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Aug-15-2000 07:47am From-APT-TPO ---
LEDA - A HIGH-POWER TEST BED OF INNOVATION AND
OPPORTUNITY*
J. David Schneider, & Richard L. Sheffield,
Los Alarnos National Laboratory, Los Alamnos, NM 87545, USAAbsrracr
The low-energy demonstration accelerator (LEDA) is
an operanonal 6.7-MeV. 100-mA proton accelerator
consisting of an injector, radio-frequency quadrupole
(RFQ), and all associated integration equipment. In order
to achieve this unprece4ented level of performance (670-
kW of beam power) from an RFQ, a number of design
innovations were required. We will highlight a number of
those more-significant technical advances, including those
in the proton injector, the RFQ configuration, the RF
klystrons, the beam stop, and the challenges of beam
measurements. In addition to identifying the importance
of rhese innovations to LEDA performance, we will
suamaize the plans for further testing, and the
possibilities for addition of more accelerating structures,
including the planned use of very-low-beta super-
conducting structures. LEDA's current and upgradable
configuration is appropriate for several future high-power
accelerators, inclu4mg those for the transmutation of
radioactive waste.
1 INTRODUCTION
As an integrated accelerator, LEIA made an impressive
number and magntude of technical advances (l, possible
only because several critcal subsystems showed
performance beyond the normal range. We will address a
number of those advances that were critically important
for reaching this performance level.
2 DESIGN FEATURES
2.1 Injector
The LEDA injector was developed (2] and refined over a
number of years, and built directly on improvements
learned from previous projects, capitalizing most
significantly on the development of the microwave-
powered ion source [3j from Chalk River Laboratories.
We also made a number of changes for this injector that
resulted in improved performance.
In an interest of simplification and improving
reliability. we eliminated all electromucN at high potential.
This required designing a special foil-lined insulator (made
of polypropylene that isolates the high-potential ion-
source chamber from the grounded ECR solenoid
magnets. Success in this isolation also reared that the
ga, fend to the ion source be at high (near-atmospheric)
pressure to avoid breakdown and discharge along the gas
feed rube The microwave feed also includes an in-linewaveguide break, where a spacer of about 3-mm thuckness
scparates the grounded WG from the continuing fmedline
into the source chamber {picture of WG break?)
We've continued the tradition (begun on the FMIT test
stand of 1978-1985) of using a precision-shaped cmiuter
surface (spherically convergent Pierce) and a single-gap
extractor. This configuration is mechanical simple ardl
provides a high-qualtty, low-divergence beam. We've used
both a tnode and teirode configuration for the electron-trap
assembly. Although the tiode provided more beam
current, most of our operations have been with the tetrode
configuration because of better protection of the electron
trap, and improved beam quality and stability.
During tuneup and commissioning, it is conveient to
operate with variable current levels and differing values of
duty factor. For this, we used first a contnuously variable
collunator, and than an insertable unit with three different
sized apertures. Both work, but [he engineering challenge
is to provide proper cooling (caster with the fixed-apcrure
device). There are some additional operanonal challenges
because the insertion of an interceptive device particularlyy
upstream of the first LEBT solenoid) adversely affects
extractor high-voltage reliabihty.
Variable beam pulsing required some .evclopment with
this microwave ion source. The successful approach
proved to be that of using a high-voltage current
modulator in the anode circuit feeding the 2.45-GIZ
magnetron. This modulator provides completely arbitrary
pulse lengths and duty factors, with short, clean
tralntis.
We recently completed a brief test of a circularly
polarized waveguide feed into the ion-source chamber.
Thin showed the expected improved coupling into the
plasma, curning in half (to 400-500 W) the microwave
power needed for operation..
Even though the LEDA RFQ was designed to accept a
less-convergent input beam (-60 mra4) than previous
RFQs, we found that very close spacing is requud
between the last LEBT solenoid and the RFQ input. Use
of an electron trap near the RFQ entrance gave benefits,
including improvement of current mea.surement [4]
LEDA operations demonstramd sustained high-current.
stable operation with well over 140 mA extracted from
the ion source. This was coupled with high powcr and gas
efficiency, and a most-impressive proton fraction (90%).
ion-source excitation with isolated microwave power at
2.45 GHz eliminates the troublesome and lifetime-
luruing internal filament. The resulting stable, uniform
plasma of low temperature provides a low-emittanec beam
(<0.2 7t mm mrad) that has proven appropriare for RFQ
injection.EIVED
C 1 3 2000
SsTT-668 P 004/006 F-199
5 5 66 -7443
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Schneider, J. David & Sheffield, Richard L. LEDA - A HIGH-POWER TEST BED OF INNOVATION AND OPPORTUNITY, article, August 1, 2000; New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc722393/m1/4/: accessed March 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.