X-ray generation by laser ablation using MHz to GHz pulse bursts

Ultrashort pulsed laser sources generating pulse trains (bursts) with intra-burst repetition rates in the MHz and the GHz regime enable an efficient production of microstructures with a high surface quality. However, x-ray radiation can be generated during the laser micromachining using large intensities of the laser radiation and its interaction with the ablation cloud or high-density plasma. Therefore, the authors report on the interaction of bursts with a wavelength of 1030 nm and pulse durations of 0.24 and 10 ps with intra-burst repetition rates of 65 MHz (MHz-burst mode) and 2.5 GHz (GHz-burst mode) as well as a combination of both burst modes, called BiBurst mode, with stainless steel, and the x-rays are generated. The x-ray dose rates determined in the respective burst modes are compared and discussed with those of conventional ultrafast laser radiation (single-pulse mode). Furthermore, a theoretical model is used to calculate the expected x-ray dose rates. In the investigated parameter range, the highest dose rates of more than 105μSv/h are determined at a specific burst setting. Compared to the single-pulse mode, significantly higher dose rates are determined using the burst mode with the same total intensity. Based on the results of this study, it can be stated that the interaction of ultrafast laser radiation in the burst mode with a generated ablation cloud or high-density plasma plays a major role in x-ray generation and the resulting x-ray dose rates.