LOW ENERGY IMPROVEMENTS TO THE FERMILAB
400-MEV LINEAR ACCELERATOR
D. E. Young, V. Dudnikov, M. Popovic, C. W. Schmidt and D. Sun
Fermi National Accelerator Laboratory *, Batavia, IL 60510, USA
Abstract
Improvements in the Fermilab operating 400-MeV linear
accelerator injector are required to achieve the beam
intensity and emittance requirement of the Proton Driver
design study [5]. It has been determined that these
requirements can be achieved by replacing the components
in the Linac below 10 MeV. An improved H
-
ion source with
an electrostatic transport to a two-section Radio-Frequency
Quadrupole (RFQ) accelerator, with the RFQ sections
separated by a magnetic five-dimensional phase-space
imaging system as used in an earlier Fermilab/SAIC PET
Project, and a new 10-MeV drift-tube linac cavity have been
studied. It appears possible that an H
-
intensity of 4.5x10
13
ions per pulse with an improvement in beam emittance from
the present system can be achieved with the proposed
changes.
1 INTRODUCTION
The Fermilab Proton Driver Project proposes to replace the
Booster accelerator with a rapid-cycling high-intensity 16-
GeV synchrotron that would increase the proton beam
intensity in the Main Injector by a factor of four. The Linac
currently operates with an H
-
beam current of 50 mA and a
pulse length of 30 µsec to give an intensity of 9x10
12
ppp.
The required intensity to achieve the Proton Driver goal is an
intensity of 3.5x10
13
ppp. Consequently a study was done to
determine if the intensity and pulse length in the present
Linac could be upgraded by a factor of four to meet the goals
of the Proton Driver.
In 1993 the Fermilab Linac was upgraded from an energy
of 200 MeV to 400 MeV by replacing the last four drift-tube
tanks with side-coupled, 805-MHz modules (CCL) operating
at a higher accelerating gradient. The design of the CCL was
based on a beam current of 50 mA and a pulse length less
than 100 µsec. An experiment was performed in 1999 to
determine whether the CCL could operate at an intensity
required for the Proton Driver design [1]. The ion-source,
preaccelerator and drift-tube linac sections were restored to
the proton mode of acceleration. It was determined that a
beam of nearly 90 mA at a pulse length of 90 µsec could be
accelerated to 400 MeV, confirming that the CCL could
accelerate beams of the required magnitude. The experiment
also confirmed that a brighter beam, i.e. higher intensity and
smaller emittance, would be desirable. Table 1 shows a
consistent set of measurements of the beam emittance as a
function of energy along the Linac at a beam intensity of 30
mA. The data show that considerable
____________________
* Work supported by the U.S. Department of Energy under
contract No. DE-AC02-76CH03000.
emittance growth occurs below 10 MeV. The emittance
dilution in the low energy beam transport systems for
H
-
beams has been widely studied and the Radio-
Frequency Quadrupole (RFQ) accelerator accepted as
the best choice for acceleration of intense beams up to
an energy where the drift-tube linac can continue the
acceleration. Observe from Table 1 that additional
emittance growth occurs in the 10 MeV linac tank. This
is understood as misalignments in the quadrupoles in
the low energy section of the cavity. The 10 MeV
cavity was originally built at Fermilab as a prototype
with the expectation that it would be replaced at a later
time. By increasing the injection energy into the cavity
and improving the alignment of the drift tubes, it is
expected that the beam emittance at 10 MeV could be
improved.
PREACC
OUT
LINAC
IN
TANK 1
OUT
LINAC
OUT
ENERGY
(MEV)
0.75 0.75 10 400
INTENSITY
(mA)
54 49 33 30
EMITTANCE
/π (mm-mrad,
95%, norm.)
1.3 2.6 5.2 7.8
Table 1 Emittance for H
-
Beams
It is proposed that the entire low energy end of the
present 400-MeV Linac be replaced to include an RFQ
and a new 10-MeV drift-tube cavity. In order to
maintain the redundancy of a stand-by ion source, it is
proposed to divide the RFQ into a low-energy section
with its ion source and low-energy beam transport
(LEBT) injecting into a second RFQ section to the full
RFQ energy. The RFQ sections are connected by a
magnetic five-dimensional phase-space imaging system
such as that used in the Fermilab/SAIC PET Project [2]
and referred to in this study as the double alpha system.
The stand-by system with its ion source, LEBT, and
low-energy RFQ would include a single alpha magnet
and its matching quadrupole magnets. By deactivating
the alpha magnet in the failing ion-source system and
activating the alpha magnet in the stand-by system, a
new ion source can be readily brought online.
Separating the RFQ sections also allows ancillary
equipment, such as beam chopper, diagnostic
equipment, and plasma neutralization devices to be
installed in the medium energy transport system
(MEBT). A sketch of this proposed low energy system
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Proceedings of the 2001 Particle Accelerator Conference, Chicago
with the remaining systems in the linac is shown in Figure 1.
2 ION SOURCE AND LEBT
Changing to an RFQ allows the possibility of improving
the present H
-
magnetron ion source or considering other H
-
sources to achieve the desired beam intensity and quality. A
review of high-intensity negative ion sources for accelerators
is given by one of the authors at this conference [3]. It is
completely feasible to upgrade the SPS source for the
required intensity, duty factor and beam quality without
Figure 1. Proposed low-energy improvement sketch.
sacrificing the reliability and availability from its proven past
performance. An extraction voltage from the ion source of
50 keV is proposed for extracting sufficient current from the
source and allowing for a short electrostatic focusing
structure which closely couples the source to the following
RFQ. The increased energy of the H
-
ions allows easier
injection and greater transmission through the RFQ.
