Proceedings of IBIC2012, Tsukuba, Japan

THCB01

ELECTRON-LENS TEST STAND INSTRUMENTATION PROGRESS*
T. A. Miller†, J. Aronson, D. M. Gassner , X. Gu, A. Pikin, P. Thieberger,
C-AD, BNL, Upton, NY, 11973, U.S.A.
Abstract
In preparation for installation of Electron Lenses [1]into RHIC, planned for late 2012, a working test stand (or“test bench”) is in use testing the performance of the gun,collector, modulator, instrumentation and controls. While
testing & operating the instrumentation, both progress andpitfalls were encountered. Results are presented fromissues including ground loop signals generated by the
DCCTs, static magnetic field interference, competing
YAG screen illumination techniques, YAG crystal
damage during beam operation, performance of the four
quadrant beam scraper electrodes, and challenges inmeasuring beam current in conductors. Working
knowledge and insight into each of these systems has
been gained through difficulties leading to success. These
insights are presented with supporting data and images.

Ground Current Problems in Pulsed ModeAs the 5kV CPS has its collector side grounded, acurrent transformer was installed in the ground connection
in an attempt to measure ground return currents frombeam loss to the grounded vacuum structure. Aside fromthe ringing on the pulses (discussed in the next section), a
large distortion of the pulse was measured in the gun CT
that was exactly compensated by an opposite distortion in
the ground CT. Note that in the waveforms shown in Fig.
1, the summation (gold) of the gun (red) and ground(blue) current pulses exactly equaled that measured at thecollector (yellow).

The major components of this test bench [2] include anelectron gun (tested from 500ns to DC, 1.0A, 5keV) andan electron collector, normal & superconducting
solenoids, a 5kV collector power supply (CPS) between
gun & collector, and a 10kV fast modulator controlling
the gun’s anode. The system is equipped with pulse &DC current transformers at the gun and collector power
supply, a beam profile monitor composed of both YAG
crystal screen & camera and a pin hole Faraday cup
intercepting the scanned electron beam, a four quadrant
halo monitoring electrode array mounted just upstream of
the collector, and an ion collecting Faraday cup electrodewithin the collector just downstream of the electron
reflector [3]. As a complete overview of the diagnostics
used was presented last year [4], details and experiences
learned during recent system tests are presented in theirrespective sections below.

CURRENT MEASUREMENT
Beam current measurement is made with both DCCTs
and pulse CTs placed on the conductors near the gun andcollector connections, as shown in Fig. 1. The DCCT is a
Bergoz IPCT-C-02 providing a resolution of 10uA over arange of 1-2000mA through a 2.7” aperture. It issensitive from DC down to 100us pulses. The pulse CT isa Pearson model 6585 with 1V/A ratio, 50Ω output, 1.5ns
rise time and a 0.3%/µs droop. This arrangement
provides measurement of 1 – 5µs pulses without anoticeable droop and pulses longer than 100µs out to DC.
However, ringing of the current pulse edges in the
conductors obscures pulse shapes over the first 15µs.
____________________________________________* Work supported by B.S.A, LLC under contract No. DE-AC0298CH10886 with the U.S. Department of Energy.
†tmiller@bnl.gov

Figure 1: Current measurement system layout. Groundreturn currents found during pulsed beam operation
impede attempts to measure beam loss currents.
The onset of the beam pulse current is believed to besupplied by stray capacitance of the floating instrument
racks containing the gun power electronics on the cathodeHV deck. This capacitance is shown in Fig. 1 as Cstray.
An attempt to compensate for this undesired return path to
the CPS via ground was made by installing a 5µF HVstorage capacitor just before the gun CT, as shown in Fig.
1 as Ccomp. This completely eliminated the groundcoupled beam pulse current, except for the ringing, andbrought agreement between the gun and collector CTs.

Ringing in Pulsed Beam Current Measurements
Initially, the fast edges (~50ns) of the beam pulse
caused ringing in the currents measured by the gun,collector as well as ground CTs. Attempts were made to
mitigate this ringing by adding RC filters (0.1µF, 50Ω) toeither side of the 50Ω RG213 HV coaxial cable bringing
power from the CPS to the cathode deck. However, theseattempts have proved unfruitful.
Further attempts were made, such as providing acapacitive bypass around the ceramic break, shown in Fig.

