Uniformization of the transverse beam profile by means of nonlinear focusing method

Abstract

It has been known that uniformization of a beam with a Gaussian profile is possible utilizing odd-order nonlinear focusing in the beam transport system, and this has recently been employed for uniform beam irradiation. Here, we have theoretically studied uniformization of the transverse beam profile using nonlinear-focusing forces produced by multipole magnets in detail. In the case where the nonlinear field of the multipole magnet is given by an infinite power series, all the odd-order multipole strengths required for uniformization of a Gaussian beam and the extent of the resultant uniform region have been expressed using the Twiss parameters. We have shown the principle of uniformization using even-order nonlinear fields. We have also actually demonstrated the transformation of a beam with an asymmetric distribution into one with a uniform distribution by utilizing nonlinear focusing, especially with the sextupole and octupole fields. The validity of the formulas presented here was confirmed through particle-tracking simulations. A practical method to realize a uniform profile using beam collimation and octupole focusing is also presented.

Transformation of the Beam Intensity Distribution and Formation of a Uniform Ion Beam by Means of Nonlinear Focusing

“…The central part of the distribution is made uniform at a gradient of 600 m −4 . This strength is about 30% weaker than in the case of perfect uniformization using an ideal multipole force [2]. The uniform region is surrounded by higherintensity peaks induced through folding the Gaussian tail.…”

“…We considered an actual layout of the beam transport system at the JAEA AVF cyclotron, equipped with octupole magnets whose axial length is 0.30 m [2]. Only the horizontal degree of freedom of the beam motion was considered here so that numerical results could be compared with the analytical results.…”

“…According to Ref. [2], the ontarget spatial distribution ρ t of the beam focused by the octupole magnet can be determined analytically. It is given as follows:…”

“…The transverse intensity distribution of the beam can be transformed into a uniform one through the nonlinear focusing force produced with multipole magnets (mainly, octupole magnets) in the nonlinear focusing method [1,2]. Actually, the uniform beam formation has been realized in several facilities using octupole [3][4][5] and sextupole magnets [6].…”

“…The nonlinear focusing method is attracting attention as a new technique for uniform irradiation with high-intensity beams in accelerator-driven neutron facilities: The use of multipole magnets is developed or planned in many high- power neutron facilities now in operation or under construction [7][8][9][10][11]. The requirement of the nonlinear force for the uniformization and the width of the resultant uniform distribution were already studied theoretically [2]. In the present paper, the transformation of the transverse beam intensity distribution by nonlinear focusing is explored systematically.…”