progress status of beam-based alignment in the undulator ......rf-gun acc1 acc2 acc3 acc4 acc5 bc2...
TRANSCRIPT
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Progress status of beam-based alignment in the undulator section
P.Castro
ACC1 ACC2 ACC3 ACC4 ACC5RF-GUNBC2 BC3
BeamACC6
BYPASS
UNDULATORDUMP
COLLIMATOR
ACC7 SEED
Beam
FLASH
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• motivation• principles of BBA• results of quad.-monitor offset• remanent dipole field in quads• undulator deflection
Overview of the talk:
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Goal:align all quadrupoles between undulator modulesto get straight trajectory in undulator section
to increase the overlap between electrons and photonsso that the SASE process can take place in the whole undulator section
1) Motivation
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required SASE 400 SASE 500 SASE 600 SASE 700 SASE 800 SASE 900
I2,Z, RMS2 Tmm2 < 30 4 1.2 4.2 1.7 7.9 9.8
I2,Y, RMS2 Tmm2 < 30 7 8.2 6.9 7.0 3.2 2.2
30 Tmm2 corresponds to 10μm at 1GeV
4.5 m
Undulator fabrication tolerances
Measurement by J.Pflüger et al.
beam path = double field integral
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DESY Jahresbericht 1999 Measurement by J.Pflüger et al.
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beam
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orbit simulation for quadrupole misalignment of 0.1 mm RMS
10 seeds (horizontal plane)
for so-called optics v4 ( with k1=7.14 m-1 and k2=-6.14 m-1 )
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beam
BPM BPMBPM
BPM
BPM
BPM
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2) BBA in undulator: steps (overall plan)
1) “ballistic alignment”, i.e., align quadrupoles to the beam
2) measure dispersion created ONLY in undulator section and correct it
it needs to measure relative offset between quadrupoles and BPM (or wire-scanners)
it needs masking of incoming disperionand needs very precise BPMs
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beam
BPM
Principle of “ballistic alignment”(Note: it works if there is ONLY pure quadrupole fields)
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measure beam positionand move quad
BPM
Principle of “ballistic alignment”
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measure beam positionand move quad
BPM
Principle of “ballistic alignment”
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measure beam positionand move quad
BPM
Principle of “ballistic alignment”
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measure beam positionand move quad
BPM
Principle of “ballistic alignment”
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measure beam positionand move quad
all quads are steering free
no dispersion generated by quads
BPM
Principle of “ballistic alignment”
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An alternative procedure: with quads OFF
beam
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Principle of “ballistic alignment”
it needs to measure relative offset between quadrupoles and BPM (or wire-scanners)
(with more precision than achieved with survey instruments)
we have done it using “quadrupole beam-based alignment” (BBA)
(Note: it works if there is ONLY pure quadrupole fields)
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Beam
x
x
Gradient<0
Gradient>0
Gradient<0
Gradient>0
Principle of quad. BBA
quad center
BPM
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Beam
x
Beam
x
x
Gradient<0
Gradient>0
Gradient<0
Gradient>0
Principle of quad. BBA
Gradient<0
Gradient>0
quad center
quad center
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Beam
x
Beam
x
BeamX=0
x
Gradient<0
Gradient>0
steering free
Gradient<0
Gradient>0
Principle of quad. BBA
quad center
quad center
quad center Gradient>0
Gradient<0
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0.6 A
-1.4 A
-3.4 A
-5.4 A 0.059 ± 0.005 mm
Example ofquad BBA
beam UND1
BPMWS
Q21
UND2
BPM
UND3
BPM
UND4
BPM
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beam UND1
Q21
UND2 UND3 UND4
Therefore,I always requestgood BPMsalso downstreamthe undulator
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0.6 A
-1.4 A
-3.4 A
-5.4 A 0.059 ± 0.005 mm
Example ofquad BBA
beam UND1
BPMWS
Q21
UND2
BPM cross: beam position with respect toquadrupole reference
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Vertical beam profile measured with wirescanner near quad
position of the wire [mm]-0.023 ± 0.020 mm
Gaussian fit
we measure the beam position with respect to wirescanner reference
advantage of wirescanner:- no scale error in the full range (±4.5 mm)
recommended: on-crest acceleration to have symmetric (Gaussian) profiles
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0.6 A
-1.4 A
-3.4 A
-5.4 A 0.059 ± 0.005 mm
Example ofquad BBA
beam UND1
WS
Q21
-0.023 ± 0.020 mmwirescan
OFFSET
Δ = -0.167 ± 0.020 mm
offset between:- wirescanner reference and - quad position reference
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Summary of the quad. BBA in undulator
are the measurements reproducible?are the offsets quad-to-wirescanner stable?
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horizontal
vertical
Oct.2005
Jan.20069.Jun.0610.Jun.06 Aug.2006
Measuredoffsetbetweenquad. andwirescanner
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Possible sources of errors:• beam angle at wirescanner
beam
with quads off the systematic error is smaller
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Possible sources of errors:• quad. magnetic center “moves” as function of its current
quad
. mag
neti
c ce
nter
quadrupole
Measurement by Y.Holler et al.