ISBN 978-3-95450-119-9
Overview and Commissioning of Facilities1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○INTRODUCTIONTHCB01

Proceedings of IBIC2012, Tsukuba, Japan

1 at the interface between the green and black portions(ground and collector) of the accelerator. The effect wasevident but only as a decrease in characteristic frequencyof the ringing. The amplitude was unchanged.

New Gun CT

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○To improve measurement of the beam current pulse, a
new pulse CT, labeled “Gun2” in Fig. 1, (also a Pearson
6585) was installed very close to the gun, and encircling
both the cathode heater and HV common conductors, as
the current is split between both conductors. The result,shown in Fig. 2 (cyan), evidences greatly reduced ringing
in the measured pulse compared to the gun (red) &collectors (yellow) CTs.

Figure 2: Waveform from the new CT placed near the gun(blue – increased amplitude) installed just below theconductor feedthroughs; compared to those (red &yellow) placed ~1m away along the conductors.
The amplitude of the new CT (blue) is exaggerated
compared to the original gun (yellow) and collector (red)traces by a factor of five. However, a much higher
frequency component is observed and distorts pulses lessthan 1µs long. As a point of reference, the operating
pulse length of the working Electron Lens [1] in pulse
mode is to be ~0.5µs. Testing is planned with filtering.

Ground Loops
The current measurement system is equipped with acalibration current source, ILX Lightwave LDP-3811,
capable of up to 0.5Adc at 10µA resolution and 0.1 –1000µs in pulsed mode. With RG58 signal transport out
to the CT enclosures and a 50Ω resistor in the loop
through the CTs, a sharp calibration pulse can bepropagated out to the CTs. This system is used for bothoffline calibration as well as system health checks during
operation with beam.
At one point, the current measurement system wasbeing tested while the electron gun was shut down. A
noise of about 50mV with a repetitive nature wasobserved from the pulse CTs. This corresponds to a
primary current of 50mA (1A/V). Our goal is to besensitive to 1mA of beam current. At that time, most of
the critical power supplies were shut down. Further

investigation showed that the fundamental frequency ofthe noise was ~7kHz. This noise was also found on thecalibration loop even with the calibration source
disconnected. The source of the noise was identified
when the noise disappeared with the removal of power tothe DCCTs. The power supply is a linear supply and nosuch noise was found in the dc power.
The DCCT measures a DC current by its saturation of
an AC-excited magnetic core. As the excitation oscillator
runs at ~7kHz, the DCCT became suspect. However, itcorrectly measured calibration pulses within its response
range. Adding to the confusion, this noise problem
disappeared during the next run of the accelerator.
Due to the danger of high voltage floating platforms on
which racks of electronics ride, safety-grounding relays
are employed to ground these platforms when themachine is shut down. Ground loops often pick up noisethat can be introduced into connected circuits. Thissystem has several of these grounding relays. One such
relay, labeled “RR” in Figure 1, is used to ground the HVcathode deck where the gun electronics are installed. It
was later found that while this relay grounds the cathodedeck, a large ground loop is formed from the CPS over its
HV RG213 cable, through the CTs, to ground via the RR
relay and back to the CPS via earth ground. This allowed
the noise within the IPCT to be induced into this groundloop and measured by all the pulse CTs in the loop. This
is why the problem disappeared when the machine wasoperated, as the safety ground relay (RR) was open.

Static Magnetic Field Interference
As the electron gun and entire beam transport aresubmersed in a solenoidal magnetic field, there are straystatic magnetic fields that the instrumentation must deal
with. The cross section of the solenoids can be seen in
Figure 1 as blue rectangles around the beam pipe. A dc
output offset of 156mV was observed in the DCCT
installed in the gun circuit. This corresponds to 31.2mA(200mA/V), which exceeds the range of the zero offset
correction on the DCCT. The static magnetic fields were
therefore studied.
The stray magnetic field inside the CT enclosure wasmeasured to be 14 Gauss, while simulations predict fields
as high as 36 Gauss near the CTs in the final Electron
Lens machine. A magnetic shield was made from a sheetof 0.030” thick Mu Metal and was made to wrap aroundthe DCCT. The field inside the shield was measured to be~1 Gauss. This reduced the dc output offset to 33mV. A
design is being developed for a two-layer shield of ironand Mu Metal for the final Electron Lens installation.

