Proceedings of IBIC2012, Tsukuba, Japan

MOPB55

ELECTRON CLOUD MEASUREMENTS USINGSHIELDED PICKUPS AT CESRTA∗
J.P. Sikora† , J.A. Crittenden, J.S. Ginsberg, D.L. Rubin, CLASSE, Ithaca, New York, USA
Abstract

Figure 1: Cloud electrons enter the SPU through an arrayof small holes in the beam-pipe wall. The pickup is biased
at +50 V and the signal amplified by +40 dB before being
recorded by a digital oscilloscope.
Signals from the SPU are amplified and sent to an Agilent DSO6054 oscilloscope where they are digitized andaveraged for 8k traces. The accelerator timing system provides triggers to the oscilloscope [3].

INTRODUCTIONCOMPARING DATA AND SIMULATION

An important feature of the Shielded Pickup (SPU) are
the holes that connect its vacuum space to the that of thebeam-pipe, as shown in Fig. 1. The holes have a depth todiameter ratio of about 3:1 that is effective in reducing thestrength of the direct beam signal [1]. The holes also limit
the angular acceptance of the detector, so that it is sensitive
primarily to electrons with vertical trajectories.
Initial cloud electrons are generated when synchrotron
radiation strikes the walls of the vacuum chamber producing photo-electrons. These electrons will then strike thewall and produce secondary electrons. There are three categories of secondary electrons [2]. The first is that of the true
secondaries – where the incident electron interacts with thematerial in a non-trivial way and secondaries are produced.
The two other categories are from scattering of an incident
electron: elastics – where the energy of the secondary isequal to that of the incoming electron – and rediffused –electrons with energies up to the incident electron energy.
Each of the categories has a corresponding secondary emission yield value, denoted by: δts , δe and δr for true secondaries, elastic and rediffused respectively.

Most of our experiments have focused on two bunchdata, where bunches of equal population are injected into
the storage ring with different spacings. As shown in Fig. 2,
the signal from the second bunch is much larger than thefirst. The second bunch signal is dominated by electrons
that were generated by the first bunch being kicked into thedetector by the second bunch. So the signal after the second bunch represents a sample of the electron cloud thatwas produced by the first bunch.
The decay of the electron cloud is revealed by the superposition of a number of measurements with different bunch
spacings. Through a comparison of these signals with simulations, the different spacings have shown different sensitivities to the three categories of secondary electrons.
Synchrotron radiation is simulated at the locations of theSPU with the program Synrad3D [4] that includes beampipe geometry and multiple diffuse scattering of photons
throughout the CESR lattice. The simulation program
ECLOUD [5, 6] uses the azimuthal photon absorption distribution to generate photo-electrons based on a parameterization of quantum efficiencies and photo-electron energies. The production of secondary electrons is based on
the relative contributions from the three categories: true,rediffused and elastic. The vacuum chamber geometry anddetector response is included in the model.

∗ This work is supported by the US National Science Foundation PHY-0734867, PHY-1002467 and the US Department of Energy
DE-FC02-08ER41538, DE-SC0006505.
† jps13@cornell.edu

ISBN 978-3-95450-119-9
Miscellaneous and others

1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○The Cornell Electron Storage Ring has been reconfigured as a test accelerator (CESRTA) with positron or electron beam energies ranging from 2 GeV to 5 GeV. An areaof research at CESRTA is the study of the growth, decay
and mitigation of electron clouds in the storage ring. Electron Cloud (EC) densities can be measured with a ShieldedPickup (SPU), where cloud electrons pass into the detector
through an array of small holes in the wall of the beampipe. The signals produced by SPU have proved to be very
useful in establishing the mitigating effect of different vacuum chamber surfaces - including differences in quantum
efficiency as well as secondary and elastic yield. This has
been accomplished through the careful comparison of observed signals with the output of the EC simulation code
ECLOUD. We present example comparisons of data andsimulation that show the sensitivity of the measurements
to secondary and elastic yield. In addition, some data has
been acquired using a solenoid to produce a longitudinalmagnetic field at the SPU. We will also present our current
understanding of the effect of a longitudinal magnetic field
on SPU signals.

MOPB55

Proceedings of IBIC2012, Tsukuba, Japanc 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Figure 2: The SPU signal from two bunches of positrons,
36ns apart.
A comparison of measured signals and simulations withan uncoated aluminum chamber is shown in Figs. 3 and 4.
The dotted lines are the measured signal and the points with(statistical) error bars are the simulation. The plots are asuperposition of two bunch measurements similar to Fig. 2,but with six different bunch spacings.
The overall shape of the decay of the cloud is dominatedby δ0 , the elastic yield δe as the energy of the incident electrons approaches zero (Fig. 3). Details of the signal fromthe first bunch are affected by changes in δr (Fig. 4).

Figure 4: Changing the rediffused yield (δr ) in the simulation increases the tail of the signal from the first bunch.
In the upper plot, a near zero value of δr in the simulation
does not match the data well. A larger value of δr givesa much better match and also affects the amplitude of thelater second bunch signals.

