Proceedings of IBIC2012, Tsukuba, Japan

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MEASUREMENT OF NANOMETER ELECTRON BEAM SIZESWITH LASER INTERFERENCE USING IPBSM
Jacqueline Yan, Sachio Komamiya, Masahiro Oroku, Taikan Suehara, Yohei Yamaguchi,
Takashi Yamanaka, University of Tokyo, Tokyo, Japan
Yoshio Kamiya, ICEPP, University of Tokyo, Tokyo, Japan
Sakae Araki, Toshiyuki Okugi, Toshiaki Tauchi, Nobuhiro Terunuma, Junji Urakawa, KEK,
Ibaraki, JapanAn e- beam size monitor, called the “Shintake Monitor”(or “IPBSM”), is installed at ATF2’s virtual interaction
point (“IP”). It plays a crucial role in achieving ATF2’s
Goal 1 of focusing the vertical e- beam size (σy*) down toa design value of 37 nm, using an ingenious technique ofcolliding the e- beam against a target of laser interference
fringes. σy* is derived from the modulation depth of theresulting Compton signal photons measured by adownstream γ detector. IPBSM is the only existing device
capable of measuring σy* as small as 20 nm with better
than 10% resolution, and can accommodate a wide rangeof σy* up to a few μm by switching between lasercrossing angles θ = 174° , 30°, and 2° - 8° according tobeam tuning status.
The effects of several major hardware upgrades havebeen confirmed during beam time, such as throughsuppressed signal jitters, improved resolution and stablemeasurements of σy* down to about 150 nm by Feb 2012.
The aims of the extensive 2012 summer reform
implemented upon the laser optics are higher reliability
and reproducibility in alignment. Our goal for the autumn
2012 run is to stably measure σy* < 50 nm. This paper
describes the system’s design, role in beam tuning, andvarious efforts to further improve its performance.

INTRODUCTION
The International Linear Collider (ILC) holds great
potential for detection and detailed research of new
physics beyond the Standard Model. Clean e-e+ collisions
enable observations of the most fundamental processes
free of synchrotron radiation loss. However ILC faces
stringent demands for luminosity, expressed here as:n N 2 frep
(1)
L= b
H
4πσ x*σ y*Dnb : no.of bunches, N : bunch population,
frep : repetition rate, HD: disruption parameter.
It is apparent from the Gaussian beam cross section in
the denominator that beam focusing is crucial forachieving high luminosity. The design IP beam sizes for
ILC are (σ*x, σ*y) = (640, 5.7) nm[1]. At the Accelerator
Test Facility 2 (ATF2), a FFS test facility for ILC locatedin KEK, “Goal 1” is to to verify the “Local Chromaticity
Correction” scheme by demonstrating focusing of σy* to
37 nm, the design size energy-scaled down from ILC[2].
“Goal 2” is to nm precision beam trajectory stabilization
under such a small σy*. IPBSM, installed at ATF2`svirtual IP, is the only existing device capable ofmeasuring σy* as small as 20 nm. Its outcomes areindispensible for Goal 1, and thus for realization of ILC.

Figure 1: Location of Shintake Monitor in the ATF(2)beamline [3,7].

Measurement Scheme of IPBSM
Figure 2 shows the schematic layout of IPBSM, which
consists of laser optics, a  detector, and DAQ
electronics[5]. The pulsed laser beam is split into twopaths by a half mirror, then made to cross to form
interference fringes at their focal points matched to IP.
The phase between the paths, controlled by a piezoelectric
stage, is scanned relative to the e- beam traversing thefringes perpendicularly. A downstream  detector
measures the modulation depth (M) of the resulting
Compton scattered photon signal at each phase. M is large
for focused beams, and small for dispersed beams (Fig. 3).
Laser fringe intensity is expressed using intensity ofmagnetic field (B), averaged over time as:
Bx2 + By2 = B 2 (1+ cosθ cos2ky y)(2)x and y are coordinates perpendicular to e- beam. ky =ksin(θ/2) (θ:laser crossing angle, λ:laser wavelength) is
wave number component normal to fringes. Assuming
Gaussian distribution, N, number of Compton photons, isrelated to beam centre y0 and σy* as[4].
N∝∫

