Losses at Extraction
140
140
120
120
100
100
80
80
60
TPSG diluter
x (mm)
x (mm)
The extraction of the CNGS beam from the SPS in LSS4 will result in beam losses in the extraction
channel, in particular on the diluter TPSG4 which is designed to protect the MSE septum element
from damage in the event of a badly steered or unsynchronised extraction. The loss mechanisms at
extraction from SPS LSS4 are the scraping of transverse beam tails in H and V planes, on the tight
septum aperture, and particles in the “beam-free” extraction kicker gap, which will be swept across
the TPSG4. The settings for the extraction are given in Table [LSS4TAB].
60
40
40
20
20
MSE septa 1 - 6
0
4029
4033
4037
4041
4045
4049
4053
4057
4061
0
4065
s (m)
Figure [LSS4FIG]. Circulating and extracted beam envelopes for the extraction system in LSS4.
The ±4  beam envelope is plotted around the orbit/trajectory.
Table [LSS4TAB] LSS4 settings for the CNGS extraction at 400 GeV/c.
Setting
Unit
Value
MPSH41402.41402[KICK]
mrad
0.0000
MPLH41672.41672[KICK]
mrad
0.5004
MPLH41994.41994[KICK]
mrad
0.3394
MPSH42198.42198[KICK]
mrad
0.1438
MKE1.F[KICK]
mrad
0.1100
MKE2.F[KICK]
mrad
0.1210
MSE418.F[KICK]
mrad
2.0828
Bump amplitude at BPCE.41801
mm
31.0500
Bump amplitude at QFA.41810.
mm
33.0000
Kicker Voltage
kV
50.0000
Opening at TPSG
mm
49.4000
Upstream MSE Girder position
mm
47.0000
Downstream MSE Girder position
mm
110.0000
1
Losses from scraping of tails
The losses from scraping of the beam tails are estimated in the following section, by calculating the
worst-case horizontal and vertical aperture, and estimating the part of the Gaussian distribution
which would be scraped on the aperture limit. The beam emittance was estimated from wire scanner
profile measurements made on the CNGS beam, fitted to Gaussian distributions. The normalised
emittance was estimated in this way to be 6.15 mm.mrad in the horizontal plane, and 4.25
mm.mrad in the vertical plane, with the energy spread p equal to 0.36 10-3 (1 ) value. For the loss
estimations, the values assumed are a factor of 2 above this. The various assumptions made
concerning all the tolerances etc. are given in table [LSS4ASS1]. In general a factor 2 is assumed
with respect to the measured or specified value.
45
Ex sig
40
Stability
35
Kicker
2E+06
losses [p+]
clearance [  ]
Table [LSS4ASS1] Assumptions on tolerances etc. for CNGS extraction at 400 GeV/c.
Quantity
Unit
Value
Horizontal normalised emittance
12.0
 mm.mrad
Vertical normalised emittance
10.0
 mm.mrad
MKE overshoot / ripple
%
±1.5
SPS orbit stability (H/V)
± 0.2

TPSG/MSE setting-up precision and
mm
± 2.0
dynamic mechanical tolerances
Total
30
1E+06
Losses
25
20
8E+05
15
10
4E+05
5
0
4035
4040
4045
4050
4055
s [m]
0E+00
4060
Figure 3.4.8: Horizontal aperture and losses. The minimum aperture after all tolerances is still
above 5σ. The total losses is smaller than 2106 p, i.e. smaller than 510-8 level, which seems
negligible. The 10-3 loss level is reached for an error in the beam trajectory of ~5.8 mm.
2
Ex Sigma
16
3.2E+02
Stability
Total
14
losses [p+]
clearance [s]
18
Losses
12
2.4E+02
10
8
1.6E+02
6
4
8.0E+01
2
0
4035
4040
4045
4050
4055
0.0E+00
4060
s [m]
Figure 3.4.9: Vertical aperture and losses. The minimum aperture after all tolerances is still above
6σ. The total losses is smaller than 3102 p, which is negligible. The 10-3 loss level is reached for an
error in the beam trajectory of ~5.9 mm.
