Efficient low debris hard x-ray source based on intense, femtosecond laser irradiation on multi-walled carbon nano-tubes

Abstract

Efficient coupling of laser energy is one of the primary concerns for devising laser based photon and charged particle sources with potential applications in a wide field of research interests. We report a two orders of magnitude efficient moderately hard x-ray (50 - 300 keV) source based on multi-walled carbon nanotubes (MWNT) irradiated by moderately intense (10 15 - 1017 Wcm-2) ultra-short laser pulses. This is also accompanied by a three orders of magnitude reduction in ion debris in comparison with conventional metallic targets making these sources operationally safe. The bremsstrahlung measurement reveals a two orders of magnitude increment in x-ray flux from MWNT. Contrary to expectation that the rise in "hot" electron temperature leads to an increment of emitted ion energies form the plasma, a monotonic reduction of ion energies with increasing laser intensity for MWNT is noticed. Angle resolved ion flux measurement reveals an extremely divergent ion emission from MWNT with an evident three orders of magnitude reduction in ion flux. Based on the scaling laws for resonance absorption, our experimental data remarkably matches with theoretical predictions based on electrostatic calculations. This confirms the localized enhancement of the laser electric field near the tip of the MWNTs yield localized hot spots in the expanding plasma sheath layer leading to a non-planar expansion. This is characterized by a decrease in ion accelerating potential as well as a divergent ion emission, as observed in experiments. © 2010 SPIE.