A STUDY OF EMITTANCE MEASUREMENT AT THE ILC
L. Jenner, Liverpool University, Cockcroft Institute, Daresbury Laboratory, Warrington, U.K.
D.Angal-Kalinin, CCLRC,ASTeC, Cockcroft Institute, Daresbury Laboratory, Warrington, U.K.
G. Blair, I. Agapov, J. Carter, L. Deacon, John Adams Institute at RHUL, London, U.K.
M. Ross, A. Seryi, M. Woodley, SLAC, Stanford, California, U.S.A.
At the ILC, the luminosity will depend on maintaining the emittance 20% measurement error gives: 5.219 2.699 [10 -8 m]
5% measurement error gives: 5.957 0.523 [10 -8 m]
(especially in the vertical plane) delivered by the damping rings. An 400
400
important part of this challenge is to remove any x-y beam couplings that 350
350
arise from the linac and to measure the emittance. 300
300
• We propose to use laser-wires (LWs) in the Beam Delivery System (BDS) 250 250
to measure the beam sizes for both emittance diagnostics and coupling
N
N
200 200
correction. 150 150
• The accuracy of the beam size measurement depends upon several 100 100
factors such as errors from beam jitter, spurious dispersion functions, 50
50
coupling of the beams, laser pointing stability and the beta functions at the
0
LW locations. 0 2 4 6 8
-8
10 0
1 2 3 4 5 6 7 8 9 10
Input Emittance: (6.000)x[10 m] Input Emittance: (6.000) [10 -8
m]
YN
• The effects of these errors on the emittance measurement accuracy and YN
its implications for the skew correction procedure are considered. The difference in reconstruction for 5% and 20% error in the measurement at the LW.
0.8
Emittance / Emittance (reconstructed)
• The emittance is measured by the LWs 0.7
f x,y • This information is iteratively fed-back to 0.6
the orthogonal SQs such that the 0.5
emittance reaches a minimum. 0.4
• The accuracy of this procedure is in part 0.3
dependent on how well the true emittance 0.2
can be reconstructed from the laserwire 0.1
measurements.
0
0 5 10 15 20 25 30
Measurement Error
(%)
The reconstructed emittance for different values of the measurement error.
• A traditional wire scanner
Optical layout of the proposed skew correction and emittance cannot withstand the thermal
measurement section at the ILC. LW-Laser Wire, SQ-Skew Quadrupole load of a 250 or 500 GeV beam,
as is expected at the ILC.
5
The skew correction procedure works 5
x 10
• A laserwire can make a non-
invasive measurement of a
emittance measured at laserwires
by iterating through the SQs and 4
scanning across small range B-field beam by bringing a laser into
3
Y = A*(X-B) 2+C
settings, until the measured emittance A = 1.4447e+006+-7.31e+004 collision with it and then
2 B = 0.002686+-0.000344
at the LWs is minimised, see right. C = 16.6+- 5.77 measuring the forward-scattered
RMS = 82207
1 N = 27 Compton photons and
• If the resolution of the 2 DOF
/N
DOF
= 1.393
electrons, see right.
0
measurement is too poor, the
process can diverge. -1 x 20 mm
total length = 114.6 m
x x 0 0.25% (500 GeV beam)
-2
This problem can often be
energy BPM • LW Compton events have been
compensated for in part by making -3
simulated using a dedicated full-
more measurements, or by increasing -4
-0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2 simulation program (BDSIM), in order to
the number of steps in the scan; both skew quad strength MPS energy collimator
determine the energy losses along the
are time consuming undertakings. beam-line and to identify possible
12.5 mm
locations of Compton detectors.
The contributions of measurement vacuum chamber
Relative Error on Pessimistic Optimistic •
Ф=15 mm OD
For a 250 GeV beam and using the
Parameter Estimate Estimate errors on the beta functions, ΔE/E = ±10%
window trajectories baseline ILC BDS optics,
spurious dispersion, beam jitter,
β function at the LW 3% 1% approximately 98% of the Compton
laser spot size, laser pointing error
scattered photon energy exits at the
LW readout error 2% 1% etc to the total measurement error. laserwire
detector (ε) downstream energy diagnostics
Laser spot waist 10% 10% Brett Parker’s BMP dipoles (20) chicane, shown left.
• Considering the optimistic
Laser Pointing Jitter 15μm, 10% 0.5μm, 10% value of the measurement error
Beam Jitter 1.0 σe, 10% 0.5 σe, 10% is 23.8%, it can be seen that in
general the procedure works
Residual Dispersion 2.5mm, 10% 0.5mm, 20% satisfactorily.
CONCLUSIONS
Beam Energy Spread 1.5x10-3, 20% 1.5x10-3, 1%
Total Error in Y >100% 23.8%
This study is still preliminary, however
it is clear that the machine-related errors
4.5
Iterative Skew Correction are significant and may dominate those
Procedure
Initially the beam is set up such that coming from the LW itself; further work
4
the ratio of the projected (E2) to
intrinsic (Ey) emittance is 3.8. 3.5
is necessary.
Measured E2 / Ey
• This ratio is arrived at from linac The extraction of the LW signal is also
3
simulations and representative of 2.5
the expected quantity of x-y 2
under investigation, using full simulation
coupling in the ILC beam.
1.5
Up to about 113 TeV of energy per bunch will tools.
After two scans of each skew reach the detector which will require installing
quadrupole, the ratio is about 1.3.
1
shielding. In principle the scattered electrons can The energy-diagnostics chicane provides
The error bars here are large due to also be detected downstream of the LW. The
0.5
-1 0 1 2 3 4 5 6 7 8 9
energy loss due to these electrons is shown.
a good location for a detector for the
the large measurement error. Skew Quadrupole Scan
Number Compton scattered photons, where a very
large signal will emerge.