Starting with the discovery of x-ray lasers in 1984, laser-created plasmas remained for almost a decade, the only medium in which large amplification of soft-x-ray radiation could be obtained. In this paper the recent first demonstration of large soft-x-ray amplification in a discharge-created plasma column, realized utilizing a fast capillary discharge to collisionally excite the 46.9 nm transition of Ne-like, Ar is reviewed. Results of the parametrization of the Ar IX discharge-pumped amplifier, the study of the dynamics of its plasma column, and the measurement of the time history of the laser pulse are reported. Prospects for laser operation at shorter wavelengths are also discussed. 0 1995 American Institute of Physics.

High-temperature iT, > 150 e'V), small-diameter (--200 J..lm) plasma columns have been efficiently generated by very fast (13 ns rise time, 28 ns full width at half maximum) pulsed discharge excitation of capillary channels filled with preionized gas. Discharges in argon-filled capillaries at currents between 20 and 60 kA produced plasmas with Ar x-Ar XIV line emission, in which the degree of ionization was controlled by the magnitude of the current pulse. The characteristics of these plasmas differ from those created by vacuum discharges in the same capillaries and approach those necessary for soft-x-ray amplification in low-Z elements. PACS numberts): 52.80. -s, 42.55.Vc

of the convoluted nature of the dendritic morphology, 4 it is highly probable that the rare-earth phase is also interconnected in the plane transverse to the growth direction. Consequently, the rare-earth phase is thought to be present as an interconnected skeieton network throughout the Terfenol-D matrix phase. The presence of a ductile skeleton network would be expected to enhance strength by retarding crack propagation throughout the brittle matrix. As X is lowered from 2.0, the volume fraction of ductile rare-earth phase increases and the strength is improved. ln general, the dark microconstituent in Fig. 1 is pure rare-earth metal when its transverse dimension is small but becomes a eutectic mixture of rare-earth metai and Terfenol-D phases when its dimension increases, such as at boundary trijunctions. This eutectic mixture becomes more dominant at the lower values of X, and a reduced ductility of the eutectic mixture over the pure rare-earth phase is probably responsible for the falloff of the slope of the strength curve in Fig. 2 We have measured the electron yields of 7 glow-discharge cathode materials under bombardment by neon and argon ions with energies between 1 and 20 keY. The surfaces of the samples were condiiioned by operating the materials as cold cathodes in a high-voltage glow discharge before the electron yield measurement. The materials studied are oxidized magnesium, oxidized aluminum, a molybdenum-aluminum oxide sintered composiie, molybdenum, stainless steel, copper, and graphite. The dependence of electron yield on ion velocity was found to be approximately linear for all materials and gases, with the slopes being strongly material dependent. The corresponding giow-discharge current intensities were observed to have a supralinear dependence on the electron yield. The results are relevant to the design and modeling of cold cathode high-voltage glow discharges.The phenomenon of secondary electron emission from a solid surface under ion bombardment has been studied extensively, regarding its importance in gas discharges, electron beam generation, plasma surface interactions, and ion beam current measurements. For example, in high~voltage cold cathode glow-discharge electron guns the electron yield of cathode surfaces is a key factor in determining the electron be.am current densities and generation efficiency. 1 Careful measurements of the electron yield have mostly been performed for atomically clean, flat surfaces in ultrahigh vacuum conditions. 2 -7 However, these results usually do not

The secondary electron emission coefficient of materials for helium ion bombardment in the energy range 0.5-20 keV was measured for the surface conditions of cathodes in high voltage glow discharges. The materials studied are oxidized aluminum, oxidized magnesium, a molybdenum-aluminum oxide sintered composite, mo!ybdenum, stainless steel, copper, gold, and graphite. Each sam pte was surface conditioned by operating it as cathode of a helium glow discharge shortly before the electron yield measurement. The results are relevant to the modeling of glow discharges and the design of cold cathode electron guns.In cold cathode glow discharges a large fraction of the electron emission is due to the ion bombardment oft he cathode surface.1 Consequently, the value of the secondary element emission coefficient of the cathode surface plays an important rote in determining the discharge characteristics. In the particular case of the generation of high-energy electron beams by high voltage glow discharges, the electron beam current density and generation efficiency depend on the secondary electron emission coefficient.2 Also knowledge of the secondary emission coefficient is necessary to model many fundamental aspects of glow discharges. J, 4

Abstract-We report the generation of high-current-density (20 A/cm 2 ) pulsed electron beams from high-voltage (48-100 kV) glow discharges using cathodes 7.5 em in diameter. The pulse duration was determined by the energy of the pulse generator and varied between 0.2 ps and several microseconds, depending on the discharge current; The largest electron beam current (900 A) was obtained with an oxidized aluminum cathode in a helium-oxygen atmosphere. An oxidized magnesium cathode produced similar results, and a molybdenum cathode operated at considerably lower currents. A small-diameter ( < 1 mm) well-collimated beam of energetic electrons of very high current density ( > 1 kA/cm 2 ) was also observed to develop in the center of the discharge. Electrostatic probe measurements show that the negative glow plasma density and the electron beam current have a similar spatial distribution. Electron temperatures of 1-1.5 eV were measured at 7 em from the cathode. The plasma density (8.5 • 10ncm-3 at 450 A) was found to depend linearly on the discharge current. In discharges at high currents a denser and higher temperature plasma region was observed to develop at approximately 20 em from the cathode. We have modeled the process of electron beam generation and predicted the energy distribution of the electron beam. More than 95 percent of the electron beam energy is calculated to be within 10 percent of that corresponding to the discharge voltage.