A comparative study of the ionic keV X-ray line emission from plasma produced by the femtosecond,picosecond and nanosecond duration laser pulses

We report here an experimental study of the ionic keV X-ray line emission from magnesium plasma produced by laser pulses of three widely different pulse durations (FWHM) of 45 fs, 25 ps and 3 ns, at a constant laser fluence of ∼1.5 × 10⁴ J cm^− 2. It is observed that the X-ray yield of the resonance lines from the higher ionization states such as H- and He-like ions decreases on decreasing the laser pulse duration, even though the peak laser intensities of 3.5 × 10¹⁷ W cm^− 2 for the 45 fs pulses and 6.2 × 10¹⁴ W cm^− 2 for the 25 ps pulses are much higher than 5 × 10¹² W cm^− 2 for the 3 ns laser pulse. The results were explained in terms of the ionization equilibrium time for different ionization states in the heated plasma. The study can be useful to make optimum choice of the laser pulse duration to produce short pulse intense X-ray line emission from the plasma and to get the knowledge of the degree of ionization in the plasma.     

Non-equilibrium ionization and electron density in the coronal plasma during solar flares

Summary The Li-like to He-like ion population ratio for calcium at the onset of solar flares is observed to be about 60% higher than in steady-state ionization balance. The measurement of the duration of this initial period of transient ionization and of the population ratios of adjacent ionization states allows an estimate of the electron density of the coronal plasma at flare onset. The density found in this study, within 1·10⁹ and 7·10⁹ cm⁻³ is comparable with the density typical of pre-flare active regions.     

Three-dimensional simulation of laser–plasma-based electron acceleration

A sequential three-dimensional (3D) particle-in-cell simulation code PICPSI-3D with a user friendly graphical user interface (GUI) has been developed and used to study the interaction of plasma with ultrahigh intensity laser radiation. A case study of laser–plasma-based electron acceleration has been carried out to assess the performance of this code. Simulations have been performed for a Gaussian laser beam of peak intensity 5 × 10¹⁹ W/cm² propagating through an underdense plasma of uniform density 1 × 10¹⁹ cm^− 3, and for a Gaussian laser beam of peak intensity 1.5 × 10¹⁹ W/cm² propagating through an underdense plasma of uniform density 3.5 × 10¹⁹ cm^− 3. The electron energy spectrum has been evaluated at different time-steps during the propagation of the laser beam. When the plasma density is 1 × 10¹⁹ cm^− 3, simulations show that the electron energy spectrum forms a monoenergetic peak at ~14 MeV, with an energy spread of ±7 MeV. On the other hand, when the plasma density is 3.5 × 10¹⁹ cm^− 3, simulations show that the electron energy spectrum forms a monoenergetic peak at ~23 MeV, with an energy spread of ±7.5 MeV.     

Determination of the negative hydrogen ion concentration in a cesium-hydrogen discharge

The concentration N _H ⁻ of negative hydrogen ions in a low-voltage cesium-hydrogen discharge plasma N _H ⁻ is determined from experiments with laser radiation absorption caused by the photodetachment of electrons from the H⁻ ions. The resolution of a setup measuring the relative absorption is ≥10⁻⁵ for a signal-to-noise ratio of ∼10⁻² or less. A heated-cathode diode is used to initiate the discharge at a voltage of U≤10 V and a current density of j≤5 A/cm² (hydrogen pressure p _{H 2} is equal to several torr, and the cesium concentration in the plasma N _Cs ⁽⁰⁾ ∼10¹⁴ cm⁻³). The absorption due to the photoionization of excited Cs atoms is shown to be negligible. The measured concentration N _H ⁻ of the H⁻ ions is 10¹²–10¹³ cm⁻³. Experimental results are consistent with the theory.     

