Konstantin Nikolaevitch Koshelev

X-ray lasing as a result of an induced instability in an ablative capillary discharge

We report lasing of the CVI Balmer-α line at 18.22 nm using the new technique of an induced MHD instability in an ablative capillary discharge. A large spike of this line during the second half-cycle of the discharge is observed. The spike is identified as amplified spontaneous emission (ASE), and enhancements are derived for capillaries of lengths between 15 and 50 mm. We explain the ASE as a result of charge exchange processes which occur between ions produced in the neck of the MHD instability and low temperature ions present in the same plasma column. To emphasize this instability, we implemented a new method by using a straight capillary with a waved structure imprinted inside. The results are encouraging and an exponential gain length is achieved with GL = 4.5 for a 40 mm length capillary. Other evidence to support the observation is realised when a plane multilayer mirror with reflectivity < 30% is introduced resulting in amplification.

Removal of amorphous C and Sn on Mo:Si multilayer mirror surface in Hydrogen plasma and afterglow

Removal of amorphous carbon and tin films from a Mo:Si multilayer mirror surface in a hydrogen plasma and its afterglow is investigated. In the afterglow, the mechanism of Sn and C films removal is solely driven by hydrogen atoms (radicals). Probabilities of Sn and C atoms removal by H atoms were measured. It was shown that the radical mechanism is also dominant for Sn atoms removal in the hydrogen plasma because of the low ion energy and flux. Unlike for Sn, the removal mechanism for C atoms in the plasma is ion-stimulated and provides a much higher removal rate.

Enhancement of laser plasma extreme ultraviolet emission by shockwave-laser interaction

A double laser pulse heating scheme has been applied to generate plasmas with enhanced emission in the extreme ultraviolet (EUV). The plasmas were produced by focusing two laser beams (prepulse and main pulse) with a small spatial separation between the foci on a xenon gas jet target. Prepulses with ps-duration were applied to obtain high shockwave densities, following indications of earlier published results obtained using ns prepulses. EUV intensities around 13.5 nm and in the range 5–20 nm were recorded, and a maximum increase in intensity exceeding 2 was measured at an optimal delay of 140 ns between prepulse and main pulse. The gain in intensity is explained by the interaction of the shockwave produced by the prepulse with the xenon in the beam waist of the main pulse. Extensive simulation was done using the radiative magnetohydrodynamic code Z*.

Enhancement of laser plasma EUV emission by shockwave–plasma interaction

Laser plasmas have been produced using a xenon gas jet as a target and a pre/main pulse heating scheme in which the two laser foci are separated in space. Extreme ultraviolet intensities around 13.5 nm and in the range 5–20 nm have been recorded, and an up to five fold increase in intensity has been measured. The gain in intensity is explained by the interaction of the shock wave produced by the pre-pulse with the xenon in the focus of the main pulse.

High brightness EUV sources based on laser plasma at using droplet liquid metal target

We present the study of a source of extreme ultraviolet (EUV) radiation based on laser plasma generated due to the interaction of radiation from a nanosecond Nd : YAG laser with a liquidmetal droplet target consisting of a low-temperature eutectic indium–tin alloy. The generator of droplets is constructed using a commercial nozzle and operates on the principle of forced capillary jet decomposition. Long-term spatial stability of the centre-of-mass position of the droplet with the root-mean-square deviation of ~0.5 μm is demonstrated. The use of a low-temperature working substance instead of pure tin increases the reliability and lifetime of the droplet generator. For the time- and space-averaged power density of laser radiation on the droplet target 4 × 1011 W cm-2 and the diameter of radiating plasma ~80 μm, the mean efficiency of conversion of laser energy into the energy of EUV radiation at 13.5 ± 0.135 nm equal to 2.3% (2π sr)-1 is achieved. Using the doublepulse method, we have modelled the repetitively pulsed regime of the source operation and demonstrated the possibility of its stable functioning with the repetition rate up to 8 kHz for the droplet generation repetition rate of more than 32 kHz, which will allow the source brightness to be as large as ~0.96 kW (mm2 sr)-1.

