MEASUREMENTS OF INTRA-BEAM SCATTERING AT LOW
EMITTANCE IN THE ADVANCED LIGHT SOURCE
J. Byrd, J. Corlett, H. Nishimura, D. Robin, S. De Santis, C. Steier, A. Wolski,Y. Wu, LBNL, Berkeley, CA, USA
K. Bane, T. Raubenheimer, M. Ross, J. Sheppard, T. Smith, SLAC, CA, USA
Abstract
The beam emittance at the interaction point of linear
colliders is expected to be strongly influenced by theemittance of the beams extracted from the damping rings.
Intra-beam scattering (IBS) potentially limits theminimum emittance of low-energy storage rings, and thiseffect strongly influences the choice of energy of damping
rings [1]. Theoretical analysis suggests that the NLC
damping rings will experience modest emittance growth at
1.98 GeV, however there is little experimental data of IBS
effects for very low-emittance machines in the energy
regime of interest.
The Advanced Light Source (ALS) at Lawrence
Berkeley National Laboratory is a third-generationsynchrotron light source operating with high-intensity,
low-emittance beams at energies of approximately 1 - 2GeV, and with emittance coupling capability of 1% orless. We present measurements of the beam growth in
three dimensions as a function of current, for normalized
natural horizontal emittance of approximately 1 - 10 mmmrad at energies of 0.7 - 1.5 GeV, values comparable tothe parameters in an NLC damping ring.
Using a dedicated diagnostic beamline with an x-ray
scintillator imaging system, measurements of thetransverse beamsize are made, and bunch length
measurements are made using an optical streak camera.
Emittance growth as a function of bunch current isdetermined, and compared with preliminary calculationestimates.

For user operations, the machine is typically operated
with emittance coupling of 3 - 6 %, to increase theTouschek beam lifetime. The machine is capable of muchsmaller coupling, and values below 1% may readily beachieved.
Correction of closed orbit distortion and verticaldispersion is achieved through use of the 96 BPM's, 94
horizontal correctors, and 72 vertical correctors. BPM'shave approximately 1 micron resolution (multi-turn, 190
averages in approximately 0.5 sec.) Beam-based alignmentis used in most quadrupole locations, and a closed-orbit
distortion of approximately 50 µm is typical in the arcs,
and <10 µm in the straights.
Table 1: ALS parameters
Energy0.7
1.0/ GeV
Natural energy spread
2.84/ x 10-4
Normalized natural
1.0
2.9
emittance / 10-6 m rad
Damping times, x,y,E 147, 216, 52, 74,
/ ms
128
44

1.5
61015, 22,
13

1 ALS MACHINE PARAMETERS
1.1 General Parameters
The Advanced Light Source (ALS) is a third generationsynchrotron radiation facility at Lawrence Berkeley
National Laboratory, California. The machine, ofcircumference 196.8 m, operaties typically between 1.5 to
1.9 GeV, with 1 - 2 mA per bunch in approximately 300
bunches in multibunch mode, with bunch length typically
15 ps at 1.5 GeV. Up to 35 mA per bunch may be stored
in two-bunch mode. Bunch volume is increased throughthe use of harmonic cavities in typical operations, in orderto improve the Touschek lifetime. The RF system is 500
MHz, and momentum compaction 1.6 x 10-3. Beamparameters for energies at which IBS studies wereperformed are shown in table 1. Figure 1 shows the triplebend achromat (TBA) cell, with lattice functions.

Figure 1: ALS cell and lattice functions.

1.1 Machine set-up

2

The machine is set-up by correcting the orbit including
RF frequency, setting tunes to optimal values determinedfrom known non-linear dynamics, setting chromaticities,
and iterating on tune and orbit corrections. Orbit
correction is to quadrupole centers as determined by beambased alignment.
A model of the lattice is generated from an orbit
response matrix using the code LOCO [2,3]. Residual
beta-beating can be corrected to less than 1% rms, and istypically 3%. Figure 2 shows residual beta-beat measured
at 1.5 GeV without additional correction, with an rms
value of 1.3% in the horizontal and 2.2% in the vertical.
Measured dispersion is shown in figure 3, showing atypical residual vertical dispersion of 5.2 mm rms.
Emittance measurements made with an x-ray beamline(see section 2.2) agree to better than 5% with calculated
natural emittances.

DIAGNOSTICS

2.1 Diagnostic beamline 3.1
We use an x-ray beamline (designated 3.1) dedicated foruse as an imaging diagnostic, with a source point in thefirst dipole magnet of a TBA cell. The dipole bend angle
is 10°, and the beamline source point is downstream of
the center tangent. At this point ßy varies rapidly, ßx issmoothly varying with distance along the orbit, and thedispersion is relatively small. Lattice functions at thesource point are approximately ßx = 0.4 m, ηx = 0.03m,
ßy = 19 m, ηy = -0.01 m. Dispersion is readily determinedat the source point by measuring beam centroid motion as
a function of rf frequency. The beamline location in an arc
cell is shown in figure 4.

