Takahiro Yuyama

Useful technique for analysis and control of the acceleration beam phase in the azimuthally varying field cyclotron

We have developed a new technique for analysis and control of the acceleration beam phase in the cyclotron. In this technique, the beam current pattern at a fixed radius r is measured by slightly scanning the acceleration frequency in the cyclotron. The acceleration beam phase is obtained by analyzing symmetry of the current pattern. Simple procedure to control the acceleration beam phase by changing coil currents of a few trim coils was established. The beam phase width is also obtained by analyzing gradient of the decreasing part of the current pattern. We verified reliability of this technique with 260 MeV (20)Ne(7+) beams which were accelerated on different tuning condition of the cyclotron. When the acceleration beam phase was around 0 degrees, top of the energy gain of cosine wave, and the beam phase width was about 6 degrees in full width at half maximum, a clear turn pattern of the beam was observed with a differential beam probe in the extraction region. Beam phase widths of ion beams at acceleration harmonics of h=1 and h=2 were estimated without beam cutting by phase-defining slits. We also calculated the beam phase widths roughly from the beam current ratio between the injected beam and the accelerated beam in the cyclotron without operating the beam buncher. Both beam phase widths were almost the same for h=1, while phase compressions by a factor of about 3 were confirmed for h=2.

Transformation of the Transverse Beam Intensity Distribution by Sextupole Focusing in a Transport Line

We investigate the transformation of the transverse intensity distribution of a charged-particle beam focused by one or two sextupole magnets in a beam transport line. It is expected, from the equation of transverse motion, that the beam is deflected toward one direction and thus deformed due to the second-order force of the sextupole magnet. Such a sextupole-induced deformation of the beam distribution has been studied analytically in detail. The beam centroid displacement and the change of the beam size are determined using the first-order and second-order moments of the intensity distribution function. In order to verify the theoretical consideration, numerical simulations and experiments were performed at the cyclotron facility of Japan Atomic Energy Agency. We experimentally demonstrate that an ion beam with a uniform transverse distribution can be formed using two sextupole magnets.