Effect of interparticle interactions on radiative lifetime of photoexcited electron-hole system in GaAs quantum wells

The paper reports on an investigation of changes in the photoluminescence linewidth and lifetime of excitons and electron-hole plasma over a wide range of densities between 3×10⁷ and 3×10¹² cm⁻² at a temperature of 77 K in GaAs/AlGaAs quantum wells. The roles played by thermal ionization of excitons at low densities of nonequilibrium carriers, exciton-exciton and exciton-electron collisions, and ionization of excitons at high pumping power densities have been studied. Zh. éksp. Teor. Fiz. 112, 353–361 (July 1997)     

ϑ-pinch in extrinsic InSb

The compression of an electron-hole plasma, caused by a ?-pinch in extrinsic InSb of 140 K, was investigated by measuring the absorption of 10·6 μm radiation and the change of the magnetic flux in the sample. A plasma density of 5 × 10¹⁵ cm^?3 was hereby found. The temporal development of the plasma density indicates that the electrons and holes resulted from impact ionization in the electric field when the magnetic field was low. Since the electric field is highest at the sample surface, the ionization was limited to the outer region of the sample and the plasma was transmitted to the inner sample volume by the ?-pinch.     

Regimes of laser plasma expansion at optical breakdown in the normal atmosphere

Propagation regimes of a plasma (fast ionization wave, laser-supported radiation wave, and laser-supported detonation wave) generated by laser radiation in a wide range of intensities (5 × 10⁸?10¹¹ W/cm² ) are described. The regimes were analyzed on the basis of the calculated dependence of the propagation velocity on the laser radiation intensity. The lower bound of the velocity was used for the fast ionization wave. Calculation results agree with experimental data and show that the plasma propagates as a fast ionization wave in the above range of intensities.     

Gas-filled electron diode based on a glow discharge

Stable ignition and sustention of a pulsed discharge with a current of up to 180 A and duration of 12 μs at a pressure of 10⁻¹–10⁻² Pa are achieved in a glow-discharge plasma cathode with the help of an auxiliary initiating discharge. An electron emission current density of up to 100 A/cm² and accelerating voltageof 15 kV are obtained in a gas-filled diode based on this type of a plasma cathode. An electron beam witha neutralized space charge can be transported almost without losses in a weak axial magnetic field alonga plasma channel formed due to the gas ionization by the accelerated electrons over a distance of up to 30 cm.     

Measuring photoionization cross-sections of excited atomic states

A new method of photoionization cross-section measurement based on the observation of saturation in the ion yield as a function of radiation intensity has been proposed. The photoionization cross-sections for the 6²P_1/2 and 6²P_3/2 levels of Rb atoms of the fundamental and second harmonics of ruby laser radiation are measured, with a tunable pulsed dye laser used for excitation. The following values of cross-sections are obtained: (1.7±0.4)·10⁻¹⁷ cm² and (1.5±0.4)·10⁻¹⁷ cm² for the levels 6²P_3/2 and 6²P_1/2, respectively, ionized by radiation of v₂=14403 cm⁻¹, and (1.9±0.5)·10⁻¹⁷ cm² for the 6²P_3/2 level ionized by v₂=28806 cm⁻¹ radiation.     

Thermal stability and radiation hardness of SiC-based schottky-barrier diodes

Pt/W/Cr/SiC Schottky-barrier diodes that retain good electrophysical parameters up to 450°C are studied. With the Auger electron spectroscopy (AES) method, it is shown that the thermal stability is provided by using a multilayer metal composition that ensures the metal/SiC interface stability. The surface-barrier structures obtained are tested for radiation hardness. They are irradiated by fast neutrons with a fluence of 4.42×10¹⁵ n/cm² and attendant γ radiation with a dose of 8.67×10⁵ R in the concentration range of N _d-N _a=10¹⁶−5×10¹⁷ cm⁻³. Irreversible modifications of the structures at N _d-N _a≤8×10¹⁶ cm⁻³ are found. The degradation of the parameters is inversely proportional to the doping level.     

