Uniform beam intensity redistribution in the LENS nonlinear transport line

Abstract

The Low Energy Neutron Source (LENS) at Indiana University is producing neutrons by using a 7 MeV proton beam incident on a Beryllium target. The Proton Delivery System is currently being upgraded [1], [2]. A new AccSys Technology,Inc. DTL section [3] will be added to increase proton beam energy from 7 to 13 MeV. A 3 MeV RFQ and 10 MeV DTL will be powered by two 1.25 MW klystrons. The goal of this upgrade is a 13 MeV,25 mA proton beam with duty factor greater than 3%. At this power level it becomes increasingly important to make a proton beam that is uniformly distributed to prevent excessive thermal stress at the surface of the Be- target. To achieve this goal two octupole magnets are being implemented in each LENS beam transport line. In this paper we discuss the experimental results of the beam intensity redistribution as well as some features inherent in tuning of the nonlinear beamline and our operational experience.

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UNIFORM BEAM INTENSITY REDISTRIBUTION IN THE LENS
NONLINEAR TRANSPORT LINE*
A. Bogdanov#, V. Anferov, M. Ball, D. V. Baxter, V. P. Derenchuk, A. V. Klyachko, T. Rinckel,
K. Solberg, IUCF, Bloomington, IN 47408, U.S.A.
Abstract
The Low Energy Neutron Source (LENS) at Indiana
University is producing neutrons by using a 7 MeV
proton beam incident on a Beryllium target. The Proton
Delivery System is currently being upgraded [1], [2]. A
new AccSys Technology, Inc. DTL section [3] will be
added to increase proton beam energy from 7 to 13 MeV.
A 3 MeV RFQ and 10 MeV DTL will be powered by two
1.25 MW klystrons. The goal of this upgrade is a 13
MeV, 25 mA proton beam with duty factor greater than
3%. At this power level it becomes increasingly important
to make a proton beam that is uniformly distributed to
prevent excessive thermal stress at the surface of the Be-
target. To achieve this goal two octupole magnets are
being implemented in each LENS beam transport line. In
this paper we discuss the experimental results of the beam
intensity redistribution as well as some features inherent
in tuning of the nonlinear beamline and our operational
experience.
INTRODUCTION
The Low Energy Neutron Source at Indiana University
is an accelerator driven pulsed neutron source [4].
Neutrons are being generated in a (p, nx) reaction on a
beryllium target. LENS is capable of providing cold
neutrons as well as neutrons in the MeV energy range
with a pulse length variable from 10 μs up to ~1.4 ms.
The intensity of these neutron beams is suitable for
applications in material science, studies of neutron
moderators, neutron technologies and instrumentation [4]
and also for neutron radiation effects research (NRERP)
[5]. Two individual target assemblies are installed for
these primary applications, a high energy, high average
flux target for the NRERP users and a target surrounded
by a custom moderator tailored to generate cold and very
cold neutrons for neutron scattering applications. Both
target assemblies share the same design of the beryllium
target.
To provide a competitive flux of neutrons the facility
will eventually use a 25 mA, 13 MeV proton beam with
more than 3% duty factor. To date the facility was
routinely running at 7 MeV, 25 mA with duty factor up to
0.6%. Available average beam power is ~ 1 kW. A series
of upgrades [1], [2] will bring the power level to 10 kW
(average) and 325 kW (peak). As a part of these upgrades
two beam transport lines providing a proton beam for
each target station have been constructed. A new DTL
section will be added to increase proton beam energy up
to 13 MeV from the current AccSys PL-7 Linac [6].
High beam power imposes a significant thermal stress
on the beryllium target. The target was designed to handle
an average beam power of 32.5 kW. However the average
power density must be limited by 650 W/cm2 to avoid
excessive thermal stress in the target. Clearly, even 1 kW
of average power available today can exceed 650 W/cm2
limit if a beam is focused into 1 cm2 spot. To achieve the
required power density the beam will be uniformly
distributed over 50 cm2 and incident on the target
mounted at 45º to the beam. Beam spreading is being
performed by octupole magnets installed at specific
locations along the beam transport lines. The first
experimental results of the beam spreading were obtained
in March 2007. These results confirm the effectiveness of
the method and will be discussed in this contribution.
LENS BEAM TRANSPORT LINES
Layout
The layout of the first beamline of the LENS facility is
shown in Fig. 1. The first beam transport line was in
operation since the initial commissioning of the LENS
facility. Experiments for beam spreading were performed
on this first beamline. The second beamline starts with an
achromatic 60 degree bend composed of two 30 degree
___________________________________________
*Work supported by the National Science Foundation (grants DMR-
0220560, DMR-0320627), the 21st Century Science and Technology
fund of Indiana, Indiana University, and the Department of Defense.
#abogdano@indiana.edu

