Fermilab Snapback Workshop

1
Mini Workshop OnDecay And Snapback In
Superconducting Magnets 

• Decay and Snapback Compensation in the Tevatron
• Mike Martens
• November 8, 2002

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Tevatron Parameters
Synchrotron with superconducting magnets. Collide
36 proton bunches on 36 pbar bunches. Radius
1 km Energy 150 Gev to 980
Gev FODO lattice bmin 30 m, bmax 100 m
Run with tunes at 20.575 (vertical) and 20.585
(horizontal). h 1113 RF frequency 53.14
MHz Bucket spacing 18.8 nsec Low Beta
regions at B0 and D0. b is 35
cm Electrostatic separators to separate the
proton and antiproton orbits. 772 dipole magnets
with B 4.4 Tesla _at_ 1000 GeV.
3
Standard Cell in the Tevatron Lattice
F
D
TQF
Horz BPM
TQD
Vert BPM
TSF
TSD
Tevatron Dipole
Tevatron Quad corrector
(There are 772 Tevatron dipoles)
Tevatron Sextupole corrector
Tevatron Quadrupole
Tevatron Beam Position Monitor
F
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Tevatron Ramp Cycle
Low beta Un-squeeze
Inject porotns pbars
980 Gev flattop
Low beta Squeeze
Tev Energy
90 Gev Reset
150 Gev Front Porch
150 Gev Back porch
Time

• Typical times for Tevatron store
• 150 Gev Front porch 2 hours
• 980 Gev Flattop 12-24 hours
• 150 Gev Back Porch 1 minute
• 90 Gev Reset 20 seconds

• Typical times for dry squeeze
• 150 Gev Front porch 10 minutes
• 980 Gev Flattop 15 minutes
• 150 Gev Back Porch 1 minute
• 90 Gev Reset 20 seconds

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Tevatron Ramp Reset Cycle
Normal sequence Remove colliding beam at 980
Gev and at low beta. Low beta unsqueeze Ramp Tev
from 980 Gev down to 150 Gev. Ramp down to 90
Gev, then back to 150 Gev. Ramp up to 980
Gev. Low beta squeeze. Wait for 15 minutes. Low
beta unsqueeze. Ramp down to 150 Gev. Ramp down
to 90 Gev, then back to 150 Gev. Inject protons
and pbars for next store. Accelerate beam to 980
Gev. Low beta squeeze. Store beam in collisions
for 12 to 24 hours.
Dry squeeze
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Tevatron Ramp Reset Cycle
After Tevatron quench (or turn off) Ramp six
times (no low beta squeeze) (150 Gev gt 980
Gev gt 150 Gev gt 90 Gev gt150 Gev) Ramp up to
980 Gev. Low beta squeeze. Wait for 15
minutes. Low beta unsqueeze. Ramp down to 150
Gev. Ramp down to 90 Gev, then back to 150
Gev. Inject protons and pbars for next
store. Accelerate beam to 980 Gev. Low beta
squeeze. Store beam in collisions for 12 to 24
hours.
Dry squeeze
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Tevatron Ramp Reset Cycle
(Future) Normal sequence (No dry
squeeze) Remove colliding beam at 980 Gev and at
low beta. Low beta unsqueeze Ramp Tev from 980
Gev down to 150 Gev. Ramp down to 90 Gev, then
back to 150 Gev. Ramp up to 980 Gev. Low beta
squeeze. Wait for 15 minutes. Low beta
unsqueeze. Ramp down to 150 Gev. Ramp down to 90
Gev, then back to 150 Gev. Inject protons and
pbars for next store. Accelerate beam to 980
Gev. Low beta squeeze. Store beam in collisions
for 12 to 24 hours.
Dry squeeze
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Tevatron Ramp Cycle
980 Gev
Current (amps)
150 Gev
90 Gev
Time (secs)
4440 Amps 1 Tev
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Tevatron Ramp Cycle
980 Gev
Start of ramp
Current (amps)
Time (secs)
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Chromaticity during operations
High chromaticity results in some beam loss on
ramp (15). Afraid to reduce chromaticity
because of possible transverse instabilities. Kee
p chromaticities at 8 units (horz and vert) on
front porch Increase chromaticites to 12 units
just before ramp. Keep chromaticity at 15 to 20
units on the ramp. Chromaticity drifts are
created by drifting b2 (sextupole) fields in
dipoles. b2 compensation scheme keeps
chromaticity constant at 150 Gev and on the
snapback (sort of.) Future Use transverse
dampers and reduce chromaticity to 2-3
units. Eliminate dry squeeze cycle to reset
sextupole fields. Improve tune and chromaticity
control during snapback.
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Chromaticity in the Tevatron

• Chromaticity is defined as

where
and is a result of sextupole fields
where
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Chromaticity in the Tevatron

• Relation between sextupole fields as b2 harmonic.

