Collection and focusing of laser accelerated ion beams for therapy applications

Experimental results in laser acceleration of protons and ions and theoretical predictions that the currently achieved energies might be raised by factors 5–10 in the next few years have stimulated research exploring this new technology for oncology as a compact alternative to conventional synchrotron based accelerator technology. The emphasis of this paper is on collection and focusing of the laser produced particles by using simulation data from a specific laser acceleration model. We present a scaling law for the “chromatic emittance” of the collector—here assumed as a solenoid lens—and apply it to the particle energy and angular spectra of the simulation output. For a 10 Hz laser system we find that particle collection by a solenoid magnet well satisfies requirements of intensity and beam quality as needed for depth scanning irradiation. This includes a sufficiently large safety margin for intensity, whereas a scheme without collection—by using mere aperture collimation—hardly reaches the needed intensities.

Figure 1

Schematic principle of depth scanning with energy broadened ion beam (courtesy U. Weber).

Figure 2

Depth dose profiles for protons in water with initial fluences ${10}^9/{\mathrm{cm}}^2$ and different incoming energy widths ($0\dots \pm 10\%$).

Figure 3

Schematic drawing of electron and ion densities after some femtoseconds of laser irradiation for radiation pressure acceleration (RPA). Compression of electrons by radiation pressure occurs until balance with the charge separation electric field is reached, which accelerates foil ions as a whole.

Figure 4

Maximum proton energy versus laser intensity. The black squares represent results from the 2D-PIC simulations described below; they follow an effective scaling of ${ϵ}_i\sim I_0^{0.8}$. The upper and lower model limits are shown for comparison.

Figure 5

Spectral yield of protons as a function of energy and for different production cone angles.

Figure 6

Calculated values of $B_z$ through solenoid versus distance.

Figure 7

Sample trajectories through solenoid collector and $\Delta E/E=\pm 0.05$.

Figure 8

Phase space projection at crossover ($\Omega =40\mathrm{mrad}$).

Figure 9

Emittances for variable $\Delta E/E$ ($\Omega =43\mathrm{mrad}$).

Figure 10

Emittances for different opening angles into solenoid ($\Delta E/E=\pm 0.05$).

Figure 11

Contour plots for usable intensities of 200 MeV protons as a function of selected production cone angle and energy window, with curves of two examples for constant chromatic emittance (required emittance) (red solid: $ϵ=5\mathrm{mm}\mathrm{mrad}$; red dashed: $ϵ=50\mathrm{mm}\mathrm{mrad}$).

Figure 12

Contour plots for usable intensities of $400\mathrm{MeV}/\mathrm{u}$ carbon ions with curves of constant chromatic emittance.