Chirped-pulse amplification (CPA) technology has been significantly advancing the development of ultra-short and ultra-intense laser. Several laser facilities aiming at 10 PW-level output power are built or under construction, such as ELI, Vulcan-10 PW, Apollon-10 PW, and SULF-10 PW, which will create unprecedented extreme physical conditions in the lab and strongly motivate the studies of laser-driven particle acceleration, x/gamma ray radiation, laboratory astrophysics, laser-driven nuclear physics etc.
Recently, in a paper published in High Power Laser Science and Engineering , Vol. 10, Issue, 4 (A. X. Li, C. Y. Qin, et al., Acceleration of 60 MeV proton beams in the commissioning experiment of SULF-10 PW laser, High Power Laser Science and Engineering, 2022, 10(4): 04000e26), a research group from Shanghai Institute of Optics and Fine Mechanics (SIOM) reports the experimental results in the commissioning phase of the 10 PW laser beamline of Shanghai Superintense Ultrafast Laser Facility (SULF), achieving high-energy proton beams with energies up to 62.5 MeV.
The commissioning experiment of the SULF-10 PW beamline was carried out on the ultrafast sub-atomic physics (USAP) platform, focusing on laser-proton acceleration using plain Cu and plastic targets. The laser energy of 72±9 J is directed to a focal spot of ~6 μm diameter (FWHM) within 30 fs pulse duration, yielding a peak power of 2.4 PW and intensity around 2.0×1021 W/cm2 on target.
Figure 1(a) shows the sketch of the experimental setup. The proton cut-off energy as a function of target thickness of Cu-foil is shown in Fig. 1(b), measured by Thomson Parabola spectrometer (TP1) along the target normal direction and by both TP2 and RCF stacks along the laser propagation direction. It can be clearly seen that in both directions, the proton cut-off energy increases when the foil thickness increases from 1 μm to 4 μm, and then decreases with larger thickness, corresponding to an optimum value at 4 μm. This trend agrees with the previously reported results from TNSA-produced proton beams, where the effects of electron reflux and pre-pulse induced plasma expansion at the target rear side results in an optimum target thickness for proton acceleration. One notices that the proton energies along the target normal direction are much higher than those along the laser direction for all target thicknesses. Typical proton spectra for various target thicknesses are illustrated in Fig. 1(c), showing the broad-energy-spread distribution. For 4-μm-thick foils, the average cut-off energy of protons is 60 MeV according to the data from 3 shots, where the highest one achieves 62.5 MeV. This is among the state-of-art results in proton acceleration using femtosecond lasers according to the previous reports.
Fig.1. (a) The sketch of the experimental setup. (b) The proton cut-off energy as a function of the target thickness of Cu-foil along the laser direction and the target normal direction. (c) Typical proton spectra for various target thicknesses.
Laser-driven proton acceleration using nanometer-thick plastic (CH) foils is also investigated. Figure 2 shows proton beam profiles for plain CH targets with three different thicknesses of 30 nm, 40 nm and 70 nm, respectively. Clear ring-like profiles appear for l=30 nm and 40 nm on all RCF layers, indicating that protons are not effectively accelerated since ionization and pre-expanding of the nanometer-thick targets driven by pre-pulses may lead to relativistically transparent plasma. Filamented structure emerges when the target thickness increases to 70 nm, which is possibly associated with Weibel instability and is an obvious sign that the plasma is still opaque rather than transparent. The results with nanometer-thick targets show that the laser contrast of SULF-10 PW beamline is not sufficient to drive effective acceleration schemes such as RPA and acceleration using structural target. Proton energies beyond 100 MeV are expected after further optimization of the temporal contrast and focal spot of SULF-10 PW beamline.
Fig.2. Proton beam profiles for plain CH targets with three different thicknesses of (a1-a4) 30 nm, (b1-b4) 40 nm and (c1-c4) 70 nm, at selected proton energies of 4.8 MeV, 7.2 MeV, 11.6 MeV and 15.9 MeV, respectively.
The above experimental results demonstrate the capabilities of the SULF-10 PW beamline, e.g., both ultrahigh intensity and relatively good beam contrast. Further optimization for these key parameters is underway, where peak laser intensities of 1022-1023 W/cm2 are anticipated to support various experiments on extreme field physics.