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Lab Course: Ion Sources
Larry Lamm
Research Professor
Technical Director of the NSL
Winter 2008
Accelerators in the lab
• Van de Graaff accelerators
– KN (3.7 MV) and JN (1.5 MV) are “singleended”, meaning the ion source is inside the
terminal, producing positively charged beams.
– FN (10.6 MV) is a “tandem” accelerator, with
the terminal electrode in the center of the
accelerator, requiring negatively charged
beams external to the accelerator.
Accelerators
FN (10.6 MV)
KN (3.7 MV)
JN (1.5 MV)
Helium Ion Source (HIS)
• Produces
negatively charged
3
4
He and He
beams for injection
into the FN
Tandem Van de
Graaff accelerator.
Helium Ion Source (HIS)
• The heart of the HIS
helium ion source is the
duoplasmatron,
manufactured by
Peabody Scientific. The
details of ion beam
production within the
duoplasmatron are a bit
complicated, but the
basic idea can be simply
described.
Helium Ion Source
• Filament
Legs (Ni)
used to
supply
high
current
(60A) to
Filament
(W)
Filament Legs (Ni)
Filament (W)
Helium Ion Source
• The
filament is
surrounded
by low
pressure
helium gas
(3He or
4He)
Low Pressure Gas (He)
Helium Ion Source
• The
filament
glows
white hot,
emitting
electrons
which
ionize the
helium
gas.
Filament (W)
Helium Ion Source
• Solenoidal
magnet
field helps
focus
electrons
to enhance
the plasma
production.
Solenoid Magnet
Helium Ion Source
• Positively
charged
helium ions
are “pulled”
out of the
source
through a
small
aperture.
Beam Aperture
Helium Ion Source
• This is
done
using an
electric
field
produced
by the
Extractor
Electrode,
at about
(-)15 kV.
Extractor Electrode (-15 kV)
Helium Ion Source
• The
diverging,
positively
charged
helium
beam is
then
focused
by the
Einzel
Lens.
Einzel Lens
Helium Ion Source
• The beam is
focused into
the lithium
charge
exchange
cell, where Li
metal is
heated to
form a vapor.
Lithium Charge Exchange
Cell
Helium Ion Source
• The lithium
charge
exchange
cell operates
at -15 kV, so
the beam
accelerates
into the cell.
Lithium Charge Exchange
Cell (-15 kV)
Helium Ion Source
• The lithium
vapor charge
exchanges
with the
positively
charged
helium beam,
giving up
electrons to
the beam
particles.
Lithium Charge Exchange
Cell (-15 kV)
Helium Ion Source
• Some beam
particles
become
negatively
charged (-1),
accelerating
out of the
charge
exchange
cell.
Lithium Charge Exchange
Cell (-15 kV)
Helium Ion Source
• An electrode
at ground
helps focus
the beam for
transmission
to the FN
Tandem
accelerator.
Ground Electrode
Helium Ion Source
• Brute force technique
• Difficult environment to stabilize
• Not much beam (a few microAmps at
best), but better than none
• Other materials are available for charge
exchange (alkali metals), but Li is probably
safest.
SNICS Ion Source
• Source of Negative Ions by Cesium
Sputtering
• Workhorse source for FN Tandem,
producing all beams except helium
• Highly reliable, sufficient beam output,
recently upgraded from single to multicathode (major upgrade)
SNICS Ion Source (Old)
• We have
recently
upgraded our
SNICS Ion
Source with a
Multi-Cathode
version. This is
a picture of the
old, single
cathode
SNICS.
MC-SNICS Ion Source
• This is a
picture of an
MC-SNICS Ion
Source (not
ours).
Principle of
operation is
the same as
the old, single
cathode
version.
MC-SNICS Ion Source
• This is 40
element
cathode
wheel. We
have a 40
element and a
20 element
wheel. More
about this
later.
SNICS Ion Source
• The cathode is
loaded with a
powder
containing the
material from
which a beam
is desired. We
make these as
needed in the
lab.
SNICS Ion Source
• Cesium is
heated in the
reservoir to
produce vapor,
which rises up
into the
source.
SNICS Ion Source
• Some of the
cesium
adheres to the
tip of the
cathode,
which is
cooled. Some
hits the hot
surface of the
ionizer.
SNICS Ion Source
• The cesium that
hits the ionizer
is “boiled” away
and an electric
field focuses
this cesium to
strike the tip of
the cathode,
which sputters
material out of
the powder.
SNICS Ion Source
• The sputtered
material
leaves the
cathode tip
and passes
through the
cold cesium
on the surface.
SNICS Ion Source
• The sputtered
particles pick
up electrons
from the cold
cesium, and
some become
negatively
charged and
are extracted
from the
source.
SNICS Ion Source
• Easy to use, very reliable.
• Prolific beam production by FN standards
MC-SNICS Ion Source
• The multicathode
version allows
us to switch
quickly (a few
seconds) to
another
cathode, a
process that
once took
several hours.
MC-SNICS Ion Source
• This is
especially
important for
the AMS
program. Work
still needs to
be done for
this process.
RF Ion Sources
• KN and JN accelerators use RF (Radio
Frequency) ion sources
• Glass bottles (cylindrical) with low presure
gas, excited with power from oscillating
electric field
• Plasma discharge, much like neon signs,
but more intense.
RF Ion Sources
• Positively charged
beams
• Significantly (1000
times) more beam
than negative ion
sources.
• But, much lower
energies.