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Chromaticity in the Tevatron

• Relation between b2 and chromaticity.

at 150 Gev
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b2 compensation in the Tevatron

• Changes in sextupole from b2 are corrected using
  the chromaticity correction sextupoles TSF and
  TSD.
• On the front porch the b2 compensation is a
  logarihmic function of time at 150 Gev
• b2 b2i m ln(t c).
• At the start of the ramp (the snapback) the b2
  compensation is a quartic function of time
• b2 b2 (at start of ramp) 1 - 2(t/T)2 (t/T)4
   .

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b2 compensation in the Tevatron

• Parameters of b2 compensation are a function of
  flattop time, TFT, and back porch time, TBP
• b2i -.04 ln(TBP/60)
• - .161 - .0277 ln(TBP /60) ln(TFT)
•  
• m .342 - .02082ln(TBP) - ln(TFT)
•  
• c 0
• The b2 snapback time T is 6 seconds.

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Chromaticity in the Tevatron

• The total chromaticity has several components
• ?Total ?Natural ?Dipoles ?Sext corr
  ?Measured
• Or
• ?Dipoles ?Measured - ?Sext corr - ?Natural
• ?Natural -29 units (from optics calculations)
• ?Dipoles ?b2 drift ?b2 geometric
• ?Sext corr ?TSF ?TSD ?CSFB2 ?TSDB2
  

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Estimated b2 drift in Tevatron
Measurement of chromaticity as function of time
at 150 Gev with the chromaticity compensation
active
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Estimated b2 drift in Tevatron
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Estimated b2 Drift in Tevatron
Estimate the b2 drift in dipoles from amount of
current needed in the chromaticity sextupoles.
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Estimated b2 Drift in Tevatron
Estimated b2 correction on front porch
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Comparison of b2 from Tevatron and magnet
measurements
Estimated b2 correction on front porch compared
to magnet measurements
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Comparison of b2 from Tevatron and magnet
measurements
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Comparison of b2 from Tevatron and magnet
measurements
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Estimated b2 Drift in Tevatron
Estimated b2 correction on front porch
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Comparison of b2 from Tevatron and magnet
measurements
Estimated b2 correction on ramp compared to
magnet measurements
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Tevatron tunes
Tune Shift of a pbar bunch from 2 head on
collisions
Tunes Keep tunes at 20.575 (vertical) and 20.585
(horizontal) Keep tunes constant to within 0.002
on front porch and at low beta Keep tunes
constant to within 0.005 during ramp.
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Tevatron tune drift
Measured tune drift at 150 Gev
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Tevatron coupling drift
Measured coupling drift at 150 Gev
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Orbit offset in SF and SD
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Orbit offset in Dipoles
This means that there must be an average
horizontal offset of 0.632 mm in the Tevatron
dipoles if the time varying sextupole fields are
to explain the horizontal tune drift at 150 Gev.
If the feeddown hypothesis explaining the tune
drift is correct then the TSF and TSD circuits
contribute 8 and 13 to the horizontal tune
drift respectively while the dipoles contribute
79 to the horizontal tune drift.
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Orbit offset in Dipoles
This means that there must be an average
horizontal offset of 0.923 mm in the Tevatron
dipoles if the time varying sextupole fields are
to explain the vertical tune drift at 150 Gev. If
the feeddown hypothesis explaining the vertical
tune drift is correct then the TSF and TSD
circuits contribute 2.6 and 26 to the vertical
tune drift respectively while the dipoles
contribute 76 to the vertical tune drift.
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Orbit offset in Dipoles
Measured horizontal orbit (11/6/02) Average
offset is 1.2 mm in BPMs at SF magnets
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Orbit offset in Dipoles
Figure 8 Vertical DFG settings at 150 Gev in
mrad. At 980 Gev the maximum current allowed in
the dipole correctors (/- 50 amps) corresponds
to a corrector angle of /- 0.1265 mrad at 980
Gev. Thus we see that several correctors are
running at too high of a value to be scaled
directly up the ramp to 980 Gev. We also see a
systematic offset in the DFGs in the E2-3, A1-2,
and B1-2 sectors of the Tevatron. This is
probably due to rolled dipoles in these regions
caused by movement of the tunnel floor in these
regions of the Tevatron.
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Orbit offset in Dipoles
With dipoles rolled by 1.4 mrad, there will be a
0.56 mm maximum vertical beam offset in the
dipoles which leads to coupling drift from the
drift in b2
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Future

• Better measurement of ? during snapback
• Understand Tevatron lattice
• Comparison to magnet measurements
• Re-visit b2 drift as function of flattop time and
  back porch time
• More precise control of ?
• Use reference magnet?