Multiple-charge beam dynamics in an ion linac. Page: 1 of 8
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MULTIPLE-CHARGE BEAM DYNAMICS IN AN ION LINAC
P.N. Ostroumov, J.A. Nolen, K.W. Shepard c V
Physics Division, Argonne National Laboratory, EP 12
9700 S. Cass Avenue, Argonne, IL, 60439Abstract
There is demand for the construction of a medium-
energy ion linear accelerator based on superconducting
rf (SRF) technology and capable of producing several
hundreds kilowatts CW for beams from protons to
uranium for use as a production accelerator for a rare
isotope facility (RIA). At present, however, the beam
power available for the heavier ions would be limited by
ion source capabilities. To overcome this limit, we have
studied the possibility of accelerating multiple-charge-
state (multi-Q) beams through a linac. We show that
such operation is made feasible by the large transverse
and longitudinal acceptance which can be obtained in a
linac using superconducting cavities. Multi-Q operation
provides not only a substantial increase in beam current,
but also enables the use of two strippers, reducing the
size of linac required. Since the superconducting (SC)
linac operates in CW mode, space charge effects are
essentially eliminated except in the ECR-RFQ region.
Therefore an effective emittance growth due to the
multi-charge beam acceleration can be controlled.
1 INTRODUCTION
A preliminary design and beam dynamics study has
been performed for the RIA driver linac structure
discussed elsewhere [1,2]. The schematic view of the
linac is shown in Fig.1.
ECR
RFQ Low-4 SRF St. I
12 keV/u 160 keV/u - 2
12.3MeV/u
85.5 MeV/u
High-R SRF03=0.61
P-=0.49
Figure 1: General design of the Linac.
The linac contains three main sections: a "pre-
stripper" section up to the first stripping target at 12.3
MeV/u, a medium energy section defined and separated
by the stripper targets and a high energy section with a
maximum uranium energy 400 MeV/u. Total voltage of
the linac is 1.36 GV. The pre-stripper section consists ofan ECR ion source followed by mass and charge
selection, an initial linac section consisting of an RFQ
and 96 low-beta independently-phased SRF cavities. The
middle section is based on 168 intermediate-beta SRF
cavities. The high-energy section consists of 172
elliptical cavities designed for three different velocities.
The heaviest ions, which are not fully stripped at the first
stripper, will be stripped a second time at -85 MeV/u.
The charge state distribution of uranium ions is centered
at the charge state q6=+75 at 12.3 MeV/u. Five charges
encompassing -80% of the incident beam after the first
stripper will be accelerated simultaneously in the
medium-p3 section. After the second stripper, 98% of the
beam is in five charge states neighbouring qo = 89, all of
which can be accelerated to the end of the linac.
The effective energy-gain gradient of uranium beam
taking into account the cryostat filling varies from 1.6
MeV/u-m' to 5 MeV/u-m". Transverse beam focusing
over all of the driver linac is provided by SC solenoids.
The length of the focusing period depends on the
resonator type.
The behaviour of uranium multi-Q beam has been
studied both by analytical and numerical methods. The
effects of various factors, such as beam mismatch,
misalignments, accelerating field errors and others on
the effective emittance growth of a multi-Q beam are
discussed.
2 BEAM DYNAMICS
2.1 Longitudinal beam dynamics
When a particle with a charge state, q, and mass
number, A, traverses an accelerating cavity with length,
L, and electric field E=E,(z)cosa, the energy gain per
nucleon AW is determined by the expression
AW,-, = q eET( 1,/JG )Lc cosps , where T(/,f/,) is the
transit time factor, Ea is the average accelerating field of
the cavity and qp, is the synchronous phase, QG is the
geometrical beta of the cavity. The transit time factor
(TTF) is a complicated function of both the field
distribution and the particle velocity. At low energy, the
particle velocity may change appreciably during the
passage through the multiple gap cavity. For this reason,
the TTF is most conveniently calculated numerically.
We define the synchronous phase for a given particle
traversing a given field with respect to that rf phase
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Ostroumov, P. N.; Nolen, J. A. & Shepard, K. W. Multiple-charge beam dynamics in an ion linac., article, August 15, 2000; Illinois. (https://digital.library.unt.edu/ark:/67531/metadc721041/m1/1/: accessed April 17, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.