Machine characterisation
Period 9 (December 2009)
Period 10 (January - October 2010)
v.01 (23/12/10)
Content
 Buncher zero-crossing procedure
 On image analysis
 BC1/2 cresting
 BC2 crest as a function of BC1 phase
 Dispersion measurements
 FC bunch charge measurements
 Steering
 Beam energy measurement
 RF phase shifters
 AP practice
2
3
4
6
6
6
7
7
8
8
1
Buncher zero-crossing procedure
Variation of the TOF procedure for zero-crossing of the buncher was introduced and successfully tested.
Below – two plots when the scope trace (1.3GHz) reading were taken at peak (old method) and at zerocross. The latter allows to find the buncher zero-cross phase more accurately and faster.
Dt - Dt0
Dt - Dt0
40
30
y = 3.5727x - 522.05
30
20
20
10
10
0
0
-10 130
140
150
-10 125
160
130
135
140
145
150
155
-20
-20
-30
-30
-40
-40
-50
-50
Zero-cross measurement (#1549)
Peak measurements (#1549)
Buncher zero-crossing with analogue LLRF (#2166).
TOF difference between 1.3GHZ clock and the BPM-02 signal was measured at Pb=100W and 1000W.
Procedure:
1) measure TOF at 100W and 1000W
2) vary buncher phase until TOF at 1000W is equal to TOF at 100W.
3) re-measure TOF at 100W
4) repeat steps (2) and (3) until TOF is equal at 100W and 1000W within 1-2ps.
TOF difference v buncher phase (#2166)
100
initial
TOF difference, ps
80
60
40
Step 1
20
Step 2
Step 3
0
0
5
10
15
20
25
30
35
Phase off zero-cross, deg
2
40
Image analysis
The RMS beam size increases with the image intensity when the beam size is small, e.g. when the slit
image is observed on YAG, see below. YAG feature or what ? The error in RMS size evaluation is
greatly diminished when the beam size is larger. However, some increase in measured beam size is still
noticeable and consistent with the increase of the “small” beam sizes.
RMS beam size v train length
#1589 (INJ-3 image of INJ-2 slit)
0.35
INJ-3 : Xrms v image intensity
# 1587
2.5
0.3
2
Xrms, mm
Xrms, mm
0.25
0.2
1.5
0.15
1
0.1
0.5
0.05
0
0
1
2
3
4
T, us
5
6
7
8
0
0
3
100
200
300
400
T, us
500
600
700
800
BC1 and BC2 cresting
#1768
Clear BC1 crest seen only using low energy edge of beam. Done at 40 pC and BC2 = OFF.
2.4
2.3
2.2
2.1
2
High Energy [A]
INJ-DIP-01 [A]
1.9
Centre [A]
1.8
Low Energy [A]
1.7
1.6
1.5
250
260
270
280
290
300
310
BC1 phase [degrees]
Below : ASTRA simulations at #1768 conditions : the energies are given at the exit from the booster;
curves relate to high, low, mid energies of the energy spectra as functions of BC1 phase.
Note: except for low energy part of the spectrum, there is no expected COSINE dependence ( at 30ophase
, the beam energy would be ~ 0.6MeV lower compared to peak energy).
After March 2010, we changed our cresting procedure to the so called “ temporal procedure” as found in
the wiki at:
http://projects.astec.ac.uk/ERLPManual/index.php/Cresting_booster_cavities_%28temporal_procedure%
29
4
The main differences are that we crest at as low bunch charge as possible (LA=0.01) and instead of
looking for the maximum deviation of the beam on INJ-YAG-05, we keep the INJ-DIP-01 current the
same, mark the position of the beam at eg/ -20 and scan the beam through in phase until the beam returns
to the same position. The crest is then the midpoint of the two phases which steer the beam to the same
position on INJ-YAG-05. This thus assumes a symmetrical curve of BC1 phase versus energy. As seen in
Figures 1 and 2, at 40 pC, this is not the case. However, Figure 3 below, is ASTRA simulations carried
out at 1 pC bunch charge.
Figure 1: ASTRA simulations at 1 pC
As can now be seen, the middle energy of the bunch now has a symmetric distribution. Assuming this is
what people use when noting the position of the beam on the screen, the crest is now correct between
ASTRA simulations and practice on the real machine.
