Proceedings of IBIC2012, Tsukuba, Japan

MOPB60BEAM DIAGNOSTICS FOR AREAL RF PHOTOGUN LINAC
K. Manukyan#, G. Zanyan, B. Grigoryan, A. Sargsyan, V. Sahakyan, G. Amatuni,
CANDLE, Yerevan, Armenia
Abstract

INTRODUCTION
The beam diagnostic section for AREAL linac [1] canbe divided into two sections
x Gun section diagnostics
x Linac diagnostics
The commissioning of the AREAL linac will proceed
in two stages: in a first step (phase 1), the gun section
with additional diagnostics will be put into operation. For
phase 2 the full accelerator will be assembled.
The beam transport line between the electron gun andthe first accelerating structure (energy < 4 MeV) has alength of almost 0.95 m, which is completely occupied bydiagnostic devices. In this diagnostic line electron beam
charge, position, transverse profile, emittance (phase 1),energy and energy spread must be measured. In a gun
commissioning phase the gun section with additional
diagnostics (pepperpot) will be put into operation. The
schematic layout of the gun section with diagnostics ispresented in Fig. 1.

BEAM CHARGE
The task for the beam current and charge measurement
is to verify that the charge produced at the cathode iscompletely transported along the whole beamline to thebeam dumps. In AREAL linac the charge of individual
bunches will be measured using Faraday Cups andIntegrating Current Transformers (ICT) [2] (see Fig. 1, 2).
Three Faraday cups are intended to be used to collect andmeasure the beam charge. Two of them will be installedin the end of the spectrometer arms. The third one is aninsertable faraday cup which will be installed in the gun
section. Electrical connections are made to the base of theFaraday cups, terminating in a BNC connector. Theoutput signal is integrated on the oscilloscope and divided
by the termination to give a reading of the charge.
Specifications of Faraday cups are presented in Table 1.
Table 1: Specifications of ARIAL linac Faraday cups.
Non insertable

Insertable

Cup Diameter (mm)

15.1

9.5

Cup Length (mm)

75

69.5

Maximum Power (W)104Impedance

50 ΩSignal Output

BNC

Also two in-air ICTs are planned to be used as nondestructive devices for electron bunch charge monitoring.
Typical installations include bellows, a wall current
bypass and an electromagnetic shield covering the ICT
completely.

BEAM POSITION

Figure 1: Layout of AREAL gun section with diagnostics(phase 1).
After gun commissioning the pepperpot will be moved
to the end of the linac. Also high resolution Cavity BPM
will be installed in the straight arm of the gun section. In
linac diagnostics it is foreseen to measure electron beam
charge, position, emittance, energy, energy spread andlongitudinal profile. The schematic layout of the linac
with diagnostics is presented in Fig. 2.

Three BPMs are intended to be installed in AREAL
linac. High resolution Cavity BPM (CBPM) will be
installed in the gun diagnostic line in phase 2. It will
provide about 1~3 Pm resolution for electron beam
trajectory measurements.
A cylindrical “pillbox” with conductive walls of lengthl and radius R resonates at its eigenfrequencies

f mnp

1
2S P0H 022§ jmn · § pS ·¨¸ ¨¸ ,© R ¹ © l ¹with jmn - the n-th zero of the Bessel function Jm of orderm, p is an integer number. For the application as BPM thelowest transverse magnetic fundamental TM010 monopoleand TM110 dipole modes are of interest. The X and Y

ISBN 978-3-95450-119-9
Overview and Commissioning of Facilities

1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Advanced Research Electron Accelerator Laboratory(AREAL) based on photocathode RF gun is underconstruction at CANDLE. The basic approach to the newfacility is the photocathode S-band RF electron gun
followed by two 1 m long S-band travelling wave
accelerating sections. Linac will operate in single bunchmode with final beam energy up to 20 MeV and the bunchcharge 10 –200 pC. In this paper the main approaches andcharacteristics of transverse and longitudinal beamdiagnostics are presented.

