Study of Compact Medical FFAG Accelerators- Radial Sector Type -

T. Misu, Y. Iwata, A. Sugiura, S. Hojo, N. Miyahara, M. Kanazawa, T. Murakami, and S. YamadaABSTRACT We have studied the various conditions and limitations for achieving compact Fixed-Field Alternating-Gradient (FFAG) accelerators to be widely used in heavy-ion cancer therapy. For the case of a normal-conducting FFAG accelerator, our linear calculation indicates 12-cell radial sectors with a field index of 10.5 as a suitable configuration. We found that its ring circumference can be as small as 70 m and that triple-cascade rings are needed to accelerate a carbon beam from 40 keV/u to 400 MeV/u. Viable radial-sector designs are possible with circumference factor C significantly lower than the value 4.45 previously quoted. FFAG Workshop 2004

The Heavy Ion Medical Accelerator in Chiba (HIMAC) has shown the therapys effectiveness after treating over 1800 patients. However, such carbon-beam medical accelerators are rather large in size and expensive, which prohibit the widespread use of carbon-beam radiotherapy. Therefore, there is a need to develop medical carbon-beam accelerators, that are compact, low-cost and simple to operate.

Since the fixed field of FFAG allows a higher magnetic field than synchrotrons, a more compact design may be achieved. The higher repetition rate of FFAG is expected due to its time-independent field structure. With such a high repetition rate, one may expect much shorter irradiation time by increasing beam intensity and a highly controlled delivery of the dose when using the latest irradiation techniques such as the spot-scanning method.

Widespread Use of Carbon-Beam Radio-TherapyFFAG Workshop 2004

HIMAC at NIRSFFAG Workshop 2004

Constraints a maximum magnetic field of 1.9 T a ring circumference of 70 m or less (Circumference Factor C of 3.3 or less) an orbit excursion of 1 m or less a length of straight section per cell of 1 m or moreFFAG Basic ParametersFFAG Workshop 2004

Analysis with Linear Optical Model Circumference FactorFFAG Workshop 2004

0 20 40 60 80 0 20 40 60 80 0 20 40 60 80 100 5 4 3 2 1Field Index kCircumference Factor C1st Stability Region2nd Stability Region1st Stability Region2nd Stability Region1st Stability Region2nd Stability Region(a) Singlet (b) Doublet (c) D-F-D TripletComparison of Stability Regions FFAG Workshop 2004Cell Number : 12 (All solutions are shown.)

1st Stability Region2nd Stability Region3rd Stability Region1st Stability Region2nd Stability Region|by | (m)E=400MeV/uBmax=1.9T20.04

13.36

6.68

Ring Radius at Bmax=1.9T (m)Circumference Factor16.70

10.02

3.34 Stability Regions of Radial-Sector Doublet FFAGs (N=8) (All solutions are shown.)For higher order stability regions (phase advance>180), b function becomes large.Although radial-sector lattices residing in higher order stability regions are interesting, they are regarded as questionable configurations.FFAG Workshop 2004Feasibility study of 1st order stability regions only

1st Stability Region of Radial-Sector Doublet FFAGsLarge k and small ring radius are preferred.Minimum radii increase with field index k.To find parameters satisfying ring radius < 11 m and small orbit excursion, we chose N = 12.Field Index k 0 5 10 15 20 5 10 15 20 5 10 15 2015

10

5

0RingRadius (m)N=8N=12N=16 (All solutions are shown.)FFAG Workshop 2004Bmax = 1.9 T

0 1 0 1 0 1 23 2 1 0Drift Length per Cell (m)Orbit Excursion (m)Momentum Ratio: 2.0Momentum Ratio: 3.0Momentum Ratio: 4.0Orbit Excursion vs. Drift Length of Radial-Sector Doublet FFAGs (Only solutions with radius < 11m are shown.)FFAG Workshop 2004

ECR1st FFAG2nd FFAG Normal-Conducting Radial-Sector FFAG ConfigurationECRb (m)20.5 mFFAG Workshop 200410

5

020

10

30

20

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00 10 20 30Angle (deg.)0 20 40 60 80LE-FFAGME-FFAGHE-FFAG

1. Possibility of achieving a broad-band high accelerating gradient rf cavityLE-FFAG: 0.22 1.88 MHz, 3.3 kVME-FFAG: 0.89 3.00 MHz, 18.4 kVHE-FFAG: 1.99 3.10 MHz, 45.5 kV

2. Tune shift due to the fringing field LE-FFAG: momentum ratio of 12.27ME-FFAG: momentum ratio of 4.18HE-FFAG: momentum ratio of 2.15

3. Dynamic aperture of each ring LE-FFAG: beam emittance of 108 p mm-mradME-FFAG: beam emittance of 8.8 p mm-mradHE-FFAG: beam emittance of 2.1 p mm-mrad Technical ConcernsFFAG Workshop 2004

0 1 2 3 4 5Frequency (MHz) 600

400

200

0

-200

-400

Cavity Impedance (W) Estimation of Cavity Parameters using FINEMETFFAG Workshop 2004

2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3.0 2.5

2.0

1.5

1.0

0.5Average Orbit Radius (m)Vertical TuneInjectionExtractionFFAG Workshop 2004 Estimation of Vertical Tune Shift due to Fringing Field LE-FFAG -Since gradient-field production by pole gap variation is the cause of tune shift, hybrid-type magnets, which produce radial gradient field by combining both coil-current distribution and magnetic pole gap, may be considered.

Dynamic ApertureFFAG Workshop 2004Transmission rate in the LE-FFAG ring was estimated as 20 percent.

Design SummaryIn this study, some of the stability regions were surveyed to search for the appropriate cell number N that gives the maximum value of k, while keeping the ring circumference less than 70 m. (2) Viable radial-sector designs are possible with circumference factors C significantly lower than the value 4.45 previously quoted. (3) Our preliminary design shows a radial-sector doublet FFAG consisting of triple-cascade rings, in each of which the orbit excursion is kept below 80 cm. A ring circumference of 69.2 m (corresponding to C = 3.3), which we believe is nearly the optimum size for the radial-sector type, is realized with normal conducting magnets of 1.93 T. (roughly the same size as the compact synchrotron design) Considering compactness and cost, the radial-sector FFAG design do not show its clear advantage over the compact synchrotron design. However, one of its merits, high repetition rate, can give the possible advantages over synchrotrons. Therefore, the further improvements of the cavity performance have been mainly pursued as a key to achieve high repetition rate.FFAG Workshop 2004

Improvements of Cavity Performance - R&D -Estimation of Core Impedance - Shape-dependence and size-dependence of core impedance were determined. - Impedance of racetrack-shape core is greater than that of circular-shape core.Development of Viable Cooling Methods - One-side (indirect) water-cooling method was employed to keep cavitys impedance high. - Cooling and endurance tests have been performed. - Indium-bonding method was successfully tested (still expensive). - Inexpensive one-side cooling methods are further investigated. [underway](3) Investigating the influence of fringing field [omitted] - Cavitys impedance is often influenced by the presence of fringing field, especially in tight lattice structure. - Avoid the reduction of shunt impedance by taking into account the influence appropriately.(4) Development of New MA Cores [underway] - Reduce rf power and/or number of cores by increasing core permeability.FFAG Workshop 2004