Laser-wavelength dependent characteristics of protons emitted from the palladium plasma

Abstract

The experimental findings regarding laser wavelength effects on the proton emission from a palladium plasma are reported. A palladium target is irradiated with fundamental, second, and third harmonics of a nanosecond pulsed laser in a broad laser fluence range (1.3–39.7 J/cm2). Time-of-flight traces of the proton pulses, measured with a planar ion collector, are used to estimate protons yield and kinetic energy. These are important parameters to consider when injecting a proton pulse into a high-energy accelerator. It was found that below the fluence of ∼10 J/cm2, the proton yield is higher for shorter wavelengths, but it does not depend on the wavelength at higher laser intensities and is ∼2 × 109 proton/pulse. For all three wavelengths, proton yield initially grows at a considerably quicker rate but slows down as fluence increases. For 355 nm and 532 nm wavelengths, the proton kinetic energies are comparable and are in the range of 280–800 eV. But the protons are 2 to 4 times less energetic in the case of 1064 nm wavelength. The disparities in proton yield and kinetic energy are attributed to the wavelength-dependent laser energy absorption in the target and plasma. The persistence of proton yield, for the successive laser pulses fired at the same target location, implies that the palladium can serve as a proton source for extended operation if ablated in suitable vacuum pressure.