Monoenergetic Proton Beams Accelerated by a Radiation Pressure Driven Shock

We report on the acceleration of impurity-free quasimononenergetic proton beams from an initially gaseous hydrogen target driven by an intense infrared ($\lambda =10\mu \mathrm{m}$) laser. The front surface of the target was observed by optical probing to be driven forward by the radiation pressure of the laser. A proton beam of $\sim \mathrm{MeV}$ energy was simultaneously recorded with narrow energy spread ($\sigma \sim 4\%$), low normalized emittance ($\sim 8\mathrm{nm}$), and negligible background. The scaling of proton energy with the ratio of intensity over density ($I/n$) confirms that the acceleration is due to the radiation pressure driven shock.

Figure 1

Raw and processed proton spectra for varying peak density $n$ and vacuum intensity $I$ showing scaling of peak proton energy $E_{\max }\propto I/nc$ [MeV]. Parameter $I/n$ shown to the right of the respective raw images. Shots taken with (a) $I=6.4$, $n=6.1n_{\mathrm{cr}}$, (b) $I=5.5$, $n=6.1n_{\mathrm{cr}}$, (c) $I=5.9$, $n=7.6n_{\mathrm{cr}}$, (d) $I=5.7$, $n=8.0n_{\mathrm{cr}}$ ($I$ in units of ${10}^{15}\mathrm{W}{\mathrm{cm}}^{-2}$). (e) Background subtracted (solid lines) and also corrected (dashed lines) spectra. Heights of corrected spectra adjusted to match those of raw lineouts. Lineout corresponding to (b) reduced $4\times$ to fit on the same scale.

Figure 2

Proton energy scaling as a function of expected energy due to hole boring, $E=4I/nc$. For $8n_{\mathrm{cr}}>n>4n_{\mathrm{cr}}$, peak proton signal increases with increasing $I/n$. For $n<4n_{\mathrm{cr}}$ beams were produced with larger variation in energy (below the observed scaling), and are not included. No beams observed above detector limit for $n>8n_{\mathrm{cr}}$. Vertical bars represent rms energy spread of each shot.

Figure 3

Shadowgraphy of the interaction for $n=4n_{\mathrm{cr}}$ at (a) $\sim 30\mathrm{ps}$ and (b) 200 ps after the first main pulse interacts with the gas. Laser entered from left, with the silhouette of the gas nozzle shown below.

Figure 4

PIC simulation showing (a) Proton density map and (b) proton spectra as seen by simulated spectrometer taken 12 ps after the start of the interaction.