Proceedings of IBIC2012, Tsukuba, Japan

MOPB73

BEAM SIZE AND INTENSITY DIAGNOSTICS FOR A SRF
PHOTOELECTRON INJECTOR
R. Barday∗ , A. Jankowiak, T. Kamps, A. Matveenko, M. Schenk, F. Siewert, J. Völker,
Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Germany
J. Teichert, Helmholtz-Zentrum Dresden-Rossendorf, Germany

A high brightness photoelectron injector must be developed as a part of the BERLinPro ERL program [1, 2].
The injector is designed to produce an electron beam with
100 mA average current and a normalized emittance of
1 mm·mrad. Prior to reaching the final gun/cathode design
a staged SRF gun development program has been undertaken which began with the operation of a fully superconducting injector utilizing a lead photocathode. This will befollowed by a normal conducting CsK2 Sb cathode capableof generating high current beams. In the first stage we havemeasured the fundamental beam parameters bunch charge,
beam energy and energy spread with a special focus on themeasurement of the transverse beam profiles. We will also
discuss our plans for the beam characterization at high currents.

of copper in the dispersive arm with a width of 2 mm and a
thickness of 4 mm.
Measuring the low energy electrons utiliting a Faraday
cup can be challenging due to backscattering of primaryelectrons and secondary electron emission. The number ofbackscattered electrons per incident electron for primaryelectrons with energy of 1 MeV impinging on a copper target is about 0.23 [6]. The secondary and backscatteredelectrons can be trapped if the Faraday cup has a sufficientlarge ratio of length to diameter, which is commonly notpractical. Alternatively the Faraday cup can be electricallybiased to suppress low energy electrons, which is the casein our beamline. The bunch charge is measured with an
integrating current transformer (ICT).
Cryomodule

Warm part

CathodeDrive Laser Slit
PortICTINTRODUCTION
In the first stage of the high brightness photoinjector
R&D [3, 4] the electron beam is produced from a Pb photocathode in a superconducting RF gun cavity operating at
1.3 GHz. A superconducting lead photocathode was initially directly deposited on the back wall of the gun cavity and irradiated by a UV laser operated at 258 nm with
2-3 ps pulse duration. The maximum average beam current extracted from this gun was 50 nA corresponding to abunch charge of about 6 pC. The maximum beam energy ofabout 2.0 MeV was achieved at a peak field on the cathodeof 22 MV/m.
In the second version of this type of the SRF gun, a thinlayer of lead with a diameter of 5 mm and a thickness ofabout 400 nm was deposited on a Nb plug, which can be inserted into the backplane of the cavity. The plug is vacuum
sealed with an indium gasket. The advantage of this design is a possibility to change the photocathodes and totest different deposition methods. The gun cavity with thecoated plug was tested up to the peak fields on the cathodeof 35 MV/m without laser irradiation [5].

DIAGNOSTICS BEAMLINE
A schematic overview of the diagnostics beamline isshown in Fig. 1. A dipole spectrometer magnet is used
to measure the beam energy and the energy spread. To improve the energy resolution we installed a vertical slit made∗ roman.barday@helmholtz-berlin.deSRFCavity0BendingMagnetYAGScreen

Beam
Dump

Stripline
SC Solenoid

1

YAG-Screen
Faraday Cup

2

YAG-Screen

3Beam
Dump

4m

Figure 1: Schematic view of the diagnostics beamline.
An electron beam is visualized by the image of the scintillation light emitted by the cerium-doped yttrium aluminium garnet (YAG:Ce) crystal with Ce-doping concentration of 0.2 %. YAG is a vacuum friendly material witha short decay time and high light output of ∼35 photons
per keV of absorbed energy. In order to avoid accumulation of the absorbed electric charge in the crystal, a thin,
10 nm, Indium Tin Oxide (ITO) coating was applied. ITO
has high electrical conductivity and is optically transparent. Initial tests with an uncoated YAG crystal irradiatedby low energy electrons showed damage, which look likefine scratches shown in Fig. 2.
To provide a better control of the electron beam andto avoid collisions between the Faraday cup and the viewscreen (which happened in the first version of the beamline with a YAG screen installed perpendicular to the Faraday cup), the YAG:Ce was incorporated into the Faradaycup (Fig. 3). In this design the free standing view screenwith a diameter of 32 mm and a thickness of 100 µm was
mounted at a 45 degree angle with respect to the incomingISBN 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 ○AbstractMOPB73

