Intense Beam Experiments at the University of Maryland
Electron Ring (UMER)
R. A. Kishek, P. G. O'Shea, M. Reiser, B. Beaudoin, S. Bernal, Y. Cui, A. Diep,
D. Feldman, M. Glanzer, T. F. Godlove, I. Haber, J. Harris, H. Li, J. Neumann,
B. Quinn, M. Qurius, M. Snowel, A. Valfells, M. Virgo, M. Walter, R. Yun,
Y.Zou
Institute for Research in Electronics and Applied Physics, Univ. of Maryland, College Park, MD 20742-3511, USA
Abstract. A detailed understanding of the physics of space-charge dominated beams is vital for many modern
accelerators. In that regard, low-energy, high-intensity electron beams provide an excellent model system. The
University of Maryland Electron Ring (UMER) has been designed to study the physics of space-charge dominated
beams with extreme intensity in a strong focusing lattice with dispersion. At 10-keV, 100 mA, the UMER beam has a
generalized perveance of 0.0015. Though compact (11-m in circumference), UMER is a very complex device. An
update on construction and early experimental results is presented.
intensities in a circular lattice, namely, the University
of Maryland Electron Ring (UMER) [3]. In addition,
we have a number of side experiments, particularly,
the Long Solenoid Experiment (LSE) [4], designed to
study longitudinal beam physics in a straight
geometry. These machines are designed to serve as a
low-cost research tool for building higher intensity
accelerators for a variety of applications, such as
heavy-ion fusion drivers, the low energy end of high
intensity electron linacs, ion booster synchrotrons,
muon colliders, and spallation neutron sources.
INTRODUCTION
There has been an increased interest in space
charge effects in recent years. In linear accelerators,
the demand for ever lower beam emittance and
increased current has meant that space-charge driven
emittance growth and halo formation can be a
consideration at energies as high as 100 MeV in some
electron linacs proposed for fourth generation light
sources [1]. Circular machines, on the other hand,
have been limited to much lower intensities than linacs
due to destructive resonances which can severely limit
the achievable number of turns. With space charge, the
resonances are no longer caused by the interaction
between the single particle orbits and the harmonics of
the field errors. Instead, a resonance can occur when
the frequency of a collective beam mode coincides
with one of the harmonics of the error frequency
spectrum.
The unknown territory in the extreme space-charge
dominated regime is very challenging and should
provide a wealth of new phenomena. The UMER
facility will allow us to investigate emittance growth
due to conversion of free energy, halo formation, and
equipartitioning in a circular machine [5]. So far, these
effects have only been studied in linear transport lines.
In addition, UMER will permit experimental
investigations of longitudinal-transverse coupling and
beam profile changes resulting from dispersion; the
behavior of bunch ends, resonance traversal; the
longitudinal resistive wall instability; and other effects
in the space-charge dominated regime that are
currently inaccessible.
At the University of Maryland, we have a tradition
of using low-energy electron beams as model systems
for studying space charge phenomena that are of
general interest in intense beam systems [2].
Previously, all of our research had been done with
straight-line systems. Our motivation to develop a ring
system is twofold. There are many interesting
phenomena in intense beam physics that evolve over
longer distances than our 5-m linear solenoid channel.
Furthermore, a ring with both strong focusing and
dispersion would also allow us to study resonance
issues in a realistic setting.
We are also investigating the possibility of using
UMER and related systems for laboratory studies
related to galactic dynamics. Many of the dynamical
processes involved in galactic formation and evolution
appear to have analogs in the physics of high intensity
beams [6].
Currently we are constructing one major facility for
conducting experiments in beam physics at extreme
CP642, High Intensity and High Brightness Hadron Beams: 20th ICFA Advanced Beam Dynamics Workshop on
High Intensity and High Brightness Hadron Beams, edited by W. Chou, Y. Mori, D. Neuffer, and J.-F. Ostiguy
© 2002 American Institute of Physics 0-7354-0097-0/02/$ 19.00
319
KEY UMER FEATURES
Table 1 lists
someUMER
relevantFEATURES
parameters of UMER.