∞
−∞

⇒ N=

⎛ ( y − y )2 ⎞10
⎟ ( Bx2 + By2 )dy
exp ⎜⎜ −2σ y2 ⎟⎠
2πσ y
⎝
2
N0
1+ cos ( 2k y y0 ) cosθ exp −2 ( k yσ y )
2

{

(

(3)

)}

M, interpreted as ratio of amplitude to average, iscalculated in Eq. (4) from N+ and N- , max. and min.,respectively, of signal intensity. Figure 3 relates M andσy*, calculated as in Eq. (5). Here, measurable range isdetermined by the laser fringe pitch “d” (Table 1)[5, 7].

ISBN 978-3-95450-119-9
Accelerator Technologyc 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Abstract

1

TUCC04

Proceedings of IBIC2012, Tsukuba, JapanM=

N+ − N−= cos(θ ) exp (−2(kyσ y )2 )N+ + N−(4)σy =⎛ cos(θ ) ⎞d
2 ln ⎜
⎟
2π
⎝ M ⎠

(5)

d=

π
λ
=
k y 2sin(θ / 2)

Laser
Optical Delay Line
Scattered Photon

Gamma Detector

Laser Light
Bending Magnet
Virtual IP
Electron Beam

Laser Interference Fringe

measurable range was widened to encompass σy* as large
as several μm by adding continuously adjustable lower θmodes. A new multi-layer CsI(Tl) detector separates
signal from BG by taking advantage of their clearly
distinct energy spectrum due to the much lower beamenergy at ATF2. Instead of the FFTB method of shifting
the e- beam using steering magnets, at ATF2, tuning of
the lower energy e- beam is maintained effective by
scanning laser fringes relative to a fixed e- beam.
Owing to these accommodations, IPBSM at ATF2 iscapable of similar resolution as FFTB despite challenges
such as much lower beam energy, single photon energy,
and repetition rate.

yxzOverall Layout of the Laser Optics
Figure 3: Relationship between beam size and modulation
depth.
Modulation Depth

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Figure 2: Schematic layout of IPBSM[6].

Table 2: Typical e- Beam and IPBSM Parameters: ATF2
vs FFTB [2, 4]
FFTB
ATF2
Beam energy
46.6 GeV
1.28 GeV
1 photon energy
8.6 GeV
15 MeV
rep. rate
30 Hz
1.56 Hz (3 Hz)
e- / bunch1 x 1010
1 x 1010
Bunch length
3 ps
16 ps
(σx*, σy*) at IP(900, 60) nm(2200, 37) nm
Laser wavelength 1064 nm
532 nm (SHG)
Range for σy*40-720 nm
20 nm-6μm

2deg. mode
4deg. mode
8deg. mode
30deg. mode
174deg. mode

1
0.8
0.6
0.4
0.2
0

10

102

10 3
104
Beam Size [nm]Figure 4: Plot of modulation depth vs σy* for each θ
mode[6].
Table 1: Observable σy* Vary with Fringe Pitch, Which
Depends on θ
Crossing
174 deg
angle θFringe pitch 266 nm
dMeasurable 20- 110 nm
σy *30 deg

8 deg

2 deg

1.028 μm

3.81 μm15.2 μm

80- 400 nm 360 nm-1.4 μm

1.2 ~ 6μm

Comparison with IPBSM at FFTB
Shintake Monitor had first been put into practical usage
at Final Focus Test Beam (FFTB ) at SLAC, where it had
succeeded in measuring σ*y ~ 70 nm with 10% resolution,
whereas the design size was 40 nm[2,4].
Table 2 shows the major changes in IPBSM relatedparameters from FFTB to ATF2. In order to produce
narrower fringe pitches to measure the smaller ATF2 σ y*,was halved by SHG from 1064 nm to 532 nm. The

