Laser-produced Magnesium Plasma Research Articles

A comparative study of the inner-shell and the ionic line radiation from ultra-short laser-produced magnesium plasma

The inner-shell radiation (K-α) and the ionic line radiation (He-α) from magnesium plasma, generated by the interaction of a 3 TW, 45 fs laser, have been studied simultaneously using an x-ray crystal spectrograph. The effect of the variation of the laser intensity and its offset from the best focus position has been studied. He-α and K-α x-ray yields are found to scale with the laser intensity as IL1.5 and IL0.6 respectively. The K-α x-ray conversion shows a maximum at the best focus and reduces symmetrically on either side of the best focus position, whereas the He-α conversion peaks when the target is placed before the focused laser beam. The angular distribution for the He-α as well as the K-α emissions shows a maximum in the forward direction and the intensity reduces with the increase in angle θ with respect to the target normal as cosα θ. The value of α is 0.7 and 3 for He-α and K-α respectively. The experimentally observed variation of the He-α line conversion for different laser parameters has been explained by considering the change in preformed plasma conditions, and the variation in the K-α emission has been explained by considering hot electron generation and their propagation in the bulk solid target. The plasma conditions prevalent during the emission of the x-ray spectrum were identified by comparing the experimental spectra with the synthetic spectra generated using the spectroscopic analysis code PrismSPECT. The results will be useful in designing laser-produced plasma x-ray line radiation sources of photon energy in the range of 1–2 keV, for its potential use as a probe pulse in x-ray backlighting, or time-resolved x-ray diffraction studies.

Pressure dependence of enhanced x-ray line emission from laser-produced plasmas expanding in an ambient gas

An experimental study of parametric dependence of space-resolved x-ray line emission from laser-produced magnesium plasma expanding in a helium background gas has been performed for different pressures up to 125 mbar. A strong enhancement in MgXI He-α resonance (1s2 S10P11 at 9.17 Å) and intercombination (1s2 1S0−1s2p3P1 at 9.23 Å) line emission is observed in the expansion region extending up to ∼ 6 mm from the target. At the optimum background gas pressure of about 40 mbar, there is an order of magnitude increase in the x-ray line intensity compared to that observed for a gas pressure of few mbar. The results of measurements of the distance of the peak emission region from the target and width of the emission zone for different pressures are presented, and the role of the background gas for the observed behavior is discussed.

XUV diagnostics of colliding laser-produced magnesiumplasmas

Charge exchange collisions between highly charged Mg ions incolliding laser-produced magnesium plasmas are studied. Pinholephotography and XUV spectroscopy are used as diagnostic tools.XUV pinhole pictures taken at early stages give moreinsight into the expansion dynamics of the colliding magnesiumplasmas. Spectroscopic studies show selective population of then = 3 levels of Mg IX ions which results in enhancementof respective line intensities. Theoretical calculations alsogive large cross section as high as 10-15 cm2 forthese charge exchange collisions when the relative velocities ofthe colliding ions are of the order of 107 cm s-1.

Time evolution of colliding laser produced magnesium plasmas investigated using a pinhole camera

Time resolved studies of colliding laser produced magnesium plasmas are performed using a pinhole camera. A ruby laser pulse is split into two beams using a movable glass wedge and focused onto a planar target or targets placed at 90° to each other. A gated pinhole camera provides an orthogonal view of the collision. Measurements with an aluminum filter allowed identification of soft x-ray production zones. A good interpenetration of the two plasmas was observed in the 90° target geometry, because of higher relative velocities, than in the case of laterally colliding plasmas. The appearance of the collision region depended strongly on the power density and on the separation of the foci on the target surfaces.

Space- and time-resolved soft x-ray emission from laser-producedmagnesium plasma

Soft x-ray emission from plasmas produced by ablation from amagnesium target employing a ruby laser is studied using a grazingincidence spectrograph in the spectral region 3-12 nm. Emissionintensities are investigated for different ionic lines as a function ofposition, time after the maximum of the laser pulse and laser irradiance.A gated pinhole camera is employed for studying the evolution of theplasma at early stages. The propagation distances and velocities aremeasured. Our results show that the line intensities of all ionic linesincrease with laser irradiance until a saturation plateau is observed athigh irradiance levels. The increase in ionization and intensitysaturation with varying laser irradiance seems to suggest stronginteraction of the laser pulse with the dense plasma formed near thetarget within the pulse duration.

Charge-exchange collisions in interpenetrating laser-produced magnesium plasmas

Charge-exchange collisions are one of the effective pumping methods for soft X-ray lasers. Experiments are performed to investigate charge-exchange collisions between highly charged Mg ions in colliding laser-produced magnesium plasmas. Pinhole photography and XUV spectroscopy are used as diagnostic tools. Spectroscopic studies show selective population of n = 3 levels of Mg IX ions, which results in enhancement of respective line intensities. Theoretical calculations also give a large cross section as high as 10−15 cm2 for these charge-exchange collisions when the relative velocities of the colliding ions are of the order of 107 cm s−1. XUV pinhole pictures are taken in early stages, which give more insight into the expansion dynamics of the colliding magnesium plasmas.

