A Non-Scaling Radial-Sector Fixed Field Alternating Gradient (FFAG) Ring for Carbon Cancer Therapy - eScholarship

2004 
A Non-Scaling Radial-Sector Fixed Field Alternating Gradient (FFAG) Ring for Carbon Cancer Therapy Eberhard Keil 1 , Andrew Sessler 23 and Dejan Trbojevic 45 ∗ CERN, CH1211 Geneva 23, Switzerland Lawrence Berkeley National Laboratory, Berkeley CA 29720, USA Brookhaven National Laboratory, Upton NY 11973, USA Abstract. A non-scaling radial-sector FFAG is investigated as a machine to produce 2 × 10 9 particles of C 6+ per pulse, at an energy of 400 MeV. This is accomplished by having an ECR ion source (producing C 4+ at 40 keV per nucleon), followed by an RFQ (that accelerates to a few MeV/u) and then a rapidly cycling synchrotron or linac that takes the carbon ions from 1 MeV/u to 31 MeV/u. The carbon is then fully stripped and accelerated in one FFAG to 119 MeV/u and then in a second FFAG to 414 MeV/u. The top FFAG has a radius of only 8.1 m and an aperture of 20 cm. The magnets are superconducting and have a maximum pole tip field of 5.3 T. The fields are linear, so the dynamic aperture is large. On the other hand, because the FFAG is non-scaling the tunes vary during acceleration and the rate of acceleration must be rapid enough to pass through resonances without unacceptable degradation of the beam. Keywords: Cyclotron, cancer hadron therapy, FFAG cyclotron PACS: 29.20.Hm, 29.27.Eg, 41.75.Ak, 41.85.Lc, 87.56.By INTRODUCTION A very interesting study of a series of scaling radial-sector FFAGs (three machines) for carbon cancer therapy has been undertaken by Misu et al.[1]. The interest in FFAG machines arises because of the medical interest in the latest therapy technique of spot-scanning which requires (about) 200Hz repetition rate which can not be achieved with a conventional synchrotron[2]. The major difficulties, encountered by Misu et al with the use of scaling FFAGs were: The radius of the top FFAG is rather large (11 m). The aperture of magnets is rather large (65 cm). The dynamic aperture in the low energy FFAG is rather small (80% of the beam is lost during acceleration). The rf frequency is forced to be rather low (in the megahertz range so as to cover the aperture). In this contribution we develop a scheme that overcomes these difficulties. We propose the same ECR ion source, but then follow it immediately with an RFQ that accelerates the ions to a few MeV/u. Then we invoke a rapidly cycling synchrotron, operating at 200 Hz, or a linac, to accelerate from the output of the RFQ to 31 MeV/u. It should not be difficult to cycle the synchrotron so rapidly as the top energy is very small and, hence, the inductance of the synchrotron magnets is greatly reduced from earlier machines that operated at 60 Hz and went to a few GeV. Then, we propose two non-scaling FFAGs that operate over a range, in each, where the ion momentum changes by a factor of two. By the use of superconducting magnets we address difficulty no. 1. The non-scaling (in contrast with a scaling) FFAG addresses difficulty no. 2. A non-scaling FFAG is almost linear and hence has a large dynamic aperture, so difficulty no. 3 is addressed (but not actually used in this design as we propose a synchrotron or linac (not an FFAG) at low energy (where the losses were large in the Misu et al design). Finally, because the aperture is reduced, compared with the Misu et al design, higher frequency rf may be employed. e-mail Eberhard.Keil@t-online.de e-mail AMSessler@lbl.gov Supported by the U.S. Department of Energy under Contract No. DE-AC03-76SF0009 e-mail Trbojevic@bnl.gov Supported by the U.S. Department of Energy under Contract No. DE-AC02-98CH10886
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