MOPB60

Proceedings of IBIC2012, Tsukuba, Japanc 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Figure 2: Layout of AREAL linac with diagnostics (phase 2).
offset dependent (as well as the bunch charge dependent)
TM110 dipole modes provide information about beamdisplacements. The cavity fundamental mode TM010,
which is at a lower frequency compared with dipolemodes, provides the reference signal to obtain the bunchcharge and phase. For more details we refer to [2-3].
For the beam position monitoring after accelerating
sections two stripline type BPMs [2-3] will be installed in
AREAL linac (Fig. 2). Stripline BPMs (SBPM) are well
suited for short bunch observation. A stripline pick-up
consists of a transmission line of several cm length,
having at both ends a feed-through matched to 50 Ω.
Using three low energy corrector magnets, a beam
position feedback for the AREAL linac will be
implemented (see Fig. 1, 2). The corrector magnets musthave maximum peak field of about 35 G and maximum
10 cm magnetic length.

TRANSVERSE BEAM PROFILE
In the AREAL linac (for energies up to 20 MeV) only
scintillating crystals will be installed, because the othersystems (such as OTR screen or the wire scanner) would
not generate sufficient signal-to-noise ratio. Profile
monitors (scintillation screen monitored by CCD camera)
are almost always the simplest beam profile measurement
devices [2, 4]. A full horizontal/vertical profile can beproduced by a single beam pulse. The image is digitized,projected onto orthogonal axes, and fitted with anappropriate function. A “background image” (obtainedwithout beam) is subtracted from the data. The calibration
is obtained using reference grid lines etched directly onthe screen.
Four optical screen monitors will allow the control,monitoring and optimization of the transverse beamenvelope in the AREAL linac. Two of them will beplaced in the low energy diagnostic line (Fig. 1) and thenext two - after the accelerating section (see Fig. 2).
The YAG: Ce scintillation screens of 30x30 mm2 and20Pm thick will be used for the profile measurements.
The screens are mounted to a ladder and positioned in thebeam with a vacuum feedthrough by a stepper motor
actuator. The position is monitored by an absolute
encoder mounted to the feedthrough. The scintillators will
be mounted at right angle to the beam. A mirror reflects
the light out of the vacuum chamber.
The readout system of the screen monitors consist of
two selectable optical branches: an overview system thatimages an area covering 28 × 37 mm2 and a system thatcovers the central 5 × 8 mm2. The overview camera will
be used during initial commissioning and covers an arealarge enough to detect unfocused or misaligned beams. It
has a projected pixel size of 23 μm. The camera thatimages the central part of the screens has a projected pixel
size of 4.5 μm.
The radiation will be detected with a room-temperature
monochrome CCD sensor digitized with a 12-bit ADC.

TRANSVERSE BEAM EMITTANCE
As a diagnostic of the first step of commissioning,when only gun will be installed, it is planned to use apepperpot technique for emittance measurement. Becauseof space charge dominated, low energy beam this method
is most convenient. Detailed analyze of principles of workof pepper pot technique are described in [5].
A tungsten “pepperpot” mask of 5 mm diameter, with150 μm hole spacing and 20 μm hole diameter is planned
to be used. The mask thickness of 0.5 mm stops electrons
with energies up to 4 MeV and should thus provide
background suppression. YAG:Ce have been selected as
scintillation material to visualize the electron beamlets,
which are passing through the “pepperpot” holes. Their
predominant emission in the green spectral range (535 –550 nm) is well matched to the maximum sensitivity of
CCD cameras. For least scattering of electrons in thematerial and highest spatial resolution, screen thicknessesof 50 μm at 5 mm diameter have been chosen. Both,“pepperpot” mask and scintillation screen are mounted on
stainless steel sliders inside of a 900 mm long DN100CFUHV chamber. Cable winches, which are driven by a pairof rotary feedthroughs and deflection rollers at back endof the monitor, allow travel of the sliders along the entire
length of the UHV chamber. In this way, “pepperpot” andscintillation screen can be moved at any location along
the emittance monitor and the distance between both canbe freely selected depending on the divergence of theelectron beam. “Pepperpot” mask and scintillation screen
can both be moved out of the beam, when reaching a“parking position” at the front end of the UHV chamber.
An out-coupling mirror (90 mm in diameter, λ/20
planarity) at the back end of the monitor deflects thetransmitted light from the scintillation screen under anangle of 90° out of the UHV chamber.
At the second stage, when the gun section
commissioning will be completed the pepperpot will beISBN 978-3-95450-119-9
2