3 DOUBLE ALPHA MAGNET MEBT
Until recently dividing an RFQ into a number of sections
to accelerate intense beams was considered unreasonable due
to the difficulty of matching the beam between the sections.
In the PET Project [2] a beam of
3
He
+
ions were accelerated
to 1 MeV in a 212-MHz RFQ. At 1 MeV the ions were
stripped with a gas jet stripper to form
3
He
++
ions which were
then focused and injected into a 425-MHz RFQ. The
stringent condition of matching the ion beam between the
two RFQ sections was done by using a magnetic five-
dimensional phase-space imaging system. In this system all
particles take the same time to pass through the transport line
independent of energy or transverse size and angle so the
transverse and longitudinal motions are uncoupled at the
entrance to the second RFQ. The performance of this MEBT
in the PET Project confirmed the calculations and operated
successfully. A similar MEBT using two 270
o
dipole
magnets and five quadrupoles is proposed in this application.
Using the design code PARMTEQ, RFQs have been
designed to accelerate a high intensity, low emittance
beam to the entrance of the linac. The first RFQ accepts
a 50-keV DC beam from the ion source, bunches it, and
accelerates the beam to 1 MeV. The second RFQ
accelerates the beam from 1 MeV to 2.23 MeV. The
design allows a beam intensity of 115 mA to be
accelerated with a transmission efficiency of better than
90%. To include the effect of possible emittance growth
between the two RFQs, the input emittance of the
second RFQ is 20% larger than the emittance out of the
first RFQ.
The code TRACE 3D [4] was used to design the
double alpha phase-space imaging system which would
transform the H
-
beam from the match-point at the
output of the first RFQ to the match point into the
second RFQ. Figure 2 shows the MEBT elements
consisting of the two alpha dipoles and five
quadrupoles. The apertures of the dipole magnets have
been increased a small amount over that used in the
PET magnets, but the edge angles have not been
altered.
Figure 2. MEBT Elements Consisting of Two Alpha
Dipoles and Five Quadrupoles.
Figure 3 shows a good match into the second RFQ for
a beam intensity of 80 mA. The small emittance
mismatch is well within the acceptance of the RFQ
where an emittance growth of 20% has been allowed.
Although this calculation demonstrates the feasibility of
using such a system, an R&D effort will be required to
validate the calculations and to derive the parameters
for a physical design.
4 10-MeV DT CAVITY REDESIGN
By raising the energy into the first cavity of the linac
from 0.75 MeV to 2.23 MeV the first 18 drift tubes
which have a 2-cm bore diameter and quadrupole
lengths of 1 inch and 1 1/4 inch will be eliminated. The
new DTL then starts with drift tubes having a 2.5-cm
bore and a quadrupole length of 1.75 inches. Thus the
problem of misalignments will be significantly
mitigated and the cell transit time improved to give a
better acceleration rate. The capability of the structure
to accelerate higher intensity and better quality beams
will be enhanced.
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Proceedings of the 2001 Particle Accelerator Conference, Chicago
5 PARAMETERS AND GOALS
Table 2 lists the parameters and performance goals of the
low-energy improvements to the 400-MeV Fermilab Linear
Accelerator to achieve the required intensity of the Proton
Driver Project [5].
6 REFERENCES
[1] M. Popovic, et.al. “High Current Proton Tests of the
Fermilab Linac,” Linac 2000, SLAC, Monterey.
[2] D.J.Larson, et.al. “Ion Optical Design of the BRF-FNAL-
SAIC-UW PET Accelerator,” PAC97, Vancouver.
[3] V.Dudnikov. “30 Years of High Intensity Negative Ion
Sources for Accelerators,” this conference.
[4] K.R. Crandall and D.P. Rusthoi. “Trace 3-d
Documentation,” LA-UR-97-886.
[5] “The Proton Driver Design Study,” Fermilab-TM-2136.
Figure 3. TRACE Calculation Showing Match to RFQ for a Beam Intensity of 80 mA.
ION
SOURCE
LEBT/
CHOPPER
RFQ-1 540
o
MEBT
RFQ-2 MATCHING
SECTION
DTL CCL
TYPE H
-
ELECTRO
-
STATIC
VANE DOUBLE
ALPHA
VANE 3 QUADS
1 BUNCHER
DRIFT-
TUBE
COUPLED
-
CAVITY
OUTPUT ENERGY
(MeV)
0.05 0.05 1 1 2.23 2.23 116 400
OUTPUT CURRENT
(mA)
115 115 102 102 97 93 86 86
OUTPUT CHOPPED
CURRENT (mA)
115 80 72 72 68 65 60 60
Emittance
(π mm-mrad 95%)
1.2 2 2.3 2.6 2.8 3
FREQUENCY
(MHz)
201 201 201 805
PULSE LENGTH
(msec)
90 90 90 90 90
Table 2. Parameters and Goals for the Performance of the New Configuration of Components for the Low Energy.
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Proceedings of the 2001 Particle Accelerator Conference, Chicago