The laser source, located in a separate laser hut, uses
SHG to create 8 ns (FWHM) Nd:YAG Q-switch laser
pulses at λ = 532 nm with an intense peak power of 164
MW. The laser beam is delivered to IP through a 30 m
transport line containing intermediate mirrors andexpanders/reducers to adjust spot size and divergence.
After emerging onto the upright “vertical table” at IP,
the laser beam is divided by a 50% beam splitter into
upper and lower paths which are focused tightly by lenses
at IP, where they cross to form interference fringes. The
e- beam, also focused to a waist at IP, is collided against
these fringes, then dumped safely. Meanwhile Compton
signal photons proceed into the downstream γ detector.
Depending on the targeted σy*, “mode switching”
between θ = 174° , 30°, and 2° - 8° (continuously
adjustable) is carried out by remote control of moverstages carrying mirror actuators. Meanwhile, weakened
laser light is admitted to diagnostic devices e.g. PSDs,
PIN-PDs, and CCD cameras for real-time monitoring ofalignment, phase, timing, intensity and profile.

PERFORMANCEIPBSM and Beam Tuning
Measurement by IPBSM commences after σy* has been
tuned below about 3 µm, confirmable by wire scanners.
Multi-knob tuning of beam trajectory may affect shower
energy deposit into the γ detector, thus degrading
resolution. We check deviation of signal photon path bysliding in a movable collimator with a 10 mmϕ center,and if necessary adjust beam orbit or collimation.

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Proceedings of IBIC2012, Tsukuba, Japan

Expectations
IPBSM is capable of measuring σy* from 20 nm to afew μm with resolution better than 10%, assuming 90
bunch measurements, S/N = 3.5 and 50 % bunch-bybunch BG fluctuation[6]. The expected accuracy formeasuring the design beam size is−0

37 ± 2 ( stat.)+4 (syst.) nm(6)

The following subsections describe (a) statistical errors,
the source of signal jitters that hinder M detection, and (b)systematic errors, which cast a lower measurable limit forσy* by smearing fringe contrast, thus under-evaluating M.

Statistical Errors
It becomes more challenging to suppress statisticalerrors below 10% as σy* is focused down very small,since the stringent beam optics implemented to realize
sudden focusing may generate extra BG and lower S/N.
Measurement precision of a smaller σy* is impacted moreheavily by Compton signal fluctuations [6,7]. The major
sources are described as follows[6,7].
Detector Resolution: Resolution of the detector, which
separates signal from BG using their distinct shower
development, may become degraded when energy
spectrum is altered due to (1) Change over time in gainor light collectivity (2) unexpected BG levels and sources(3) photon energy loss due to collision with collimators(4) fluctuation in beam size or trajectory.
Laser Timing and Intensity: Because 8 ns (FWHM)
laser pulses interact with much shorter 16 ps e- beampulses, even a few ns change in laser timing will trigger
pulse-to-pulse inconsistency in the laser power felt by thebeam during its passage. Fluctuations in laser timing andtotal power are monitored using TDC and PD,
respectively, and are measured during typical operation ismeasured to be 1 - 2%, with precision better than 1 %.
ICT Monitor Resolution: By normalizing signal
energy by beam current measured using an “integrated
current transformer (ICT) monitor”, the impact frombeam current jitter is suppressed down to the monitor
resolution, measured to be about 2.5 - 5%.

