Molecular radiation in recombining helium plasma

Helium-plasma radiation stimulated in a longitudinal pulsed discharge with preionization (P_He=30–350 torr, U=10–15 kV, I 1 kA) has been studied. Two broad bands with =370 and 600 nm were detected in the region = 230–800 nm. Helium-plasma relaxation processes were considered. It was found that an important role in the formation of excited states of helium molecules can be played by Penning processes, which lead to the effective formation of atomic and molecular helium ions in the plasma. What excited states are responsible for the broadband radiation in the region, however, remains an open question.V. D. Kuznetsov Siberian Physicotechnical Institute, Tomsk State University. Translated from Izvestiya Vysshik Uchebnykh Zavedenii, Fizika, No. 11, pp. 20–29, November, 1992.     

Conditions for XUV amplification considering recombination in clusters

3 cm^-l for a wavelength of λ_R≈200 Å in a time ≲1 ps can be expected. The measurable gain G_real depends on the cluster density N_cl. For N_cl≈10¹⁶ cm^-3 we expect G_real≳20 cm^-1. Received: 30 October 1997     

Population inversion in recombining hydrogen plasma

The collisional-radiative model is applied to a recombining hydrogen plasma in order to investigate plasma conditions in which a population inversion between the energy levels of hydrogen can be generated. Population inversion is expected in plasmas for which three-body recombination makes a large contribution to the recombining processes and the effective recombination rate is larger than a critical value for a given electron density and temperature. Calculated results are presented in figures and tables.     

Parametric amplification and harmonic generation in the VUV and XUV

To overcome limitations in the high harmonic conversion efficiency, the possibility of parametric amplification in the VUV and XUV is considered. Potential approaches, including parametric gain transfer from low- to high-order processes, are discussed. Experimentally, a near two photon resonant parametric gain scheme in xenon, excited by femtosecond KrF laser radiation at _p=248 nm, was investigated, leading to signal (_s) and idler (_i) emissions at 149 nm and 760 nm according to 2_p=_s+_i. For the signal field at 149 nm an exponential increase with density and length is obtained. By tuning the excimer laser radiation slightly towards the two-photon resonance, a macroscopic amplification of about 50 at 149 nm was measured. In addition, a sum-mixing signal at 106 nm according to _sum=2_p+_i, showing a similar exponential increase as the 149-nm signal, is observed, which indicates a coupling of both processes with a gain transfer by the common idler field. PACS 42.65.Ky     

X-ray spectroscopy diagnostics of a recombining plasma in laboratory astrophysics studies

The investigation of a recombining laser plasma is topical primarily because it can be used to simulate the interaction between plasma jets in astrophysical objects. It has been shown that the relative intensities of transitions of a resonance series of He-like multicharged ions can be used for the diagnostics of the recombining plasma. It has been found that the intensities of the indicated transitions for ions with the nuclear charge number Z _n ~ 10 are sensitive to the plasma density in the range N _e ~ 10¹⁶–10²⁰ cm^–3 at temperatures of 10–100 eV. The calculations performed for the F VIII ion have determined the parameters of plasma jets created at the ELFIE nanosecond laser facility (Ecole Polytechnique, France) in order to simulate astrophysical phenomena. The resulting universal calculation dependences can be used to diagnose different recombining plasmas containing helium-like fluorine ions.     

Laser cooling of recombining electron-ion plasma

A method of producing and confining ultracold electron-ion plasma with a strongly nonideal ion subsystem is considered. The method is based on the laser cooling of plasma ions by the radiation resonant with the ion quantum transition. A model is developed for the laser cooling of recombining plasma. Computer simulation based on this model showed that the ion nonideality parameter can be as large as ～100. The data obtained demonstrate that the production of ultracold nonideal plasma is quite possible.     

Width and shift of the He II Balmer- line in a dense z-pinch plasma

The measurement of the width and shift of the He II Balmer-a\alpha line is reported in this paper. The monochromator used in the detection system was calibrated in wavelength by using a low-density hollow-cathode lamp emitting the He II Balmer-a\alpha line. The effect of the radial gradients on the shift and width is also corrected.     

Possible isotope enrichment due to resonance trapping in a recombining plasma

The effect of resonance trapping is shown to produce a transient isotope enhancement of specific charged states of a low abundance isotope during the recombination and decay of a highly ionized plasma by reducing the recombination rate of the more abundant isotope. Such an effect might lead to an efficient isotope separation process, particularly for low vapor pressure elements.     

Structure of the XUV emitting regions in a laser produced plasma

The XUV spectra emitted by plasma produced by focusing a ruby laser on plane targets of carbon and beryllium have been observed with a novel stigmatic spectrograph fitted with a toroidal mirror. The observations have been made both side-on on the expanding plasma and end-on looking inside the crater. In the latter case the intensity emitted is very large and the lines broadened indication electron densities approacing the critical density at 2.2×10²¹ cm^-3. The relative intensity of resonance lines, free-bound and free-free continua and He-like and Li-like satellites has been determined. In particular the latter satellites appear to arise only from the region inside the crater.     

