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.
Highlights
In charged-particle beam applications, uniform irradiation is frequently required to bring about homogeneous irradiation effects all over a sample with a large area or on a large number of samples simultaneously
We demonstrate that it is possible to produce a well-uniformized distribution utilizing an octupole focusing field when the tail of a Gaussian beam is truncated
Equations (9) indicate that a beam with a larger initial emittance and/or a larger beta function are practically advantageous to uniformization in a wider area by using a lower multipole magnetic field
Summary
In charged-particle beam applications, uniform irradiation is frequently required to bring about homogeneous irradiation effects all over a sample with a large area or on a large number of samples simultaneously. As an alternative uniform-irradiation method, the nonlinear-focusing method using multipole magnets is promising This method is superior to the above methods in that we can perform high-uniformity irradiation at a constant particle fluence rate over the whole area of a large target. It is based on the idea proposed by Meads that the transverse intensity distribution can be smoothed by an octupole lens [5]. We demonstrate that it is possible to produce a well-uniformized distribution utilizing an octupole focusing field when the tail of a Gaussian beam is truncated.
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