A high-energy, high-yield proton beam with a good homogeneous profile has been generated from a nanosphere target irradiated by a short (30-fs), intense (7 × 10 20 W=cm 2) laser pulse. A maximum proton energy of 30 MeV has been observed with a high proton number of 7 × 10 10 in the energy range 5-30 MeV. A homogeneous spatial profile with a uniformity (standard deviation from an average value within 85% beam area) of 15% is observed with the nanosphere dielectric target. Particle-in-cell simulations show the enhancement of proton cutoff energy and proton number with the nanosphere target and reveal that the homogeneous beam profile is related with a broadened angular distribution of hot electrons, which is initiated by the nanosphere structure. The homogeneous spatial properties obtained with the nanosphere target will be advantageous in developing laser-driven proton sources for practical applications in which high-quality beams are required.

A proton energy spectrometer system is composed of a time-of-flight spectrometer (TOFS) and a Thomson parabola spectrometer (TPS), and is used to characterize laser-accelerated protons. The TOFS detects protons with a plastic scintillator, and the TPS with a CR-39 or imaging plate (IP). The two spectrometers can operate simultaneously and give separate time-of-flight (TOF) and Thomson parabola (TP) data. We propose a method to calibrate the TOFS and IP by comparing the TOF data and the TP data taken with CR-39 and IP. The absolute response of the TOFS as a function of proton energy is calculated from the proton number distribution measured with CR-39. The sensitivity of IP to protons is obtained from the proton number distribution estimated with the calibrated TOFS. This method, based on the comparison of the simultaneously measured data, gives more reliable results when using laser-accelerated protons as a calibration source. The calibrated spectrometer system can be used to measure absolutely calibrated energy spectra for the optimization of laser-accelerated protons.