FFAG-ERIT R&D 06/11/06 Kota Okabe (Kyoto Univ.) for FFAG-DDS group

Neutron source for BNCT FFAG-ERIT scheme Requirements from BNCT( Boron Neutron Capture Therapy ): In order to remedy the tumor of 10cm 2, 2*10 13 neutrons are needed. If we assume that remedy time is 30 minutes => Flux    cm 2 sec. Accelerator as a neutron source ; Energy is low, but beam current is very large (I > 40mA [CW]) ERIT : Emittance-Energy Recovering Internal Target The stored beam is irradiated to the internal target, it generates the neutron in the storage ring. The beam energy lost in the target is recovered by re-acceleration. Technically hard and expensive Feature of ERIT scheme Beam current reduced by storage the beam in the ring.

Overview of FFAG-ERIT accelerator system

Emittance growth in storage ring Using an internal target in the ring, the beam emittance can be increased in 3-D directions by multiple scattering and straggling. In this reason, the storage ring require to large acceptance. In ERIT scheme, however, the beam emittance growth can be cured by Ionization Cooling effect.

Ionization cooling (1) The rate equation of beam emittance passing through a target material is,LongitudinalHorizontalVertical Cooling term Heating term 0 Wedge TargetAcceleration Cavity When the wedged target is placed at dispersive point, can be possible.

Ionization cooling (2) Energy loss rate dE/dx from Bethe-Bloch formula ( 9 Be target) In the light orange area (5~11MeV) the neutron is stable generated For example, target thickness ~ 5  m Energy loss :  E t ~ 35 keV 10 MeV proton beam Energy loss :  E t ~ 46.8 keV 7 MeV proton beam

Requirement for FFAG storage ring Large acceptance momentum acceptance dp/p ~ 5 [%] (from RF bucket height) transverse acceptance > 1000 [  mm mrad] Length of straight section (to install large RF cavity(width 54cm)) The numbers of sectors is few, length of the straight section is easy to guarantee. To be the compact which can be installed in the hospital Mean radius (r 0 ) ~ 2 [m]

Spiral sector type FFAG

Magnetic field calculation (TOSCA) Lattice parameters Cell num. = 8 Open sec. angle = 45 deg Open F angle = 13.5 deg Packing fac. = 0.3 Average radius = 1.8 m Spiral sector type FFAG 7MeV proton beam The size of the accelerator becomes small compared with the radial sector type.

Spiral angle and k value optimization 26 o, k = 2 30 o, k = 2 35 o, k = 2 26 o, no clamp, k = 2 35 o, k = 1.7 Optimized parameter K value = 1.7, Spiral angle = 34 deg (With field clamp) x ~ 1.68, y ~ 1.20 We optimize k value and spiral angle.

Acceptance study Gap 14 [cm] Horizontal Vertical ~7000π mm-mrad~1400π mm-mrad Gap 14 [cm]~7000π [mm-mrad], ~1400π [mm-mrad] Gap 17.5 [cm] ~6100π [mm-mrad], ~3200π [mm-mrad] Gap 20.0 [cm] ~7000π [mm-mrad], ~2400π [mm-mrad] Hori. Acceptance, Vert. acceptance

Ionization cooling simulation Initial condition Transverse Hori. emittance = 15 pi [mm mrad], Vert. emittance = 15 pi [mm mrad], Matched twiss para. Longitudinal dE = 0, Inial RF phase 10 deg. ( moved from synchronous phase.) ICOOL Using TOSCA field map Fluctuations in the energy : Vavilov distributions Multiple scattering : Moliere distribution Particle num. = 1000 Be target is rectangle (no wedge). Target thickness = 5  m RF amplitude V rf = 400 kV, (mom. Acceptance ~ 4%)

Surviving turn number Result of magnet gap 14 [cm] Mean surviving turn num. 192 turn Suv.rate = particle num. (at turn) / initial particle num

RMS emittance and energy spread An analytical solution and the simulation results are corresponding well while a little the beam loss. Particle of the large amplitude is lost as the turn number increases. Emittance is saturated

Vertical acceptance - dependent ~1400π [mm-mrad] ~2400π [mm-mrad] ~3200π [mm-mrad] Vertical beta ~ 1.35 [m]

Discussion From simulation results, the most cause of beam loss is heating of the vertical direction. It is difficult to achieve strong focusing the vertical direction in the spiral type lattice. In this reason, radial sector type suitable for ERIT?

Radial sector type

Magnetic field calculation (TOSCA) FDF lattice F-Mag. = 6.4[deg], D-Mag. = 5.1 [deg], F-D gap 3.75[deg], F-Clamp gap = 1.9[deg], Clamp thick = 4[cm] Mean radius = 2.35[m] 11MeV proton beam x ~ 1.75, y ~ 2.23 FD ratio ~3

Vertical beta function & acceptance Vertical acceptance ~ 3000π [mm-mrad]Vertical beta ~ 0.83 [m] Tracking results used TOSCA field. (Horizontal acceptance > 7000π [mm-mrad])

Surviving turn number Mean surviving turn num. 810 turns

RMS emittance and energy spread An analytical solution and the simulation results are corresponding well while beam loss is few.

Vertical beta function - dependent  y = 0.83[m] : y = 2.22, Mean surviving turn num. 810 turn  y = 0.75[m] : y = 2.32, Mean surviving turn num. 910 turn

Summary It is important to suppress overheating of the vertical direction to increase the surviving turn number. Radial sector is more suitable than the spiral sector type for controlling the heating of the vertical direction. The surviving turn number exceeded 900 turns for radial sector type by controlling the tune. It might have to care for the longitudinal and horizontal direction trying increase the number of turns any further.