Proceedings of IBIC2012, Tsukuba, Japan

MOPB56

ELECTRON CLOUD MEASUREMENTS USING A TIME RESOLVEDRETARDING FIELD ANALYZER AT CESRTA∗

Abstract
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, decayand mitigation of electron clouds in the storage ring. With aRetarding Field Analyzer (RFA), cloud electrons pass into
the detector through an array of small holes in the wall ofthe beam-pipe. The electrons are captured by several collectors, so that the electron flux can be measured as a function of horizontal position. Up to now, we have time averaged the collector currents to provide DC measurements.
We have recently designed and constructed a new Time Resolved RFA (TR-RFA), where the collector currents can beobserved on the time scale of nanoseconds. We present
a summary of the design, construction and commissioning of this device, as well as initial beam measurements atCESRTA.

tors [5]. The current of electrons into the detector is timeaveraged and recorded. The devices installed at CESRTA
have segmented collectors that allow this electron current
to be measured as a function of horizontal position [6].
These devices have been useful in measuring the effectiveness of mitigation techniques, the horizontal distribution of
the electron cloud and in constraining EC model parameters [7].
The Time Resolved RFA (TR-RFA) combines the properties of these two devices by providing a time resolved
collector signal and a grid that can be used to obtain additional energy information. Figure 1 shows a cross sectionof the beam-pipe with a TR-RFA detector. The middle ofthree grids is used to provide the retarding field. There arenine collectors, each 0.6 cm wide and 7.5 cm long, etchedon a kapton flex circuit that are connected directly to individual SMA feed-throughs as shown in Fig. 2. During
data taking, the collectors are biased at +50 V to prevent
secondary electrons from leaving the collector surface.

INTRODUCTION
The Time Resolved Retarding Field Analyzer (TR-RFA)
is a natural extension of two other instruments that havebeen very successful in the study of electron clouds (EC):
Shielded Pickups (SPU) and Retarding Field Analyzers(RFA).
The SPU is simple device for sampling the time resolved
growth and decay of the electron cloud [1, 2]. The SPU
measures the flux of cloud electrons into the beam-pipewall by allowing a sample of this current to pass into thedetector through an array of small holes. The holes also
isolate the SPU electrode from the direct beam signal –the depth to diameter ratio of the holes of about 3:1 provides significant attenuation [3]. This is especially important when recording time domain data. Time domain information has been used to map the growth and decay ofthe electron cloud, measure the effectiveness of mitigation
techniques and to constrain the parameters used in electron
cloud simulations [4].
In the case of a Retarding Field Analyzer (RFA), cloud
electrons also enter the detector through an array of holes.
In addition, a retarding grid can be biased negatively tosuppress low energy electrons from reaching the collec∗ Work supported by the US National Science Foundation
PHY-0734867, PHY-1002467, the US Deptartment of Energy
DE-FC02-08ER41538, DE-SC0006505, and the Japan/US Cooperation Program† jps13@cornell.eduFigure 1: A cross section of the TR-RFA shows the geometry of the grooved beam-pipe extrusion and the relative
location of grids and collectors.

INITIAL BEAM TESTS
Early in 2012, two prototype TR-RFAs were installed atCESRTA in beam-pipe of round cross section, one made ofbare aluminum, the other with the vacuum surface coatedwith TiN. They were located in two of four chicane dipolemagnets, replacing standard RFAs that had been in use previously. A bias of +50V was applied both to the retarding
grid and the collectors during initial data taking. Cables
25 cm long route each collector signal to cascaded pairs ofISBN 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 ○J.P. Sikora† , M.G. Billing, J.V. Conway, J.A. Crittenden, J.A. Lanzoni,
X. Liu, Y. Li, D.L. Rubin, C.R. Strohman, CLASSE, Ithaca, New York, USA
M.A. Palmer, Fermilab, Batavia, Illinois, USA
K. Kanazawa, KEK, Ibaraki, JapanMOPB56

Proceedings of IBIC2012, Tsukuba, Japanother signals either remain steady or increase.
Generally, the performance of the prototypes was successful although the direct beam signal is significant. Therewas also a resonance in the detectors due to details ofgrounding that could be excited by the direct beam signal.
Initial data was taken only after some effort was made toreduce the ringing of the detector with termination of thegrounded grids and filtering of the retarding grid bias.

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Figure 2: Bottom view of the nine collectors on a kapton
flex circuit. Each collector is connected to an SMA feedthrough. Collectors are numbered in order of horizontal
position, with 1 toward the outside of the storage ring.

Figure 3: A test chamber with a TR-RFA installed on roundbeam-pipe. The thin pipe on the right connects to a small
ion pump. The thin pipe on the left carries wires for biasing
the grids.
Mini-Circuits ZFL-500 amplifiers for a voltage gain of 100.
The amplified signals are sent out to an array of Agilent
DSO6054L oscilloscopes where data is averaged for 8ktraces. The accelerator timing system provides the scopeswith a trigger on each turn of the stored beam.
Initial results can be seen in Fig. 4, where the signal fromthe TR-RFA in the bare aluminum chamber is shown withand without a dipole magnetic field of 790 G. The beam in
the storage ring consisted of a 10 bunch train of positronsat 5.3 GeV with 14 ns spacing. A witness bunch is alsopresent 112 ns after the train. Without magnetic field, collector 4 (near the horizontal center of the beam-pipe) has
the largest signal. When the field is turned on, all of thesignals are reduced in amplitude, but collector 4 is reduced
more than the others. This suppression of the central signal with magnetic field has also been seen in measurementswith standard RFAs [8].
The peak voltage of each collector is plotted for each chicane magnet setting between zero and 790 Gauss in Fig.5.
The signal in all collectors is reduced considerably evenwith low magnetic fields. This is probably due to the deflection of the low energy photo-electrons (produced bysynchrotron light) so that they do not propagate out into
the region of detector sensitivity. As the magnetic field increases, the signal from collector 4 decreases steadily as