Systematic Errors
Systematic errors are interpreted using “modulation
reduction factors” C i (i = 1, 2, ....). These smear laser
fringe contrast and reduce Mmeas from its theoretical valueas Mmeas = CαCβ.......Mideal i.e. over-evaluate σ y.meas as
σ2y.meas + |ΣlnCi|/2ky2. It is crucial to estimate the effectfrom these laser and/or beam related factors and correct
y,meas accordingly. Certain factors affect only 174 degmeasurement of σy* < 50 nm (see Table 3[6,7]).
Laser position and profile at IP: Laser crossing is
generally well aligned to e- beam center at IP within 0.1σlaser using mirror actuators with < 50 nm resolution. Fine
readjustment is conducted in between scans while
monitoring jitters or drift. Meanwhile, profile imbalance
between the two laser paths, due to lens misalignment,
cast significant local bias on fringe intensity.
Relative position jitter: Fluctuation in relative position
between beam and laser smears the M curve. By the time
we measure σy* < 50 nm, beam position jitter, likely frommagnet vibrations or unstable extraction from damping
ring, will be monitored by O(nm) resolution “IPBPMs”
Fringe tilt: A tilt between the e- beam and the transverse
plane on which laser fringes form can be observed fromthe relative offset between the two misaligned laser spots
upon the final focal lenses after laser paths have been
initially overlapped with beam position on a screen at IP.
Spherical wavefronts: If the e- beam collides at a point
offset from laser waist, it would feel “distorted” fringes,
due to the finite curvature of the spherical Gaussian
wavefronts. Movers attached to the 174 deg final lenses,
is expected to scan and align laser focal point to IP within
100 µm precision when measuring the smallest σy*.
Change of beam size within fringe: Measurement of an
ultra focused e- beam, whose waist is tuned exactly to IP,
is vulnerable to fluctuation in σy* inside the finite
longitudinal length of laser fringes. Alignment precisionmust be reinforced for the heavily impacted 174mode.
Table 3: Upper Limits of Each M Reduction Factor
Predicted for Measuring the Design σy* = 37 nm at ATF2.
Assumed here are nominal laser and ATF2 beam
parameters, as well as implementation of specificcorrection functions for the sensitive 174 deg mode usedin this case [6]Modulation reduction factor
Total power imbalance
Relative position jitter
Fringe tilt
Alignment (t, z)
Spatial coherence
Spherical wavefronts
Beam size growth within
fringe37 nm at 174 deg> 99.8 %> 98.0 %> 97.2% (tilt < 1 mrad)(> 99.6%, > 99.1 %)> 99.9%> 99.7%> 99.7%

Major Systematic Errors in Recent Beam Time
Table 4 summarizes M reduction factors analysed using
recent measurements of σy* near 500 nm. These error
sources were assessed through the 2012 summer upgrade.

ISBN 978-3-95450-119-9
Accelerator Technology

3c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○M detection requires precise laser position alignment,
and a sufficiently focused laser waist to be matchedstrictly to IP. First, laser spots are overlapped with ebeam spot within O(10 µm) precision on a screen monitor
inserted into IP. Following this is the finer “laserwirescan”, where one laser path at a time is scanned transverse
to e- beam using mirror actuators, and laser position is set
to the peak of the resulting Gaussian Compton signal.
Similarly, a “z-scan” resolves longitudinal offset toachieve the sharpest fringe contrast i.e. deepest
modulation. Laser Q-switch timing is synchronized withbeam timing using precise digital delay modules. Stable
operation requires suppression of timing jitter < 1 ns.
After completing all spatial and temporal alignments,
IPBSM continuously feeds the σy* it measures using
interference scans back to the beam focusing process.

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Proceedings of IBIC2012, Tsukuba, Japan

Here, one of the most dominant bias factors is profile
imbalance between upper and lower laser paths. This canbe inferred from the difference in peak energy and sigma(or σlaser) in Compton signal of a pair of laserwire scans.
Another major bias factor is the tilt of laser fringes
relative to e- beam. The alignment precision confirmablewith eyes after constructing laser paths at IP is about a
few mm, corresponding to a tilt of 5 – 20 mrad. Assuming(according to recent status) precision of 3 mm, and IP
σlaser ~ σx* ~ 10μm, this corresponds to a maximum Ctilt of
99 % and 83% for 8and 30modes, respectively.
Assuming design IP σlaser and σx* for 174  mode,
suppression of tilt to 1 mrad would improve Ctilt to better
than 97%, or 1-2 nm in σy* blow-up.