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quadrupole field (transversally)
quad field + dipole field = quad field (shifted)
x0
By x0
By
x0
By
By (x) = g x BoBy (x) = g x + Bo
g is function of current
center: 0 ? – Bo/g
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Measurements of the quad center vs current (on spare quad)
quad field + dipole field = quad field (shifted)
x0
By x0
By
x0
By
fit function: xmeas = xo – Bo/g
“constant” quad. offset
xo
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x-33
0
By
Typical quad field
10 A
Measurement by Y.Holler et al
x
yz
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x-33
0
By
0 A
Measurement by Y.Holler et al
x
yz
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x-3 30
By
-1 A
dipole field
Measurement by Y.Holler et al
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-1 A
Measurement by Y.Holler et al
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“moving” quad. center
remanent dipole field when large current is applied
after degauss B = 0
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Is this remanent field the same for all quadrupoles ? NO
How large is the remanent dipole field in the quadrupoles INSTALLED ?
beam UND1
BPM
UND2
BPM
UND3
BPM
UND4
BPM
Q6 to correct back
Q5 to +100, then -1.9 A (quad. field compensated)
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black orbit: Q5 quads are degaussed reference orbitred orbit: Q5 quads set to +100 A then to -1.9 Agreen orbit: Q6 moved to correct orbit to reference
beam UND1
BPM
UND2
BPM
UND3
BPM
UND4
BPM
Q6 to correct back
Q5 to +100, then -1.9 A (quad. field compensated)
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Quadrupole shift applied to compensate remanent dipole field
red: after cycling to 100 A then to -1.9 Agreen: after cycling to -100 A then to +1.9 A
two measurements:
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Q5UND1
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Q5UND2
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Q5UND3
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Q5UND4
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Q5UND5
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Q6UND1
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Q6UND2
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Q6UND3
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Q6UND4
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Q6UND5
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from the position shifts of the quadrupole:
The integrated remanent dipole field in the quadrupole
(shown in previous slide)
relationship:
the remanent integrated quadrupole field:
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Measured remanent dipole field in undulator dipoles
RMS = 0.08 T.mmmax B = 0.11 T.mm
field amplitude:
statistically, there isno differencebetween x and y plane
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SIMULATION: effect of remanent dipole fields in quads
for k=0 (quadrupole gradient = 0)
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SIMULATION: effect of remanent dipole fields in quads
for optics “variant 1” (k1 = 10.9 m-1 ,k2 = -10.6 m-1)
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Recommendation: degauss the quadrupoles in the undulator
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In the past we tried several times to get beam through undulator:
- with quadrupoles off
- with quadrupoles off + degaussed
…unsuccessfully.
only when horizontal undulator steerers are used
to get beam through the undulator with quads off and degaussed:
5) Beam deflection in the undulator section
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hor. steerers degaussed and off
quads degaussed and off
steering-free quads
quads off
z [m]
beam
minimize beam angle at undulator entrance
August 2006
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smallest beam angle at undulator exit: for I = -3.6 A
wirescanner results:
• wire 5UND6 has an offset -1.6 mm w.r.t. the other wires • wires have systematic offset 0.4 mm (with a std.dev. 0.1 mm)
• beam parallel to undulator axis very small incoming beam angle
it looks like that:
BPM 9EXPwirescanners
• same deflection in each undulator segment
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smallest beam angle at undulator exit: for I = -3.6 A
steerer effect: 50 µrad / 1 A (at beam energy of 680 MeV)
integrated dipole field = 0.113 T.mm/A = 1.13 G.m/A
for -3.6 A integrated dipole field = 0.41 T.mm = 4.1 G.m
average field = 0.41 T.mm / 5 m = 82 µT (or 0.82 G)
(next slide)
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Measurement of undulator steerer strength (with wirescanners)
this steerer ΔI = 1 Adeflection = 50±1 µrad
steerer effect: 50 µrad / 1 A (at beam energy of 680 MeV)
integrated dipole field = 0.113 T.mm/A = 1.13 G.m/A
calculated using magnetic model (undulator+steerer) : 0.114 T.mm/Aby M. Tischer
fit
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smallest beam angle at undulator exit: for I = -3.6 A
steerer effect: 50 µrad / 1 A (at beam energy of 680 MeV)
integrated dipole field = 0.113 T.mm/A = 1.13 G.m/A
for -3.6 A integrated dipole field = 0.41 T.mm = 4.1 G.m
average field = 0.41 T.mm / 5 m = 82 µT (or 0.82 G)
(next slide)
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required SASE 400 SASE 500 SASE 600 SASE 700 SASE 800 SASE 900
I2,Z, RMS2 Tmm2 < 30 4 1.2 4.2 1.7 7.9 9.8
I2,Y, RMS2 Tmm2 < 30 7 8.2 6.9 7.0 3.2 2.2
30 Tmm2 corresponds to 10μm at 1GeV
4.5 m
Undulator fabrication tolerances
Measurement by J.Pflüger et al.
beam path = double field integral
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SIMULATION: effect of -3.6 A on ALL undulator steerers (in a drift space)
for k=0 (quadrupole gradient = 0)
beam on axis
angle > 1 mrad
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SIMULATION: effect of -3.6 A on ALL undulator steerers (in a drift space)
for k=0 (quadrupole gradient = 0)
beam offset = 2 mm , angle = -0.5 mrad
vacuum chamber
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Wire position range with part of the beam seen:
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recommendation: apply -3.6 A to all undulator steerers
Outlook:next step of BBA: measure dispersion created in undulator
magnetic measurement of undulator segment in tunnel (by J. Pflüger et al)
improve BPM resolution (by N. Baboi et al)
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This work would not have been possible without the help of:
B. FaatzM. SachwitzO. HenslerN. BaboiD. NölleJ. ThomasU. HahnM. TischerY. HollerL. GumprechtM. Blöcher
and many more…
THANK YOU !