Rick Baartman
LTNO Beam Optics
LTNO Beam Optics
1 Background
The current setup was inherited from the Oak Ridge
``Unisor'' experiment. This includes all optics from the point at which the
beam is deflected upwards by the Unisor electrostatic mirror. Besides the
mirror, there are 4 identical quadrupoles, powered in a symmetric triplet
arrangement (outer 2 are wired together, as are the inner 2). Downstream of
the triplet is an iris that is reduced to a diameter of 2 mm when the
target is cooled to below 4K. From the iris to the target (cold finger)
there is a drift of 1 metre. The target is as large as 1 cm
diameter. Electrostatic steering plates located near the iris are used to
centre the beam on target. There is a beam stop near the target which can
be inserted laterally by way of a long mechanical feedthrough.
2 Theoretical Performance
Though the cold finger is 1 cm dia., the
experiment is compromised if more than a few percent of the radioactive
beam misses it. For this reason, and because without a nearby collimation
system, the beam has ``soft'' edges (due to scattering and aberrations in
the 40 metres of beam transport), the beam is tuned to achieve a nominal
size of 7 mm dia. With a 2 mm dia. aperture 1 m distant, this means the
acceptance is 3.5 pmm-mrad. Since the surface ionization source
emittance is roughly 7 pmm-mrad, this means the transmission with
iris closed is at best on the order of 25%.
3 Performance to date
With iris closed, transmission of the order of
25% has indeed been observed. However, consistent performance with
radioacitive beam on the cold finger has been sometimes difficult. There
have been 2 reasons: unreliable diagnostics and an incomplete understanding
of the optical properties of the electrostatic mirror. The result has been
that when performance was poor, it was difficult to pinpoint the cause.
Regarding the mirror optics, recent experiments have been performed which
show there to be a surprisingly large focusing effect. This has yet to be
confirmed with electric field tracking, but there is now agreement between
theoretical and running tunes.
Regarding diagnostics, the problem is mostly related to the difficulty of
working across the thermal barrier necessary for achieving the low
temperatures. The beam stop is attached to a long mechanical feedthrough
and so its position is not always known with sufficient confidence. This
has a major impact because during setup with a stable beam, if the beam
stop is poorly centred when inserted, then when it is turned out for RIB
running, some of the radioactive beam could miss the cold finger.
As well, signals from the beam stop are not as well-shielded and so are
noisier than for other ISAC beamstops. This requires that setup beams be
rather high intensity ( ~ 100 nA). At this level, any surfaces near
the beam, which are not grounded or have insulating coating, can charge up
sufficiently to steer the beam. This can cause the test beam to behave
differently from the ( < 1 pA) RIB. There have been indications of such
behaviour.
4 Possible Improvements
4.1 Consistency
The LTNO cold finger represents a small target
at the end of a long drift. Ideally, such an acceptance limited, background
sensitive experiment ought to have its own phase space selection
slits. This is the case for example for the 8p
experiment. Lacking these, the slits at the start of the transport section
after the separator are used instead. There are 2 disadvantages to
selecting phase space this far from the experiment: effects of scattering
with the background gas are worse, and one is more sensitive to equipment
malfunction such as too much power supply ripple.
Slits can be placed in the last 2 diagnostic boxes before the
mirror. Ideally, the optics should be arranged such that the cold finger is
at the image of one of the slit assemblies. At present, not enough is known
about the mirror aberrations to say with certainty that this is
possible. If it is not, the mirror should be replaced with a standard ISAC
electrostatic bend section.
One needs to calculate an electric field map for the
electrostatic mirror and track particles through it to obtain first order
optics plus higher order aberrations, then to check whether existing optics
can be used to image a set of slits at the cold finger. If so, then only a
set of slits needs to be built, identical to the existing slits in
LEBT. These activities would only require a few weeks of a beam physicist's
time, plus of course technician's time to build the slits.
If the mirror cannot image the slits, then new optics are to be designed,
replacing the mirror with a standard ISAC bend section. These activities
would require 1 or 2 man-months of a beam physicist's time, plus
approximately the same amount of technical resources as to build a new
experimental station.
4.2 Intensity
There is no principle reason that the acceptance of
the LTNO be small compared with the source. The optics could be redesigned
to achieve near 100% transmission without increasing the size of the cold
finger or increasing the heat load. However, this would require a focusing
element between the iris and the cold finger, and would necessitate a
redesign of the cryostat as well.
The required activities are similar to those in the
pessimistic scenario above for replacing the mirror, but as well would
include a redesign of the LTNO station itself.
5 Conclusion
Much progress has been made in achieving consistent
results from LTNO. However, the present apparatus will never be as easy to
tune as other ISAC LE experiments. Adding slits will make a moderate
improvement to consistency, for moderate cost/disruption. Changing the
optical design could yield a factor of 4 improvement in intensity, but for
large cost.
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On 2 May 2003, 09:55.