Achievement of essentially full spatial coherence in a high-average-power soft-x-ray laser Y. Liu, 1,4 M. Seminario, 2 F. G. Tomasel, 2 C. Chang, 3,4 J. J. Rocca, 2 and D. T. Attwood, 1,3,4 1 Applied Science and Technology Graduate Program, University of California, Berkeley, California 94720 2 Department of Electrical and Computer Engineering, Colorado State University, Fort Collins, Colorado 80523 3 Department of Electrical Engineering and Computer Science, University of California, Berkeley, California 94720 4 Center for X-ray Optics, Lawrence Berkeley National Laboratory, Berkeley, California 94720 Received 21 July 2000; published 5 February 2001 We report an observation of essentially full spatial coherence in a high average power soft-x-ray laser. Rapid coherence buildup due to strong refractive antiguiding in a long plasma column is experimentally demon- strated. This allows the generation of fully coherent, milliwatt-level average power soft-x-ray radiation by a tabletop device. The peak brightness of this laser reaches 2 10 25 photons s -1 mm -2 mrad -2 within 0.01% spectral bandwidth, making it one of the brightest soft-x-ray sources available. DOI: 10.1103/PhysRevA.63.033802 PACS numbers: 42.55.Vc Applications such as high-resolution microscopy, interfer- ometry, lithography, and holography motivate the develop- ment of advanced light sources at soft-x-ray wavelengths. Current approaches for the generation of high brightness co- herent radiation in this spectral region include undulators at modern synchrotron radiation SR facilities 1, high-order harmonic generation HHG of optical lasers 3,4, and soft- x-ray lasers 2. The degree of spatial coherence of radiation plays a critical role in many of the most important applica- tions. For SR sources, high spatial coherence is achieved at the expense of photon flux by spatial filtering 1. HHG sources driven by coherent optical lasers have shown high spatial coherence with average powers reaching the order of microwatts by using a phase-matching technique 4. Com- pared with SR and HHG sources, soft-x-ray lasers have sub- stantially higher pulse energy and narrower linewidth. How- ever, to date they have been characterized by rather low spatial coherence 5–9. The demonstration of nearly full spatial coherence has been one of the main goals of soft-x- ray laser research. Soft-x-ray laser beams are generally limited to single-pass or double-pass amplification of spontaneous emission ASE through the plasma. From the van Cittert–Zernike theorem 10, a high degree of spatial coherence from an ASE-based laser can be achieved when the gain medium has a Fresnel number less than unity. However, this is difficult to achieve in a plasma column, unless some forms of spatial filtering are used 11,12. Here we experimentally demonstrate that re- fraction in a plasma with sharp density gradients can reduce the effective transverse source size significantly and result in essentially full spatial coherence. Although theoretical re- sults have suggested that refractive antiguiding and gain guiding along a long plasma column could result in im- proved spatial coherence 13–15, in previous experiments the coherence buildup was limited to values significantly be- low full coherence 7,16. In the present work, we utilized fast capillary discharge excitation to produce plasma col- umns with both very high axial uniformity and the length-to- diameter ratio exceeding 1000:1, in which strong refractive antiguiding makes it possible to achieve essentially full spa- tial coherence with a plasma column length of 36 cm. The laser beam in our experiments is generated by exci- tation of an Ar-filled capillary channel with a fast discharge current pulse that rapidly compresses the plasma to form a dense and hot column with a large density of Ne-like ions 17,18. Collisional electron impact excitation of the Ne-like ions produces a population inversion between the 3 p ( 1 S 0 ) and 3 s ( 1 P 1 0 ) levels, resulting in amplification at 46.9 nm. The experiments are conducted utilizing aluminum oxide capillary channels 3.2 mm in diameter and up to 36 cm in length, filled with preionized Ar gas at a pressure of 59 Pa. The plasma columns are excited by current pulses of 25 kA peak amplitude, with a 10–90 % rise time of approxi- mately 40 ns. The setup is similar to that used in previous experiments 19,20. The excitation current pulse was pro- duced by discharging a water dielectric capacitor through a spark gap switch connected in series with the capillary load. The laser is very compact and occupies a table area of only 0.41m 2 . Efficient extraction of energy is obtained by op- erating the laser in a highly saturated regime. The laser pulse energy increases nearly exponentially as a function of plasma column length, until the beam intensity reaches the gain satu- ration intensity at a plasma column length of about 14 cm 18. For longer plasma columns, the laser average pulse energy increases linearly with length from 0.075 mJ for a plasma column 16 cm in length to 0.88 mJ ( 2 10 14 photons/pulse) for a plasma column length of 34.5 cm 20. Average laser powers of 3.5 mW are obtained when operating the laser at a repetition rate of 4 Hz. The spatial coherence of a quasimonochromatic light FIG. 1. Schematic representation of the experimental setup used in the two-pinhole interference coherence measurements. PHYSICAL REVIEW A, VOLUME 63, 033802 1050-2947/2001/633/0338025/$15.00 ©2001 The American Physical Society 63 033802-1