Proceedings of the 14th Annual Meeting of Particle Accelerator Society of Japan
August 1-3, 2017, Sapporo, Japan

PASJ2017 THOM07

PYECLOUD SIMULATIONS OF THE ELECTRON CLOUD FOR THE
J-PARC MR
B.Yee-Rendon∗ , R. Muto, K. Ohmi, M. Okada, K. Satou, M. Tomizawa and T. Toyama
KEK/J-PARC, Tsukuba, Ibaraki, Japan
SIMULATIONS

Abstract
Recently Electron Cloud (EC) simulations were done using an updated version of the computational model developed at KEK. The results obtained were consistent with themeasurements during the slow extraction operation modefor the J-PARC Main Ring (MR). Additionally, new EC simulations were performed using PyECLOUD program. This
code was created at CERN and it is the update version ofECLOUD program. The main advantage of the PyECLOUD
code with respect to the one at KEK is the adjustable numbers of the macro-particles which allowed to manage thelarge amount of electrons created when the multipactor condition is reached. In addition, this work used a more accurate
model for low energy electrons than the previous simulations, consequently, the EC density simulated has a goodagreement with the one measured in the surveys. Additionally, due to the continued upgrade in beam power at fast
extraction mode for the MR, the EC could reappear again inthis scheme, therefore, PyECLOUD simulations were done
to estimate the EC density at these beam conditions

INTRODUCTION
The MR of J-PARC accelerates protons to the energy of
30 GeV in two modes: Fast Extraction (FX) for the Neutrino Experimental Facility and Slow Extraction (SX) for
the Hadron Experimental Hall [1]. The presence of the EC
at SX mode is a main concern for the successful operationand the power upgrade of the machine. Several EC surveys
were done to understand the conditions to enhanced the EC
build-up and provided countermeasures [2, 3], in addition,
EC simulations were developed to corroborate our assumptions about the source of this phenomenon and estimate its
impacts for the upgrade conditions [4–7].
The simulations of the EC started with a code developed
at KEK from Ohmi for the KEK Photon Factory [8], theprogram was used to estimate the EC build-up for the JPARC accelerators [4–7]. The latest results agreed with theEC observations and support the idea of the microbunch
structure as one of the main conditions for EC build-up [7].
Furthermore, a new study of the EC was done using thecode PyECLOUD developed at CERN [9], one of the advantages of this program is that allows to handle the exponential
increase of the electron during the EC build-up and the possibility to use with the PyHeadtail code to estimate the effectsof the EC in the transverse beam instability [10].
∗ byee@post.j-parc.jp

PyECLOUD is the update version of the code
ECLOUD [11], it have been extensively used to reproduce the EC measurements in the CERN accelerators.
The general description of the PyECLOUD can be foundin previous works [9]. Table 1 shows the main parameters
used in this simulations, these values were similar that theone used in the last study [7]. The major improvements
were adopted similar model for the Secondary Electron
Yield (SEY) and the energy spectrum of the true secondaries
electrons.
Table 1: Simulations Parameters for the JPARC MRParameters

Units

Value

EnergyGeV30
Beam PowerkW 37 (SX) and 500 (FX)
Circumferencem1567.5
Beam pipe radiuscm6.5
rms bunch size
cm0.5
Ionization cross section
M barn2R00.7E0eV150δmax
1.7s1.35
EmaxeV287
σtrue
1.082
µtrue
1.663
Vacuum pressure at 5 ms1µPa
0.2
Vacuum pressure at 75 ms1 µPa
0.9Time stepns1
The SEY employed is defined asδtot al==δelastic + δtrue√√Eδmax s Ema
E − E + E0 2x
R0 ( √) +(1)√Es − 1 + ( Ema)s
E + E + E0x

where R0 , E0 , δmax , s, Emax are the parameters obtained
from the fit for the elastic and the true secondaries contributions [12]. The first part correspond to the electrons that interact in elastic way with the chamber wall (δelastic ) and thesecond one the electron has a complex interaction with theatoms of the materials the so called true secondaries (δtrue ).
These values are strong dependence of the material and thegeometry of the beam pipes. Figure 1 shows the curves of
the SEY used for MR J-PARC simulations, additionally, a1- 197 -The time after debunching starts (P3).

Proceedings of the 14th Annual Meeting of Particle Accelerator Society of Japan
August 1-3, 2017, Sapporo, JapanPASJ2017 THOM07

subplot for small range of electron energy is included toobserve more clear the contribution of the elastic part.

Figure 1: The total SEY (δtot al , green solid line) is the sumof the contribution for the elastic part (δelastic , blue dot line)
and the true secondaries (δtrue , red dot line).
The energy spectrum of the true secondaries electrons isrepresented by a log normal distribution [9]dntrue1(ln(E) − µtrue )2=√exp −2dE
2σtrue2πEσtrue(2)

σtrue and µtrue were from the fit and they are strong
dependence of the material of the beam pipes. In this work,
the values used were obtained from measurements of otherexperiment [13].
In addition, the EC build-up at FX were analyzed to estimate the potential risk during this acceleration mode.

