The beams of the ECR sources
the complain of the users
The only goal of most builders
of ECR sources, challenging each others, is to get as much as
possible ions of the highest possible charge state. They usually
set, just at the exit of the magnet selecting the charge
state, a Faraday cup as large as possible, and try to get the
maximum currents by tuning parameters of the sources.
The users take
the ions on the focal point of the magnet and try to direct the beam on
their targets to get a beam spot as well characterized as possible.
This is the time at which troubles come.
As everybody knows that the ions are extracted from a plasma of large
size, in which the ions may not be uniformly prepared, the users are
not expecting to get a good emittance and accept to spent quite a
long time to get an approximate beam spot by tuning
lenses, steerers and so on in an erratic way until they get an
In our lab we have been carried out experiments in various places through the worldand with
various ECR sources and found that each time we try to direct the
beam directly from the focal point of the selectring magnet the beam
was about 2 cm lower than expected. Finding the same trouble with fully
different sources build with different people made us puzzled.
then build a special mechanical design allowing to move a large
fluorescent target in any direction from the focal point of the
magnet and found that actually there were three different beams coming
from this point in three different direction approximately at 120 d°.
We always found that there was always one of these
beams pointing to the ground in the right mechanical axis of the
magnet and two other beams pointing, approximately, to the top on
the right and left direction.These finding obviously led us to think
about the well know three branches star imaging the octupolar character
of the confined plasma. We succeded in usung the first of
these beams by directlng the pipes of the beam line to the ground but
it was much more difficult to use the two others.
presented these results in a meeting of the European network on the
Multicharged beam we organized in Versailles few years ago. There was a
lot of dicussions, some colleagues agreeing and claiming that they
actually already observed more than one beam while others who never got
any experimental evidence for an effect disagreeing. We realized
that those who never observed this three beams used
actually a solenoid just before the binding magnet while all the others
directly sent the extracted beam from the plasma to the magnet . It may
then be possible that the solenoid in a quite complex way may
extracted beams .