Monoenergetic High-Energy Ion Source via Femtosecond Laser Interacting with a Microtape

Intense laser-plasma ion sources are characterized by an unsurpassed acceleration gradient and exceptional beam emittance. They are promising candidates for next-generation accelerators towards a broad range of potential applications. However, the laser-accelerated ion beams available currently have limitations in energy spread and peak energy. Here, we propose and demonstrate an all-optical single laser scheme to generate proton beams with low spread at about 1% level and hundred MeV energy by irradiating the edge of a microtape with a readily available femtosecond petawatt laser. Three-dimensional particle-in-cell simulations show that when the electron beam extracted from both sides of the tape is injected into vacuum, a longitudinal bunching and transverse focusing field is self-established because of its huge charge (about 100 nC) and small divergence. Protons are accelerated and bunched simultaneously, leading to a monoenergetic high-energy proton beam. The proposed scheme opens a new route for the development of future compact ion sources.

Popular Summary

Laser-driven particle accelerators offer one of the most promising approaches to the next generation of advanced accelerators due to their large acceleration gradients and small size. In recent years, researchers have made remarkable advances, continuously breaking the record of proton energy in experiments. However, the distributions of energy spectra are still exponentially decaying, which is a major obstacle to cancer therapy and other applications, where energy spread is required to be only about 1% of the peak-energy value. Generation of ion beams with such a low-energy spread is extremely challenging with current mechanisms. Here, we propose a novel method to robustly achieve such high-quality proton beams.

In our method, a femtosecond laser pulse is targeted at an edge of a microscale metal tape. The laser drags the abundant number of electrons from the tape, accelerating them to the rear edge of the tape where they form a strong longitudinal electric field. The field, in turn, accelerates protons from hydrocarbon and water-vapor contaminants and simultaneously bunches them up. Our 3D simulations show that a proton beam with a peak energy greater than 100 MeV, energy spread of about 1%, and particle number in the billions can be stably generated. This is a substantial improvement compared to other known mechanisms.

Our proposed scheme opens a new route for the development of compact ion sources for applications such as cancer therapy. This novel interaction geometry also offers great opportunities for the studies of high-energy-density physics as well as sources of particles and radiation.

Figure 1

Schematics of our peeler scheme. A femtosecond laser (red and blue) is incident on the edge of a microscale tape (grey). The micron-thick tape is infinitely long across the laser focus and is a few tens of microns wide along the laser propagation direction. Abundant electrons are continuously dragged out and accelerated forward by the intense laser. When these energetic electrons arrive at the rear edge of the tape, a longitudinal bunching field is established (yellow). Protons (green dots) are simultaneously accelerated and bunched by this field, leading to a highly monoenergetic proton beam.

Figure 2

Three-dimensional PIC simulation results. Distributions of the longitudinal electric field $E_{sw}$ (a), electron density $n_e$ (b), laser electric field $E_y$ (c), and self-generated magnetic field $B_z$ (d) at $t=22T_0$ in the plane ($x$, $y$). Panels (e) and (f) show the averaged longitudinal electric field $E_x$ and transverse electric field $E_z$ at $t=70T_0$ in the plane ($x$, $z$), respectively. In panel (e), the purple sheet represents the compressed proton layer, which is situated at the negative gradient region of $E_x$. In panel (f), the colored lines correspond to the collimated trajectories of selected protons, where the color represents the relative proton energy.

Figure 3

Particle density distribution. (a,b) Electron density distributions at $t=22T_0$ and $70T_0$, respectively. (c) Proton density distribution at $t=86T_0$. The isosurface values in panels (a)–(c) are $n_e=0.75n_c$, $n_e=0.2n_c$, and $n_p=0.07n_c$, respectively; the ($x$, $y$) projections are at $z=0$ and the ($x$, $z$) projections at $y=0$, except that in panel (a), only the ($x$, $y$) projection is shown.

Figure 4

Energy gain of electrons and evolution of particle energy spectrum. (a) Energy gain of electrons from the longitudinal (${\mathrm{\Gamma }}_x$) and transverse (${\mathrm{\Gamma }}_y$) directions, defined by Eqs. (5) and (6), respectively. The color scale represents the final electron energy. (b) Energy spectrum of electrons at $t=16T_0$, $26T_0$, and $48T_0$. (c) Proton phase space at $t=66T_0$, $82T_0$, and $106T_0$. (d) Black, blue, and red solid lines represent the energy spectrum of protons at $t=56T_0$, $66T_0$, and $106T_0$, respectively. The red dashed line shows the energy spectrum of carbon ions at $t=106T_0$.

Figure 5

Peak proton energy ${\epsilon }_p$ (black asterisks) and energy spread (red triangles) with varying laser intensity, where the other parameters are kept almost the same as those of Fig. 2, except the electron density, to avoid the relativistic transparency. The dashed black line displays the best-fit scaling for peak proton energy.

Figure 6

Robustness study and comparison. (a) Proton energy spectra from the cases with different fluctuations considered, including the tape width $l_x$ (black), thickness $l_y$ (green), and the incidence angle ${\theta }_i$ (blue). (b) Proton energy spectra considering the effects of laser pointing stability. The blue and black solid lines represent the cases with misalignments of $y_d=2\lambda$ and $y_d=4\lambda$, respectively. In panels (a) and (b), the red dashed line represents the results from Fig. 4, for comparison. For each case displayed by the solid lines, except the difference given by the legend, the other parameters are the same as those described in the Figs. 2, 3, 4. (c) Electron (dashed lines) and proton (solid lines) energy spectra obtained from the TNSA (black) and RPA (blue) mechanisms with the same laser parameters.