Nonlinear matcher for prevention of emittance growth in space charge dominated beams

Nonlinear space charge forces of high intensity beam produce strong emittance growth in linear focusing channel due to mismatching of the beam profile with focusing field. To obtain matching conditions for a beam with an arbitrary distribution function, it is necessary to accept that the potential of the external focusing field is a highly nonlinear function of radius. Four vanes quadrupole structure with duodecapole field component is suggested for suppression of emittance growth in high current beam. Beam profile in proposed transport line has to be close to square instead of conventional circle cross section. Adiabatic change of electrode shapes along the channel results in transformation of initially nonuniform beam into the beam, matched with the channel with pure quadrupole field. Results of prevention of emittance growth in the bright beam with phase space density of 1.5 A/(pi cm mrad) are given.