Laser prepulse dependency of proton-energy distributions in ultraintense laser-foil interactions with an online time-of-flight technique

Abstract

Absolute energy distribution of hard x rays produced in the interaction of a kilohertz femtosecond laser with tantalum targets Rev. Sci. Instrum. 77, 093302 (2006); Fast protons are observed by a newly developed online time-of-flight spectrometer, which provides shot-to-shot proton-energy distributions immediately after the irradiation of a laser pulse having an intensity of ϳ10 18 W/cm 2 onto a 5-m-thick copper foil. The maximum proton energy is found to increase when the intensity of a fs prepulse arriving 9 ns before the main pulse increases from 10 14 to 10 15 W/cm 2 . Interferometric measurement indicates that the preformed-plasma expansion at the front surface is smaller than 15 m, which corresponds to the spatial resolution of the diagnostics. This sharp gradient of the plasma has the beneficial effect of increasing the absorption efficiency of the main-pulse energy, resulting in the increase in the proton energy. This is supported by the result that the x-ray intensity from the laser plasma clearly increases with the prepulse intensity.

Development of Laser-driven Proton Source Toward Its Applications

“…The protons produced by the intense laser are measured in the direction normal to the target. The TOF proton measurement gives on-line real time information of the proton energy distribution [16,17]. Figure 2 shows the typical interferogram of the preformed plasma on a 7.5 μm thick polyimide target using the probe beam with fundamental wavelength (800 nm).…”

“…The interferogram is obtained from a Fresnel biprism using a hundred femtosecond probe pulse at the time of ~50 ps before the pump beam [20,21]. The protons are observed with a TOF ion spectrometer [16,17]. The protons produced by the intense laser are measured in the direction normal to the target.…”

“…High-energy protons with the maximum energy of ~2.3 MeV from a 7.5 μm thick polyimide target and ~1.2 MeV from a 3 μm thick copper target are observed. Here, we employ single-shot time-of-flight (TOF) ion spectrometer and interferometer, which allow us to determine repeatability of the proton acceleration [16,17].…”

Calibration and Performance Tests of Detectors for Laser-Accelerated Protons

“…This unsegmented configuration does not offer spatial information on the particle distribution. Instead, a good relation between energy deposition and pulse amplitude is provided, both for single protons as well as for overlapping signals of large numbers of particles, as those observed in previous laser-acceleration experiments [17]. The PMT anode pulse is displayed on a fast oscilloscope and stored for subsequent analysis.…”

Laser-Driven Proton Acceleration Research and Development

“…The protons were detected by the Time-of-Flight (ToF) spectrometers, CR-39 nuclear track detectors, and Thomson parabola. The ToF spectrometers Yogo et al 2007) were installed at the target normal direction and ±22.5° from it. In order to select the proton signal and reduce noise, the ToF spectrometers contained sweeping magnets removing all but the most energetic electrons, Al filters blocking laser and other light, and…”