Figure 4: Signals from the nine collectors of the aluminumsmooth wall chamber with 10 bunches of positrons at 8mA(1.3 × 1011 particles) per bunch and one witness bunch
112 ns after the train. The upper plot is with no dipole
field; the lower plot is at 790 Gauss.

Figure 5: Under the same conditions as Fig. 4, the negative
peak voltages from each collector are plotted over the rangeof magnetic fields from zero to 790 Gauss.

ISBN 978-3-95450-119-9
2Miscellaneous and others

Proceedings of IBIC2012, Tsukuba, Japan

MOPB56SUMMARY AND FUTURE WORK
Four Time Resolved Retarding Field Analyzers havebeen installed in test chambers at CESRTA. Electron cloud
signals are measurable with short bunch trains and someinteresting effects have been see in the data as a function ofmagnetic field.
The new TR-RFAs have also be connected to the original time averaging electronics and data has been taken inthis configuration. A comparison of the two data taking
methods is in progress.
The upcoming experimental run in late 2012 will offer
an opportunity to do some systematic data taking. This will
include using the retarding grid to obtain additional energy
information. A good deal of work also needs to go into
simulation of the beam-pipe geometries and the detectors,
as has been done with the existing Retarding Field Analyzers and Shielded Pickups.

Figure 6: Signal from the nine collectors of grooved chambers with 10 bunches of positrons, 6mA (9.6 × 1010 particles) per bunch. The upper plot is from the TiN coatedchamber; the lower plot is bare aluminum.

REFERENCES[1] E. Mahner, T. Kroyer and F. Caspers, Phys. Rev. ST Accel.
Beams 11, 094401 (Sep. 2008).
[2] 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).
[3] M. Sands, “Energy Loss from Small Holes in the Vacuum
Chamber,”, PEP-253, Sept. 1977.
[4] J.A. Crittenden, et al., “Electron Clouds Modeling Results forTime-resolved Shielded Pickup Measurements at CESRTA”,
in Proc. of PAC’11, New York, NY, USA, August 2011,
WEP142, p.1752, (2011).
[5] R.A. Rosenberg, K.C. Harkay, “A Rudimentary Electron Energy Analyzer for Accelerator Diagnostics”, Nucl. Instrum.
Meth. Phys. Res., A 453, pp. 507–513 (2000).
[6] M.A. Palmer, et al, “Design, Implementation and First
Results of Retarding Field Analyzers Developed for The
CESRTA Program”, in Proc of PAC’09, TH5RFP030, Vancouver, BC, Canada, (2009).
[7] J.R. Calvey, et al., “Update on Electron Cloud Mitigation
Studies at CESRTA”, in Proc. of IPAC’11, San Sebastián, August 2011, MOPS083, p.796, (2011).
[8] J.R. Calvey, et al., “CESRTA Retarding Field Analyzer Measurements in Drifts, Dipoles, Quadrupoles and Wigglers”,
in Proc. of IPAC’10, Kyoto, Japan, May 2010, TUPD023,
p.1973, (2010).

ISBN 978-3-95450-119-9
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3c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○FOUR CHAMBER ASSEMBLY
In August 2012, four test chambers were assembled andinstalled at CESRTA in order to test their (EC) mitigation
performance. A TR-RFA was installed in each chamber.
The chamber extrusions have two round cross sections, onesmooth and the other with grooves at the top and bottom of
the chamber as shown in Fig. 1.The grooves are intended as
an EC mitigation technique in dipole magnets. Two chambers of each cross section were made, each about 66 cm
long. One pair has a surface of bare aluminum and theother pair has a coating of TiN on the vacuum surface. They
were assembled and installed with each chamber centeredin a dipole of a four magnet chicane.
The construction of the chambers required particularcare in machining the 270 holes of 1.7 mm diameter for thedetector in the grooved chamber. To avoid burs on the inner surface of the grooves, tests were made with mounting
wax and normal machining as well as Electrical Discharge
Machining (EDM) fabrication of the holes. EDM was used
for all of the chambers. The rows of holes were intentionally staggered so that the signal from each collector would
be the average over the grooved surface beneath it.
Given the large beam induced signal seen in the prototypes, the depth to diameter ratio of the holes could havebeen changed to increase attenuation. The hole geometry
was left unchanged in order to better compare the data fromthe original standard RFAs and the TR-RFAs to be installedat the same location.
Some preliminary data was taken shortly after installation. A 10 bunch positron beam at 5.3 GeV was injected
into the storage ring to 60 mA total current. Data from thetwo grooved chambers without a magnetic field is shownin Fig. 6. As expected, the signal from the bare aluminumchamber is larger than that from the TiN coated chamber.
Additional analysis is required to determine whether or notthere is quantitative agreement with the model. The direct
beam signal appears to generate ringing in the data that issimilar to that of the prototype.