Focusing and transport of high-intensity multi-MeV proton bunches from a compact laser-driven source

Laser ion acceleration provides for compact, high-intensity ion sources in the multi-MeV range. Using a pulsed high-field solenoid, for the first time high-intensity laser-accelerated proton bunches could be selected from the continuous exponential spectrum and delivered to large distances, containing more than ${10}^9$ particles in a narrow energy interval around a central energy of 9.4 MeV and showing $\le 30\mathrm{mrad}$ envelope divergence. The bunches of only a few nanoseconds bunch duration were characterized 2.2 m behind the laser-plasma source with respect to arrival time, energy width, and intensity as well as spatial and temporal bunch profile.

Figure 1

Experimental setup. After the protons are produced in the laser-matter interaction at the target (a), the divergent bunch enters the solenoid (c). For characterization of the initial beam, a RCF stack can optionally be placed in front of the solenoid at 40 mm (b). For detection of the bunch at larger distances, either a RCF stack can be used for dose analysis and transverse beam profile measurements at distances up to 2120 mm (d),(e) or alternatively a diamond detector at 2205 mm for time-of-flight measurements with also a RCF stack attached to detect the protons after passing the $20\mu \mathrm{m}$ thin diamond (f).

Figure 2

Simulated phase space and transverse beam profile of a laser-accelerated proton beam at a distance of 2.2 m behind the source. The particles are color coded due to the adjustment of the solenoid to focus protons of 10 MeV at this position.

Figure 3

Proton imprint in the RCF stack at 36.8 cm behind the source: experimental (upper) and simulation (lower sequence) results. In both cases the size of a single film is $60\mathrm{mm}\times 60\mathrm{mm}$. Additionally, an electron signal can be seen in the experimental data in the lower left quarter.

Figure 4

Measured transverse beam profile of the focused energy (a) 9.4 MeV and (b) 5 MeV at a distance to the source of 1.07 m. In both cases the spot was around 5 mm in diameter and contained (a) $4\times {10}^8$ and (b) $1.6\times {10}^9$ protons.

Figure 5

(a) Measured transverse focus profile of 9.4 MeV protons behind the solenoid at 2.12 m distance to source. The focus is about 10 mm in diameter and contains $5\times {10}^8$ protons. (b) Simulated focal spot in the corresponding RCF layer, using a realistic wire assembly for the solenoid coil, which results in an observable astigmatism of the lens. Additionally indicated are the position of the diamond detector (dashed black square) and the optional 5 mm aperture (dashed black circle) relative to the proton beam.

Figure 6

Proton imprint (in units of deposited energy) in the RCF layers for energies larger than the focused one with corresponding Bragg energies. Contrary to the expectations, a focused feature is still detectable up to the last layer.

Figure 7

Measured deposited energy (red circles) in RCFs when focusing 10 MeV protons at 2.12 m and corresponding exponential fit (blue line).

Figure 8

Normalized time-of-flight results of the diamond detector (at 2.2 m from source) for three different shots: (1a) focusing of 9.4 MeV protons, (1b) focusing of 9.4 MeV protons with a 5 mm pinhole at 2.12 m, and (1c) focusing of 5 MeV, also using the pinhole. The dashed graphs (2a)–(2c) are the corresponding expected spectra from simulations. (3b) and (3c) show the exponential decaying spectrum for the high energies obtained from the RCF data.