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Table 4: Upper limits of Dominant M Reduction Factors
for Measuring σy* ~ 500 nm at 4 deg mode, Estimatedusing Data from June, 2012
Modulation reduction factor
O(500) nm at 4 deg
Profile imbalance (t, z)(> 94%, > 89 %)Relative position jitter> 95 %
fringe tilt (t, z)> 95% (tilt < 20 mrad)
Alignment (t, z)(> 95%, > 99 %)Polarization> 98%

BEAM TIME STATUS OF IPBSM
Status in 2010 - 2011
In spring of 2010, due to tentative tuning issues, aspecial “10 × βy*” beam optics had been implemented.
Though this limited the smallest feasible σy*, it enabled
exceptionally high S/N, under which IPBSM measured
σy*= 300 ± 30 (stat.) +0 -30 (syst.) nm [6] at 8 deg mode.
In autumn 2010, βy* was restored to the nominal 0.1
mm, after which IPBSM measured σy* averaging at 280± 90 nm (stat.) at 5.96 deg[6,7]. After this laser optics was
immediately switched to commission the 30 deg mode inpursuit of smaller sizes. However M-reconstruction wasinterrupted by unexpectedly large signal jitters, then by
the Great Eastern Japan Earthquake on March 11, 2011.
While recovering the IPBSM system alongside rest of
ATF, we investigated signal jitter sources and upgradedhardware accordingly. One of major suspects was asignificant rise in BG levels, likely due to return of
nominal βy* optics. This lowered S/N to below 0.5, which
degraded detector precision. The long post-IP bending
magnet was discovered to be a major bremsstrahlung
source. An “intermediate collimator” was installed theer,
and its effectiveness was verified during the following
beam run. Instabilities in trajectory and profile for both ebeam and laser may also have caused large jitters.
We re-commissioned the interference mode for the firsttime after the earthquake by detecting M at 2mode
under 10 × βy* optics to measure σy* tuned down to 2.5 −3 μm. Due to certain issues in beam tuning, such as
having to experiment with several sets of (βx,βy*), σy*was tuned to only ~ 1 μm by the end of 2011. However
IPBSM demonstrated good resolution in continuously

measuring σy* at 2 - 8modes. A series of consecutive
scans at 5 marked the best status for this run, as σy* =1058 ± 23 (stat.) nm[7]. Meaningful data wereaccumulated for study of systematic errors and detector
resolution. More efficient scan software and display
monitors were introduced into the ATF2 control system.

Status in 2012
Beam focusing resumed in early 2012. On Feb 17, weachieved full commissioning of 30 deg mode under 10 ×βy* optics, followed by stable measurement of σy* around
150 nm under 3 × βy* optics. The best status came from10 consecutive stable scans which averaged at M = 0.56 ±0.02 (stat.), σy*= 166.2 ± 6.7 (stat.) nm. Figure 5 shows
one of these interference scans.
From there, we took advantage of the favorable laser
conditions to commence commissioning of the 174 deg
mode. At this point σy* has not been sufficiently focused,as βy* optics wasn’t at nominal. Low S/N and drifts inbeam condition made it even more difficult to reproduce
such slight M patterns. Thus time was spent in performing
various check-ups of hardware and scan methods.
A large part of beam tests was dedicated to optimizing
schemes for mode switching, optics setup, and multi-knob
tuning. At the lower 2-8 deg modes, we measured large M
of 0.8-0.9 i.e. clear fringe contrast, and used these toinvestigate systematic errors. This provided us with
important directions for the 2012 summer upgrade.

Summer Upgrade of 2012
Systematic errors concerning recent beam times arelargely due to misalignment of optical components or of
the laser beam itself. In 2012 summer, the vertical tablelaser optics was reformed extensively. The primary aims
are higher stability and reproducibility, crucial for
fulfilling ATF2 Goal 1. The new layout will also
accommodate certain upgrades for Goal 2 planned forearly 2013, e.g. installation of a new IP chamber withthree IPBPMs. The major upgrades points are as follows.
Firstly, reference lines were precisely redefined on newbase plates to guide the travelling laser path, and ensure
ideal setup of optical elements. Positioning of final lenses
is then strictly checked using CW alignment laser andtransparent targets. The focal scan system was introduced
for everymodes by adding movers to all lenses.
Consistency in laser path is particularly importantduring mode switching, as part of σy* focusing process.
Angle selection used to be executed by two sets ofrotating stages carrying mirrors, shared by all modes.
Because these mirrors are also responsible foroverlapping laser with e- beam at IP, it is difficult tomaintain laser path consistency when switching in
between modes to accommodate beam tuning. Therefore,
we replaced the rotators with a more reliable combinationof mirror on a small linear mover. With one set for eachmode, this enables independent mirror adjustments.
More effective alignment is expected to alleviate
multiple issues such as fringe tilt, non-ideal laser crossing,ISBN 978-3-95450-119-9
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Proceedings of IBIC2012, Tsukuba, Japan

Figure 5: One of the smallest σmeas* measured at
30°mode. M = 0.51 ± 0.02 (stat.), σy*= 168.5 ± 6.2(stat.) nm (Feb 23, 2012, 10 × βx*, 3 × βy*). Data isplotted as average of ten events at each phase, then χ2
fitted.

Stabilization of the Laser Optics
Stabilization and careful maintenance of theenvironment-sensitive laser system is of utmost
importance to successful operation. We regularly analyse
data collected from temperature sensors strung all aroundthe laser optics. The transport line and its mirror boxes
received insulation and anti-vibration reinforcements.
To maintain laser position stable over long operation
hours, “Beamlok”, a piezoelectric feedback device wasadded to the laser cavity, and demonstrates satisfactory
performance. Duo-lateral PSDs are installed at several
locations to monitor laser position drift and jitters. To
confirm compatibility for the upcoming beam run, each
PSD and readout DAQ electronics have been individually
recalibrated and tested for linearity and signal cross-talk.
Laser oscillation is maintained stable by regularinspections, tuning and exchange of laser cavity mirrors,
seeder and flash lamps. These also relaxed laser beam
divergence, improved pointing stability at IP, andsignificantly improved laser profile, which contributed tomeasurements of clear fringe contrast at lower  modes.

SUMMARY AND FUTURE GOALSPBSM is on its way to measuring ATF2’s design σy*while serving as a vital beam tuning tool. At present wehave achieved stable measurements of σy* > 150 nm.
Hardware upgrades have been carried out to further
suppress systematic errors. Particularly important is thereform of 2012 summer, aimed at improving laser path
reproducibility and alignment precision. In preparation forstart-up of the ATF2 dedicate runs on Oct.15, we are
currently undergoing a series of beam-off tests, such asconfirming component positioning precision andoptimizing laser spot size and divergence. The effects
from the upgrade will be further verified during beamtime, We expect to operate under significantly improved
precision and stability, and fully commission 174 deg
mode i.e. consistently measure σy* < 100 nm before year
end. This demands favorable conditions for both IPBSM
laser optics and beam tuning.
After achieving ATF’s goals, the next step is toupgrade IPBSM for utilization as the nm resolution beam
size monitor necessary for initial commissioning of the
ILC beamline. This demands major modification of laser
optics and detector to accommodate the higher beamenergy, smaller design σy*, and multi-bunch operation at
ILC. Although ATF2 is a scaled down prototype, thetechnologies verified there are directly applicable to ILC.
R&D of IPBSM is indeed valuable for realization of ILC–
like future Tev scale linear colliders.

REFERENCES[1] SB2009 Proposal Document, Rel.1.1, 2009[2] B.I. Grishanov et al., ATF2 Collaboration, ATF2 Proposal,
KEK Report 2005-2[3] T. Shintake, ”Measurement of Small Electron-BeamSpots”, Annu. Rev. Nucl. Part. Sci. 1999. 49:125-62.
[4] P.G.Tenebaum, ”Expanded Studies of Linear Collider Final
Focus Studies at the Final Focus Test Beam”, SLACReport-95-475 (1995).
[5] T. Suehara: “A nanometer beam size monitor for ATF2” inNuclear Instruments and Methods in Physics Research A
616 (2010) 1–8[6] Y. Yamaguchi, Master thesis at Graduate School of
Science, The University of Tokyo (2010)[7] J. Yan, Master thesis at Graduate School of Science, The
University of Tokyo (2011)

ISBN 978-3-95450-119-9
Accelerator Technology

5c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○offset injection into lenses, and profile imbalance due todifference in focal points between upper and lower paths.
Path length difference due to the delay line for fringe
phase control inserted into the upper path, is treated byadding a drift space created by image flopping mirrors.

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