Controversial issues

This page is dedicated to new, recent or tentative ideas in the field of ion surface interactions (or others), which may be controversial and be used as an open forum. Readers may send me contradictory claims: jpbriand920@aol.com which I would be pleased to post in this page.

Issue 1: The beams of the ECR sources or the complain of the users

The problem

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 acceptable spot.

Our experiments

In our lab we have been carried out experiments in various places through the world and with various ECR sources and found that each time we try to direct the beam directly from the focal point of the selecting 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.

We 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°. 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 findings obviously led us to think about the well know three branches star imaging the octupolar character of the confined plasma. We succeeded in using the first of these beams by directing the pipes of the beam line to the ground but it was much more difficult to use the two others.

We 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 discussions, 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 reconcentrate the extracted beams.

Comments welcome