Initial experimental results of the Small Isochronous Ring (SIR)

Abstract

The Small Isochronous Ring (SIR) is under development at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University (MSU). The ring is a small-scale experiment that simulates the dynamics of intense beams in large-scale accelerators. Its main purpose is the study of longitudinal space charge effects in the isochronous regime. A 20 to 30 keV bunch of hydrogen or deuterium ions is injected in the ring. After a variable number of turns (∼20-30), the bunch is extracted and its longitudinal profile is studied. This paper describes the hardware already designed, manufactured and installed as well as the first preliminary experimental results.

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INITIAL EXPERIMENTAL RESULTS OF THE SMALL ISOCHRONOUS
RING (SIR)*
J. Rodriguez†, E. Pozdeyev, F. Marti, NSCL, Michigan State University, MI 48824, USA
Abstract
The Small Isochronous Ring (SIR) is under
development at the National Superconducting Cyclotron
Laboratory (NSCL) at Michigan State University (MSU).
The ring is a small-scale experiment that simulates the
dynamics of intense beams in large-scale accelerators. Its
main purpose is the study of longitudinal space charge
effects in the isochronous regime. A 20 to 30 keV bunch
of hydrogen or deuterium ions is injected in the ring.
After a variable number of turns (~ 20-30), the bunch is
extracted and its longitudinal profile is studied. This paper
describes the hardware already designed, manufactured
and installed as well as the first preliminary experimental
results.
1 INTRODUCTION
The main goal of the project is to study experimentally
the effects of space charge in the isochronous regime.
Using low energy, low intensity hydrogen or deuterium
beams, we intend to simulate the beam dynamics of high
current machines. Papers [1], [2] and [3] provide
additional details on the project. Results of simulations of
the beam dynamics in the injection line and in the ring can
be found in [4] and [5] respectively.
The injection line of SIR and the four magnets of the
ring have been manufactured and installed. Beam has
been extracted, injected in the ring and detected with a
Faraday Cup after ¼ of a revolution. No systematic
experiments have been performed yet since the assembly
finished at the end of April 2003. Figure 1 shows a picture
of room where SIR is being built. Beam has been
transported through the colored segments.
2 ION PRODUCTION AND SEPARATION
A multicusp ion source is used to produce a beam
composed of H+, H2
deuterium will also be performed. In this case, the
ultimate pressure in the ring will be lower since its higher
mass will increase the effective pumping speed giving a
longer life-time of the bunches in the ring.
Biasing the source (up to 30 kV) defines the extraction
energy of the beam. The puller of the ion source also acts
as the first ground electrode of the Einzel lens that
immediately follows. Focusing as close to extraction as
possible is required due to the large angle dispersion of
the extracted beam. Typically, a voltage in the Einzel lens
of ~
? ??? ???? ??????????? ???????? ????????? ?? ?????????
length ~10 cm. Some non-linear effects have been
observed. Simulations indicate that they are mainly
+ and H3
+ ions. Experiments with
___________________________________________
*Work supported by NSF Grant # PHY-0110253.
†rodriguez@nscl.msu.edu
Figure 1: Room where SIR is being built.
0-7803-7739-9 ©2003 IEEE2649
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produced by the Einzel lens [4].
A couple of orthogonal pairs of steering plates (9 cm
long, 6 cm gap, ±1 kV voltage, up to 65 mrads kick to 30
keV H2
misalignments in the injection line.
+ beam) are the first mean to correct possible
The different mass states are separated by a dipole
magnet identical to those used in the ring [2].
The first preliminary measurements show currents in
the order of 10-100µA for different ions species. This is
enough to cover the desired range of peak currents in the
ring. Systematic experiments to characterize the extracted
beam will be conducted in the following months.
3 EMITTANCE MEASUREMENT SYSTEM
A couple of orthogonal pairs of movable slits and
Faraday Cups are used to characterize the transverse beam
parameters. This information is used to recalculate the
focusing lengths of the Einzel lens and electrical
quadrupoles needed to match the injected beam to the
close orbit solution in the ring.
Figure 4 shows an X-X’ (medium plane of the ring) and
a Y-Y’ map of a 15 keV H2
emittance
10
ε
?
estimated. Previously, other experiments were performed
where the ion source had a 2 mm diameter aperture
instead of a 1 mm one. In that case an emittance
50
mm mrad
ε π ⋅⋅
?
was measured.
The distance between slits (0.5 mm gap) is 47.5 cm.
Another couple of orthogonal pairs of steering plates
(identical to the ones right after the ion source) are located
between slits. These electrodes, together with the dipole
magnet and the inflector plates can be adjusted to correct
for possible misalignments or residual external fields.
A beam stopper will be added in this region in the
coming months. It will be covered with phosphor to be
used as a fast diagnostic tool.
+ beam from which an
mm mrad
⋅
(unnormalized) can be
π ⋅
4 MATCHING AND CHOPPING SECTION
A triplet of electrical quadrupoles, located right after
the emittance measurement box, together with the Einzel
lens and the quadrupole corrector coil in the mass
separator provide flexibility to match the extracted beam
to the close orbit solution in the ring. Refer to [4] for more
details in the capabilities of the injection line to match
different initial ensembles. The lengths of the quadrupoles
are 10.5 cm, 14 cm and 6 cm. The distance between them
is 9 cm and the gap between electrodes is 7 cm. The shape
of the electrodes is not hyperbolic but circular. 3-D
calculations do not show a significant deviation from
ideal case.
A pair of 5 cm long plates are pulsed up to ±3.5 kV to
provide the time structure of the bunches. Two grids next
to the plates are used to limit the deflection region.
Bunches as short as 30 ns are expected. At this time, no
precise measurements of bunch lengths have been
performed since a fast Faraday Cup has not been built yet.
A regular Faraday Cup located at the end of the injection
line has been used and pulses <5 µs have been observed??
A pair of 16.5 cm inflector plates (gap ?? cm), located
right after the chopper, bend the trajectory of the beam
Figure 2: First section of injection line
Figure 3: X-X’ and Y-Y’ maps (15keV H2
+)
Figure 4: Quads, Chopper, Inflector Plates
Section
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17.5o and a second pair located in between two of the
magnets of the ring, position it in its medium plane. This
second pair is pulsed to avoid deflection after completion
of the first turn. The geometry of plates is optimized to
maximize distance between deflected beam and plates and
to avoid introducing non-linear effects in the beam.
5 RING INJECTION SECTION
Besides a pair of pulsed deflector plates responsible for
bending the beam trajectory back to the medium plane of
the ring, there is a movable three position phosphor
screen. This allows interception of the beam both when it
is injected and after the first turn. Figure 5 shows the
beam being intercepted when injected.
A triangular shape in the beam can be observed.
Although, no satisfactory explanation was found yet, it is
believed that the ceramics holding the Einzel lens may be
the cause. They may be charging up or the perturbation
they introduce in the field distribution could be disturbing
the beam close to their position.
Three 500 L/s turbopumps are used to reach a pressure
of 2???-7 torr in the ring with a few mTorr pressure in the
Ion Source. One of them is located right after the ion
source, a second one in the emittance measurement box
and a third one in the first straight section between
magnets. A fourth one will be installed in one of the other
sections of the ring.
6 CONCLUSION
The injection line and the four ring magnets have been
manufactured and installed. Beam completed ¼ of a
revolution in the ring at the end of April 2003.
In the next few months, the other three straight sections
of the ring will be designed and manufactured. A Fast
Faraday Cup will be developed and tested. The control
system will be enhanced. The source of the triangular
shape of the beam will be studied and corrected.
Systematic experiments to characterize the ion source will
also be performed.
7 ACKNOWLEDGMENT
The authors are grateful to the staff of NSCL for their
suggestions and support. In particular, we would like to
thank R. Zink, R. Fontus, D. Sanderson, D. Pedtke, R.
York, A. Zeller, D. Lawton, S. Hitchcock, D. Devereaux
and A. Mond for their involvement in the design,
manufacturing and/or assembling of different parts of
SIR.
8 REFERENCES
[1] E. Pozdeyev, “A Small Isochronous Ring for
Experimental Study of the Longitudinal Space Charge
Effect in Isochronous Cyclotrons”, PAC’2001, pg.
3549.
[2] E. Pozdeyev, F. Marti, J. Rodriguez, R. York, “Small
Isochronous Ring Project at NSCL”, EPAC’2002, pg.
1395.
[3] E. Pozdeyev et al., “Report on progress of the Small
Isochronous Ring project
proceedings.
[4] J. Rodriguez, E. Pozdeyev, F. Marti, “Injection Line of
the Small Isochronous Ring”, EPAC’2002, pg. 1401.
[5] E. Pozdeyev, J. Rodriguez, “Computer Simulations of
the Beam Dynamics in the Small Isochronous Ring”,
EPAC’2002, pg. 1398.

at NSCL”, these
Figure 6: Straight section between magnets
Figure 5: Beam intercepted in phosphor screen
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Proceedings of the 2003 Particle Accelerator Conference