SINGLE BUNCH WITH SOLENOID

Figure 3: The effect of changing δ0 in the simulation isshown above. The measured signal is shown as dots; thesimulation as symbols with statistical error bars. The simulation fit is much better with a δ0 of 0.4 (lower plot).

Up to now, the flight time of cloud electrons has set thescale of photo-electron energy distributions in this analysis.
The addition of a magnetic field should give additional information about the momentum of the electrons. Coils havebeen installed that can provide a longitudinal magnetic fieldin the region of the SPU detectors. Data with longitudinalmagnetic field can be seen in Fig. 5. As the field increases
up to 30 G, photo-electrons from the beam-pipe side wall
begin to enter the detector as sketched in Fig. 7.
In Fig. 6, the peak voltage of the detector signal is plotted at each magnetic field setting over a range of ± 150 G.
This was done at several different single bunch intensities
from 1 to 10 mA ( from 1.6 × 1010 to 1.6 × 1011 electrons/bunch). There is a significant asymmetry for positive
and negative magnetic fields as might be expected with asynchrotron radiation stripe primarily on one side of thechamber. Analysis of this data is just beginning.
A simplified conceptual model of the SPU with a longitudinal field that contains the following ideas: the electrons entering the detector are those with nearly verticaltrajectories so that they pass freely through the holes in thebeam-pipe wall (see Fig. 1). As a result, with a longitudinal magnetic field the electrons entering the detector will
follow trajectories that are circles with centers in the planeISBN 978-3-95450-119-92

Miscellaneous and others

Proceedings of IBIC2012, Tsukuba, Japan

Figure 7: With longitudinal magnetic field, electrons entering the detector will follow circles in the plane of the holes.

Figure 8: A simulation of the electron cloud signal from asingle bunch in a longitudinal magnetic field of 23 Gausswith rectangular beam-pipe shows peaks that correspond tomultiples of a quarter cyclotron period.
Figure 6: The peak value of the SPU signal is plotted ateach magnetic field setting. This was done with single electron bunch intensities from 1 to 10 mA.
of the detector holes as shown in Fig. 7.
Another important constraint is that the amount of time
for non-relativistic electrons to complete a cycle (or fixed
portion of a cycle) is determined by their cyclotron period
τcyc = 2πme /qB. This time is independent of the energy of the electrons. The beam-pipe was also simplified inthis model to be rectangular. As can be seen in Fig. 7, any
electrons that come from the top surface of the rectangular beam-pipe will arrive in a half cyclotron period, thosefrom the bottom will arrive in less than a quarter period,
and those from the side wall something between a half anda quarter period. So at a given magnetic field, the arrival
time of photo-electrons is determined mostly by their pointof origin rather than their energy.
An ECLOUD simulation of rectangular beam-pipe wasrun with a realistic synchrotron light distribution that included scattered photons. Its output agreed with the conceptual model by showing three signal peaks that corresponded to photo-electrons from the midpoint of the side
wall, another from the top of the beam-pipe and a broad
peak from secondary electrons produced on the ceiling as
shown in Fig. 8.
Simulations that also included existing beam-pipe geometry (non-rectangular) did not agree with the data until ad-justments were made to the photo-electron energy distributions. Work is continuing in order to establish the simulation parameters that will yield good agreement with dataover a wide range of magnetic field values and beam energies.

REFERENCES[1] M. Sands, “Energy Loss from Small Holes in the Vacuum
Chamber,”, PEP-253, Sept. 1977.
[2] M. A. Furman & M. T. F. Pivi, “Probabilistic Model for theSimulation of Secondary Electron Emission,” Phys. Rev. STAccel. Beams 5, 124404 (Dec. 2002).
[3] J. P. Sikora, et al., “Time Resolved Measurement of Electron Clouds at CESRTA Using Shielded Pickups ,” in Proc.
of PAC’11, New York, NY, USA, August 2011, WEP195,p.1855, (2011).
[4] G. Dugan & D. Sagan, “Synrad3D Photon Propagation andScattering Simulation,” 49th ICFA Advanced Beam Dynamics
Workshop on Electron Cloud Physics, Ithaca, NY, K. Smolenski, Ed. (in press), Paper PST08.
[5] F. Zimmermann, G. Rumolo and K. Ohmi, in ICFA BeamDynamics Newsletter, K. Ohmi & M. Furman, Eds., International Committee on Future Accelerators, No. 33, p. 14-24(Apr. 2004).
[6] J.A. Crittenden, et al., “Electron Cloud Modeling Results forTime-resolved Shielded Pickup Measurements at CESRTA,”
in Proc. of PAC’11, New York, NY, USA, August 2011,
WEP142, p.1752, (2011).

ISBN 978-3-95450-119-9
Miscellaneous and others

3c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Figure 5: Data from the SPU with a 2 mA single bunch ofelectrons with a longitudinal field from 0 to 30 Gauss.

MOPB55