RESULTS
The previous work showed the relevance of the microbunch structure of the beam for the EC build up in thesimulations [7]. Smooth bunch shape (low frequencies components) produced less electron that one which have a severemicrobunch structure (high frequencies components).
Figure 2 top presents the beam current collected by theFast Current Monitor (FCT) and used as input for the simulations; on the bottom: the comparison between electron
flux measured by the EC detector (green dot line) and thecomputed using PyECLOUD (blue dot line) at the beginning of the debunching process when the beam has a smoothbunch shape are shown.
In the same way, Figure 3 shows similar data for a timeof 75 ms after the debunching started: when the EC reachedthe highest intensity and the beam distribution has a more
spiky longitudinal profile.
In Figure 2 the signal recorded for the EC detector wasonly detector noise and the calculated by the simulations wasaround 12 nA/cm2 . In constrant, Figure 3, the simulations
reproduced the peaks observed in the measurements.
In addition, comparisons between electron flux of the measurements and the simulations were done for the period ofFigure 2: Top: the beam current; bottom: the comparison of
the electron flux signals measured (green dot line) and thesimulated (blue dot line). These values corresponded to thebeginning of debunching process at SX mode of MR.

time in which the EC signal was more intense (See Figure 4).
In the case of the measurements the signal was averaged
over 100 turns, the error bars included the beam fluctuations
and detector noise. For the simulations the signal was averaged over the last 3 turns, the statistics errors were included.
Similar as Figure 3 bottom, there was in agreement between
the location of the peaks between the measurements andsimulations, nevertheless, the code underestimated the highof peak about 20% in average.
Finally, the EC buildup for the beam configuration of FX
has been under study. In this case the beam profile signal
recorded during neutrino experiments and the same vacuum
pressure from the SX studies were used.
Figure 5 shows beam current (top) and the electron flux
signal (bottom), the electron flux was lower than the produced any case at SX. Most of the electron produced forthis case were for trailing edge multipactor mechanism, however, it can be observed some smaller peaks than can notcorrespond to this mechanism, the reason is that some small
bumps were presented between the bunches of the longitudinal distribution. Those came for some noises in the mea-- 198 -Proceedings of the 14th Annual Meeting of Particle Accelerator Society of Japan
August 1-3, 2017, Sapporo, Japan

PASJ2017 THOM07

during the conversion from the voltage signal (recorded bythe oscilloscope) into current.

Figure 3: Top: the beam current; bottom: the comparison of
the electron flux signals measured (green dot line) and thesimulated (blue dot line). The values corresponded at 75 ms
after the beginning of debunching.

Figure 5: The beam profile for FX measured by the neutrinogroup at J-PARC (top) and the corresponding electron clouddensity simulated by PyECLOUD (bottom), using the samevalues of the SX simulations for the rest of the parameters.

CONCLUSION AND OUTLOOK

Figure 4: The comparison of the electron peak flux measured (red dot line) and the simulated (blue solid line). The
error bars are included

surements, the experiment could only measured one time
the longitudinal profile, that were not completed removed

PyECLOUD was able to reproduced similar results as theprevious code [7]. The code reconfirmed the importance ofthe bunch structure for the enhancing of the EC. In addition,
PyECLOUD allowed us to made a direct comparison between the signal recorded by the EC detector and simulated
one as is seen in the Figure 3. There was a good agreementin the location of the peaks (Figure 3), nevertheless, thesimulations underestimated their peak amplitude. Moreover,Figure 4 shows that the peak produced by the code were in
average 20% lower than the measurements.
This difference could be due to the models of the SEY andenergy spectrum of the secondary for this simulations. Forthe SEY, the δtrue was based in previous measurements several years ago [14], thus, the SEY curves must be changed
due to scrubbing effect, moreover, for δelastic the valuesof the copper were employed. In the case of the energy
spectrum, the parameters were obtained in other experi-- 199 -Proceedings of the 14th Annual Meeting of Particle Accelerator Society of Japan
August 1-3, 2017, Sapporo, JapanPASJ2017 THOM07

ments [13]. In addition, when the electron hit the collector
plate this produced also secondaries electrons, this effects isnot simulated.
For the case of FX (See Figure 5), in the actual operation
the bunch shape is smooth and has a high current, this conditions produced that the electrons that impacts the walls had
an energy beyond 40 keV. At that energy region the rate ofsecondary electron production is lower than one, thus, EC
build-up was not reached.
During the last year, EC simulations were developed to reproduced the signals measured during the studies and corroborate the main condition that enhanced the EC. To continueimprove the EC simulations and shed light on the conditionsof the EC several surveys were done or are under process. For
instance, the measurements of the energy distribution which
impacts the beam walls presented in this conference [3] andthe measurements of the SEY of the same materials of MR
beam walls.

[9] G. Iadarola and G. Rumolo, “PyECLOUD and build-up simulations at CERN”, Proceedings of the ECLOUD12 workshop,
5-9 June 2012, La Biodola, Isola d’Elba, Italy; G. Iadarola,“Electron Cloud studies for CERN particle accelerators andsimulation code development”, CERN-THESIS-2014-047.
[10] B. L. K. Shing et al., “ Code Development for Collective
Effects", CERN-ACC-NOTE-2017-0008.
[11] G. Rumolo and F. Zimmerman, “Practical User Guide forECloud”, CERN-SL-Note-2002-016 (AP).
[12] R. Cimino et al., Phys. Rev. ST Accel. Beams 18,051002 (2015).
[13] J. J. Scholts, D. Dijkkamp, and R. W. A. Schmitz, “Secondaryelectron emission properties,” Philips J. Res., vol. 50, pp.
375–389, 1996.
[14] M. Okada et al., “Control of the Multipactoring by the Surface
Coating of the Exciter Electrodes in J-PARC MR”, in Proceedings of the 7th Particle Accelerator Society of Japan (2010).

ACKNOWLEDGMENT
The authors thank to G. Iandarola, G. Romulo, A. Romanoand M. Schenk from CERN, K. Sakashita and M.Friend fromNeutrino group and the the members of J-PARC Main Ringfor their support on this work. This work was partially supported by JSPS KAKENHI Grant Number JP16H06288,
Grant-in-Aid for Specially promoted Research titled “Measurement of CP symmetry of neutrino by upgrading T2K
experiment”.

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