2nd beamline
1st beamline
X Octupoles Y
12º Dogleg Section
Target Station
Figure 1: Layout of the LENS first beamline.
Quads
Quads
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bending magnets with an intervening quadrupole. The
beamline continues with a nonlinear beam spreading
system which utilizes octupole magnets in a configuration
similar to the first beamline. The second beamline is
presently under commissioning. In the first experiments
on the second beamline we were able to deliver to the
target more than 20 mA of proton beam with 0.02% duty
factor. At the end of both beamlines there are two 12º
bending magnets in a dogleg configuration to limit
activation of accelerator and beamline equipment from
back-streaming neutrons.
Octupole magnets are inherently nonlinear elements
that couple the particle motion in x and y dimensions. To
minimize coupling, a beam at the entrance to an octupole
is tuned to be very narrow in one dimension and wide in
the other. Moreover, octupole magnets require very
precise positioning relative to the beam. The beam
position at the entrance to both octupoles needs to be
accurate to ±1 mm. To control the beam profile and
position we have installed beam profile monitors – harps
– at the entrance of each octupole and in the two dogleg
sections [6]. Each harp provides a beam profile in x-y
dimensions. The harp electronics readout is programmed
in a way that allows background subtraction. The 20
micron diameter of the harp wires limits the operational
range of the harps to low power level. The estimated
temperature of the wires exposed to 20 mA with a 20 μs
pulse width at 16 Hz is slightly below 1500 ºC. This
temperature is a reasonable limit for taking accurate
measurements with wires. At higher temperatures
thermionic emission of electrons disturbs the accuracy of
measurements.
As a way of monitoring the shape of a beam going to
the target we have installed a scintillator viewer on an
actuator mechanism located between two bending
magnets in the dogleg area. This viewer is a water cooled
carbon plate coated with a dusting of alumina powder.
The uniformity of the Al2O3 powder was poor and so the
viewer was only used as a qualitative assessment of the
beam profile.
Light produced by the beam hitting the viewer is
captured by a radiation resistant CID camera [7]. The
camera is connected to a computer through a frame
grabber and operates through a LabView interface. This
gives us a way to manipulate and analyze beam images.
High power beam can seriously damage equipment in
the event of undesirable beam losses. To monitor beam
losses we are developing a system based on Bergoz [8]
current transformers (CT). There is a CT installed at the
exit of the accelerator. In addition, each beamline has a
CT measuring the current of the beam going to the target.
Comparing the readouts of the CTs at the beginning and
at the end of the beamlines will be one way of estimating
beam losses.
All slits and collimators along the lines are made from
carbon to reduce the possible neutron production from
beam lost in the transport lines. To protect the beamlines
in case of the target failure we have installed fast valves,
which close the beamlines and isolates vacuum within
7 - 8 ms.
At the end of each beamline there is a diagnostic beam
stop capable of measuring proton beam current. When the
dogleg bending magnets are turned off, beam is directed
onto this beam stop.
Simulations
The basic parameters of the beamlines were defined in
the first order simulations with TRANSPORT [9]. These
simulations give us the settings of the beamline elements
that produce the required beam profiles at the entrance to
each octupole. The results of the first order simulations
are shown in Fig. 2. The 3rd order effects of octupole
focusing as well as misalignment effects were studied
with TURTLE [10]. The details can be found at [11].
EXPERIMENT ON BEAM SPREADING
Beamline Alignment
The first results of the beam spreading system were
obtained on the first beamline. Strong tolerance on the
beam position in front of an octupole required a good
alignment of the beamline. Moreover we discovered that
the proton beam was coming out from the accelerator
with a horizontal angle on the order of a few mrad. A
