PS Beam Measurements at Flat-Top Fields near Transition Energy

Beams up to 1013 protons were kept circulating for more than 100 ms at various magnet field levels near transition and the beam behaviour was observed for several radial positions. These levels corresponded notably to values of n (= γ-2 - γtr-2) in the range from 10-2 to < 10-4. They were measured using i) the synchrotron frequency, ii) the change in revolution frequency with radial position, iii) debunching times, and, close to transition, iv) the onset of the negative mass instability. Results from i), ii) and iv) agree well with standard theory and from iii) with a theory taking into account conditions close to γtr. The negative mass instability was observed even when, due to the spread in n values, only part of the beam was above transition. No losses and no global instabilities were observed down to η ∾ 5 . 10-4 (below transition). Experimental data and their explanation are presented and the interest of operating beam compressor rings close to γtr is pointed out.