Overview and Commissioning of Facilities

Proceedings of IBIC2012, Tsukuba, Japan
moved to the high energy electron beam diagnostic line
(after accelerating sections). In this section it is alsoplanned to measure electron beam emittance using
quadrupole scan technique.

MOPB60the cherenkov radiation and the light yield depend on therefractive index of the material. A high refractive index

BEAM ENERGY, ENERGY SPREAD AND
LONGITUDINAL PROFILE

V x | E xH x  Dx2V E2 .
In order to maximize the momentum resolution of thespectrometer, the dispersive contribution to beam size
should be large compared to the emittance contribution.
The energy spread measured is then:VE |VxDx.

This is achieved by providing horizontal focus at
YAG:Ce doped screen (S1), behind the bending at
spectrometer arm. The setting for the focus is not known apriori, since space charge effects will perturb the beamtransport matrix transformation, and the focus will behard to find on the S1 where the beam is widened bydispersion. As we have another screen (S2), located in the
non-dispersive straight beam path before the accelerating
section (for solenoid field scan and achievement ofminimum beam spot size), we provide horizontal focus at
S1 installing it with the same path distance from the gun
as S2. The dipole is a gradient free rectangular magnet,
which behaves like a drift horizontally, so a focus
produced at S2 and well observable there, will be moved
to S1 after switching on the bending magnet.
The same technique will be applied after accelerating
sections for energy and energy spread measurements (seeFig. 2).
The electron beam longitudinal phase profile is planned
to be measured using the partial conversion of the energyof the electron bunch via cherenkov radiation [6] into aflux of photons with the same time properties as theelectron bunch. These light pulses are then measured witha streak camera which is placed at some distance to theinjector. To obtain a good time resolution high light yield
in a small opening angle is required. The opening angle ofFigure 3: Setup of the bunch length monitor system with
the Cherenkov radiator aerogel.
results in high light yield but poor time resolution.
Aerogel is considered for this task. The setup is shown inFig. 3.

SUMMARY
The diagnostics system of the AREAL linac ispresented. Electron beam charge, position, emittance,energy, energy spread and longitudinal profile
measurement techniques, which will be applied, aredescribed in details. The gun section (phase one) ofAREAL linac will be commissioned in summer 2013.

ACKNOWLEDGMENT
Authors express thanks to colleagues from PSI andDESY for support and useful discussions.

REFERENCES[1] B. Grigoryan et al, TUPC031, Proc. of IPAC’11.
[2] P. Forck, “Lecture Notes on Beam Instrumentationand Diagnostics”, JUAS 2012.
[3] M. Wendt, MOOC01, Proc. of DIPAC’11.
[4] R. Ischebeck et al, TUPD45, Proc. DIPAC’09.
[5] V. Schlott et al, TUPC25, Proc. DIPAC’07.
[6] J. Ronsch et al, POM016, Proc. of DIPAC’05.

ISBN 978-3-95450-119-9
Overview and Commissioning of Facilities

3c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○The momentum and momentum spread of the electron
beam can be measured through dispersion created by adipole magnet. Due to limited space for diagnostics at gunsection, 90° bending magnet is chosen (see Fig. 1).
Absolute momentum measurement is given by thegeometry and calibration of the dipole and the subsequent
drift length (about 20 cm).
The energy spread can be inferred by observing thebeam in a dispersive section. The spot size is a
convolution of the emittance contribution and thedispersion contribution and can be written to first order
as: