Does presenting results in conference and publishing in unreviewed proceedings prevent better publications?
While I do not know accelerator physics in particular, for most fields and most journals, a non-reviewed publication does not count as prior publication.  If this is the case for accelerator physics as well, then putting an extended abstract in a non-reviewed conference will not block you from publishing an improved full paper in a journal later.  The prior version should be explicitly acknowledged in the journal submission.

The other reason that you might end up with less publications is internal rather than external: once you've put it out and gotten your feedback, you might not feel as strong an urge to do a formal peer-reviewed publication.  If this is the case, then you need to ask yourself whether this is actually a problem for you or not.  I, for example, have a number of publications that have never gone beyond technical reports or white-papers, but am comfortable with that because I think most of them did not need to have a full publication, and I've got enough others.  If you are a person who has difficulty motivating yourself to writing up results for peer review, however, you may want to consider the spur to publication as valuable and therefore avoid the non-peer-reviewed pre-publications.