Evanescent-wave proton postaccelerator driven by intense THz pulse

Hadron therapy motivates research dealing with the production of particle beams with $\sim 100\mathrm{MeV}/\text{nucleon}$ energy and relative energy fluctuation on the order of 1%. Laser-driven accelerators produce ion beams with only tens of $\mathrm{MeV}/\text{nucleon}$ energy and an extremely broad spectra. Here, a novel method is proposed for postacceleration and monochromatization of particles, leaving the laser-driven accelerator, by using intense THz pulses. It is based on further developing the idea of using the evanescent field of electromagnetic waves between a pair of dielectric crystals. Simple model calculations show that the energy of a proton bunch can be increased from 40 to 56 MeV in five stages and its initially broad energy distribution can be significantly narrowed down.

Figure 1

Scheme of the THz-driven evanescent-wave postaccelerator.

Figure 2

Proton velocity vs phase $\varphi$ for $v_{\mathrm{THz}}=v_{\mathrm{THz}}^{*}$ (solid line), $v_{\mathrm{THz}}<v_{\mathrm{THz}}^{*}$ (dashed line), and $v_{\mathrm{THz}}>v_{\mathrm{THz}}^{*}$ (dotted line) THz phase velocities. The interaction within one half cycle of the field is shown by the thick part of the curves.

Figure 3

Proton energies after subsequent units of a 12-stage accelerator sequence.

Figure 4

The longitudinal component of the electric field (solid line), and the absolute value of the transversal focusing/defocusing force acting on a unit charge (dotted line) vs the transversal coordinate. Please notice the more than one order of magnitude difference between the left and right scales.

Figure 5

Input ($+$) and output ($\times$) energy distribution for a proton bunch.

Figure 6

Trajectory of protons with initial transversal offset. The inset shows the longitudinal velocity distribution when transversal forces are taken into account (solid line), and when they are neglected (dashed line).

Figure 7

Output proton energy distributions for the subsequent stages of a five-stage accelerator sequence in the case of 0.5 THz frequency.

Figure 8

Output proton energy distributions for the subsequent stages of a five-stage accelerator sequence in the case of 0.25 THz frequency.