KEY
The beam is drifting at 10 keV, and 36 FODO cells
Table 1 lists some relevant parameters of UMER.
around
each turn provide transverse focusing, while 3
The beam is drifting at 10 keV, and 36 FODO cells
induction
longitudinal
In a 3
aroundgaps
each provides
turn provide
transverse focusing.
focusing, while
future
stage,
the
induction
gaps
will
also
be
used
to a
induction gaps provides longitudinal focusing. In
accelerate
the
beam
to
50
keV.
At
the
present
p
of
0.2,
future stage, the induction gaps will also be used to
the beam
is nonrelativistic.
intensity
accelerate
the beam to 50 The
keV.beam
At the
present can
β of be
0.2,
quantified
in is
a number
of ways,The
forbeam
example
usingcan
thebe
the beam
nonrelativistic.
intensity
dimensionless
% example
(defined using
in ref.
quantified inintensity
a numberparameter,
of ways, for
the
[3] as
the ratio of the
space-charge
forceχ to
the external
dimensionless
intensity
parameter,
(defined
in ref.
focusing
force
thespace-charge
beam radius).
tune
[3] as the
ratio at
of the
force toThe
the external
focusing isforceexpressed
at the beam
radius).of The
depression
in terms
% tune
as
depression is and
expressed
in wavenumber
terms of kχ asas
the plasma
p
ν
k
=
= 1 − χ , and the plasma wavenumber kp as
ν 0 k0
HIF drivers, for example, will likely
kp
= 2 χ . HIF drivers, for example, will likely
k0
operate
with 0.89 <% < 0.98. With a zero-current
operate
with per
0.89period
<χ < of
0.98.
phase advance
<J0 =With
76°,a azero-current
nominal
phase
advance
per
period
of
σ
0 = 76°, a nominal
emittance of 8n = 10 |im and beam radius a = 1 cm,
emittance
of εnthis
= 10
µm of
andextreme
beam radius
a = 1The
cm,
UMER
can attain
region
intensity.
UMER can attain this region of extreme intensity. The
intensity parameter, tune depression or beam current
intensity parameter, tune depression or beam current
can be varied in UMER over a wide range by changing
can be varied in UMER over a wide range by changing
to different
apertures sizes in the beam collimator at
to different apertures sizes in the beam collimator at
the the
exitexit
of the
gun,gun,
by by
changing
thetheanode-cathode
of the
changing
anode–cathode
spacing,
or by
thethe
beam
energy.
spacing,
or changing
by changing
beam
energy.
TABLE
1. UMER
design
specifications.
TABLE
1. UMER
design
specifications.Data
Data
____________fromfrom[3]____________
[3]
Energy
1010keV
Energy
keV
p(=v/c)
0.2
0.2
β (= v/c)
Current
<=<=
100
Current
100mA
mA
Generalized
perveance
0.0015
Generalized
perveance
0.0015
Emittance,
4x rms,
norm
Emittance,
4x rms,
norm
1010jim
µm
Pulse
Length
40-100nsns
Pulse Length
40-100
Circumference
11.52mm
Circumference
11.52
197nsns
LapLap
timetime
197
Pulse
repetition
Pulse
repetition
raterate
6060HzHz
Mean
beam
radius
Mean
beam
radius
<=<=
1 1cmcm
FODO
period
0.32
FODO period
0.32 mm
0-current
phase
advance,
σ
o
0-current phase advance, <J0
7676° °
7.6
0-current
Betatron
tune,
ν
o
0-current Betatron tune, V0
7.6
Tune Depression
>= 0.2
Tune Depression___________>=0.2_____
The electron bunch is injected into the ring at a
The
electron
injected
the the
ringinjector
at a
repetition
ratebunch
of 60isHz
or lessinto
from
repetition
rate
of
60
Hz
or
less
from
the
injector
system [7] with the help of a DC Panofsky quadrupole
system
with the
helpThe
of bunch
a DC Panofsky
quadrupole
and [7]
a pulsed
dipole.
can be extracted
within
and the
a pulsed
dipole.