Furthermore, the phase marked 0 in Figures 1 and 3 in ASTRA is the crest of the reference particle, which
remains unchanged with bunch charge at an internal ASTRA phase of -12.292. The conclusion is that the
phase used in ASTRA is the same as the phase used on the actual ALICE machine, within experimental
error.
Figure 4 below show energy spectra of the 1 pC bunch at three different phases. This distribution should
be approximately the intensity distribution we see on INJ-YAG-05.
Figure 2: Energy spectra from ASTRA at 1 pC
5
BC2 crest as a function of BC1 phase.
It has been known that the phase offset in BC1 affects the crest of BC2. Some measurements were taken
in an attempt to characterise this affect. The raw data can be found in the spreadsheet linked below:
\\dlfiles03\ALICE\Work\2010\09\26\Shift 3\ Booster phasing.xlsx
The following procedure was carried out with the laser attenuation set to 0.01. First the BC1 crest was
found. A given phase offset was then given to BC1 and the gradient of BC1 set to centre the beam on
INJ-5 with the nominal value of INJ-DIP-01 at 2.4 A (to ensure no time of flight differences due to
different beam energy). BC2 was then switched on to a “gradient” of 7 V and the crest of BC2 found
using the “temporal procedure” as written in the wiki:
http://projects.astec.ac.uk/ERLPManual/index.php/Cresting_booster_cavities_%28temporal_procedure%
29
The crest of BC2 was then recorded as a function of BC1 phase offset from -20 to 0 in steps of 2 units on
the BC1 station phase. The results are shown below.
Simulations in ASTRA were carried out to find the BC2 crest with BC1 phases set at -20, -10 and 0 with
an attempt made to keep the same energy gain from BC1.
Measurement:
Simulation (-20  -10)
Simulation (-10  0 )
0.74
0.85
0.96
Data was also taken of the BC1 grad set needed to recover the beam image on the centre of INJ-5 with a
INJ-DIP-01 current of 2.4 A. If you ignore the first data point, it fits quite well to a quadratic:
Dispersion measurements
EMI-1: Dx = 1.42m (#2059)
FC bunch charge measurements
With burst generator, when the frequency is reduced compared to nominal 81MHz, it’s possible to resolve
individual bunches on FC traces, see an example below.
6

INJ FC -1 : traces at 1/7 (black) and 1/4 (blue) of 81.25MHz frequencies (#2152). Note the FC
signal oscillation frequency matches that of the beam. LC circuit oscillations or what ?
Steering
Steering through undulator
Effect of vertical ST2 correctors on beam position on wedges
VCOR-03
VCOR-05
UP
Large
Smaller
CN
Small
Larger
Effect of horizontal ST2 correctors on beam position on wedges
DIP-04
HCOR-05
UP
Large
Large
CN
Large
near zero
Effect of HVCOR-05 is explainable by beam optics.
Horizontal steering procedure (quite effective and quick)
1) centre beam on CNWG with DIP-04
2) centre beam on UPWG with HCOR-05
Vertical steering is not that straightforward.
There is also an earlier procedure (Neil T.) in wiki.
Beam energy measurement
There was some confusion over ~10% difference in beam energy measurement on AR1-1 and EMI-1. A
direct comparison made in #2055 has shown that the difference is ~1%, i.e.
AR1-1: DIP-01 = 45.4A 27.5MeV
EMI-1: DIP-04 = 8.48A  27.85MeV.
Some analysis of the initial discrepancy is made in a note presented at ALICE physics meeting (see
ALICE physics meetings at \\dlfiles03\apsv4\Astec\Projects\ALICE\ALICE_Physics_Meeting ).
Partially, the problem is explained by mixing up total and kinetic beam energies in ALICE and EMMA
Magnet Tables. The issue of correct steering into the energy spectrometers could also contribute.
7
RF phase shifters
Beam based calibration of the buncher phase shifter was made in #2058. The results are summarised in
the plot below.
AP practice
Bad practice : to tweak something well upstream to improve something well downstream (e.g. using BC2
GS to increase charge measured on EMI FC). This leads to many changes in longitudinal and transverse
beam dynamics as clearly demonstrated by the beam images below when the BC2 GS was slightly
changed from 7.2 to 7.5.
BC2 GS=7.2
BC2 GS=7.5
8
EMI-6 (#2097)