steerer magnet was installed at the exit of the accelerator
to correct this angle. We used the harps for measuring the
position of the beam center. Harp positions were aligned
using optical surveying techniques. Each quadrupole
magnet position was adjusted until its steering effect was
less than 1 mm, as measured by the harps.
Experiment
Two goals were set for the experiment. The first goal
was to get a uniformly distributed beam on the viewer.
Figure 2: Transport calculation of the beam envelope
in the first beamline. The beam is tuned for the
octupoles.
Y
X
Oct X
Oct Y
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Secondly we wanted to analyze the performance of the
alumina powder that covers the viewer. The initial tests
with the viewer showed that the alumina powder didn’t
have enough dynamic range to prove useful in the low
power mode that is required for harps. For this reason in
the low power experiments we used beryllium oxide
ceramic as a scintillator material. In our experiment the
average beam current was reduced to ~ 100 nA and the
peak current of ~ 1mA was quite enough for measuring
beam profiles on the harps and scintillator.
The strategy of tuning the beam with the octupoles
follows. Once the misalignment of the beam line elements
was corrected, we adjusted the quadrupoles and used the
harps to verify that the beam had the required shape in
front of each octupole. By tuning the octupole magnets
we were able to observe a beam that was uniformly
distributed over 2 X 2 cm2. The effect of the octupoles on
the beam is shown on screenshots from the LabView
program (Fig. 3). On the left side of the screenshot is an
image of the beam from the CID camera with background
subtracted. On the right side is the beam profile measured
as grayscale intensity distribution along any desired line
(green lines in Fig. 3). A dip at the center of the X-profile
corresponds to a gap between 2 pieces of Be oxide that
were glued together. However the Y-profile shows the
typical distribution with “batman ears”.
CONCLUSION AND FUTURE
DEVELOPMENT
To prevent excessive thermal stress caused by a proton
beam at the surface of the Be-target we use an octupole
beam spreading system to produce a beam with uniform
intensity distribution. The experiments with low beam
power showed the effectiveness of our octupole system.
However more experiments are planned to test the system
at high power. We are also discussing an image intensifier
as a possible way to monitor beam scintillation on the
residual gas. Optimization of the imaging system includes
tests of different scintillator materials and adding camera
filters. The final goal is to get a uniform beam on the
target instead of the viewer. It can be done by retuning the
line according to simulations. To verify beam distribution
on the target we are investigating possible ways of
looking with a camera directly at the target.
REFERENCES
[1] A. Bogdanov et al., “Upgrade of the LENS Proton
Linac: Commissioning and Results”, this conference.
[2] W. A. Reass et at., “The Klystron RF Systems for the
Indiana University “LENS” Accelerator”, this
conference.
[3] AccSys Technology, Inc., 1177 Quarry Lane,
Pleasanton, CA 94566-4757.
[4] D. Baxter et al., NIM A 542 (2005) 28-31.
[5] B. von Przewoski, et. al., “Neutron Radiation Effects
Program at the Indiana University Cyclotron
Facility”, ICANS-XVII, Santa Fe NM, 2005.
[6] V. P. Derenchuk, et al., “The LENS 7 MeV, 10 mA
Proton Linac”, PAC’05, Knoxville, July 2005,
p.3200.
[7] Thermo Scientific, www.thermo.com.
[8] Bergoz Instrumentation, www.bergoz.com.
[9] PSI Graphic Transport Framework by U. Rohrer
based on a CERN-SLAC-FERMILAB version by K.
L. Brown et al.
[10] PSI Graphic Turtle Framework by U. Rohrer based
on a CERN-SLAC-FERMILAB version by K. L.
Brown et al.
[11] W. Jones, et at., “Non-Linear Beam Transport for the
LENS 7 MeV proton beam”, PAC’05, Knoxville,
July 2005, p.1704.










Octupoles
Octupoles
X
X
Y
Y
Figure 3: Beam spot on the viewer (left) and beam
profiles (right).
Octupoles OFF
Octupoles ON
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04 Hadron Accelerators
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A08 Linear Accelerators
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