YAG PROFILE MONITOR
One of the two beam profile monitors on this machineis comprised of a scintillating screen made of a Cerium
doped single crystal YAG, 0.1mm thick mounted on aretractable holder just at the entrance of the collector. A
GigE camera, AVT Manta G145B, fitted with a manual

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Proceedings of IBIC2012, Tsukuba, Japan

THCB01zoom lens, viewing the YAG screen through the hole in
the electron reflector [4] and through a 4-inch viewport at
the downstream extremity of the collector. The system isequipped with illumination light sources for inspection,
discussed in detail below.

Lens Type Comparison
The original system design incorporated a speciallyassembled zoom lens [5] using Navitar lens components,
as shown in Fig. 4. The advantage of this lens was its
insensitivity to light outside of its field of view. It’s
disadvantage was a considerably small aperture; thus
having a poor light gathering ability. Its forward apertureis only 25mm with a body length just over 300mm longand component cost totaling $1500.

body length of 120 – 208mm depending on zoom.
Although its minimum object distance (MOD) is 1.5m inmacro mode, the distance between the lens and camera
was increased in order to decrease the MOD; thus
providing space for more optical components, as
explained in the section on illumination techniques.
Tests with both lenses produced good resolution; wherea resolution test of the Sigma lens setup measured 34µm
(50µm being the goal.) The light gathering ability of thislarger aperture lens was by far the benefit. Images of
200mA beam pulses taken with the Navitar lens required
more than 3µS beam pulses with a gain of 33dB. The
high gain produced very grainy images due to noise.
Comparatively, the Sigma lens setup provided comparable
beam images at 1µs and only 3dB of gain with a muchless noise. The disadvantage of the Sigma lens is that itsuffers severely from internal reflections within the lens.
It is very sensitive to light from outside its field of view.
Hence, illumination techniques previously possible with
the Navitar lens, namely the flooding of light by an LED
ring [5], is no longer possible. Moreover, with theincreased diameter of the lens, less space is available forprojecting light through the viewport alongside the lens.
Figure 3 suggests that the viewport be mounted off centerto accommodate this.

Illumination Techniques

Figure 4: Two lenses compared. Top: Sigma f/4-f/5.6
lens with large aperture & compact body, Bottom:
Navitar lens components with small aperture but goodstray light rejection.
To avoid the YAG crystal’s emission being degradedby elevated temperatures due to absorbed beam energy, aneffort was made to find a lens with a larger aperture. A
70-300mm zoom lens by Sigma Corp., manufactured forthe consumer SLR camera market and readily available
for $200, was tested. The lens was mounted to the AVT
Manta G-145B GigE camera, by Allied Vision
Technologies; which has a Sony IXC285 2/3” type sensor(1.5 MPixels). Its forward aperture is about 50mm with a

Overview and Commissioning of Facilities

White light illumination is provided by a collimated
LED source [5]. Illumination at wavelengths that produce
YAG fluorescence has been added. This provides amethod to periodically test the health of the YAG crystal
in situ. The optics layout in Fig. 3 shows the placementof a 500nm dichroic longpass beam splitter (1.1mm
thick), Edmund Optics NT47-421, placed in the space
between the camera and lens. This is used to project anenlarged 405nm laser spot onto the YAG crystal via thecamera lens for focus determination and an in situ healthcheck of the crystal. An image of the enlarged laser spot
on the YAG crystal is shown in the upper left corner ofFig. 5. The 4mm laser oval laser beam is expanded by thecamera lens to a 11mm spot on the YAG 1.3m away.
The dichroic beam splitter reflects the 405nm light into
the optical path toward the YAG crystal and allows thebroad spectrum YAG emitted light, ~500-700nm [7], topass through to the camera while blocking any reflectedISBN 978-3-95450-119-9

3

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Figure 3: Layout of YAG profile monitor optics with UV laser, dichroic beam splitter, Sigma 70-300mm f/4-4/5.6
macro zoom lens, longpass filter, and GigE 1.5MPixel digital camera.

