Compact hadron driver for cancer therapies using continuous energy sweep scanning

A design of a compact hadron driver for future cancer therapies based on the induction synchrotron concept is presented. To realize a slow extraction technique in a fast-cycling synchrotron, which allows energy sweep beam scanning, a zero momentum-dispersion $D(s)$ region and a high flat $D(s)$ region are necessary. The proposed design meets both requirements. The lattice has two-fold symmetry with a circumference of 52.8 m, a 2-m dispersion-free straight section, and a 3-m-long large flat dispersion straight section. Assuming a 1.5-T bending magnet, the ring can deliver heavy ions ($200\mathrm{MeV}/\mathrm{u}$) at 10 Hz. A beam fraction is dropped from the barrier bucket at the desired timing, and the increasing negative momentum deviation of this beam fraction becomes large enough for the fraction to fall in the electrostatic septum extraction gap, which is placed at the large $D(s)$ region. The programmed energy sweep extraction enables scanning beam irradiation on a cancer site in depth without an energy degrader, avoiding the production of secondary particles and the degradation of emittance. Details of the lattice parameters and computer simulations for slow extraction are discussed. An example extraction scenario is presented. Qualities of the spilled beam such as emittance and momentum spread are discussed, as well as necessary functions and parameters required for the extraction system.

Figure 1

Energy sweep extraction in one acceleration cycle and the integrated dose along the path, where $B(t)$ is the magnetic flux density of the guiding magnet.

Figure 2

3D Schematic of a cancer site and scanning spots.

Figure 3

Barrier bucket and acceleration voltage pulse with its reset voltage pulse (upper). Tracks of particles that have left the trapping region encircled by a broken line (lower).

Figure 4

Essential feature of energy sweep extraction.

Figure 5

Overview of the driver ring.

Figure 6

Cell structures.

Figure 7

Lattice functions and lattice for the single super-periodicity.

Figure 8

Temporal evolutions of machine and beam parameters.

Figure 9

$V_{ac}$ and $V_{bb}$ profiles in time before extraction, where $T_s$ and ${\omega }_s$ are, respectively, the revolution time period and angular revolution frequency of the synchronous particle; $t_{rf}$ is the rising and falling time period of both voltage pulses (fixed to $35\mathrm{n}\mathrm{s}$), and $t_f$ is the flat top time period, fixed to $T_s/40$.

Figure 10

Phase plots of tracked macroparticles in phase space with $V_{ac}$ and $V_{bb}$; voltage heights are shown in relative units. The asymmetric bunch profile seen in the early time period of acceleration is generated by a combination of the reflection due to the right hand barrier voltage pulse with a relatively steep voltage rising profile and dependence of the phase velocity on $\mathrm{\Delta }p/p$.

Figure 11

Phase plots of macroparticles leaving the barrier bucket region, where the bunch core is invisible because its momentum spread is quite small. The positions of broken lines, from which particles leak, should be noted.

Figure 12

More fine structure of the phase-space plot.

Figure 13

Phase space with the $V_{ac}$ profile adjusted for extraction.

Figure 14

Integrated spill for different values of $\delta$.

Figure 15

Example of spill control for 5,000 macroparticles.

Figure 16

Equivalent circuit of the electrostatic septum and its high voltage supplies.

Figure 17

(a) Ideal and simulated voltage patterns and (b) the discrepancy between both.

Figure 18

Time table of Switch On/Off.

Figure 19

(a) Cross-section of Lambertson magnet; (b) Schematic view of the extraction area with an electrostatic septum with voltage 120 kV, gap distance 3 cm, and total length 2 m A: entrance of the septum region; B: entrance of the Lambertson magnet; (c) Extraction beam orbit and downstream; (d) Extraction orbit on the vertical plane.

Figure 20

Schematic phase plots of the circulating beam core, remaining spill, and spilled beam at the septum region and Lambertson magnet entrances, with tracking simulation results.

Figure 21

Averaged turn separation vs. extraction timing.

Figure 22

(a) Energy loss vs. kinetic energy of a projectile carbon ion and (b) Momentum change per pass vs. kinetic energy of a projectile carbon ion.

Figure 23

Geometry near the septum wire for the extraction.

Figure 24

RMS values for small scattering angle per wire.