Enhancement and spatial characteristics of kα x-ray emission from high-contrast relativistic fs laser plasmas

Abstract

We studied the hard X-ray emission and the Kα X-ray conversion efficiency (η K ) produced by 70 fs, 400 nm high-contrast femtosecond laser pulse focused on Cu solid target. At an intensity I = 2 × 1018 W • cm-2, the Cu η K can reach 1 × 10-4. Compared with the Cu η K obtained by using a 800-nm low-contrast laser pulse, the second harmonic exhibits a higher X-ray flux by a factor of 2. We show that the use of high-contrast laser pulses may be an effective method to optimize the Kα X-ray emission via the vacuum heating mechanism. In addition, the laser focusing was varied widely to give a range of intensities from 1015-10 18 W • cm-2 by target offset from best laser focus. Two individual emission peaks are obtained, one is at best focal spot and the other is at larger target offset corresponding to ~ 1015 W • cm-2. Each X-ray emission peak is corresponding to different energy absorption mechanism. © 2007 Springer-Verlag New York.