Simultaneously recorded single-pulse images of (a) the Thomson scattered signal from the electron density and (b) the transverse Xe(M) ~ 1 keV x-ray emission zone (log scale) of a stable 248 nm channel produced in a Xe cluster target are illustrated
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Figure 2. Simultaneously recorded single-pulse images of (a) the Thomson scattered signal from the electron density and (b) the transverse Xe(M) ~ 1 keV x-ray emission zone (log scale) of a stable 248 nm channel produced in a Xe cluster target are illustrated. The channel was developed at a height of 1.45 mm above the orifice of the nozzle. The x-ray camera utilized a pinhole with a diameter of 10 µm, a size that gives a limiting spatial resolution estimated to be 20–30 µm. The direction of propagation is left to right and the centre of the nozzle corresponds to the coordinates (Y, Z) = (0, 0). The Xe cluster target was produced by a cooled high-pressure pulsed-valve fitted with a circular nozzle having a diameter of 2.50 mm. These data correspond to pulse 19 (24 January 2011). The locations of corresponding features in these images are indicated by the vertical connecting lines. The sharp expansion at the longitudinal position Z 0 mm in panel (a) signals the termination of the self-trapped channel. A broad elliptical halo of ionization with a diameter of ~ 1 mm is seen in the −0.5 mm ≤ Z ≤ 1.0 mm region. The abrupt expansion of the x-ray emission in panel (b) at Z 0 mm marking the end of the channeled propagation mirrors the corresponding morphology of the Thomson image in panel (a) and the localized zones of x-ray emission coincide with matching features in the Thomson image.
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.