Simultaneously recorded single-pulse images of (a) the Thomson scattered signal from the electron density and (b) the transverse Kr(L) ~ 1.7 keV x-ray emission zone (log scale) of a stable 248 nm channel produced in a Kr cluster target
Figure 4. Simultaneously recorded single-pulse images of (a) the Thomson scattered signal from the electron density and (b) the transverse Kr(L) ~ 1.7 keV x-ray emission zone (log scale) of a stable 248 nm channel produced in a Kr cluster target. The channel was produced at a height of 1.60 mm above the opening of the nozzle. The x-ray camera utilized a pinhole with a diameter of 50 µm and had a spatial resolution estimated to be 75–100 µm. The Kr cluster target was produced by a cooled high-pressure pulsed-valve fitted with a circular nozzle having a diameter of 2.65 mm. These data correspond to pulse 519 (14 June 2012). The direction of propagation is left to right and the position of the nozzle is the same as shown in figure 2(a); the coordinates (Y, Z) = (0, 0) correspond to the centre of the nozzle. The matching locations of corresponding features in these images are indicated by the vertical connection lines. The abrupt expansion of the signal in panel (a) at Z 0 mm signals the termination of the confined propagation. A weak halo surrounding the bright zone indicating peripheral ionization is visible. The channel termination at Z 0 mm in panel (b) that coincides with the Thomson image in panel (a) is manifest.
Comparative single-pulse studies of self-trapped plasma channel formation in Xe and Kr cluster targets produced with 1–2 TW femtosecond 248 nm pulses reveal energy efficient channel formation (>90%) and highly robust stability for the channeled propagation in both materials. Images of the channel morphology produced by Thomson scattering from the electron density and direct visualization of the Xe(M) and Kr(L) x-ray emission from radiating ions illustrate the (1) channel formation, (2) the narrow region of confined trapped propagation, (3) the abrupt termination of the channel that occurs at the point the power falls below the critical power Pcr, and, in the case of Xe channels, (4) the presence of saturated absorption of Xe(M) radiation that generates an extended peripheral zone of ionization. The measured rates for energy deposition per unit length are ~ 1.46 J cm−1 and ~ 0.82 J cm−1 for Xe and Kr targets, respectively, and the single pulse Xe(M) energy yield is estimated to be > 50 mJ, a value indicating an efficiency >20% for ~ 1 keV x-ray production from the incident 248 nm pulse.