Proceedings of IBIC2012, Tsukuba, Japan

Figure 2: Photograph of the view screen after collision withthe Faraday cup on the left and scratches on the uncoatedview screen from low energy electrons on the right.

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Figure 4: Example of the solenoid scan.
beam. A 45 degree mirror located in the accelerator plane
perpendicular to the beam direction reflects the beam image onto a charge-coupled device (CCD) camera [7]. The
camera is mounted vertically below the accelerator plane tominimize its damage due to radiation. The CCD-Camera issensitive to the light within the range of 400 to 1000 nm
which has a maximum at 500 nm and is well suited for thedetection of the scintillation light from the YAG:Ce witha maximum radiation wavelength of 550 nm. The CCD
chip has dimensions of 659X494 pixels with a pixel sizeof 9.9X9.9 µm and is digitized with a 12 bit ADC.

tungsten plate
current
measurement

e beamview screen

Figure 3: Schematic view of the Faraday cup with aYAG:Ce view screen and a slit.

Transverse Beam Profile
The beam emittance can be measured using a standard
solenoid scanning technique, where the transverse beam
size is measured for different focal lengths of the solenoid.
An example of the solenoid scan is shown in Fig. 4. The
electron beam was focused to waists of ∼350 µm [8].
Additionally, the transverse emittance can be measured
using a single slit mask and a view screen while keeping
the focal length of the solenoid fixed. A horizontal slit ina tungsten plate with a width d of 0.1 mm and a thicknessof 1.5 mm provides a compromise between requirements

on a good resolution and a tolerance of the angular alignment. The stopping power of tungsten at the beam energyof 1 MeV is 19.3 MeVcm2 /g, so that we expect a sufficient
suppression of the background.
Both techniques are based on the precise beam size measurement, so, the spatial resolution of the view screens isa critical point. The scintillation mechanism involving following phases [9]:• energy conversion• thermalization of electrons and holes• energy transfer to the luminescent centers• photon emission at the scintillating sites
can limit the scintillator resolution. Here we consider only
the geometric effects and an interaction of the primarybeam with the view screen and not the processes of the light
generation. For a low energy electron beam the spatial resolution of the screen can be limited by multiple Coulomb
scattering of electrons in the screen. If we ignore single
electron scattering at large angle, the multiple scattering
angle of electrons penetrating through a material with a radiation length of Z0 is given by [10]r
13.6MeVzz
θrms =
1 + 0.038ln,
(1)
βpc
Z0
Z0
where p is the electron momentum, βc is the velocity of theparticles and z/Z0 is the thickness of the material in radiation lengths (Z0 =3.5 cm for YAG:Ce). For a point electron
source with beam energy of 1 MeV the RMS size of theelectron beam √at the exit of the screen with an effective
thickness of z= 2d can be estimated asss Z Z
13.6MeVz3022
θrms dz dz ∼∼ 53µm. (2)βpc
3Z0However, the electron beam appears for the CCD-Camera
with an RMS size ofs
13.6MeVz3yrms ∼,
(3)
βpc
12Z0

ISBN 978-3-95450-119-92Transverse Profiles, Screens & Wires Monitors

Proceedings of IBIC2012, Tsukuba, Japan

where n is the index of refraction. Equation 4 shows thatthe screen thickness has a significant influence on the spatial resolution in the horizontal plane. From this point ofview, materials with a large index of reflection, like BGO
with n=2.15, or diamond with n=2.4, provide a better spatial resolution, but the influence of the reflection index isweak. For the YAG:Ce view screen with an index of reflection n=1.82 the beam size seen by the camera for a point
electron beam is limited by the thickness of the scintillator Xeff ∼ d. This corresponds to the √minimum RMS sizein the horizontal plane xrms = Xeff / 12 ∼ 30 µm. Additionally, multiple Coulomb scattering also contributes tothe beam size, so that the screen resolution in the horizontal plane for an electron beam with a finite beam size islarger than 30 µm estimated for a point electron beam without scattering.

