Proceedings of IBIC2012, Tsukuba, JapanTUPB70

THE ATF2 MULTI-OTR SYSTEM: STUDIES AND DESIGNIMPROVEMENTS∗
J. Alabau-Gonzalvo, C. Blanch Gutierrez, A. Faus-Golfe, J.J. Garcia-Garrigos, J. Resta-Lopez
IFIC (CSIC-UV), Valencia, Spain
J. Cruz, D. McCormick, G. White, M. Woodley, SLAC, Menlo Park, California, USA
Abstract
A multi-Optical Transition Radiation (mOTR) system
made of four stations has been installed in the extraction
line of ATF2 and has been fully operational since September 2011. The system is being used routinely for beam
size and emittance measurements as well as for couplingcorrection and energy spread measurements. In this paper we present the current design and a OTR monitor future upgrade to avoid the wakefields when a simultaneous
measurement is made. Finally we report the measurements
made in ATF2 during 2011-2012.

mOTR system in the diagnostics section of the EXT line,
highlighting the WS location for comparison.

ATF2 is an extension of the Damping Ring (DR) of ATF
built at KEK (Japan) [1, 2]. ATF2 is a prototype of a Final
Focus System (FFS) for a Future Linear Collider such as
the International Linear Collider (ILC) or the Compact Linear Collider (CLIC). The first of the ATF2 goals is to generate a vertical beam size of 37 nm at the Interaction Point(IP). The second ATF2 goal it to achieve nanometer-level
beam stabilization at the IP in order to be able to demonstrate the capabilitiy of this optics design to reliably deliver high luminosities at future high-energy linear colliders. A mOTR system was installed in the ATF2 extractionline (EXT line) during autumn 2010. This system consistsof four OTR monitors [3], and is currently being used forbeam size measurement and emittance reconstruction during the ATF2 operation. The mOTR system was installednear an existing wire scanner system (WS) [4] in order tocompare the performance of both systems. The WS needs
a high number of pulses to make a measurement, resulting
in an overestimation of the size due to the beam position
and intensity jitter. Moreover, it can take several minutes
to complete a single set of beam size measurements. On
the other hand, the OTRs are capable of single-shot measurements of the beam ellipse at the beam repetition rate
(1.56 Hz). This permits reconstructing the emittance withhigh statistics and making correlated measurements such as
studying the emittance preservation during the extraction
from the ATF DR. The minimum beam size that the OTR
system can measure is about 2 µm (the 2-lobe distributionof the OTR light starts to become a dominant factor at thisscale, whereupon a different measurement scheme would
be required). The measurement resolution of this system istypically a few-percent. Figure 1 shows the layout of the∗ Work supported by: FPA2010-21456-C02-01 and by Department ofEnergy Contract DE-AC02-76SF00515

Figure 1: Layout of the ATF2 diagnostics section of theEXT line where the WS and the Multi-OTR system are located, the beam going from right to left.

CURRENT DESIGN AND FUTUREUPGRADES
The 4 OTRs are based on the design of a previous one
labeled as OTR1X [5], placed near one of the WSs in order
to compare them and to demonstrate the ability to measure the small beam sizes likely to be found after a linear
collider DR. Some of the issues presented in the old design were improved in the new one: the footprint was lowered from 55 cm to less than 30 cm, the target actuator wasplaced on the top, thus reducing the interference problems
with the supporting structure, the lens darkening and thecamera damage due to radiation was solved by using a 90
degree mirror and a lead protection shield, the targets werechanged using two made of 2 µm aluminum and two targets made of 3 to 5 µm kapton with 0.12 µm aluminum
coating.
Figure 2 shows a general overview of the new OTR design and Figure 3 shows it as it is installed in ATF2 EXTline. The yellow scotch covers the main body and the side
window to protect from light coming in from outside. Below it, the vertical and horizontal stepper motors are shown.
From the body, in the upper-left direction is the target actuator and in the upper-right direction the optical elements.
The lead shielding for preventing CCD camera damage canbe seen on the left of the OTR. At the left and right sidesof the body there are a couple of bellows that allows somehorizontal and vertical displacement of the whole OTR system.

ISBN 978-3-95450-119-9
Transverse Profiles, Screens & Wires Monitors

1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○INTRODUCTIONTUPB70

Proceedings of IBIC2012, Tsukuba, Japan
screen so that the measured beam size was wrong.
Target actuator

Mitutoyo lense bea

OTR bodyme beam

CCD camera

Stepper motors

In the ”non-operation mode” the OTR body is placed ina position such that the incoming and the outgoing beam
pipe are straight (Figure 4 up). Due to design constraints
the whole OTR body has to be lowered for operation. Thisbrings the chamber closer to the beam (Figure 4 down).
This mode of operation generates wakefields that are significant when the 4 OTRs are in measuring position, i.e.
simultaneous measurement. This generates an emittance
growth that has been measured. Figure 5 shows the wakefield effect on the vertical and horizontal size at the OTR3X
when the OTR2X is lowered from the position where thebellows are horizontal and aligned with the beam pipe (herearound 5.5 mm) to the measuring position (which is the 0reference position in the horizontal scale).
165

160

σ\DW275>MP@σx at OTR3 [μm]c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Figure 2: 3D view of the new OTR design.

