Proceedings of the 16th Annual Meeting of Particle Accelerator Society of Japan
July 31 - August 3, 2019, Kyoto, Japan

PASJ2019 FRPI022

KEK のコンパクト ERL でのビームコリメータと 60 pC 電子ビームに対するウェーク場評価
A WAKE FIELDS EVALUATION FOR BEAM COLLIMATORS AND THE 60 pC
ELECTRON BEAM AT THE COMPACT ERL AT KEK田中織雅#, A), 中村典雄 A), 帯名崇 A)， 谷本育律 A)， 宮島司 A)， 島田美帆 A), ノーベルピークノラ B)
Olga Tanaka#, A), Norio Nakamura A), Takashi Obina A), Yasunori Tanimoto A), Tsukasa Miyajima A), Miho ShimadaA),
Nora Peak NorvellB)
A)High Energy Accelerator Research Organization, KEKB)SLAC National Accelerator Laboratory

Abstract
When the high intensity particle beam passes through locations with narrow apertures such a collimator’s rods, it leads
to the creation of the unwanted wakefields. At the Compact ERL at KEK we have five beam collimators (one in theinjector line, one in the merger section, and three in the recirculation loop), which are used for the beam halo reduction
and for the beam loss localization. Those collimators consist of four cylindrical rods of 7 mm radius made of copper.
They could be independently inserted from the top, bottom, left and right sides of the beam chamber. The transverse wakefield may affect the beam emittance and the longitudinal wake field can cause the energy loss and the energy spread. In
the present study we investigated the collimator's impact to the beam. We evaluated the collimator's wake fields throughthe CST simulations. The estimated transverse kicks, longitudinal wakes, and expected energy losses are demonstrated
here.

1.

INTRODUCTION

The Compact ERL (cERL) at KEK [1] has fivecollimators (one in the injector section, one in the merger
section and three in the recirculation loop, see Fig. 1) toremove the beam halo and to localize the beam loss. An
operation at 10 mA average beam current and 1.3 GHz
repetition rate is planned in the near future. The
collimator’s wakefields are expected to play an importantrole, even when the bunch charge is increased up to 60 pC.
Current beam parameters of the cERL are summarized inthe Table 1.

Figure 1: A layout of the cERL and its collimators.
All cERL collimators consist of four cylindrical rods of7 mm radius made of copper. They could be independently
inserted from the top, bottom, left and right sides of thebeam chamber. Collimators COL1 – 3 were designed forthe straight sections, therefore they have a round chamber
50 mm radius made of stainless still. Its schematic is given
at Fig. 2.a. Note that the energy at collimators COL 1 – 2is 4 MeV (see Table 1), while the energy at rest of them is17.6 MeV. Collimators COL4 – 5 are dedicated to the arc

section, thus their chambers are elliptical 70 x 40 mmdiameter.
Table 1: cERL Electron Beam Parameters
ParameterBeam energy [MeV]:
Injector
Recirculation loop
Bunch charge [pC]
Repetition rate [GHz]
Bunch length (rms) [ps]Energy spread [%]Normalized emittance(rms) in injector [µm·rad]:Horizontal
VerticalDesign

In operation

4
17.6
60
1.3
2
<0.1

4.05
17.5
60
1.3
4.5
0.12

<3<3

2.88±0.09,
1.99±0.20

Materials used are the same. The detailed scheme can befound at Fig. 2.b.
In the present study, first, we have estimated transverse
kicks imposed by the collimator’s rods. This calculation isneeded to account for the beam blow up (emittance growth)
associated with collimator’s wake. Then, longitudinal
wakes were summarized to obtain the expected energy
losses of the beam passing through the collimator and its
energy spread. Finally, we have compared those resultswith results of the measurements using cERL beam.
Present study is helpful towards the IR-FEL upgrade ofcERL [2 – 3].

___________________________________________# olga@post.kek.jp- 1074 -Proceedings of the 16th Annual Meeting of Particle Accelerator Society of JapanJuly 31 - August 3, 2019, Kyoto, Japan

PASJ2019 FRPI022

Figure 3: A summary of the transverse wake kick factorsof the collimators.
Table 2: Expected Values of the Emittance Blow-Up for
the Collimator Half Gap 1.5 mm
βy [m][μm×rad]
1.15
27.47
1.25
19.23
0.954
34.76
0.954
6.99
0.954
6.99

Collimator
Figure 2: A schematic of the collimators with chambers
made of stainless steel and rods made of copper: a.
Collimators COL 1 – 3 for the straight sections; b.
Collimators COL 4 – 5 for the arc sections.

