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Obtaining an X-ray laser emission from plasmas, created and driven by an intense IR laser, has been pursued at the Laboratoire de Spectroscopie Atomique et Ionique (LSAI) for several years. At present, we operate various types of X-ray lasers driven by IR laser pulses of different durations (600 ps, 100 ps, and 600 ps/1 ps). A review of different techniques used at the LSAI to produce a strongly amplified emission using the collisional excitation pumping is presented. In the second part of this paper, to illustrate the potential of the X-ray lasers for applications, we present the main results obtained with an X-ray laser emitting at 21.2 nm in a study of surface defects of a niobium cathode, induced by strong electrical fields. We also describe a novel imaging interferometry device using an X-ray laser as a source and designed as a tool for high-resolution diagnostic of dense plasmas.

We present the very first experiment allowing in situ observation of surface evolution during very high electric field application. This has been achieved by soft (/spl lambda/=21.2 nm) X-ray laser Fresnel two-mirror interferometry. The surface under study was a niobium planar cathode opposite to a stainless steel blade-like anode. The applied field range on the cathode was F/spl ap/0 to 50 MV/m. The interferograms were single shots of 80 ps duration, allowing to probe the surface morphology quasi-instantaneously. The delay between two shots was 20 min while the electric field was kept on. The comparison of successive interferograms gives the Nb surface evolution over /spl sim/2 h. Thus, we observed the appearance of defects of /spl sim/3 nm high and their evolution with time. These may be the precursors of the minute breakdown which occurred later on. This technique thus makes it possible to detect the preliminary stages of a discharge.

We present results obtained with an experimental set-up allowing measurements of DC field emission currents in the range of 10 −18 to 10 −5 A. The experiments are carried out in ultra high vacuum ( P −10 Torr) and at room temperature. The detection of currents in the range of 10 −18 –10 −12 A is performed with an electron multiplier located behind a grid anode. Higher currents are measured using a bulk anode and a picoammeter. The whole system has been designed to study niobium samples from sheets used to build superconducting accelerator cavities. Measures in the low current range have shown electron emission regimes that seem not to fit the Fowler-Nordheim law. A possible mechanism for such emissions involving the surface oxide layer is proposed. Comparisons of the electron field emission from samples prepared by different surface treatments as chemical etching, electropolishing, oxide coating by anodization, are presented.

Field electron emission from extended Cu electrodes after cleaning by electric arcs has been compared with field emission from gas-covered monocrystalline W points when subjected to ion bombardment from the residual gas. With both emission systems, exposure to argon or to hydrogen in the pressure range 10-5-10-2 Pa caused two subsequent stages: an emission growth explained as the removal of adsorbed gases (stage 1), and a noisy increase of the emission explained as sharpening due to surface migration of metal atoms (stage 2). With Cu there occurred a third stage: a decrease of the emission explained as reduction of the emitters by sputtering. The time constant tau 1 of the first stage was found to be proportional to the product p-1i0-1SFN12/, p being the gas pressure, i0 the initial electron-current, and SFN the apparent emitting surface determined from Fowler-Nordheim plots. The proportionality factor is given for the metals and gases used. The completely analogous behaviour of W points and Cu electrodes in stages 1 and 2 indicates an analogy between the emitting sites in both cases.

Transient perturbed niobium surface states are observed for the first time during a long continued action of the applied DC-field owing to surface imaging by the means of an X–UV laser interferometry technique.

We present the very first experiment allowing in-situ observation of surface evolution during very high electric field application. This has been achieved by soft X-ray laser (/spl lambda/=21.2 nm) Fresnel bi-mirror interferometry. The surface under study was a niobium planar cathode opposite a stainless steel bladelike anode. The applied field range on the cathode was E/spl ap/9-50 MV/m. The interferograms where single shot recorded of 80 ps duration allowing to quasi-instantaneously probe the surface morphology. The delay between two shots was 20 min while electric field has been kept on. The comparison of successive interferograms gives the Nb surface evolution over several hours. Thus, we observed the appearance of defects a few nanometers high and their evolution with time. Such a technique makes it possible to detect the preliminary stages of a discharge.

We use x-ray laser interferometry to probe defects induced by a strong electric field on niobium surface. Niobium has been chosen on account of its frequent use in superconductive cavities of particle accelerators. The x-ray laser emits bright, 50 ps-duration pulses at (lambda) equals 21.2 nm. The beam is reflected on the niobium surface under grazing incidence. The interferometer is of the wave-front division type. Interferograms are single shot recorded, which enables to probe `instantaneous' defect morphology. We observed appearance and evolution of defects between 14 MV/m and 35 MV/m. The vertical set amplitude is of 10 - 20 nm. The defect structure has been observed to shift by 500 micrometers along the metal surface under a constant 35 MV/m electric field, during the 20 minutes time interval between two laser shots.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

An experimental set-up allowing measurements of DC field emission currents in the range of 10 -18 A to 10 -6 A in ultra high vacuum (P -10 Torr) at room temperature has been developed. The detection of currents in the range of 10 -18 A to 10 -12 A was carried out with an electron multiplier located behind a grid anode. Higher currents were measured using a massive anode and a picoammeter. The whole system was designed to study niobium samples from sheets used to build electron superconducting accelerator cavities. Comparisons of the electron field emission from samples prepared by different surface treatments as chemical etching, electropolishing, oxide coating by anodization, were done. Some of the obtained results are presented below.

The effect of various gases on prebreakdown electron emission in vacuum is studied from metallic micropoints and with gas compositions that are typical of ultra-high-vacuum systems. It was found that the presence of the gas changes preferentially the pre-exponential factor of the Fowler-Nordheim characteristics, which is related to the size of the emitting surface. The observed phenomena indicate that the combined action of several gases may create a dynamical system of adsorption and desorption leading to characteristic effects (instabilities, deviation from linearity of the Fowler-Nordheim plots, unrealistic values of the field enhancement factor) which are often ascribed to nonmetallic functions. Thus, metallic micropoints which occur in extended cathodes after sparking or arcing are possible candidates as emission sites. Since minute breakdowns are not always detectable, this statement may also hold for situations where the electrodes are thought to be virgin.

The effect on prebreakdown currents in vacuum of different gases introduced in the vessel is compared with the influence of gas desorbed from the anode surface. The experiments show that gases such as CO and N/sub 2/ reduce the pre-exponential factor in the Fowler-Nordheim equation. With anodic gas desorbed there may be a feedback to the emission current resulting in an apparent change of the slope of the Fowler-Nordheim plots. This can cause large apparent field enhancement factors beta , thus explaining unrealistically high values for metallic field emitters. >

The temperature of a prebreakdown vacuum anode spot has been measured in the visible region by a black body radiation method, hereafter called ?spectral pyrometry?. A special spectrometer was designed for this experiment. It uses a circular graded spectrum filter turning around its axis with a velocity of 3000 rpm. It is then possible to analyze incident radiation with 12 nm resolution and the sensitivity of a quantum detector. This apparatus was first used to establish the high temperature spectral emissivity distribution of anode material. Results are presented for the anode spot temperature. These seem to confirm the hypothesis of high local heating of the anode.

A particular discharge regime seems to be associated with the presence of "hot spots" on the anode surface during vacuum prebreakdown, leading to increased stability. This communication ation is concerned with the description of some features of this regime, local gas desorption, current jumps on Fowler-Nordheim plots at spot ignition.