Two-dimensional near-ﬁeld and far-ﬁeld imaging of a Ne-like Ar capillary discharge table-top soft-x-ray laser C. H. Moreno,* M. C. Marconi,* V. N. Shlyaptsev, † B. R. Benware, C. D. Macchietto, J. L. A. Chilla, and J. J. Rocca Electrical Engineering Department, Colorado State University, Fort Collins, Colorado 80523 A. L. Osterheld Lawrence Livermore National Laboratory, Livermore, California 94551 Received 17 February 1998 We have performed systematic measurements and numerical modeling of the divergence and exit beam size of a 46.9 nm Ne-like Ar capillary discharge pumped soft-x-ray ampliﬁer. Two-dimensional near-ﬁeld and far-ﬁeld measurements were obtained over a wide range of discharge parameters. The spot size and divergence of the laser beam were observed to increase signiﬁcantly with decreasing discharge pressure. Simultaneously, the beam intensity distribution changed from a single peak pattern to an annular proﬁle. These effects are shown to be the result of increased refraction of the soft-x-ray beam, caused by larger density gradients in the plasma columns of the lower pressure discharges. The spatial images are nearly cylindrically symmetric, and have less structure than those reported in the literature for laser-driven soft-x-ray lasers. Beam proﬁles syn- thesized by model calculations are found to be in good agreement with the recorded images. The obtained images in combination with the model computations clarify the origin of the different beam patterns that are observed in capillary discharge soft-x-ray laser experiments. S1050-29479809208-7 PACS numbers: 42.55.Vc; 42.60.Jf I. INTRODUCTION The observation of large soft-x-ray ampliﬁcation in the plasma of a capillary discharge 1 and the subsequent dem- onstration of a saturated discharge pumped table-top soft-x- ray laser in Ne-like Ar at 46.9 nm 2 have established a new approach for the development of compact and practical soft- x-ray lasers. In these lasers the gain medium is a hot and dense plasma column with aspect ratios approaching 1000:1, generated in a capillary channel by a fast discharge current pulse. The fast current pulse rapidly compresses the plasma to form a needle-shaped column in which lasing is obtained by collisional electron excitation of Ne-like ions 1–3. Knowledge of the near-ﬁeld and far-ﬁeld spatial distribution of the output of these lasers is of both practical and basic interest. Signiﬁcant insight in the physics of this new kind of soft-x-ray ampliﬁer can be gained from such images. Re- cently, two-dimensional near-ﬁeld imaging studies of the laser-pumped soft-x-ray ampliﬁers have been reported 4–7. For example, near-ﬁeld imaging studies of a laser-pumped Ne-like Ge soft-x-ray laser experimentally conﬁrmed that lasing in the J =0 -1 line takes place closer to the target and in a higher density region as compared with the J =2 -1 line 4. The images recorded in those studies have also shown that the soft-x-ray laser beams generated in that kind of laser- created plasmas have a great deal of structure 4,5. Herein we report the results of a near-ﬁeld imaging study of a capillary discharge-pumped soft-x-ray laser, together with far-ﬁeld two-dimensional images obtained for the same discharge conditions. Measurements performed decreasing the discharge pressure showed an important increase in laser beam refraction caused by larger density gradients in the plasma column. As the discharge pressure was lowered from 750 to 500 mTorr the beam diameter and divergence in- creased, and the beam proﬁle was observed to change from a single peak distribution with the maximum on axis to an annular-shape proﬁle. These observations are in good agree- ment with ray tracing model calculations that used the elec- tron density and gain proﬁles computed with a hydrodynamic-atomic physics model of the plasma. The computations show that the plasmas of the lower pressure discharges have signiﬁcantly larger density gradients, larger temperatures, and reduced radial opacity. II. EXPERIMENTAL SETUP The measurements were conducted in a Ne-like Ar capil- lary discharge laser emitting a single strong laser line at 46.9 nm 8. In this device, a fast current pulse with a peak am- plitude of about 37 kA having a ﬁrst half period of 72 ns generates and compresses an elongated Ar plasma column in which the necessary conditions for lasing are achieved ap- proximately 39 ns after the onset of the current. In the ex- periments we report herein, the discharges took place in a polyacetal capillary 16.4 cm long, 4 mm in diameter ﬁlled with pure Ar gas at different selected pressures ranging from 500 to 750 mTorr. The imaging setup employed to record the near-ﬁeld pat- terns is schematically illustrated in Fig. 1. The output aper- ture of the capillary discharge laser was imaged into a two- dimensional soft-x-ray sensitive detector using a 150 cm radius of curvature iridium-coated mirror. The curved mirror *Permanent address: Departmento de Fı ´sica, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Buenos Aires, Argentina. † Present address: Lawrence Livermore National Laboratory, Liv- ermore, CA 94551. PHYSICAL REVIEW A AUGUST 1998 VOLUME 58, NUMBER 2 PRA 58 1050-2947/98/582/15096/$15.00 1509 © 1998 The American Physical Society