THCB01

Proceedings of IBIC2012, Tsukuba, Japan405nm light. A second layer of protection from UV light
was added by placing a longpass dielectric filter, Thorlabs
FEL0500, with 500nm cutoff, just in front of the camera.

finally to generate three profile plots along each of thethree axes. These are shown in Figure 6 along with thebeam image they were taken from.
The beam profile images were initially taken using
beam currents around 200mA and pulse widths of 12µs
with a camera gain of 0dB. Once a problem with the gain
control was fixed, the pulse widths were reduced to 3µs
and a gain of 30dB was used (at the expense of higher
noise). When the larger aperture Sigma lens was used thepulse widths were further reduced to 0.5µs with a gain of
3dB (with less noise), as in the case of the image in Fig. 6,
but with a beam current of 725mA.

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Y-axisFigure 5: YAG fluorescent illumination. Left: 405nm
laser spot on YAG crystal via camera lens, pictured
above laser module, heatsink, and beam splitter; andRight: YAG crystal (shown in red scale for contrast)
flooded with 455nm light, pictured above the projection
LED.
Initially, a 435nm longpass colored glass filter,
Thorlabs FGL435S, was used to keep the UV light fromreaching the camera. However, this filter fluoresced
under the UV laser light and produced unwanted lightfrom the surface of the filter. This led to the use of optics
with dielectric coating where light with λ shorter than acutoff is reflected instead of being absorbed.
The first dichroic beam splitter used, as shown in lower
left corner of Fig. 5, was a thicker (3mm) model fromThorlabs. This thicker glass between the lens and camera
first required a refocusing of the lens due to the longer
effective path length, but finally degraded the resolution
to a degree that appeared grossly out of focus. The muchthinner 1.1mm optic from Edmund Optics was installed;yielding a resolution measurement of 34µm.
The laser used is a 5mW GaN diode laser, commonly
found in Blue Ray players and now in laser pointers. It
was purchased from Beam of Light Technologies [6].
This laser module is supplied with integrated current
driver electronics, as shown in the lower left corner ofFig. 5 along with its heatsink mount.

YAG Beam Profile Measurements
Custom analysis software, developed in house, allows
an image to be taken from the camera and stored in a
project file. Tools are included to identify & mark the
20mm YAG screen border (for size calibration), to findthe center of gravity of the beam spot, to overlay X, Y &θ axes (where θ is an arbitrarily selectable angle), andX-axis45° –axis

Figure 6: Profile of 10 mm beam spot on YAG with X, Y,
and θ = 45º axes superimposed and the three beam profilegraphs along each axis.

YAG Crystal Damage Incident
While propagating a 400 ~ 500mA beam of with apulse width of 80µs and a rate of 1Hz, the YAG screen
plunger was mistakenly inserted for about 3 pulses. The
beam, measuring ~1cm on the YAG crystal, damaged the
crystal to a point where it’s single crystal structure wascompromised. The normally transparent crystal sheet
became opaque white. Photos in Fig. 7 show thedamaged 30 x 0.1mm YAG crystal removed from its
holder (1), a microscope photo of the damaged area (2),the damaged crystal exposed to 455nm light (3), and an
image of a beam hitting high on the crystal showing theboundary between areas where the fluorescence capability
was destroyed (4). Figure 7.3 shows an undamaged 5mmring where the crystal was protected by the aluminum
holder and still fluoresces as expected; where most of thedamaged portion no longer fluoresces and reflects someof the incident blue light. The microscope images in Fig.
7.4 were taken of the heavily damaged area and are shownat a magnification of 15X with an inlay at 60X.
A study was done to estimate the penetration thickness
for a 5keV electron beam incident on a YAG crystal

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Proceedings of IBIC2012, Tsukuba, Japan
covered with a 100 nm thick aluminum layer. It wascalculated that the entire energy of the beam is depositedin a surface layer only about 0.2 microns thick; leading topotential thermal shock damage due to sheer stresses thatdevelop due to differential expansion of adjacent layers at
different resulting temperatures. Ranges are roughestimates based on the so-called corrected Gruen formula.
The temperature in this sublayer is predicted to have risen
to 200°C during a 1µs pulse. This translates into a severe
temperature gradient of 1010 oC/m.
1.

