T. Letardi

High-resolution x-ray spectrometer based on spherically bent crystals for investigations of femtosecond laser plasmas

Ultrashort-pulse, laser-produced plasmas have become very interesting laboratory sources to study spectroscopically due to their very high densities and temperatures, and the high laser-induced electromagnetic fields present. Typically, these plasmas are of very small volume and very low emissivity. Thus, studying these near point source plasmas requires advanced experimental techniques. We present a new spectrometer design called the focusing spectrometer with spatial resolution (FSSR-2D) based on a spherically bent crystal which provides simultaneous high spectral (λ/Δλ≈104) and spatial resolution (≈10 μm) as well as high luminosity (high collection efficiency). We described in detail the FSSR-2D case in which a small, near point source plasma is investigated. An estimate for the spectral and spatial resolution for the spectrometer is outlined based on geometric considerations. Using the FSSR-2D instrument, experimental data measured from both a 100 fs and a nanosecond pulse laser-produced plasma are pr...

Lawson-Penner limit and single passage free electron lasers performances

The limits of the performances of a single passage FEL device imposed by the Lawson-Penner law are discussed within the framework of the super mode theory.

Performance of a ten-liter electron avalanche-discharge XeCl laser device

This paper describes the laser performance of an X-ray preionized ten-liter volume, electron-avalanche discharge XeCl system, operated at dc charging voltages of up to 70 kV both with and without X-ray preionization. An output energy of more than 11J uniformly distributed over a (7×10) cm2 spot size was achieved using a multichannel spark-gap as the main discharge switch. The discharge was also successfully operated in the switchless mode, yielding a 4.8 J output energy and a 0.9% efficiency. The time-resolved gain measurements provided information on the laser discharge evolution.

Compact three-electrodes excimer laser IANUS for a POPA optical system

A novel discharge configuration has been realized with the XeCl laser IANUS, which has been developed at the ENEA Frascati center as a 1:1 laser head prototype for the 1 kW average output power XeCl system EOLUS (EUREKA EU211 Project). The patented design of the discharge region consists of two active zones which are geometrically in parallel and electrically in series. The obvious advantages of this configuration are, the presence of the intermediate high impedance electrode, suitable for prepulse application, then the absence of current return bars, so improving the recirculating gas flow uniformity, and the automatic synchronization of the two discharges very useful for a compact oscillator-amplifier configuration (POPA). A number of measurements have been done on IANUS, to have an almost complete characterization; based on these results a generalized self-filtering unstable resonator (GSFUR) has been designed and realized on the smaller gap, leaving the other active medium for the high optical quality beam amplification, in the scheme of a POPA system. The GSFUR output beam results are 1.5 times the diffraction limit, with an energy of 9 mJ in a 87 ns FWHM pulse.

Magnetic pulse compressor for prepulse discharge in spiker‐sustainer excitation technique for XeCl lasers

A high‐voltage pulse output modulator using a step‐up transformer with two stages of magnetic pulse compression circuits has been developed as a spiker for the purpose of obtaining the breakdown of the gas mixture when using the spiker‐sustainer excitation technique for pumping XeCl discharge lasers. When the transformer input is fed by a current pulse with a peak value and a full width of 1.16 kA and 1.6 μs, respectively, formed by a discharge of an initially charged 19‐kV, 57‐nF capacitance in series in the transformer primary loop, the 2.7‐nF output capacitor bank (Cp) of the spiker can be charged up to 67 kV with a (10%–90%) rise time of 59 ns. The modulator has been tested at a repetition rate of up to 600 Hz in burst mode using a resistor in parallel with Cp as the load.

