1. Beam Stability of the LHC Beam Transfer Line TI 8
  J. Wenninger, B. Goddard, V. Kain, J. Uythoven (CERN, Geneva)
Introduction
n target
n beam to
Gran Sasso
SPS
TI 8
TI 2
TI 8, the first 2.7 km long transfer line between the Super Proton Synchrotron (SPS) and the LHC was commissioned in the fall of The stability of the line is critical for injection into the LHC, since it affects emittance growth, beam loss in the LHC injection regions and setup time of the lines. The maximum injection error tolerated in the LHC corresponds to 1.5 × sbeam.
Stability studies were limited by the beam intensity that could be used during this commissioning phase, which had to be kept low in order to limit activation of the beam dump installed on the downstream end of the line which is close to the LHC tunnel..
Model Independent Analysis
Model Independent Analysis is based on a spacial-temporal analysis via Singular Value Decomposition of a matrix A holding the data history
This technique was applied to trajectory data collected over 6 hours of stable beam conditions. Due to limitations on the intensity, the single short resolution of the BPM readings was only 200 mm. A visual inspection of the trajectories did not reveal any sign of drift.
Applying MIA to the trajectory sample yields the following eigenvalue spectrum :
The associated spacial vector / trajectory is consistent with a trajectory originating at the extraction septum
In graphical form:
r.m.s. amplitude
Coherent
signal A U V W
noise…
The r.m.s. amplitude gives the beam position jitter from shot to shot. It corresponds to an amplitude of sbeam/8.
The observed amplitude is consistent with the measured ripple of the extraction septum power converter.
Matrix V contains the orbit pattern associated to each eigenvalue wi ,while the column vectors of matrix U describe the time evolution of the corresponding trajectory pattern.
There is one eigenvalue that is well above the baseline level corresponding to the noise.
Collimator alignment
During the TI 8 commissioning a beam alignment method for collimators was tested with a LHC Carbon collimator located in the upstream part of the transfer line.
Since this alignment method is sensitive to shot-by-shot beam position jitter, it was used to evaluate the stability of the beam in the transfer line.
The experiment
Principle
A collimator jaw is moved into the beam to scrape off a significant fraction of the beam. The intensity fluctuations at the downstream end of the line, normalized to the intensity in the SPS ring, reflect the position and size jitter of the beam from one shot to the next. From the knowledge of the BCT resolution it is possible to extract the intensity fluctuation due to the beam position jitter at the collimator.
Phase 1 : collimator fully open
- determine BCT resolution  5%
SPS
BCT
Phase 2 : one collimator jaw closed
- measure BCT fluctuations  11%
- scaled BCT resolution  8%
- determine contribution from beam jitter  8%
Collimator
TI8
Assuming Gaussian bunch profiles, the shot to shot beam position jitter is less than sbeam/10.
Collimator
BCT
BCT