Measurement of the spin-exchange and chemical ionization rate constants in collisions of polarized metastable 23 S 1 helium atoms with 32 S 1/2 sodium atoms

Experimental data on the spin-exchange rate constants for the He(2³ S ₁)-Na(3² S _1/2) system are reported for the first time. Measurements show that the spin-exchange rate constant is C _se = (23 ± 11) × 10⁻¹⁰ cm³ s⁻¹ and the chemical ionization rate constant is C _si = (29 ± 14) × 10⁻¹⁰ cm³ s⁻¹ at a temperature of 420 K. The results are compared with the data calculated from the rate constants.     

Characteristics of thermally stimulated depolarization phenomena in zinc-oxide ceramics for varistors

The results of experimental investigations and an analysis of the temperature dependences of the thermally stimulated depolarization current are presented for zinc-oxide ceramics suitable for use in high-voltage varistors. A model for the depolarization phenomena is proposed that takes into account charge exchange on localized electronic states on both sides of the intercrystallite potential barrier. The model is used to obtain estimates of the ionization energy and density of shallow (∼0.07 eV, 1×10¹⁷ cm⁻³) and deeper (∼0.2 eV, 1×10¹⁸ cm⁻³) bulk levels and surface-localized levels (∼0.1 eV, 1×10¹³ cm⁻²). Zh. Tekh. Fiz. 67, 60–63 (October 1997)     

Near IR laser light visualizators using nonlinear GaSe and other layered crystallites

Two methods of preparation of the devices for visualization of pulsed and continuous near-IR (near infrared) are described and the results of conversion of pulsed and continuous IR (800–1360 nm) laser radiation into the visible range of spectra (400–680 nm) by using a transparent substrate covered with the particles (including nanoparticles) of effective nonlinear materials of GaSe _x S_{1 − x} (0.2 ≤ x ≤ 0.8) are presented. Converted light can be detected in transmission or reflection geometry as a visible spot corresponding to the real size of the incident laser beam. Developed device structures can be used for checking if the laser is working or not, for optical adjustment, for visualization of distribution of laser radiation over the cross of the beam and for investigation of the content of the laser radiation. Low energy (power density) limit for visualization of the IR laser pulses with 2–3 ps duration for these device structures are: between 4.6–2.1 μJ (3 × 10⁻⁴−1 × 10⁻⁴ W/cm²) at 1200 nm; between 8.4–2.6 μJ (4.7 × 10⁻⁴−1.5 × 10⁻⁴ W/cm²) at 1300 nm; between 14.4–8.1 μJ (8.2 × 10⁻⁴–4.6 × 10⁻⁴ W/cm²) at 1360 nm. Threshold damage density is more than 10 MW/cm² at λ = 1060 nm, pulse duration τ = 35 ps. The results are compared with commercially existing laser light visualizators.     

Target plasma formation by UHF power

The properties of plasma injected into an open magnetic trap of uniform field from an independent UHF source have been investigated. Plasma is created in the UHF source at the frequency of 2400 MHz (power input 150 W) in the electron cyclotron resonance (ECR) regime at the pressure of neutral argon (10⁻⁵−10⁻²) torr. It is established that a rather quiescent target plasma with controlled density within the range of (2 × 10⁸−2 × 10¹²) cm⁻³ and temperature 2–3eV is accumulated in the trap. It turned out that plasma lifetime in the trap is determined by a classical mechanism of particle escape at the expense of collisions, at fixed value of magnetic field in the trap it practically is not changed with the variation of neutral gas pressure and reaches the value ≈ 4×10⁻³ s at the magnetic field strength in the trap equal 1600 Oe.     

Multiple charge states of titanium ions in laser produced plasma

An intense laser radiation (10¹² to 10¹¹ W/cm⁻²) focused on the solid target creates a hot (≥1 keV) and dense plasma having high ionization state. The multiple charged ions with high current densities produced during laser matter interaction have potential application in accelerators as an ion source. This paper presents generation and detection of highly stripped titanium ions (Ti) in laser produced plasma. An Nd:glass laser (KAMETRON) delivering 50 J energy (λ=0.53 μm) in 2.5 ns was focused onto a titanium target to produce plasma. This plasma was allowed to drift across a space of ∼3 m through a diagnostic hole in the focusing mirror before ions are finally detected with the help of electrostatic ion analyzer. Maximum current density was detected for the charge states of +16 and +17 of Ti ions for laser intensity of ∼10¹¹ W/cm⁻².     