High energy density z-pinch plasma conditions with picosecond time resolution

Using an X-pinch configuration, we have determined that micropinches produced by exploding-wire z pinches can have densities approaching solid density and temperatures of 0.5-1.8 keV, depending upon the wire material used. These plasma parameters, determined from x-ray spectra recorded using an x-ray streak camera, vary drastically on time scales ranging from <10 to 100 ps. Computer simulations require radiation loss to reproduce the observed plasma implosion, suggesting that a radiative-collapse hypothesis for micropinch plasma formation may be correct.     

Focussing of laser beams by means of a z-pinch formed plasma guiding system

It is shown that during a capillary discharge, plasma channels for guiding light from high-power lasers are temporarily formed. The experiments are performed with alumina capillaries of 2-mm radius and 14-cm length filled with about 100 Pa of helium, methane or hydrogen. The transmission of probe light is enhanced due to light guiding by a factor up to 20 during a time window of 5 ns. The results indicate the formation of a first channel, when the shock wave reaches the axis, and of a second channel after the reflection of the shock wave on the axis. Compared to simple focussing by a lens, in the guiding device the intensity–length product increases by almost two orders of magnitude. Guiding is achieved over 180 times the Rayleigh length. Received: 20 June 2000 / Revised version: 4 August 2000 / Published online: 6 September 2000     

Demonstration of a collisionally excited optical-field-ionization XUV laser driven in a plasma waveguide

We describe the first demonstration of a collisionally excited optical-field-ionization laser driven within a waveguide. Lasing on the 4d(9)5d-4d(9)5p transition at 41.8 nm in Xe8+ was observed to be closely correlated to conditions under which the pump laser pulses were guided well by a gas-filled capillary discharge waveguide. Simulations of the propagation of the pump laser radiation show that gain was achieved over essentially the whole 30 mm length of the waveguide.     

XUV radiation transfer through high-Z metal foil in a laser-produced plasma

Aluminum and gold foils have been irradiated with 1.05-μm, 100-psec laser pulses at an intensity of 3 × 10¹⁴ W/cm². A spatially resolved spectrum of XUV radiation from a rear-side plasma has been observed in the wavelength range from 10 to 100 Å. The spectral intensity for the Al foil decays exponentially with foil thickness and goes to zero at 3 μm. The intensity for the Au foil also decays exponentially up to 1 μm but remains almost constant from 1 to 6 μm. This result for the Au foil indicates that radiation heat conduction plays an important role in energy transport through high-Z plasmas.     

Population inversion between the ground and first excited states in a recombining hydrogen plasma

Plasma conditions for generating a population inversion between the ground and first excited states in a recombining hydrogen plasma have been investigated on the basis of the CR model. Population inversion can be expected when three-body recombination plays a dominant role; the required regions of electron density and temperature are specified. It is shown that upper bounds exist for the ground-state population density at given electron density and temperature. Larger inversion densities can be obtained between the ground and first excited states than between excited levels. Numerical results are presented.     

Backward Raman amplification in a plasma waveguide

Backward Raman amplification of a short laser pulse in a plasma waveguide is demonstrated. With a guided seed pulse of 0.8-microJ energy and a pump pulse of 345-mJ energy in a 9-mm-long optically preformed plasma waveguide, 910-fold energy amplification is achieved. Heating of the plasma by the long pump pulse is identified to be a key issue for plasma-waveguide-based backward Raman amplifiers.     

XUV generation from plasma produced by a XeCl excimer laser on a Cu target

Summary We present a detailed study of XUV and soft X-ray emission from Cu plasma produced by an excimer laser at intensitiesI _L≦8·10¹¹ W/cm². The XeCl excimer laser (ψ≈308 nm) delivers pulses with energyE _L≈2.3 J, temporal durationt _L≈100 ns and brightnessB≧10¹⁴ W/cm² sr. We recorded a spectral conversion efficiency η=0.5% eV⁻¹ forI _L=4·10¹¹W/cm² in the aluminium window at 73eV with a harder X-ray tail around ≈400eV. We also measured the dependence of X-ray signal on laser intensity and viewing angle. Experimental results have been compared with some analytical laser-plasma interaction models.     

Radiation amplification by a hydrogen plasma

We have detected and measured for the first time the amplification of a narrow-band radiation (second harmonic of an argon ion laser, λ=244 nm) propagating through a dc gas-discharge plasma in hydrogen at a pressure of 0.5–1.0 Torr. An attempt is made at explaining this phenomenon.