Development of AN XUV Amplifier.
Abstract
The feasibility of an XUV amplifier based on the recombination of a laser-produced plasma was investigated. Experiments were carried out with a Nd('+3)-glass laser generating sub-nanosecond pulses of either IR light (1.054 (mu)m, up to 100 J) or frequency-tripled UV light (0.351 (mu)m, up to 60 J). The plasma formed at the surface of a flat target was cooled at an early stage of its expansion by inserting a metallic foil near the target. The cooling of a dense plasma is known to result in a recombination cascade dominated by collisions, which tend to popuate high-lying quantum states of the recombining ion. With point-focus illumination (diam.(, )<(, )100 (mu)m), population inversions were observed in the helium -like ions Al('+11) and Mg('+10), showing possibility of gain on the transition 3('3)D-4('3)F at 129.7 (ANGSTROM) and 154.4 (ANGSTROM) respectively. We established that the inversions were caused neither by resonance mechanisms such as photopumping or charge transfer, nor by channelling of the plasma flow. We defined, after a parametric study, the optimum target design (slit-foil of an arbitrary material, 200 (mu)m wide and 200 (mu)m from the original plasma formation), and illumination conditions (UV laser of intensity 5(TURNEQ)10('14) W/cm('2)). The short wavelength laser was shown to couple its energy to the target more efficiently. With line-focus illumination (100 (mu)m x 2 mm, using specially manufactured cylindrical optics), inversion was observed in the hydrogenic ion F('+8) between levels n = 2 and n = 3. The corresponding transition, Balmer (alpha) at 80.9 (ANGSTROM), was spatially resolved over (TURN)1 mm in the direction of expansion, with single-shot recording. Numerical simulations, using existing laser-fusion code and rate equations, were in good agreement with the data when we included enhanced recombination (via cooling) and plasma opacity. The gain region travelled with the velocity of the expanding plasma (0.3-1 10('8) cm/sec). The gain could be high locally in time and space, 10('2) cm(' -1) over(, )<(, )100 psec in Al('+11) (129.7 (ANGSTROM)) and 5 cm('-1) over (TURN)200 psec in F('+8) (80.9 (ANGSTROM)). However, the plasma length (200 (mu)m and 2.5 mm respectively, in the region of highest gain) did not permit direct gain measurement from time-integrated recordings of the amplified spontaneous emission. We discuss how that goal could be reached in the future with upgraded UV pump laser (factor 5) and time-resolved diagnostic capability (< 100 psec).
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 1982
- Bibcode:
- 1982PhDT........35C
- Keywords:
-
- Physics: Optics;
- Charge Transfer;
- Laser Plasmas;
- Laser Target Interactions;
- Ultraviolet Lasers;
- Amplifier Design;
- Flow Velocity;
- Phase Transformations;
- Plasma Density;
- Space-Time Functions;
- Spatial Resolution;
- Lasers and Masers