X eff

d450

nded/sin450

through a thin beryllium vacuum window and hits a viewscreen (Fig. 6). Two different materials were investigatedas a view screen: a free standing chemical vapor deposited(CVD) diamond with a thickness of 150 µm and a freestanding YAG:Ce crystal with a thickness of 200 µm. A
mirror reflects the scintillation light into a CCD-Camera
oriented at ∼155 degree angle with respect to the incoming
beam. A CVD-diamond has an excellent thermal conductivity of ∼2000 W/(mK), which is two orders of magnitude
higher than for YAG:Ce and is well suited for the visualization of the beam with high intensity.

Be-window view screen

mirrore1550

CCD-Camera
Figure 6: Layout of the experimental setup.
In this experiment the electron gun operated with 0.1 ms
and 1.0 ms long macropulses separated by 1 s. Within themacropulse the bunch repetition rate was changed between
0.1 and 26 MHz at a bunch charge of ∼20 pC giving amacropulse charge between 0.2 and 290 nC. The light intensity for both screens was measured as a function of themacropulse charge. Both screens showed a linear dependence from the charge, but YAG provides light output twoorders of magnitude higher than diamond for the same settings of the CCD-Camera as shown in Fig. 7. An example of the beam profile measurement is shown in Fig. 8.
No optical attenuator was used between the screen and theCCD-Camera in this measurement.

Figure 5: Beam size seen by the camera in the horizontal
plane for a point electron beam.

BEAM CHARACTERIZATION AT HIGH
BEAM INTENSITY
A test of the view screens at high beam intensity was
performed at Helmholtz-Zentrum Dresden Rossendorf. An
electron beam with a momentum of 15 MeV/c passes

Figure 7: Light output intensity measured with aCVD-diamond and a YAG:Ce scintillator.

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 ○giving a spatial resolution of the screen in the vertical planeof 26 µm. Here we neglected the electron energy loss andassumed a homogeneous emission of the scintillated lightalong the scintillator.
In the horizontal plane an effective beam size seen by thecamera due to the tilt of the screen (Fig. 5) can be estimatedfor our geometry asd+δd1
Xeff = √ = √1+ √,(4)
222n2 − 1

MOPB73

MOPB73

Proceedings of IBIC2012, Tsukuba, JapanFUTURE DEVELOPMENTS AT HZBFigure 8: Beam profile measurement using a YAG:Ce viewscreen and a Gaussian fit.

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Optical Measurements
A white-light interferometer (magnification 20X) was
employed to measure the surface roughness of theCVD-diamond. Figure 9a shows an unpolished side of thediamond, which is formed from the polished silicon wafer
on which it is grown. The ridges with a transverse size ofabout 10 µm and a height of several nanometer are due tothe polishing of the silicon wafer. Figure 9b shows a polished side of the diamond, which is free of such artifacts,
but the peak-to-peak roughness is about a factor of 4 worse.
Such structures can scatter the light and have a potential toaffect the spatial resolution at a beam size comparable tothe size of the ridges. After the irradiation of the diamondscreen with an electron beam no surface damage was observed.
Area: 235.2 x 235.2 um

Sda:

0.412 mR

Area: 235.2 x 235.2 um
6.683 nm

(a)

Sda:

