The s y s t e m chosen in [2] had a potential dis t r ibut ion (1) with A = 0 (i.e., with s t rong focusing), p robably in o rde r to intensify the action on pa r t i c l e s t ravel ing close to the axial plane of the beam and reduce the number of re turned pa r t i c l e s . The p a r a m e t e r s U, D / L , C, A (D is the channel width of the sys tem) were opt imized in advance by compute r -ca lcu la t ing the t r a j ec to r i e s of ions with different initial ve loci t ies V 0 and posi t ions Y0, the angular deviat ion of the veloci ty vec tor being ~:0.02 radian. As a resu l t of this opt imizat ion the calculated number of pa r t i c l e s escaping f rom the s y s t e m was reduced to 7% (in the energy range W m a x / 2 to Wmax), while the calculated eff iciency was 92%, subject to the repeated recupera t ion of the back sca t t e r ed ions, a f te r thei r ref lect ion f rom the magnet ic field of the s toppers (magnetic m i r r o r s ) . The r e su l t s predic ted by these numer ica l calculat ions were ver i f ied by l abora to ry expe r imen t s [2]. The field of the Pos t s y s t e m was c rea ted in the f i r s t r e ta rda t ion sect ion (in which no flow of pa r t i c l e s to the wa l l swas expected), us ingten pa i r s of rods ehargedwi th r e spec t to the l inear ly increas ing potential on the axis of the s y s t e m . In the next re ta rd ing stage, in which the potential cor responded to the energy of the incoming pa r t i c l e s , the negat ively charged rods in the modules were replaced by cyl indr ical gr ids , so that the pa r t i c l e s acce le ra t ed by the deflect ing field in the t r a n s v e r s e d i rec t ion passed through the gr ids and fell on p la tes at a h igher potential than that on the axis of the module. The ave rage exper imenta l eff ic iency over the energy range W m a x / 2 to Wma x for a s y s t e m of 21 modules was 83% for 5~ of b a c k s c a t t e r e d ions, or 88% if secondary recupera t ion of the pa r t i c l e s escaping into the expander could be achieved. Increas ing the eff iciency of the Pos t s y s t e m to more than 90% is v e r y prob lemat ica l , even in r a re f i ed beams (neglecting space charge) .