The above plots show that, for the parameters indicated, losses due to regular scraping of the
transverse beam tails at extraction are expected to be well below the 10-3 level. This value is
reached when trajectory excursions / misalignments etc. reach the 5-6 mm level.
Losses from particles in the kicker rise time
The losses from particles in the kicker rise time are estimated in the following section, for the
assumptions listed in Table [LSS4ASS2]. A typical longitudinal beam profile was superimposed on
the MKE waveform, and the resulting particle distribution evaluated for the bunches which would
be expected to impact the TPSG4 diluter (for a null beam size).
Table [LSS4ASS2] Assumptions on particles in kicker rise time.
Quantity
Unit
Gaps between two PS batches
μs
Nominally empty bunches in each gap
Total beam in each gap
%
MKE kick for TPSG4 impact
%
Bunches swept across TPSG4
3
Value
1.05
210
0.7
33-77
70-160
k/ko
1.2
p+
1.E+11
1.0
1.E+10
0.8
0.6
1.E+09
0.4
0.2
1.E+08
0.0
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
time us
1.2
p+
1.E+11
Stay in SPS
Hit TPSG
k/ko
Fig. LSS4loss1. Measurement (2nd October 2005) of the intensity distribution over one SPS turn at
top energy (4340 ms from the start of the cycle) in 25 ns time bins. The total beam population was
5.08×1013 p. About 3.1×1011 p were localized in the extraction gaps (a total of about 0.7 % of the
beam). The MKE kicker waveform with 10.5 s flat-top is also shown, together with an indication
of where particles will impact the TPSG4.
Into TT40
1.0
1.E+10
0.8
0.6
1.E+09
0.4
0.2
1.E+08
0.0
0.6
0.7
0.8
0.9
1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2
2.1
2.2
2.3
2.4
2.5
time us
1.2
p+
1.E+11
Into TT40
k/ko
0.5
Stay in SPS
Hit TPSG
1.0
1.E+10
0.8
0.6
1.E+09
0.4
0.2
1.E+08
0.0
12
12.1
12.2
12.3
12.4
12.5
12.6
12.7
12.8
12.9
13
13.1
13.2
13.3
13.4
13.5
13.6
13.7
13.8
13.9
14
time us
Fig. LSS4loss3. CNGS beam population over one SPS turn, normalised MKE kick strength and
destination of the particles in the kicker gap in the limit of a beam with null beam size: (a) detail of
the rising edge, (b) detail of the falling edge. For the second extraction the losses may be reduced
with the choice of a longer pulse.
4
The results of the study (in the limit of a beam with null beam size) are:
 1.1×1011 p stay in SPS. These particles oscillate at amplitudes smaller than the machine
aperture and the oscillation will be damped by the transverse feedback before the next
extraction [Hof1]
 9×109 p (2 × 10-4 of the total beam intensity) hit the TPSG4 directly
 2×1011 p ‘badly’ extracted into TT40. These have been tracked through the TT40 aperture in
a simple model (no errors) and no losses were seen, as a result of the protection provided to
the TT40 aperture by the TPSG.
The results do not change appreciably even when the finite beam size is taken into account, since to
first order the extra beam scraped on the TPSG4 from bunches with an axis outside the diluter are
compensated for by the beam from bunches with an axis near the edge of the diluter which escapes
the TPSG4.
Conclusion
The losses in the extraction channel have been estimated for scraping of tails and for beam in the
kicker rise time swept across the TPSG4. It seems reasonable to assume that the total losses at the
TPSG4 will be at or below the 10-3 level. The largest contribution is expected to come from
particles in the MKE rise time, which also causes a significant fraction of the beam (around 510-3)
to be kicked but remain in the SPS aperture.
5