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.

To investigate the possibility of guiding a fast, heavy-ion beam with tilted curved surfaces, we study the transmission properties of a 4 MeV C+-ion beam passing through a narrow gap between a convex glass lens and concave glass lens. The ion-beam transmission of 42–59% obtained at the tilt angle θ= 3° is 20–30 times greater than that of 2% estimated from the cross section of the narrow gap between the two cylindrical lenses. The results of laser-beam-transmission experiments indicate that, even at θ= 3.4° where almost no ions pass through the gap without interacting with the cylindrical glass surfaces, approximately 10% of the ions are transmitted. The transmission probability is asymmetric with respect to the polarity of θ, indicating that the ion beam impinging on the concave surface is more easily transmitted than when impinging on the convex surface. Furthermore, no significant ion-energy loss occurs upon transmission for the angular range -3≤θ≤+ 3.4°. These results provide evidence that fast heavy-ion beams are guided by curved surfaces.

No abstract