3.1ALS: measured beta beating (response matrix analysis)
0.1x∆β /βx0.05

0Figure 4: Diagnostic beamline 3.1.

−0.05

−0.10510φ [2 π]15x0.1y∆β /βy0.05

0−0.05−0.101234φy [2 π]567891.5 GeV, 2000−11−05, rms horizontal beta beating = 1.3%, vertical = 2.2%

Figure 2: Measured ß-beat.
ALS dispersion measurement0.2

ηx [m]

0.15
0.1

The imaging optics are shown in figure 5. The opticalsystem is a Kilpatrick/Baez mirror system, which
produces a 1:1 image of the source on a bismuth
germinate crystal. At 1.5 GeV, the optical resolution of
the system determined by lifetime and beamsize
measurements is less than 10 µm. At this energy or thecorrection factor is not significant for smallest beamsizes
measured. Resolution at lower energies requires furtherstudy.
Carbon neutral density filters are used to avoid
saturation of the optics and to block visible light. An
optical microscope magnifies the visible light image fromthe crystal scintillator onto a 640 x 480 pixel CCD
camera, with pixel resolution of approximately 4 µm.
Aperture

Mirror Tank

0.05

ShieldWallSlits & Photocathode

0Source−0.05020406080100120140160180

Microscope
CCD

VerticalAdjustment

0.020.015

Elevation View

0.01
ηy [m]Image

KB mirrors

200Visible
Light
MirrorCarbon foil

Translation Stage
Screen0.005

Figure 5: Diagnostic beamline optics

0−0.005−0.010204060

80
100120
BPM s−position [m]1401.5 GeV, 2000−12−18, rms vertical dispersion = 5 mm

Figure 3: Measured dispersion.

160180200

CCD output is read into processing software and a 2-D
Gaussian fit is derived for the x,y beam distribution.
Since the betatron axes are not in the horizontal andvertical planes at the source point, the fitting routine
must find these axes before deriving σx and σ y. This effectis significant since the axes may be rotated by up to 30°,

depending on coupling. The error in the fit is typically±0.5 µm.

2.1 Streak camera
Inserting a mirror into the optical path allows visible
light to be brought onto a Hammamatsu C5690 streak
camera. Since the insertion of a mirror interrupts theoptical path for transverse beamsize measurements,
simultaneous observations of longitudinal and transverse
beam dimensions are not possible. Streak camera output
is recorded and analyzed offline. Statistical averaging isused to determine FWHM values.

3 MEASUREMENTS
3.1 Measurement technique
Measurements of transverse and longitudinal beam sizes
are made in single-bunch mode after setup in multibunchmode. A single-bunch current of 1 mA is 0.67 nC or
4.2x109 electrons. Beam current and beam dimensions arerecorded as the current decays naturally (for higher
current), and by knockout using a pulsed magnet when thelifetime becomes excessive. Coupling is minimized bycareful machine set-up and use of the four skewquardupole families in the ALS. Measurements are madefor a given coupling value first in the transverse plane,
then in the longitudinal direction. Coupling is increased
by changing quadrupole settings to move the tune to acoupling resonance. The coupling resonance is broadenedby the use of skew-quadrupoles.

3.2 1.5 GeV

Figure 7 shows the measured horizontal and verticalbunch size and bunch length as a function of bunch
current, under different conditions of emittance coupling.
Measurements were made at low coupling (2% to 6%),and at high coupling (30% to 50%). It can be seen thatthe bunch length is increased in the low-coupling case,which is evidence for intra-beam scattering.
The beamsizes at lowest current, 0.25 mA, correspondto normalized emittances of γεx=4x10-6 m-rad,γεy=0.07x10-6 m-rad, assuming natural energy spread.
For the high-coupling case, changes in transverse
dimensions as a function of current are attributed to tune
shift with current of approximately 4x10-4 per mA due tobroadband transverse impedance, moving the tune off thecoupling resonance.