RZLINE code modelling of distributed tin targets for laser-produced plasma sources of extreme ultraviolet radiation

Abstract. An integrated model is developed to describe the hydrodynamics, atomic, and radiation processes that take place in extreme ultraviolet (EUV) radiation sources based on a laser-produced plasma with a distributed tin target. The modeling was performed using the RZLINE code—a numerical code for the simulation of EUV emission by hot dense plasmas. The purpose of the simulation is to evaluate the spectral characteristics of the radiation source, conversion efficiency, source size, evaporation rate of the target, energetic, and space distribution of debris (nanoparticles, neutrals, and ions). The advantages of a distributed target in comparison with a single droplet target are also discussed.

Plasma production by means of discharge in a spherical cavity

The work is devoted to the study of plasma, appearing as a result of cumulation of shock wave with form close to spherical. The shock wave was obtained by triggering of fast discharge (dI/dt about 1012 A/s) on inner surface of cavity, made from insulator. Spherical cavity with radius 4.5 mm was filled with Ar at 80 Pa. Inductive storage with semiconductive opening switch was used as a current driver. Spherical plasma with diameter about 1 mm, emitting in vacuum ultraviolet (vuv), was detected by means of pinhole measurements with time gated microchannel plate camera, starting about 50 ns from the beginning of the discharge. vuv spectra have shown the presence spectral lines of ArV–ArVIII ions, which gives the estimation of electron temperature as 30 eV. The plasma ball reveals no instabilities, keeps its characteristics and emits vuv radiation during 300 ns. After 600 ns from the beginning of the discharge plasma emits a flux of electrons with energies about 1 keV with temporal structure about 100 ns.

Ion Beams from Axial Discharges and the X‐ray Laser Problem

Recent estimates and a few available data for charge exchange between colliding ions reveal that for specific systems cross‐sections can become considerable even at relatively low impact energies despite Coulomb repulsion. Lasing in the x‐ray region by respective charge transfer pumping thus appears feasible employing a scheme, where a suitably prepared ion beam interacts with a proper target plasma. Axial discharges can produce such beams and various configurations are discussed. Finally the possibility is analyzed that lasing recently reported for a capillary discharge is due to such a mechanism.

Dynamics of a plasma in a capillary discharge driven by a plasma focus operated in the mode of a plasma switch opening.

A compact and simple type of current driver based on a plasma focus working in the mode of a plasma opening switch was used for excitation of a capillary discharge in Ar pressures in the range 0.1-0.5 Torr. A current of up to 50-60 kA with a rise time of about 200 ns was achieved in the capillary. The Ar plasma was compressed from an initial diameter of 5 mm to a diameter of 1 mm. The dynamics of compression have been studied using time-resolved VUV pin-hole diagnostics. Time-resolved VUV spectra show that the plasma consists mainly of the Ne- and F-like ions Ar IX and Ar X. A spectral line at 468.7 A, which was identified as the transition in Ne-like Ar IX, has an anomalously high intensity compared with other 3-3 transitions of Ar IX.

Loss of Hydrogen Atoms in H 2 Plasma on the Surfaces of Materials Used in EUV Lithography

Low-pressure hydrogen is an important component of the working medium in extreme ultraviolet (EUV) projection lithography. Under the action of EUV photons and fast secondary electrons on the gas medium, plasma and atomic hydrogen, actively interacting with the surface, are produced. This interaction is very important, because it largely determines the lifetime of the multilayered EUV optics. In this study, the loss of atomic hydrogen under the conditions of a low pressure (<10 Torr) RF plasma discharge on the surfaces of materials used in EUV lithography is investigated. The surface loss probabilities of H atoms on these materials are measured. It is shown that surface recombination of atomic hydrogen goes according to the Eley-Rideal mechanism via direct recombination of H atoms from the gas phase with chemically and physically adsorbed atoms. In this case, the surface recombination probability is mainly determined by the density of chemical adsorption sites. The density of adsorption sites and the desorption energy of H atoms are estimated. The desorption energy of physically adsorbed H atoms on pure metal surfaces (or surfaces exposed to plasma) is about 0.5 eV, and the density of sorption sites is close to the surface density of atoms. This results in a high loss probability of H atoms on metals (∼0.1). