Efficient J=0-1 soft-X-ray lasing in neon-like ions at pump powers below 250 GW

2 . By using a 0.7% prepulse that precedes the main pulse by 5 ns and applying a total pump energy of 100 J or less, the J=0-1 lasing is at least one order of magnitude higher than the non-lasing background. For the 32.6-nm line of Ti, the 25.5-nm line of Fe, and the 23.1-nm line of Ni, gain coefficients of (±) 4.20.4cm^-1, (±) 3.90.3cm^-1, and (±) 3.60.6cm^-1, respectively, were measured for 2.4-cm-long curved targets, resulting in gain–length products of ∼10. Angle-resolved spectra indicate a beam divergence of 3 mrad (FWHM), typically. The space-resolved spectra show that the J=0-1 lasing lines are emitted from an approximately 60-μm-wide (FWHM) plasma region, whereas the nearby continuum emission is produced in a considerably broader plasma region of ∼250 μm. Lasing at 25.5 nm in neon-like iron was observed at a pump power as low as 180 GW (∼9 TW/cm²), with, however, considerable shot-to-shot scatter in the absolute laser output. Received: 5 September 1997/Revised version: 10 November 1997     

Formation of silicon carbide and diamond nanoparticles in the surface layer of a silicon target during short-pulse carbon ion implantation

Synthesis of silicon carbide and diamond nanoparticles is studied during short-pulse implantation of carbon ions and protons into a silicon target. The experiments are carried out using a TEMP source of pulsed powerful ion beams based on a magnetically insulated diode with radial magnetic field B _r . The beam parameters are as follows: the ion energy is 300 keV, the pulse duration is 80 ns, the beam consists of carbon ions and protons, and the ion current density is 30 A/cm². Single-crystal silicon wafers serve as a target. SiC nanoparticles and nanodiamonds form in the surface layer of silicon subjected to more than 100 pulses. The average coherent domain sizes in the SiC particles and nanodiamonds are 12–16 and 8–9 nm, respectively.     

Circular ion diode with self-magnetic insulation

The results of a study of the generation of a gigawatt-level pulsed ion beam formed by a diode with an explosive-emission potential electrode in self-magnetic insulation mode are presented. The experiments have been performed on the TEMP-4M ion accelerator operating in double-pulse formation mode: the first pulse is negative polarity (300–500 ns, 100–150 kV) and the second is positive (150 ns, 250–300 kV). The ion current density is 20–40 A/cm²; the beam consists of protons and carbon ions. To increase the efficiency of the ion current generation, a circular geometry diode is proposed. It is shown that with the new design, the plasma is effectively formed over the entire working surface of the graphite potential electrode. During ion beam generation, magnetic insulation of the electrons is achieved over the entire length of the diode (B/B _cr ≥ 3). Because of the high drift velocity, the transit time of electrons in the anode-cathode gap is 3–5 ns, whilst the transit time of C⁺ carbon ions exceeds 8 ns. This indicates low efficiency self-magnetic insulation for this geometry of diode. At the same time, it has been observed experimentally that during ion current generation (the second pulse), the electron component of the total current is suppressed by a factor of 4–5. A new mechanism of limiting the electron emission, which explains the decrease in the electron component of the total current in the circular diode with self-magnetic insulation, is proposed.     

On the possibility to realize the efficient relativistic Čerenkov microwave generator without an external guiding magnetic field

We consider the preconditions for efficient operation of the relativistic Čerenkov microwave generator in the absence of an external guiding magnetic field. The analytical expression for the length of propagation of a solid cylindrical electron beam along the drift tube is obtained in the framework of the paraxial approximation. The influence of preliminary modulation of the electron beam on the starting current of the generator and its linear efficiency is analyzed. We calculated the geometry of a relativistic Čerenkov microwave generator with an efficiency of about 30%, in which the solid cylindrical electron beam propagates over a short resonance decelerating system (with the length L ≈ 3λ, where λ is the radiation wavelength) in the absence of an external magnetic field. In experiments, the efficiency of power conversion from the high-current electron beam into the electromagnetic radiation of the E₀₁ mode reached 8%, approximately (relative to the total current of the vacuum diode), for a power of 1.2 ± 0.3 GW and a generation frequency of 4.06 GHz. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 49, No. 10, pp. 829–836, October 2006.     

Erosion of the GaAs target under irradiation by a high-power pulsed ion beam

The results of examination of the GaAs-target erosion under irradiation by a high-power pulsed ion beam are reported. In the experiments, use was made of a high-power pulsed ion source with the following parameters: ion energy — 250 keV, target current density — 350 A/cm², pulse duration — 80 ns, target energy density — up to 7 J/cm². The target erosion coefficient and its dependence on the number of successive pulses are measured. It is found that the surface roughness parameter is increased with the number of successive beam pulses. A regular structure of surface relief is observed to form in the case where the number of pulses > 20–40. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 66–70, January, 2007.     

Large-area phoswich balloon experiment for hard-X-ray astronomy

Summary We describe a balloon experiment, currently in the marking, devoted to the observation of celestial X-ray sources. The main features of the X-ray telescope are summarized as follows. It operates in the energy band from 20 to 300 keV. Its energy resolution is about 17% at 60 keV. Under the hypothesis of 10⁴ s of observing time, 3 mbar of residual atmospheric pressure and 3σ of statistical significance, the expected sensitivity of the instrument is 2·10⁻⁶ photons/cm² s keV in the (20÷200) keV energy band, corresponding to about 1 milliCrab. Its high sensitivity allows us to detect both time variability in the flux and cyclotron lines in the spectra of X-ray sources. It has a field of view of 3° FWHM and has the possibility of resolving complex fields by using multipitch modulation collimators. With such a configuration its angular resolution is about 10′. Paper presented at the 2° Convegno Nazionale di Fisica Cosmica, held at L'Aquila, 29 May–2 June 1984.     

Properties and structural state of the surface layer in a zirconium alloy modified by a pulsed electron beam and saturated by hydrogen

A pulsed action of an electron beam on a Zr-1% Nb zirconium alloy is studied. Alloy samples are irradiated by three 50-μs pulses at an energy density of 15–25 J/cm², a power of (3–6) × 10⁴ W/cm², a current density of 10–50 A/cm², and an electron energy of 18 keV. This method of processing is found to modify the surface layer of the alloy without changing the structure-phase state of its volume. This surface modification increases the hydrogen saturation resistance of the alloy.     

Generation of hard breaking radiation on the MIG system with a plasma opening switch

This paper presents the results of experiments on the generation of pulsed γ radiation of average photon energy ∼1.2 MeV and duration ≤20 ns on the MIG system with a plasma opening switch. Operating modes with a maximum dose downstream of the anode of (1–2)·10¹³ and 10¹² R/s over an area of 2–3 and 100 cm² are obtained, respectively. Institute of High Current Electronics, Siberian Branch of the Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 26–30, December, 1999.     

Long-pulse KrCl laser with a high discharge quality

The discharge quality and optimum pump parameters of a long-pulse high-pressure gas discharge excited KrCl laser are investigated. A three-electrode prepulse–mainpulse excitation circuit is employed as pump source. The discharge volume contains a gas mixture of HCl/Kr/Ne operated at a total pressure of up to 5 bar. For a plane–plane resonator, the divergence of both output laser beams is measured. A low beam divergence of less than 1 mrad is measured as a result of the very high discharge homogeneity. A maximum laser pulse duration of 150 ns (FWHM) is achieved for a pump duration of 270 ns (FWHM) and a power density of 340 kW cm^-3. Pumping the discharge under optimum conditions employing a stable resonator results in a maximum specific energy of 0.45 J/l with a laser pulse duration of 117 ns and an efficiency of 0.63% based on the deposited energy. PACS 42.55.Lt; 52.25.-b; 52.59.Ye     

Ultrashort electron beam and volume high-current discharge in air under the atmospheric pressure

Conditions are studied under which an electron beam and a volume discharge with a subnanosecond rise time of a voltage pulse are produced in air under atmospheric pressure. It is shown that the electron beam appears in a gas-filled diode at the front of the voltage pulse in ∼0.5 ns, has a half-intensity duration of ≤0.4 ns and an average electron energy of ∼0.6 of the voltage across the gas-filled diode, and terminates when the voltage across the gap reaches its maximum value. The electron beam with an average electron energy of 60 to 80 keV and a current amplitude of ≥70 A is obtained. It is assumed that the electron beam is formed from electrons produced in the gap due to gas ionization by fast electrons when the intensity of the field between the front of the expanding plasma cloud and the anode reaches its critical value. A nanosecond volume discharge with a specific power input of ≥400 MW/cm³, a density of the discharge current at the anode of up to 3 kA/cm², and specific energy deposition of ∼1 J/cm³ over 3 to 5 ns is created.     

Spatio-temporal characterization of double plasma mirror for ultrahigh contrast and stable laser pulse

An ultrahigh contrast laser pulse of over 10¹¹ for 6 ps before the main pulse was achieved by employing a double plasma mirror installed at the end of a 100 TW Ti:sapphire laser system. Spatial beam qualities such as focusability and stability were found to be extremely sensitive in the range of 14–360 J/cm² on the double plasma mirror, while ultrahigh contrast was maintained. At the fluence of 90 J/cm² the focusability of the ultrahigh contrast laser was not degraded, and the stability was very close to that obtained without the double plasma mirror when the 2-dimensional normalized standard deviation and the correlation function for several laser beam profiles were analyzed. These results are requisites for carrying out relativistic laser-plasma interactions with ultrahigh contrast laser pulses, enabling the use of ultrathin solid targets.     

Fast electron energy deposition in aluminium foils: Resistive vs. drag heating

The high current electron beam losses have been studied experimentally with 0.7 J, 40 fs, 6 10¹⁹ Wcm^-2 laser pulses interacting with Al foils of thicknesses 10-200 μm. The fast electron beam characteristics and the foil temperature were measured by recording the intensity of the electromagnetic emission from the foils rear side at two different wavelengths in the optical domain, ≈407 nm (the second harmonic of the laser light) and ≈500 nm. The experimentally observed fast electron distribution contains two components: one relativistic tail made of very energetic (T _h ^tail ≈ 10 MeV) and highly collimated (7° ± 3°) electrons, carrying a small amount of energy (less than 1% of the laser energy), and another, the bulk of the accelerated electrons, containing lower-energy (T _h ^bulk=500 ± 100 keV) more divergent electrons (35 ± 5°), which transports about 35% of the laser energy. The relativistic component manifests itself by the coherent 2ω₀ emission due to the modulation of the electron density in the interaction zone. The bulk component induces a strong target heating producing measurable yields of thermal emission from the foils rear side. Our data and modeling demonstrate two mechanisms of fast electron energy deposition: resistive heating due to the neutralizing return current and collisions of fast electrons with plasma electrons. The resistive mechanism is more important at shallow target depths, representing an heating rate of 100 eV per Joule of laser energy at 15 μm. Beyond that depth, because of the beam divergence, the incident current goes under 10¹² Acm^-2 and the collisional heating becomes more important than the resistive heating. The heating rate is of only 1.5 eV per Joule at 50 μm depth.     

Structure and optical properties of carbon films obtained by multipulse pulsed laser deposition

We have obtained carbon thin films on silicon and glass substrates with multipulse pulsed laser irradiation of graphite under vacuum (p ≈ 2.6 Pa) using a high-frequency series of nanosecond laser pulses (τ = 85 ns, λ = 1060 nm) with pulse repetition frequency f ≈ 10–20 kHz and laser power density q ≈ 15–40 MW/cm². We established the optimal laser power density and laser pulse repetition frequency for obtaining amorphous nanostructured diamond-like films.     

Experimental and computational study of the passage of an electron beam through the curvilinear element of the Drakon stellarator system in a tenuous plasma

Results are presented from the first stage of studies on the passage of an electron beam with energy 100–500 eV in a magnetic field of 300–700 Oe through the curvilinear solenoid of the KRéL unit, the latter being a prototype of the closing segment of the Drakon stellarator system, in the plasma-beam discharge regime. The ion density at the end of the curvilinear part of the chamber, n _i ≈8×10⁸–10¹⁰ cm⁻³, the electron temperature T _e ≈4–15 eV, and the positions at which the beam hits the target for different distances from it to the electron source are determined experimentally. The motion of the electron beam is computationally modeled with allowance for the space charge created by the beam and the secondary plasma. From a comparison of the experimentally measured trajectories and trajectories calculated for different values of the space charge, we have obtained an estimate for the unneutralized ion density of the order of 5×10⁷ cm⁻³. Zh. Tekh. Fiz. 69, 22–26 (February 1999)     

Excitation of nuclei in a hot, dense plasma: Feasibility of experiments with 201Hg

It is shown that in a plasma produced on the surface of a sample consisting of a natural mixture of mercury isotopes, ∼10⁴−10^{5 201}Hg nuclei can be excited into the low-lying isomeric level 1/2⁻ (1.561 keV) by an ultrashort laser pulse with energy ≈1 J, duration ≈200 fs, and intensity ≈10¹⁶ W/cm² and the lifetime of the level can be determined. Possible mechanisms leading to the excitation of ²⁰¹Hg nuclei by photons and electrons in a dense, hot plasma are examined and the cross sections of the processes are estimated. Schemes for detecting the effect are proposed. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 5, 312–316 (10 September 1997)     

Measurement of laser light thomson-scattered from a cooling electron beam

First results are presented from an experiment scattering laser light from a relativistic electron beam. The 5 cm diameter continuous electron beam of 28 keV kinetic energy and 2.6 A current presents an electron gas of a density of 8×10⁷ cm^–3, from which 20 ns pulses of laser light (490 nm) were scattered at a repetition rate of 15 Hz and an average power of 20 mJ per pulse. The Doppler-shifted wavelength of photons backscattered under 180° was analysed with a Fabry-Perot interferometer. This technique provides, for the first time, a non-destructive measurement of the velocity distribution in an electron beam radially resolved in space. The results presented here comprise the direct measurement of the absolute electron energy and the degree of space-charge compensation in the electron beam. The determination of an upper bound of 10^–2 for the ratio of longitudinal to transverse electron temperature implies the first direct measurement of a flattened velocity distribution.     

Wide-aperture electric-discharge XeCl lasers

Experimental investigations have been performed for aXeCl laser with the active region aperture equal to 6.5×7 and 15×15 cm. To pump the laser, two types of generator were used: a generator connected in a double-loop circuit with a peaking capacitor and a multichannel spark gap switch and a magnetic thyratron generator. The working mixtureNe−Xe−HCl was preionized with soft x rays and a low-current electron beam, providing an initial electron density ranging from 10⁶ to 5·10¹³ cm⁻³. Conditions in which a homogeneous space discharge is initiated have been studied. It has been investigated how the degree of ionization of the gas and the aperture of the active region affect the discharge and lasing characteristics of the laser. The laser efficiency reaches 4% with an energy of ≈ 6 J extracted from one liter of the active region. Institute of High-Current Electronics, Siberian Division of the Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 54–59, April, 2000.     

Experiments on two-step heating of a dense plasma in the GOL-3 facility

This paper presents the results of experiments on two-stage heating of a dense plasma by a relativistic electron beam in the GOL-3 facility. A dense plasma with a length of about a meter and a hydrogen density up to 10¹⁷ cm⁻³ was created in the main plasma, whose density was 10¹⁵ cm⁻³. In the process of interacting with the plasma, the electron beam (1 MeV, 40 kA, 4 μs) imparts its energy to the electrons of the main plasma through collective effects. The heated electrons, as they disperse along the magnetic field lines, in turn reach the region of dense plasma and impart their energy to it by pairwise collisions. Estimates based on experimental data are given for the parameters of the flux of hot plasma electrons, the energy released in the dense plasma, and the energy balance of the beam-plasma system. The paper discusses the dynamics of the plasma, which is inhomogeneous in density and temperature, including the appearance of pressure waves. Zh. éksp. Teor. Fiz. 113, 897–917 (March 1998)     

All-fiber millijoule energy and nanoseconds pulse operation of a high beam quality multi-stage pulse-pumped Yb-doped amplifier cascade

We report on an all fiber Master Oscillator Power Amplifier (MOPA) structured pulsed-pumped fiber-amplifier seeded by laser diode and modulated by current with an output of several nanojoule energy at 100 Hz repetition rate. To suppress ASE, pulse-pumped technology was adopted in four stages of amplification. By means of this technology, repetition could be adjusted freely without change of pump current which is different from continuous pump. 80 μJ output was achieved in 18 ns pulses in the 15 μJ amplification stage. Moreover, we achieved over 1.2 mJ/pulse with pulses of 10 ns between 1 Hz to 100 Hz repetition in the 30 μm core amplification stage with output beam quality of M ² ≈ 1.4. Energy was limited by launched pump power.     

Effects of irradiation parameters on metal surface erosion by pulsed ion beams

Potential application of high-power ion beams of submicrosecond and microsecond durations with the initial particle energy from 50 to 1000 keV and power density 10⁷–10⁹ W/cm² to ensure metal surface erosion are investigated. Evaporation is treated as a major erosion mechanism and the erosion coefficient is taken as an efficiency indicator. Dependences of the erosion coefficients of several metals on beam parameters obtained via calculations using a technique based on the solution of thermal conductivity equation with phase transitions are presented. The ion species, their initial energy, current pulse duration and power density are used as the beam parameters controlling the result of irradiation. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 49–54, August, 2007.