2.

INFLUENCE
WAKEOFTHE

COLLIMATOR

2.1 Transverse wake and emittance growth
Let us consider first transverse wakefields created by thevertical rods of the collimator. The summary of simulation
results together with analytical calculations isdemonstrated at Fig. 3. We have estimated the emittanceblow-up for the 60 pC electron bunch at cERL using
analytical expression [4]:Δε yε y0

= 1+

β yσ ω2− 1,ε y0

COL1
COL2
COL3
COL4
COL5

=4=4
= 17.6= 17.6= 17.6Δεy/εy0[%]
1.05
0.84
3.82
1.61
1.612.2 Longitudinal wake and energy spread
Now, let us consider a problem of longitudinal wakefields excited by collimators. The values of the wake-loss
factor were evaluated numerically through the CST
simulation for half-gap values in the range from 0.1 to 1.5mm. For the analytical description, the following equation
was considered [6]:k =(1)Z 0cbln   .
2π 3/ 2σ z  a (3)where the value Δ is the transverse emittance growthwith respect to the initial emittance. The rms of thecentroid kicks caused by the longitudinally varying field
could be found as follows [5]:σω =Q rmsk ⊥ y0 .
E/e(2)In Eq. (2) the value
is the beam energy at the locationof collimator (see Table 1). The value = 60
is theis the beam centroid offset,bunch charge. The value
and lastly, the value
is the rms kick factor, estimatedfor the bunch head-tail difference in the kick. For Gaussianbunch= /√3.
Expected values of the emittance blow-up due to thecollimator half gap 1.5 mm are summarized in Table 2.
Since for 60pC per bunch and burst mode of theoperation the emittance growth effect is expected to besmall, let us treat longitudinal wake.

Figure 4: A wake-induced energy spread for different
values of the collimator half gap and bunch lengths 2 ps(blue line) and 4.5 ps (red line).

- 1075 -Proceedings of the 16th Annual Meeting of Particle Accelerator Society of Japan
July 31 - August 3, 2019, Kyoto, Japan

PASJ2019 FRPI022

In Eq. (3) the value= 120 is the impedance of thefree space, = 3 ∙ 10/ is the speed of light,
isthe bunch length, = 25
is the vacuum duct’s half
aperture, andis the collimator’s half gap.
The wake-induced energy spread for Gaussian bunch
due to one collimator for the 60 pC bunch is the following:
σ E 0.4 × 60 [ pC ] × k [V pC ](4)=.
17.6 [ MeV ]E
The factor 0.4 appears since the rms energy spread of aGaussian bunch is given by krms = k 2 3 − 1 ≈ 0.4k .
The dependence of the energy spread on the collimator’shalf gap for the designed (2 ps) and current (4.5 ps) bunchlength is demonstrated at Fig. 4. Now, when all impacts aresummarized, let us demonstrate measurement results andthey comparison with theoretical predictions.

3.

is theHere
is the number of data points, andvalue of the i-th data point. The rms beam size is given by:

1 659
2
N i ( xi − xc ) .
(6)
N i =1
Results of the processing of all six measurements aredemonstrated at Fig.6. The rms beam size is not changed
significantly within the error bar except in the case of the1.5 mm half gap. This is consistent with the impedance
evaluation. Apparently the beam core was found to bedamaged when the collimator’s half gap become 1.5 mm.

σx =BEAM-BASED MEASUREMENTS

3.1 Energy spread measurement
For the measurement of the energy spread caused by thecollimator’s longitudinal wake, we used COL3 locatedbefore first arc entrance, CAM13 located between COL3
and the entrance and CAM15 located just in the middle ofthe arc (see Fig. 1). CAM13 needed to monitor the thebeam spot, which was successively cut by collimator’s rods.
The measurement itself was done by CAM15. To do so,first, we have restored the history of the quadrupole
magnets to have the best beam spot at the COL3 location.
Then we have degaussed all quadrupoles of the first arc
between CAM13 and CAM15 to make the dispersion
maximized. We have measured the dispersion to be 2.41 m.
The default energy spread was σE/Edefault = σz/η = 0.117%.
It is a ratio of the rms beam size to the dispersion. However,in the following we care only the change of the energy
spread, not its default value.
Next step was to insert the collimator COL3. We used
two horizontal rods, because the beam spot at thecollimator location is known for its vertical beam halo.
Therefore, we have avoided an influence of the halo on ourenergy spread measurement. We have performed themeasurement for the half gap values 2 mm, 1.5 mm, 2 mm,2.5 mm, 4 mm, COL out correspondingly. Related rms
beam sizes and beam profile peak positions were recorded
at CAM15. The raw data of the beam profile was fitted
automatically by the python routine at the control room.
Then we have processed it manually using Gaussian fitting
routine and weight analysis. An example of themeasurement data processing could be found at Fig. 5.
Here the upper image is a CAM15 beam spot, the blue
curve at the bottom plot is the raw data, the red line is its
Gaussian fitting, and the magenta mark denotes the peak
position with respect to the data weight.
Weight analysis [7] gives the following expression forthe profile peak position:
659
1 659
xc =  xi Ni , where N =  Ni .
(5)
N i =1i =1