An implementation of the CsK2 Sb cathode will allow aproduction of the electron beam with an average current ofup to 100 mA and bunch charge of 77 pC at 1.3 GHz repetition rate. YAG:Ce crystal provides a high light output andis suitable for the beam visualization at low intensity, but
it has a moderate spatial resolution [11]. YAG:Ce exhibitsa decay time of ∼70 ns, which is longer than the distance
between the bunches at 1.3 GHz. At a charge density of∼ 0.01 pC/µm2 a saturation blurring appears due to limited amount of Ce sites available for emission. We want tostudy the influence of the Ce activator concentration varying from 0.1 to 0.2 % in the YAG crystal on the spatial resolution and saturation effect which occurs at high chargedensity. We want to install a screen holder with five different view screens (Fig. 10) to compare a spatial resolution of a standard YAG:Ce, LuAG:Ce, CRY19 (trade namefrom CRYTUR), CVD-diamond with a wire scanner serving as a reference signal. The wire scanner utilizes a tungsten wire with a diameter of 40 µm. At high beam intensitya CVD-diamond screen is preferable and we want to study
the influence of the radiation damage on the efficiency ofthe radiator at high intensity.
CRY19
YAG:Ce(100 µm, 0.1% Ce) (50 µm)

YAG:Ce(100 µm, 0.2% Ce)

0.698 mR
27.14 nm(b)

Diamond(150 µm)

LuAG:Ce(50 µm)Tungsten wire
(40 µm)Figure 10: Holder for the view screens investigation.

-6.681 nm
Sq:
Sa:
St:

2.227 nm
1.628 nm16.87 nm

Points: 230400
QUARTIC

480x480
1/2" CCD
1.0X Body
No Relay
520 nm Phase
20X

Area: 235.2 x 235.2 um

Sda:0.174 mR
1.375 nm(c)-27.07 nm
Sq:
Sa:
St:

9.036 nm6.450 nm64.81 nm

Points: 230400
QUARTIC

480x480
1/2" CCD
1.0X Body
No Relay
520 nm Phase20X

Area: 235.2 x 235.2 um

Sda:

The transverse beam size at high currents will be alsomeasured using a double slit system with a shield to protect
the precise slit at high beam power based on the Cornell
design [12].

0.130 mR
0.880 nm(d)

ACKNOWLEDGMENTS
The authors acknowledge fruitful discussions withA. Bondarenko, A. Burrill, G. Kube and D. Böhlick.
Work supported by Bundesministerium für Bildung undForschung and Land Berlin.
-1.377 nm
Sq:
Sa:
St:

0.459 nm0.299 nm13.50 nm

Points: 230400
QUARTIC

480x480
1/2" CCD
1.0X Body
No Relay
520 nm Phase
20X-0.877 nm
Sq:
Sa:
St:

0.293 nm0.234 nm2.224 nm

Points: 230400
QUARTIC

480x480
1/2" CCD
1.0X Body
No Relay
520 nm Phase
20X

Figure 9: Unpolished (a) and polished (b) sides of theCVD-diamond; front (c) and back (d) sides of the YAG:Cecrystal.

REFERENCES[1] A. Jankowiak et al., Proceedings of LINAC’10, TUP007,p. 407.
[2] J. Knobloch et al., Proceedings of IPAC’12, MOPPP015,p. 601.
[3] T. Kamps et al., Proceedings of IPAC’11, THPC109,p. 3143.

ISBN 978-3-95450-119-9
4Transverse Profiles, Screens & Wires Monitors

Proceedings of IBIC2012, Tsukuba, Japan

MOPB73[4] A. Neumann et al., Proceedings of IPAC’11, MOODA03,p. 41.
[5] J. Sekutowicz, Proceedings of EuCARD’12.
[6] R. Ito et al., Rad. Phys. Chem. 42 (1993) 761.
[7] http://www.baslerweb.com[8] J. Völker et al., Proceedings of IPAC’12, TUPPD051,p. 1518.
[9] P. Lecoq et al., Inorganic Scintillators for Detector Systems,(Springer Verlag, Berlin-Heidelberg, 2006).
[10] G.R. Lynch et al., NIM B58 (1991) 6.
[11] G. Kube et al., Proceedings of IPAC’12, WEOAA02,p. 2119.

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

5c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○[12] I.V. Bazarov et al., Phys. Rev. ST Accel. Beams 11 (2008)
100703.