The SNS front-end, an injector for a high-power hydrogen-ion accelerator Page: 1 of 3
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THE SNS FRONT-END, AN INJECTOR FOR A HIGH-POWER
HYDROGEN-ION ACCELERATOR *
Ernest Orlando Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA 94720, USA
The Spallation Neutron Source** (SNS) will be an ac-
celerator-based facility in Oak Ridge, TN, delivering
pulsed neutron beams to experimenters. Negative hydro-
gen ion-beams are generated and pre-accelerated in a 2.5-
MeV linac injector, or front end (FE), accelerated to 1
GeV energy by a linear accelerator system, converted into
protons and accumulated in a ring accelerator, and then
directed towards a mercury target to generate the neu-
trons. The proton beam arrives at the target in bursts of
less than 1 ps duration and with more than 1 MW average
power. The front end has been built and commissioned by
LBNL in Berkeley; shipment to ORNL is essentially com-
plete. This paper provides an overview of FE major de-
sign features and experimental results obtained during the
commissioning process. The SNS-FE can be viewed as a
prototype of a high-current, high duty-factor injector for
other accelerator projects or, without the elaborate
MEBT, as an independent 2.5-MeV accelerator for vari-
The SNS accelerator systems are comprehensively dis-
cussed elsewhere . They aim at delivering intense pro-
ton-beam pulses of less than 1- s duration to the spalla-
tion target at 60-Hz repetition frequency and with an av-
erage beam power of 1.44 MW. The 1-ms long H- macro
pulses that are accelerated by the linac to 1-GeV energy
have to be chopped at about 1 MHz frequency into 'mini
pulses' of 645-ns duration, with 300-ns gaps.
LBNL has built the front end (linac injector) with its main
beamline elements consisting of ion source, low-energy
beam-transport section (LEBT), RFQ accelerator, and me-
dium-energy beam-transport section (MEBT), as well as
the ancillary systems needed to operate them. The beam-
line, without ancillary systems, is shown in Fig. 1. Some
parts of the SNS front end, i.e. the rf power system for the
RFQ and the MEBT chopper structures and their power
supplies, were supplied by LANL; some diagnostic elem-
ents and associated electronics by LANL and BNL, and
personnel from these laboratories as well as from SNS-
ORNL participated in the commissioning. The SNS
Front-End project has been described in detail elsewhere
with an ample collection of references [2, 3] and the pres-
ent paper emphasizes the latest design features and com-
Beam chopping is performed by two separate chopper
systems located in LEBT and MEBT, respectively. The
LEBT chopper removes most of the beam power during
the mini-pulse gaps, and the MEBT chopper reduces the
rise and fall time of the transported beam to 10 ns.
The main nominal parameters for the SNS Front End
are listed in Table 1. The front end was assembled and
commissioned at the Integrated Testing Facility at LBNL
was shipped to ORNL in June/July, 2002.
II. ION SOURCE AND LEBT
The ion-source is shown in Fig. 2. The discharge
plasma is sustained by pulsed 2-MHz-rf power and con-
fined by a multi-cusp magnet configuration. Pulse ignition
is facilitated by maintaining a stationary low-density plas-
ma, driven by a separate 13.56-MHz generator. A mag-
netic dipole filter separates the main plasma from a
smaller H- production region where low-energy electrons
help generating copious amounts of negative ions. A
heated collar, equipped with eight cesium dispensers, sur-
rounds this H- production volume, and a very thin (about
1/2 mono layer) coating of cesium on the collar and out-
let-electrode surfaces enhances the extracted beam current
by a factor of three for a given rf power.
* Work supported by the Director, Office of Science, Basic Energy Sciences, US Department of Energy, Contract No. DE-AC03-76SF00098.
** SNS is a collaboration of six US Laboratories: Argonne National Laboratory (ANL), Brookhaven National Laboratory (BNL), Thomas Jefferson
National Accelerator Facility (TJNAF), Los Alamos National Laboratory (LANL), E. O. Lawrence Berkeley National Laboratory (LBNL), and Oak
Ridge National Laboratory (ORNL). SNS is managed by UT-Battelle, LLC, under contract DE-AC05-000R22725 for U.S. Department of Energy.
angle of about 30 against the LEBT axis to compensate
Figure 1. The 9-m long SNS front-end beamline. for this effect. The angle can be adjusted as needed.
Table 1. FES Nominal Performance Parameters
The outlet plate of the ion source contains a dipole-
magnet configuration that creates a transverse deflecting
field across the extraction gap, separates the extracted
electrons from the ion beam and steers them towards a
'dumping' electrode biased at 5 kV with respect to the
outlet plate. Because this dumping field steers the ion
beam as well, the entire plasma generator is tilted at an
Output energy (MeV)
H- peak current:
MVIEBT output (mA)
Nominal ion-source output,
assuming 80% RFQ transmission (mA)
Output normalized transverse rms emittance (It mm mrad)
Output normalized longitudinal rms emittance (It MeV deg)
Macro pulse length (ms)
Duty factor (%)
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Keller, R. The SNS front-end, an injector for a high-power hydrogen-ion accelerator, article, February 1, 2002; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc740419/m1/1/: accessed April 25, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.