The
bunch
can
be
extracted
first turn or after any number of turnswithin
with a
the system
first turn
after any
of the
turns
with line
a
that or
duplicates
the number
features of
injector
system
thatthat
duplicates
the features
the injector
except
the electron
gun is of
replaced
by a line
large
except that the electron gun is replaced by a large
diagnostic chamber with phosphor screen, emittance
meter and energy analyzer [4, 8].
diagnostic chamber with phosphor screen, emittance
meter
and complete,
energy analyzer
[4, 8].magnetic lattice will
When
the ring
consist
of over 140 quadrupoles, dipoles and steering
When complete, the ring magnetic lattice will
magnets.
focusing gradients
and steering
bending
consist ofThe
overtypical
140 quadrupoles,
dipoles and
fields
are
on
the
order
of
5
Gauss/cm
and
10
Gauss,
magnets. The typical focusing gradients and bending
respectively.
The
use
of
iron-based
magnets
fields are on the order of 5 Gauss/cm and 10 Gauss,is
impractical
such use
low fields.
Therefore,magnets
the UMER
respectively.for The
of iron-based
is
magnets
are
based
on
an
iron-free
printed
circuit
(PC)
impractical for such low fields. Therefore, the UMER
design
while
fabricated
to acircuit
very (PC)
high
magnets[9]arewhich,
based on
an iron-free
printed
tolerance,
lowers
the costto ofa the
design [9]considerably
which, while
fabricated
veryproject.
high
Atolerance,
new feature
that we lowers
have introduced
is a
considerably
the cost of recently
the project.
quadrupole
withthatelectronically
adjustable
skewness
A new feature
we have introduced
recently
is a
quadrupole
with allows
electronically
adjustable
skewness
(roll).
The feature
us to both
correct for
residual
(roll). Thein
feature
us to
bothalso
correct
residual
skewness
our allows
system,
and
to for
deliberately
skewness skewness
in our system,
andlocation
also toin deliberately
introduce
at various
the ring for
introduce
skewness
at various
beam
dynamics
studies
[10]. location in the ring for
beam dynamics studies [10].
The design of the electron gun and simulations of
The design ofare
thedescribed
electron gun
and simulations
its performance
elsewhere
[11-12]. ofA
its
performance
are
described
elsewhere
[11-12].
A
grid near the cathode can be used to generate
grid
near
the
cathode
can
be
used
to
generate
perturbations as means of investigating instabilities.
means
investigatingthe
instabilities.
Inperturbations
addition, weashave
beenofinvestigating
possibility
In addition, we have been investigating the possibility
of producing ultra-short (1 ns) current pulses on top of
of producing ultra-short (1 ns) current pulses on top of
the main pulse. We have achieved this by using a
the main pulse. We have achieved this by using a
nitrogen
laser as a photocathode drive laser on the
nitrogen laser as a photocathode drive laser on the
dispenser
1). This
This will
will allow
allowisistoto
dispenser cathode
cathode [13]
[13] (Fig.
(Fig. 1).
create
perturbations
on
the
beam
that
are
well
located
create perturbations on the beam that are well located
inintime
and
space.
The
evolution
of
such
a
perturbation
time and space. The evolution of such a perturbation
will
processes, e.g.
e.g.
will allow
allow us
us to
to study
study dissipative
dissipative processes,
resistive
resistivewall
wallphenomena.
phenomena.
FIGURE1.1.Combined
Combined photoelectron
photoelectron and
FIGURE
andthermionic
thermioniccurrent
current
(uppertrace).
trace). Note
Note the
the sharp
sharp current
(upper
current spike
spike corresponding
correspondingtoto
thephotoelectron
photoelectron pulse.
pulse. The
The lower
the
lower trace
trace isis the
thegun
gunvoltage
voltage
pulse.
pulse.
To allow detailed comparison between theory and
To allow detailed comparison between theory and
experiment, UMER will have a comprehensive set of
experiment,
UMER will have a comprehensive set of
beam diagnostics. Each of the 13 diagnostic stations
beam
diagnostics.
Eachhave
of the
13 diagnostic
stations
around the ring will
a phosphor
screen
and
around
the
ring
will
have
a
phosphor
screen
and
capacitive beam position monitor [14]. In addition, fast
capacitive
beam
position
monitor
[14].
In
addition,
fast
current monitors and resistive beam position monitors
current
monitors
and
resistive
beam
position
monitors
will also be installed. A sophisticated diagnostic endwill
also be
A sophisticated
diagnostic The
endchamber
hasinstalled.
been fabricated
by FM Technologies.
chamber
beenemittance
fabricatedmeters
by FMofTechnologies.