Figure 5: An energy spread measurement data at CAM15:
beam spot (top), raw data and its fitting (bottom).

Figure 6: A horizontal beam size at CAM15 with respect
to the horizontal collimation.
3.2 Influence of the beam halo
The location of the collimator COL3 (before the arcentrance, Fig. 1) is known for the considerable vertical- 1076 -Proceedings of the 16th Annual Meeting of Particle Accelerator Society of Japan
July 31 - August 3, 2019, Kyoto, Japan

PASJ2019 FRPI022

beam halo [8]. The correspondent beam spot at CAM13 isshown at Fig.7. Thus, by inserting vertical rods of thecollimator COL3, one can study the beam halo influenceon the energy spread and consequently manage the beamquality by removing the halo by the collimator.

was set to 5.31 mm, 5.75 mm, 6.25 mm, and COL3 outcorrespondingly. Then the proper data processing gave
results, presented at Fig. 8. One can see the horizontal
beam size is slightly decreased with the cutting away thevertical beam halo. This means that the beam halo
increases the energy spread.

4.

Figure 7: A beam spot at CAM13: without (top), and with(bottom) the collimation.

Figure 8: A horizontal beam size at CAM15 with respect
to the vertical collimation.
To study this effect, we performed the samemeasurements as was described in the previous section.
Although the vertical collimation was allowed. A series offour measurements was done. The collimator’s half gap

CONCLUSION AND OUTLOOKWe have performed a study of the collimator’s wakeinfluence on the 60 pC electron beam at cERL. It should betaken into account for an intense short bunch, when aconsiderable beam collimation is required. We haveestimated the expected emittance growth due tocollimator’s wakefield under current operational
conditions at cERL. It is a few percent or less.
Collimators are also generating the wakefield effect thatbrings an additional energy spread. At cERL we find it tobe 0.0028 % at 17.5 MeV. Easy to account for the energy
loss per bunch at one collimator. It is= ∥ =168.7nJ
for the 60 pC bunch. This value is acceptable since themachine is operated in burst mode currently. The CW
operation (1.3 GHz, 80 mA, 2 ps) with 219.3 W power loss
might require an additional machine protection.
Experimentally we have found, that for current beam
parameters (bunch charge is 60 pC (burst mode), bunch
length is 4.5 ps, energy spread is ~0.12%, beam energy is
17.5 MeV (injector energy is 4.05 MeV) even if one put
the half gap of the collimator up to 2 mm, the emittanceand energy spread are not affected so much. Thus weapproved the beam collimation at cERL.
Considering future cERL upgrade to the IR-FEL, apossibility of consequent degradation of the FELperformance should be taken into account. Correspondentpower loss was obtained as 13.7 W (81.25 MHz, 5 mA, 2ps). In this paper, we have shown numerical, analytical andmeasurement results on the collimator’s wakefields thatwill be important for the next step operation.

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[2] R. Kato et al., “ Development of high repetition middle
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[3] O. Tanaka et al., “High bunch charge operation of cERL for
infrared free electron laser test”, in these proceedings,FRPI025.
[4] M. Dohlus, T. Limberg, “Impact of Optics on CSR-RelatedEmittance Growth in Bunch Compressor Chicanes”, in Proc.
of 21st Particle Accelerator Conference (PAC05), Knoxville,USA, May. 2005.
[5] S. Di Mitri, PhysRevSTAB 13,052801 (2010).
[6] S. Heifets and S. Kheifets, “Coupling impedance in modernaccelerators”, Rev Mod Phys 63, 631 (1991).
[7] P.R. Bevington and D.K. Robinson, “Data Reduction andError Analysis for the Physical Sciences”, McGraw-Hill,(1992).
[8] O. Tanaka et al., Phys. Rev. Accel. Beams, 21 (2018) 024202.

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