A simple high-resolution on-line x-ray imaging crystal spectrograph for laser–plasma interaction studies

A simple, high-resolution x-ray imaging crystal spectrograph is described for on-line measurements and analysis of x-ray emission from laser-produced plasmas in the wavelength range of ∼5–12 Å. The spectrograph uses a planar TAP crystal for dispersion and a phosphor-coated optical fiber plate followed by an image intensifier and a charge-coupled device camera for detection. The spectrograph covers a wavelength range of ∼1.4 Å in a single shot about a desired wavelength setting with a spectral resolution of ∼0.012 Å. It also has a design flexibility to cover the full spectral range of 5–12 Å in a single shot with a spectral resolution of ∼0.040 Å. The spectrograph is particularly suited for studies involving low vacuum environment, e.g., plasma interaction with a background gas, laser interaction with a gas jet, etc., which preclude the use of other on-line detectors such as open-ended microchannel plates. An example of space resolved x-ray emission from laser-produced magnesium plasma expanding in a helium gas is also presented.

Study of diffraction efficiency of a free-standing transmission grating in keV spectral region using laser produced plasmas

X-ray line emission from a laser produced plasma has been used to study relative diffraction efficiency of a gold microstructure transmission grating in the keV spectral region. Measurements of X-ray intensity of the Mg XI 1s 2–1s2p line emission at λ=9.2 Å obtained from a laser produced magnesium plasma were made from the zeroth to fourth diffraction order. The contribution of continuum and line radiation at other wavelengths to the zeroth order was suppressed by using appropriate X-ray filter. Enhanced diffraction efficiencies observed for the first to fourth orders relative to the zeroth order are found to be in agreement with theoretical calculations considering the effect of partial X-ray transmission through the gold bars.

A simple XUV transmission grating spectrograph with sub-ångström resolution for laser-plasma interaction studies

A simple XUV transmission grating spectrograph operating in the wavelength range 3-90 Å with sub-ångström resolution without using any XUV imaging optics is described. This is based on a free-standing gold microstructure grating of 2000 Å period in normal incidence geometry. A spectral resolution of 0.6 Å is obtained by optimizing the slit aperture, the source-to-grating distance and the grating-to-detector distance. Electron temperatures deduced from analysis of the spectrum of a laser-produced magnesium plasma using a plasma spectroscopic code are consistent with theoretical considerations. The spectral range can be extended to higher wavelengths by using a larger detector. The simple geometry and ease of operation of the spectrograph should make it useful in many experiments such as those involving plasma-based XUV lasers and odd-harmonic generation in the XUV spectral region using ultra-short laser pulses.

Three-dimensional number density mapping in the plume of a low-temperature laser-ablated magnesium plasma

Simultaneous optical absorption and laser-induced fluorescence measurements have been used to map the three-dimensional number densities of ground-state ions and neutrals within a low-temperature KrF laser-produced magnesium plasma expanding into vacuum. Data is reported for the symmetry plane of the plasma, which includes the laser interaction point at a delay of 1 μs after the ∼30 ns KrF laser ablation pulse and for a laser fluence of 2 J cm −2 on target. The number density distributions of ion and neutral species within this plane indicate that two distinct regions exist within the plume; one is a fast component containing ions and neutrals at maximum densities of ∼3×10 13 cm −3 and ∼4×10 12 cm −3, respectively and the second is a high-density region containing slow neutral species, at densities up to ∼1×10 15 cm −3.

Space-resolved Spectral Diagnosis of Line-shaped Laser-produced Magnesium Plasmas

Space-resolved spectra of line-shaped laser-produced magnesium plasmas in the normal direction of the target have been obtained using a pinhole crystal spectrograph. These spectra are treated by a spectrum analyzing code for obtaining the true spectra and fine structures of overlapped lines. The spatial distributions of electron temperature and density along the normal direction of the target surface have been obtained with different spectral diagnostic techniques. Especially, the electron density plateaus beyond the critical surface in line-shaped magnesium plasmas have been obtained with a fitting technique applied to the Stark-broadened Ly-α wings of hydrogenic ions. The difference of plasma parameters between those obtained by different diagnostic techniques is discussed. Other phenomena, such as plasma satellites, population inversion, etc., which are observed in magnesium plasmas, are also presented.

Identification of hydrogenlike and heliumlike transitions in the spectrum of laser-produced magnesium plasmas

Nonresonance spectral lines of Mg xii and Mg xi emitted by magnesium laser-produced plasmas have been observed in the extreme-vacuum-ultraviolet region and their transitions classified. As many as eight beams of the Omega laser system of the Laboratory for Laser Energetics at the University of Rochester were linearly focused onto magnesium-coated flat targets to produce linear plasma radiation sources from 3 to 6 mm long. The spectra were photographed end-on with a grazing-incidence spectrograph. The identified Mg xii lines are classified as 2s–3p, 2p–3d, 2s–4p, 2p–4d, and 3d–4f transitions. The identified Mg xi lines are classified as 1s2s–1s3p, 1s2p–1s3d, 1s2p–1s4d, 1s3p–1s4d, and 1s3d–1s4f. Ionization differentiation is based on comparing the present data with other magnesium spectra emitted from plasmas produced by different sources or differing in the ionization conditions. Transition classification is based on comparing the experimental wavelengths with the calculated values and studying the correlation between observed line intensities and the calculated oscillator strengths.