The
chamberhas
houses
the slit-wire and
chamber
houses
emittance
meters of energy
the slit-wire
and
pepper-pot
types;
a retarding-field
analyzer
pepper-pot types; a retarding-field energy analyzer
320
with eV resolution for energy and energy spread
measurements; a movable phosphor screen with 1.5
meter travel for insertion into the complete transport
line; and a Faraday cup for current measurement.
around the entire perimeter of the ring during the first
turn. This phased installation is scheduled to be
completed by mid-2003.
After closure, we intend to attempt multi-turn
operation, targeting 100 turns at low current (10 mA)
and 10 turns at high current (100 mA), while
maintaining beam emittance to within a factor of 4. At
the end of that phase, we wish to upgrade UMER to a
fast cycling synchrotron that accelerates the beam to
50 keV over 50-100 turns to study resonance crossing.
DEVELOPMENT OF ENERGY
ANALYZER (EA)
To measure the energy spread of the beam, we
have successively evolved the design of a parallelplate electrostatic retarding voltage energy analyzer
(EA) [4,8]. The first improvement consisted of adding
a collimating cylinder attached to the retarding mesh.
The cylinder collimates the particle trajectories such
that they are normally incident on the mesh, hence
reducing any errors from a transverse angle or spread
in velocities. Simulations show the resolution of the
device is about 8 eV (at 10 keV), and the measured
values of the energy spread are just slightly greater
than this value. This is a factor of 5 improvement over
the previous design.
ACKNOWLEDGEMENTS
We wish to acknowledge Richard York and his
colleagues at Michigan State University as well as FM
Technologies, Inc. for their assistance with the design
and construction of UMER components. This project
is fully supported by the US Dept. of Energy grant
numbers DEFG02-94ER40855 and
DEFG0292ER54178.
REFERENCES
Note: in the following, PACOl means "Proceedings of the
2001 Particle Accelerator Conference, Chicago, IL, edited by
P. Lucas and S. Weber, IEEE Cat. No. 01CH37268" (2001).
1. P.O. O'Shea and H.P. Freund, Science, 292, 1854 (2001)
2. Martin Reiser, Theory and Design of Charged Particle
Beams, (New York: John Wiley & Sons, Inc., 1994).
3. P.O. O'Shea, et. al., MM A 464, 646 (2001); also
PACOl, p. 159; http://www.ireap.umd.edu/umer
4. Y. Cui et al., PACOl, p. 2976.
5. R.A. Kishek, P.O. O'Shea, M. Reiser, Physical Review
Letters 85, 4514 (2000).
FIGURE 2. Simlon simulation of the latest EA Design,
showing the structure of the device and the equipotentials, as
well as some representative particle trajectories.
6. R. A. Kishek at al., PACOl, p. 151.
While the measured energy spread meets our
specification of 20 eV, we are continuing to improve
our diagnostic.
By electrically separating the
collimating cylinder from the retarding mesh [4], we
gain additional control over the focusing of the
trajectories, and simulations predict an extra factor of
10 in the resolution of the EA, i.e., down to 10"4! This
3rd-generation EA (Fig. 2) is currently being tested.
7. S. Bernal et al., PACOl, p. 2129.
8. A. Valfells et al., PACOl, p. 3582.
9. W.W. Zhang, H. Li, S. Bernal, T. Godlove, R.A. Kishek,
P.O. O'Shea, M. Reiser, V. Yun, and M. Venturini,
Physical Review ST-AB 3, 122401 (2000).
10. H. Li et al., PACOl, p. 1802.
11. D. Kehne, et al., NIM, A464, 605-609 (2001).
12.1. Haber et al., PACOl, p. 2952; also these proceedings.
PLANS
13. D. Feldman et al., PACOl, p. 2132.
At present (May 2002), we are engaged in a phased
installation of the ring. The first two our of 18 ring
sections are currently being installed, and we are
therefore about to complete 40 deg. shortly. The
installation is done gradually to permit us to take
measurements using the end diagnostic chamber
14. J. Harris et al., PACOl, p. 1387.
321