Composition of a non-self-sustained discharge plasma in an N2: O2: H2O mixture at atmospheric pressure

The plasma composition of a discharge sustained by a pulsed ionization source of μs duration is computed. It is shown that, within a time interval of ∼10⁻⁶ s after the ionization pulse, the dependences of the ion densities on the electric field and ionization source power show features that should be taken into account when developing laser systems for controlling electric discharges in long air gaps. The effect of the plasma composition on the efficiency of electron photodetachment from negative O ₂ ⁻ ions is investigated by the example of a discharge initiation system consisting of two lasers with different pulse durations and wavelengths. Plasmochemical processes under conditions of enhanced electron photodetachment from negative O ₂ ⁻ ions are simulated. It is shown that photodetachment can increase the electron density for a time of <10⁻⁵ s.     

Femtosecond laser plasma in CaF<Subscript>2</Subscript> crystal microchannel and effective generation of characteristic X-ray radiation

The dependence of the characteristic X-ray radiation yield from CaF₂ crystal on the formed microchannel depth under highly intensive (I ∼ 3 × 10¹⁵ W/cm²) laser pulses with different contrast was obtained. The maximum of the characteristic X-ray radiation yield at these experimental conditions corresponded to the microchannel depth of 30–50 μm. The efficiency of the laser radiation conversion to the characteristic X-ray radiation increased from 6 × 10⁻⁸ for the surface up to 10⁻⁷ in the microchannel. The dependence of the characteristic X-ray radiation yield on the viewing angle showed that the source of X-ray radiation was located near the surface inside the microchannel.     

Experimental observations of the stepped Cu-atom ionization in the active medium of a Cu laser

An optogalvanic effect in a pulsed copper-vapor laser is discovered and investigated. It is found that the generalized deexcitation rate constant of the resonance level to the ionization state of the active medium is <σv>∼ (3±1)·10⁻⁷ cm³·s⁻¹. An optothermal effect caused by the hysteresis of the optogalvanic phenomenon is found experimentally. The feasibility of measuring the stepped ionization constants of resonance and metastable states for a number of metal atoms in a pulse-discharge plasma is discussed. Institute of Physics of Semiconductors of the Siberian Branch of the Russian Academy of Sciences; Institute of Atmospheric Optics of the Siberian Branch of the Russian Academy of Sciences; Institute of High-Current Electronics of the Siberian Branch of the Russian Academy of Sciences; Tomsk State University. Translated from Izvestiya Vysshikh Uchebhykh Zavedenii, Fizika, No. 8, pp. 57–62, August, 1999.     

Ionization Dynamics in a Pulse Disturbed Magnetically Confined Helium Plasma

The equilibrium of a magnetized Helium plasma is disturbed by a pulsed Trivelpiece-Gouldwave. The electrons obtain the energy by linear collisionless wave absorption. The relaxation phenomena of density and energy are explained in terms of two relaxation times τ_E, τ₁ and a quantity giving the additional ionization. These quantities are derived from a small signal fluid model based upon energy and particle balance equations. In the experiment they are taken from the transient curves of Langmuir-probe current, optical line radiation and the noise power at the electron cyclotron frequency. The experimental conditions are: Helium-gas, p = 1 …? 5 Pa, T_e = 4 eV, n = 1 …? 5 · 10¹⁰ cm^?3, B = 6,5 · 10^?2 T, 27 MHz rf plasma source, low frequency fluctuation level < 1%, classical losses. The energy relaxation time …?_E = 10 …? 15 μs is given by inelastic collision losses. The ionization time constant τ₁ is related to the instantaneous ionization frequency during the transient state. It shows a high value at the very beginning of the pulse which must be explained by a tail formation in the distribution function and enhanced radial losses becoming Bohm-like in the transition phase.