2.

THCB01Figure 8: Signals from four quadrant halo monitor
electrodes – signature of longitudinal beam oscillation.

CONCLUSION

3.

4.

With an installation of two electron lenses in RHIC
planned by the end of 2012, the availability of the test
bench approaches its end. What remains are a destructive
test of a YAG crystal, the completion and testing of apinhole scanning beam profile monitor (subject left for a
future publication) and testing of a prototype BPM [8].

Figure 7: Damaged YAG 1. damaged Al coated YAG; 2.
microscope image of damaged area; 3. fluorescence ofdamaged crystal under 455nm light. 4. top less-damaged
area still partially sensitive to beam (20mm YAG screenboundary shown as solid line).

Plans for a Destructive YAG Test
In an attempt to find the non-destructive thermal limit
affecting YAG fluorescence, the beam pulse length will
be incremented while analyzing the YAG image profiles
until the peak begins to flatten due to the thermal limitingof fluorescence. Then attempting to find a thermal limit
causing permanent loss of fluorescence, profiles of YAG
images will be analyzed while alternately taking images
with the safe beam pulse length and an increasingly long
beam pulse until the response of the crystal at the safe
pulse length begins to be permanently altered.

FOUR QUADRANT HALO MONITOR
This instrumentation system includes a four quadrant
scraper plate, or halo monitor. The four plates are circularquadrants with a 33.5mm diameter central aperture.
Although conceived for the aid of steering & alignment aswell as downstream beam loss detection, they have
proved invaluable in determining machine parameters by
clearly showing conditions of longitudinal beam
oscillations between the gun and collector. Figure 8
shows the signals from the four plates during one such
instance.

The authors would like to thank J. Hock, K. Hamdi, C.
Liu, B. Sheehey. Special thank to Matthew Stetski forgroundwork done during the summer of 2011 on theimage analysis program. We appreciate the support fromB. Schoepfer and members of the EBIS group as well as
members of the controls group, especially A. Fatma, A.
Fernando, L. Hoff, R. Olsen, & C. Theisen, andrecognize the support of the Accelerator Components &Instrumentation Group, especially N. Baer, J. Carlson, T.
Curcio, B. Johnson, J. Kelly, D. Lehn, J. Siano, D. Von
Lintig, & A. Weston.

REFERENCES[1] W. Fischer, et al, “Status of the RHIC Head-On BeamBeam Compensation Project”, Proceedings of PAC 2011,
New York, NY, USA, THP055[2] X. Gu, et al, “The E-Lens Test Bench for RHIC BeamBeam Compensation”, Proceedings of IPAC 2012, New
Orleans, LA, USA, WEPPD084[3] A. Pikin, et al, “Structure and Design of the Electron Lens
for RHIC”, PAC ’11, New York THP100, p. 2309; JACow,https://www.jacow.org[4] D. Gassner, et al, “RHIC Electron Lens Test Bench
Diagnostics”, DIPAC 2011, Hamburg, Germany, MOPD04,
p. 38; JACoW, https://www.jacow.org[5] T. Miller et al, “RHIC Electron-Lens Beam Profile
Monitoring” BIW 2012, Newport News, VA TUPG039[6] Beam of Light Technologies, 10117 SE Sunnyside Rd.
Suite F, MB 324, Clackamas, OR 97015
http://www.z-bolt.com/[7] Y. Zhu, et al, “Investigation of the Optical Properties of
YAG:Ce Phosphor”, Lighting Research Center, Rensselaer
Polytechnic Institute, 21 Union St. Troy, NY 12180[8] P. Thieberger, et al, “Conceptual Design of a High
Precision Dual Directional Beam Position Monitoring
System for Beam Crosstalk Cancellation and Improved
Output Pulse Shapes”, BIW 2012, Newport News, VA
MOPG024

ISBN 978-3-95450-119-9
Overview and Commissioning of Facilities

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Copyright ○ACKNOWLEDGEMENTS