Crossing transition at RHIC

1
Crossing transition at RHIC

• V.Ptitsyn, N.Abreu, M. Brennan, M.Blaskiewicz, W.
  Fischer, C. Montag, R. Lee, S.Tepikian

2
RHIC transition crossing parameters
Unfortunately the RHIC lattice design could not
avoid the transition crossing. Making the
transition energy less that the injection energy
would require too long dipole magnets and too
large dispersion function. Except protons, all
RHIC species has to cross the transition during
the acceleration ramp. Low acceleration rate of
superconducting RHIC makes the transition
crossing more challenging.
Here the parameters relevant for the transition
crossing of gold ions in Run-7
3
Choice of a1
Importance of nonlinear chromatic effect at RHIC
transition is stressed by the large value of
nonlinear time. During that time there are
particles in the beam which undergo the unstable
motion because the time of the transition
crossing depends on the particle momentum. J.Wei
studies on the RHIC design stage showed that
(without gt jump) 70 of the beam will be lost
and more than 60 emittance increase is expected.
Calculated a1 dependence on lattice choice
(different b) and chromaticities
a1 depends both on the linear lattice and on the
chromaticity. In order to eliminate this
chromatic effect one needs a1
-1.5. Unfortunately, b3m lattice brings the
dynamic aperture problem. Presently, b5m
lattice is used at the transition with a1
-0.3 Measurements of a1 showed a good
agreement with predicted design value
(M.Blaskiewicz et al)
4
gt-jump at RHIC
gt-jump is applied to minimize the time the beam
spends in nonlinear and nonadiabatic areas of
synchrotron motion, thus minimizing the beam
losses and longitudinal bunch area increase.
What jump amplitude is needed? To overcome
nonlinear chromatic effect, the jump has to be at
least Larger jump amplitude provides more
safety margin. For the jump time the shorter is
the better. Present gt-jump at RHIC changes gt
by about 1 unit in 40 ms. Enhancement of the
transition crossing rate by factor 60.
Gamma
70
80
90
Time after accramp, s
5
gt-jump realization
First order matched gt jump scheme. Used in each
RHIC sextant.
Family of jump quadrupoles (gt) placed in the
dispersive section (arc). At 90o betatron phase
advance between the quads, the dispersion and
beta-function perturbations remain
local. Betatron tune compensation family (qt) is
set at the area of small dispersion.
Design and realization of the gt-jump has been
done by J.Kewisch, C.Montag, S.Peggs, S.Tepikian,
and D.Trbojevic
6
gt jump optics distortion
In RHIC the phase advance between gt-quads is
82o Thus b-function and dispersion distortions
leak to the rest of the ring. Nevertheless at
optimal settings the optics distortion is
acceptable. The betatron tune excursions during
the jump are within 0.003.
2
Chromaticities also experience jump because of
changes in the optics. A scheme, involving
different sextupole families is under development
which should allow to adjust and control this
jump (C.Montag) Important for the transverse
instability control .
1
Chromaticity
70
80
90
Time,s
-1
-2
7
As result of the gt jump application and fine
tuning of betatron tunes and chromaticities the
beam losses through the transition region may be
done to less than 1
transition
In Run-8 the transition in Blue and Yellow rings
happened in different time, since in Yellow ring
IBS-suppression lattice, with higher gt was used.
8
Quadrupole oscillations after transition

• Even with optimally timed gt jump just after
  transition a bunch length starts oscillations.
• This leads to a bunch area increase which causes
  the rebucketing (at the storage energy) to be
  less effective .
• Possible reasons for those oscillations
• Beam self-induced field
• Remaining chromatic non-linearity (h1)
• Both those effects can cause a mismatch of the
  bunch area right after the transition crossing

The decision was made to develop a damper to
address this problem.
9
Results successful commissioning during the
2008 d-Au run
Quadrupole feedback application
N.Abreu et al

• Amplitude of the 4th RF harmonic of wall current
  monitor signal is shown as a function of the time
  (left plot).
• Longitudinal bunch area over the energy ramp
  (right plot).
• The longitudinal bunch area at the end of the
  energy ramp is 10 smaller when the feedback
  system in on.

10
Remaining issues of longitudinal dynamics at the
transition

• Recent simulations showed possible bunch area
  increase due to interactions with HOM of RF
  cavities.
• Above 1.2e9 Au ion/bunch the clear indication of
  the
• quadrupole coupled bunch instability was
  seen as bunches oscillates at different phases
  after crossing the transition.

These observed Coupled Bunch Modes can not be
damped with the feedback as it is. For those
modes the development of new feedback system
will be required.
11
Beam radius control at the transition area

• Different types of RF loops dominate the RF
  controls in different RHIC rings in the
  transition area.
• In Blue ring a constant mean radial orbit
  is maintained by the radial loop.
• In Yellow ring the ring-to-ring synchro
  loop maintains the same longitudinal phase
  between Blue and Yellow beams.
• In the transition region, tiny difference in the
  bending field between Blue and Yellow rings can
  lead to considerable radial orbit excursion.

transition
Ramp to ramp variation of Yellow beam radial
orbit excursion 0.1-1mm Corresponding bending
field error dB/B 2.6e-5
Mean radial orbit, mm
Bending field feedback is under development and
will be tested next run.
12
Transverse instabilty at the transition region

• Presently limits the ion beam intensity that can
  be accelerated in RHIC at 1.1e9 Au/bunch.
• The instability is very fast. Growth time (ten(s)
  of ms) is considerably smaller than synchrotron
  period (gt130 ms). Should be similar to the beam
  break-up in linacs.
• Most probably, the instability correlates in time
  with the chromaticity crossing 0.
• The instability was first observed with small
  number of bunches in early RHIC runs and was
  cured by the application of octupoles (amplitude
  dependent betatron tune spread).
• The instability reappeared at high number of
  bunches (gt90) and at higher bunch intensity. In
  this case it shows clear dependence on the bunch
  position in the train.
• Bunch losses vary along the bunch train
• Transverse emittance blowup varies along the
  bunch train
• Current explanation of these effects
• the electron cloud, accumalted in the
  beam with large number of bunches, lowers the
  instability threshold and introduce the
  dependence of instability strength on bunch train
  position.

13
Instability, as seen by the button BPM, affects
tails of bunch mini-trains
103 bunch pattern with two mini-gaps. Time, when
the instability happens also depends on the bunch
position in the train. Closer to the beginning
of the train -gt later instability development
14
Bunch intensity transmission through the
transition for 103 bunches with two gaps.
Beam losses increase towards the end of bunch
mini-trains
gap
gap
15
Evolution of instability frequency content
6.4ms (500 turns) between traces
Distinctive feature Instability arises at 300
MHz, then the instability power moves
continuously to higher frequencies as
the instability develops
16
Snapshots of vertical bunch centroid versus
longitudinal coordinate
Bunch 111 (last bunch in the train) 4ms between
traces
17
What can be done against the instability?

• Better chromaticity control through transition
  region (tools for better chromaticity
  measurements chromaticity jump control)
• RF counter-phasing (suggested by V. Litvinenko).
• Aimed to keep longer bunch throughout the
  transition region, reducing both electron cloud
  formation and bunch charge density.
• Development of the high-bandwidth feedback
  system.
• Beam scrubbing

18
Summary

• Almost completely beam loss free transition
  crossing is done at RHIC with the gt-jump.
• Longitudinal quadrupole oscillations after the
  transition have been successfully addressed by
  the feedback, minimizing the longitudinal bunch
  area growth.
• Transverse instability at the transition, which
  presently limits the RHIC ion beam intensity, is
  under detailed exploration.