Figure 4: The OTR body is lowered to enable the target tointercept the beam.

155
150
145
140








01

234
Y OTR2 [mm]56






<275>PP@Figure 5: Effect in OTR3X beam size due to wakefields
generated in OTR2X when lowering it.
Figure 3: OTR as it is installed in the ATF2 EXT line, thebeam going from right to left.

In addition, a calibration lamp was installed, consistingin a bellow that permits the movement in and out the beam
pipe of a small lamp which is subjected with a ceramic tube
for isolation and feeded by a feedthrough BNC. Anotherupgrade with respect to the original design is a zoom system that can set a variable Field of View. This was motivated by the difficulty to find the beam in operation and thefact that sometimes the spot was slightly bigger than theIn order to avoid this effect without having to redesign
the whole OTR body, a modification of the target holderthat brings down the intersection between the optics line
and the target has been proposed. The mechanical layoutof the new holder is compared with the current one in Figure 6. In this way the whole body has to be lowered only
by 1.52 mm instead of 7 mm, thus reducing wakefield effects. The target itself has to be modified as well by having the mirror surface in the opposite side as in the current
design and a ramp has to be added in order to avoid interference of the actuator with the OTR body when the target

ISBN 978-3-95450-119-9
2Transverse Profiles, Screens & Wires Monitors

Proceedings of IBIC2012, Tsukuba, JapanTUPB70is extracted. The microscope lens also has to be changed
depending on the last design of the target holder. Theseimprovements are planned to be implemented in autumn
2012.

−5

3.5x 10Size at Diagnostics section of EXT line

the coupling. An automatic procedure based on a response
matrix algorithm has also been implemented in the ATF2
control system and works well when the coupling is low.
Moreover, the mOTR system has been used to measure
the beam energy spread giving a value consistent with thenominal one. This measurement was made by changing
the dispersion in the OTR location and measuring the consequent growth of the beam size. The mOTR is also helpful
to other studies such as investigating the source of the emittance growth after the DR beam extraction.

3

Vertical size [m]

2.5

20.5

2X
TRO
TR
3X

OO

TRO 0X
TR1X

1.5

1

Figure 8: OTR (OTR#X) and WS (MW#X) beam size comparison. Model in solid line. This set of measurements wasmade in the ATF2 run period of December 2011.

SUMMARY
165

170

175
180
Position along the line [m]

185

190

Figure 7: OTR (OTR#X) beam size measurement compared with the model, in solid line. This set of measurements was made in the ATF2 run period of March 2012.

EMITTANCE RECONSTRUCTION AND
OTHER STUDIESEmittance measurements are currently performed withthe mOTR system with success. The performance of thesystem has been compared with that of the WS. Figure 7
shows a set of measurements of the vertical beam size compared with the optical model while Figure 8 shows a measurement comparison with the WS system.
Apart from the emittance reconstruction the mOTR system is being used routinely for cross-plane coupling correction with success. The usual correction method consistsin scanning in intensity four skew quadrupoles installed upstream of the OTRs and then reconstructing the emittancewith the mOTR system. The optimal set of skew intensities
is the one which minimises the vertical emittance and thus

The mOTR is working well and is being used routinely
in beam emittance reconstruction and in coupling correction. Improvements are planned in both hardware and software including studies in the 4D emittance reconstruction,
single-shot coupling correction and automatic beam finding using the data from the nearer BPMs.

REFERENCES[1] ATF2 Group, “ATF2 Proposal Vol.1,CERN-AB-2005-035”,(August 11, 2005).
[2] P. Bambade et al. (ATF Collaboration), “Present status andfirst results of the final focus beam line at the KEK Accelerator Test Facility”, Phys. Rev. ST-AB Vol. 13, (April, 2010).
[3] J. Bosser et al., “Optical transition radiation proton beam profile monitor”, Nuclear Instruments and Methods in Physics
Research A238, (1985), 45-52.
[4] H.Hayano et al., Wire Scanners for Small Emittance Beam
Measurements in ATF, XX International Linac Conference,Monterey (2000).
[5] Marc Ross et al., A Very High Resolution Optical Transition
Radiation Beam Profile Monitor, SLAC-PUB-9280, (July,2002).

ISBN 978-3-95450-119-9
Transverse Profiles, Screens & Wires Monitors

3c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Figure 6: Current target and target holder in the left, newproposal in the right.