On space charge dominated beam transport without emittance growth

Abstract

Possible mechanisms of beam emittance conservation in high current transport lines and in RF linacs are discussed. Invariance of beam emittance is treated as a problem of proper matching of the beam with focusing and accelerating channels. To obtain matching conditions for a beam with an arbitrary distribution function in transport channel, it is necessary to accept that the potential of the external focusing field is a highly nonlinear function of radius. The solution for external potential is obtained from the stationary Vlasov’s equation for beam distribution function and Poisson’s equation for electrostatic beam potential. Gradual change of nonlinear focusing field results in adiabatic transformation of the beam with initial nonlinear distribution into the beam, matched with the linear focusing channel. Alternating‐gradient focusing structure with higher order multipole component creates better matching of the beam with the channel than pure quadrupole structure. Beam emittance growth in linear accelerator is studied for a drift tube linac with solenoid focusing. It is shown that beam phase space distortion in RF linac can be eliminated by the appropriate choice of the value of focusing field with respect to acceleration gradient. An analytical approach is illustrated by results of a particle‐in‐cell simulation.