X-ray radiation from ions with K-shell vacancies

Abstract New types of space resolved X-ray spectra produced in light matter experiments with high intensity lasers have been investigated experimentally and theoretically. This type of spectra is characterised by the disappearance of distinct resonance line emission and the appearance of very broad emission structures due to the dielectronic satellite transitions associated to the resonance lines. Atomic data calculations have shown, that rather exotic states with K-shell vacancies are involved. For quantitative spectra interpretation we developed a model for dielectronic satellite accumulation (DSA-model) in cold dense optically thick plasmas which are tested by rigorous comparison with space resolved spectra from ns-lasers. In experiments with laser intensities up to 10 19 W/cm 2 focused into nitrogen gas targets, hollow ion configurations are observed by means of soft X-ray spectroscopy. It is shown that transitions in hollow ions can be used for plasma diagnostic. The determination of the electron temperature in the long lasting recombining regime is demonstrated. In Light-matter interaction experiments with extremely high contrast (up to 10 10 ) short pulse (400 fs) lasers electron densities of n e ≈3×10 23 cm −3 at temperatures between kT e =200–300 eV have been determined by means of spectral simulations developed previously for ns-laser produced plasmas. Expansion velocities are determined analysing asymmetric optically thick line emission. Further, the results are checked by observing the spectral windows involving the region about the He α -line and the region from the He β -line to the He-like continuum. Finally, plasmas of solid density are characteristic in experiments with heavy ion beams heating massive targets. We report the first spectroscopic investigations in plasmas of this type with results on solid neon heated by Ar-ions. A spectroscopic method for the determination of the electron temperature in extreme optically thick plasmas is developed.

Self-filtering unstable resonator operation of XeCl excimer laser

A diffraction-limited laser beam with a pulse energy of 120 mJ and brightness up to a magnitude order of 1014W . cm-2. Sr-1has been achieved in an X-ray preionized discharge XeCl laser by using a self-filtering unstable resonator. The near- and far-field laser properties have been examined.

XeCl discharge diagnostic by holographic interferometry

A direct measurement of the electron-density time evolution in the active medium of a self-sustained XeCl laser discharge has been performed by means of the holographic interferometry technique. The time and space resolved behaviour of the discharge seems to confirm that the halogen depletion instability and the hot spots developing in the cathode region are the basic mechanisms which determine the premature termination of the output laser pulse.

Shadow monochromatic backlighting: Large-field high resolution X-ray shadowgraphy with improved spectral tunability

The shadow monochromatic backlighting (SMB) scheme, a modification of the well-known soft X-ray monochromatic backlighting scheme, is proposed. It is based on a spherical crystal as the dispersive element and extends the traditional scheme by allowing one to work with a wide range of Bragg angles and thus in a wide spectral range. The advantages of the new scheme are demonstrated experimentally and supported numerically by ray-tracing simulations. In the experiments, the X-ray backlighter source is a laser-produced plasma, created by the interaction of an ultrashort pulse, Ti:Sapphire laser (120 fs, 3–5 mJ, 10 16 W/cm 2 on target) or a short wavelength XeCl laser (10 ns, 1–2 J, 10 13 W/cm 2 on target) with various solid targets (Dy, Ni + Cr, BaF 2 ). In both experiments, the X-ray sources are well localized spatially (∼20 μm) and are spectrally tunable in a relatively wide wavelength range (λ = 8–15 A). High quality monochromatic (δλ/λ ∼ 10 −5 –10 −3 ) images with high spatial resolution (up to ∼4 μm) over a large field of view (a few square millimeters) were obtained. Utilization of spherically bent crystals to obtain high-resolution, large field, monochromatic images in a wide range of Bragg angles (35° < Θ < 90°) is demonstrated for the first time.

X-ray contact microscopy using an excimer laser plasma source with different target materials and laser pulse durations

Soft X‐ray contact microscopy (SXCM), using a pulsed X‐ray source, offers the possibility of imaging the ultrastructure of living biological systems at sub‐100 nm resolution. We have developed a table‐top pulsed plasma X‐ray source for this application, generated by a large‐volume XeCl laser, achieving a good conversion efficiency to ‘water‐window’ X‐rays (hν≈280–530 eV).