3.2 0.7 GeV
Growth rates from intra-beam scattering are expected toincrease very rapidly at lower energies, and at 0.7 GeV we
expect IBS effects to be strong.
Due to suspected limitations in the resolution of the xray optics at such low energies, the transverse beamsizes
were not recorded during this set of measurements.
Bunch length was measured for different RF voltages,
as a function of beam current, on and off couplingresonance. Figure 8 shows the variation of bunch length
under these conditions. The increase in bunch length inthe off-resonance case is taken as evidence of IBS

4 COMPARISON WITH THEORY

At 1.5 GeV intra-beam scattering is not expected to besignificant, and it is known that potential well distortionand microwave instability are significant [4]. No
significant effects attributable to IBS were observed.
Figure 6 shows bunch length as a function of current fordifferent coupling factors and RF voltage. Increase oftransverse bunch sizes is observed and can be consistentlyexplained by energy spread increase due to microwave
instability. No difference in bunch length was observed
between low-coupling and full-coupling conditions.
PRF=10 kW
40

on coupling resonance
Bunch length, rms / ps

3.3 1.0 GeV

30

PRF=100 kW
20

10Accurate IBS calculations require knowledge of thecontribution to energy spread from the microwave
instability, and also bunch lengthening from potential
well distortion.
IBS calculations using both the Bjorken-Mtingwa andthe Piwinski formalism's have been made for the ALS
lattice under assumed conditions of potential well
distortion and energy spread from microwave instability at
1.5 GeV. They predict a small effect due to IBS underthese conditions, a result consistent with measurements.
At lower energies, however, the potential well
distortion and energy spread from microwave instability
are not well characterized in the ALS. Assuming nocontribution from these effects, we calculate the transverse
emittances, and the energy spread, to increase by a factorof 3 to 5 over 0-10 mA per bunch, resulting in a bunchvolume increase of 5 to 11. We observe an increase ofapproximately 6.

5 DISCUSSION

00246810Bunch current / mAFigure 6: Bunch length measurements at 1.5 GeV

The desire for low emittance at relatively low energy in
the NLC damping rings has lead to IBS studies atmachines with some similar parameters. Table 2 shows
relevant parameters for the NLC damping rings design.

Transverse beamsize, rms / µm

1008060

4030 % coupling horizontal beamsize
30 % coupling vertical beamsize
2 % coupling horizontal beamsize2 % coupling vertical beamsize

200024681012Bunch current / mA

Bunch length, rms / ps

403020106 % coupling
30 % coupling

0024681012Bunch current / mA

Bunch length, rms / ps403020103 % coupling
full coupling
0024681012

Bunch current / mA

Figure 7: Horizontal, vertical bunch size, and bunch length measurements at 1.0 GeV, as a function of bunch current,
and a variety of emittance coupling settings.

50REFERENCES

Prf=5 kW, low-coupling
Prf=5 kW, high-coupling40

Prf=50 kW, low-coupling

Bunch length, rms / ps

Prf=50 kW, high-coupling

30

2010002468

10

Bunch current /mA

Figure 8: Bunch length measurements at 0.7 GeV
Table 2: NLC main damping ring parameters
Energy1.98/ GeV
Natural energy spread
9.1/ x 10-4
Normalized natural
2.3
emittance / 10-6 m radDamping times, x,y,E
4.79,
/ ms
5.03, 2.58
While the some of the parameters at the ALS do notmatch those of the NLC damping rings (especially thedamping times), the emittance is similar and comparison
with theory will allow better understanding and confidence
in calculations. In addition, differences in the impedance
between machines complicate comparisons, particularly
due to potential well distortion and microwave instability.
Emittance growth has also been studied at the Accelerator
Test Facility at KEK, Japan [5], under conditions similar
to the design parameters for an NLC damping ring.
More detailed calculations of IBS are currently inprogress. Measurements to date are consistent withpreliminary calculations. For future measurements, weplan to have available additional optical diagnostic, withdifferent source point parameters. Data from multiple
observation points allows separation of energy spread andtransverse emittance. Continued measurements areplanned, and measured data will be used to compare withtheory and allow improved understanding and confidence
in theoretical predictions.

[1] "Zeroth-Order Design Report for the Next Linear
Collider", The NCL Design Group, SLAC Report
474, May 1996.
[2] J. Safranek, Nucl. Instr. and Meth. A388, 27 (1997)[3] C. Steier, D. Robin, "Fully coupled analysis of orbit
response matrices at the ALS", EPAC 2000, Vienna.
[4] J. Byrd, "Observations of single bunch collective
effects in the ALS", Beam Instability Workshop,
ESRF, March 2000.
http://www.esrf.fr/machine/myweb/machine/Workshop/BIW/PROC/wg_single/ALS_SB_collfx.pdf[5] J. Urakawa, "Experimental Results and TechnicalResearch and Development at ATF (KEK)", proc.
EPAC 2000, Vienna, 2000.

ACKNOWLEDGEMENTS
Work supported by the U.S. Department of Energy
under contract No.s DE-AC03-76SF00098 (LBNL), DEAC03-76SF00515 (SLAC).