Transcript PowerPoint-Präsentation

 Known evidence:
E-cloud instability is one of the main single bunch
intensity limitations in the SPS for the LHC beam.
 How would the electron cloud instability threshold change if the
injection energy into the SPS was raised to 50-70 GeV/c ?
 Answer to this question is not clear :
 Higher energy means more rigid, and therefore more stable, beam
 At higher energy the beam gets transversely smaller, which
enhances the pinch of the electrons as the bunch goes through them
 The matched voltage is lower at higher energy, which translates into
a lower synchrotron tune (destabilizing)
 HEADTAIL simulations neeeded to answer the question!
Model with uniform E-cloud
- full overview on the parameters -
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The bunch is always assumed to be matched to ist bucket
Simulations are done in dipole field regions because the electron cloud is
mainly located in the SPS dipoles.
Main implications of the assumptions
• Longitudinal emittance 0.35 eVs and rms bunch length 0.3 m:
* Matched voltage scales like ||/ and is re-adjusted for the simulations at different
energies
• Normalised transverse emittances: ~3.0 m implies that transverse
beam sizes scale like  -1/2
Model with uniform E-cloud
Overview on the instability thresholds
Instability thresholds as:
• Bunch intensity when the e-cloud density is fixed  decreases with energy!
Model with self-consistent e-cloud
HEADTAIL simulations
1/
E-cloud build up
threshold
For dmax=1.4 the instability threshold decreases like 1/ till it levels off at the value of
the build up threshold
 For momenta > ~100 GeV/c, e-cloud build up and instability thresholds become equal.
• Experiment at the SPS was attempted last year (24 October 2006) to
verify this scaling law:
– Chromaticity correction on the ramp to see whether it would make the
beam (4 batches) unstable
– Chromaticity correction and 200 MHz voltage ramp down to 500 kV to
observe beam stability at flat top (450 GeV/c)
– No instability was observed
– HEADTAIL simulations also show no expected instability with the high
voltage used along the ramp and the blown up longitudinal emittance at
450 GeV/c.
• More measurements are planned this year at the SPS, trying to set up a
magnetic cycle with an intermediate flat top at energy that could be
between 50 and 70 GeV/c.
We could attempt to study the ECI threshold having chromaticity
corrected and matched bunch at this intermediate energy (to reproduce
the conditions of the simulations).
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Benchmark with Ohmi‘s code PEHTS is also ongoing to establish the
correctness of the HEADTAIL prediction
Where we are standing:
The bunch at 270 GeV/c is more unstable than the bunch at 40 GeV/c
Thresholds estimated from the figures:
– 40 GeV/c  7 x 1011
– 270 GeV  2 x 1011
•
HEADTAIL predicts thresholds about a factor 10 lower. We are verifying
possible mistypings, parameters and model.
Countermeasures (I):
Reducing the chamber size...
The table shows the average electron cloud central density (m-3) for nominal beam current
(1.1 x 1011) at 50 or 70 GeV/c
 The beam is unstable in all the cases with electron cloud! (threshold is about 1.5 x 10 12 m-3)
dmax
size
nominal
0.75 x
0.5 x
1.3
1.4
1.6
2 x 1012
2.5 x 1012
4 x 1012
1.5 x 1012
3 x 1012
1.5 x 1012
Countermeasures (II):
Reducing the dmax or acting on beam parameters
dmax <1.3 there is no electron cloud and therefore, no instability for the
nominal LHC beam (even keeping the present pipe size!)
• If
 Efficient scrubbing, NEG (or different kind of) coating on surfaces
 Grooved surfaces seem to reduce the SEY (example, courtesy W. Bruns)
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
• Perhaps
injecting into the SPS with a higher longitudinal emittance?
Summary
• E-cloud single bunch instability in the vertical plane is presently an
intensity limitation in the SPS
• The scaling of ECI thresholds with energy, as predicted by
HEADTAIL simulations, is not favorable under conservation of
longitudinal emittance and normalized transverse emittances
• This can be overcome by
– suppressing the e-cloud (smaller chamber radii, NEG or grooved surfaces)
– injecting into the SPS with larger ez
• Verification of the scaling law: done, ongoing or planned.
– Benchmark with experiments (attempted once in the SPS in October
2006, but no effect observed due to the high voltage during ramp, more
MDs planned for this year)
– Benchmark with another ECI code (PEHTS, K. Ohmi, KEK)
– Look for mode coupling when crossing the ECI threshold