
4
5
-0.5 0 0.5 1
1
2
3
4
5
particles with small amplitudes
Transverse Profile
Par
ticl
e D
ensi
ty →
Transverse Axis Transverse Axis
Transverse Axis Transverse Axis
Bea
m A
xis
→
Corresponding Trajectory
Injection – Simplest Model
5 10 15 20 25
-1
-0.75
-0.5
-0.25
0.25
0.5
0.75
1
5 10 15 20 25
-1
-0.75
-0.5
-0.25
0.25
0.5
0.75
1
5 10 15 20 25
-1
-0.75
-0.5
-0.25
0.25
0.5
0.75
1
Injection Septum Magnet
Injected Particles
Machine Circumference
Tra
nsve
rse
Dis
plac
emen
t
Simple Injection with a Bump ~ a few turns
Injection Septum Magnet
Injection Bump Magnets
• Use Magnet Bump– Equilibrium Orbit near Septum
during Injection (reasonable amplitudes)
– Remove bump after Injection so particles miss Septum
• OK for injecting a few turns• Not for the 100's required on ISIS
H- Charge Exchange Injection ~ many turns
• Inject an H- Beam
• Bring together H- with circulating beam in dipole
• Strip to H+ with foil (~2% loss H-, H0)
• Circulating beam Passes through foil (20 times)
• Inject on top of circulating beam over 100's of turns
• Allows HI Beam! Injection Bump Magnets
Stripping Foil
2 e- + H
H
H
Re-circulating H
Injection Painting
5 10 15 20 25
-1
-0.75
-0.5
-0.25
0.25
0.5
0.75
1
5 10 15 20 25
-1
-0.75
-0.5
-0.25
0.25
0.5
0.75
1
5 10 15 20 25
-1
-0.75
-0.5
-0.25
0.25
0.5
0.75
1
5 10 15 20 25
-1
-0.75
-0.5
-0.25
0.25
0.5
0.75
1
• To introduce a range of oscillation amplitudes during injection:
1. Vary Injection Point 2. Vary Equilibrium Orbit
• On ISIS do both1. Vertical Plane2. Horizontal Plane
• Inject current at constant rate– Vary amplitude non linearly
with time
1. Vary Injection Point
2. Vary Equilibrium Orbit
Injection on the Falling Magnet Field
• Inject before field minimum
• Field is 'too high'– Beam circulates on smaller orbit
• As field drops, orbit moves out– At 0.0 ms is on design orbit
• Inject at one point on inside radius– Movement of orbit gives painting
• Best configuration:– Painting process exploits B[t]
– Gives more time for trapping
Inject
Eqm Orbit Moves
Out
0.005 0 0.005 0.01 0.015Times
0.2
0.3
0.4
0.5
0.6
0.7
dleiFT
Dipole Field vs Time
Extraction
Acceleration
Trapping
Injection
Injection Details
• Accumulate 2.8x1013 protons over 150 turns
• Stripped with alumina foil – 0.3 μm thick
– 120 mm x 40 mm
– 98% efficient (2% H0, H-)
– Protons pass through foil ~20 times
• Injection Bump Magnets
– Single turn, 14 000 A
– Pulsed (45 mr)
• Septum– 6 turn, 4000 A
– DC (285 mr)
Extraction of Beam
• At Extraction– Protons Circulating at 800 MeV (=0.84)– 3.2 kJ per pulse– Two bunches with 200 ns gap
• Extraction System – 3 fast kicker magnets
deflect the beam into …
– a septum magnet
which lifts it into the EPB
• Kickers need to be fast to avoid beam loss– Go from zero to full field between passage
of bunches
Extraction Details
• Kickers– 3 units give 15 mr kick
– 0 to 0.04 T in < 210 ns
– Single turn, 5000 A
– 0.5 m long
• Septum – ~ 8 m downstream
– 8 Turn, 8900 A
– DC
– 1.8 m long (21 degrees)
– Lifts beam out of machine
Measurements of Transverse Motion
Transverse oscillation observed at one point in ring over 30 turns
Measurements of Transverse Motion
-150 -100 -50 0 50 100 150-1
0
1
2
3
4
5
6
7
8
9High Intensity Horizontal Scan: R5HPM1, 2.5E13 ppp
Position (mm)
Det
ecto
r S
igna
l (V
)
-100 -50 0 50 100
-400
-200
0
200
400
600
800
1000
1200
1400
High Intensity Horizontal Scan: R5HPM1, 2.5E13 ppp
Position (mm)
Tim
e (
us
)
-150 -100 -50 0 50 100 150-1
0
1
2
3
4
5
6
7
8
9High Intensity Vertical Scan: R6VPM1, 2.5E13 ppp
Position (mm)
Det
ecto
r S
igna
l (V
)
-100 -50 0 50 100
-400
-200
0
200
400
600
800
1000
1200
1400
High Intensity Vertical Scan: R6VPM1, 2.5E13 ppp
Position (mm)
Tim
e (
us
)
Development of Horizontal and Vertical Profiles -0.5 – 1.5 ms
Why is a High Intensity Machine Difficult to Run?
• Must Have Tight Loss Control on a High Power Beam– Mean Power: Injection 16 kW → Extraction 160 kW
– Fractional losses must be low
– Minimal Activation for Maintenance
– Prevent damage
• Losses on a High Intensity Beam are not easy to control!– Complicated loss mechanisms which are very sensitive to many parameters
– E.G. Beam Distributions, Correction Dipoles, Quadrupoles, RF, Linac, etc.
• Some important aspects of beam physics are not yet fully understood
Ring Tuning and Loss Control
• Loss control– Absolute levels
– Location
– Time ~ i.e. Particle Energy
• Monitoring– 40 Beam Loss Monitors
– Beam Toroids
– 5 Profile Monitors
– 30 Position Monitors
– Wire scanners, scintillators …
• Optimising Handles– Correction Dipoles (14)
– Correction Quadrupoles (20)
– Injection System
– RF System
– Extraction System
– Linac
• Many 100's of parameters
Diagnostics ~ Some Machine Details
So What’s New on a 20 year old Machine?
• Plenty of R&D Underway– Important for ISIS ~ to achieve optimal running with major upgrade – Important for New Machines
• Main Topics– Longitudinal Trapping i.e. DHRF upgrade and optimisation– Transverse Space Charge and Related Losses– Instabilities– Loss Control
• Involves– New Diagnostics & Measurements– Computer Simulation/Theory– Improved Beam Control and Manipulation (Software/Hardware)
• Paves the way for MW ISIS Upgrades
Closing Comment …
• First injected beam into the ring in 1984
– 20 years on ISIS is still a world leader
• Enormous credit to those who designed, built, commissioned machine
– Few things stay ahead for so long …
• Still upgrading, improving … to get to 0.24 MW
• Next steps 1 MW, 4 MW …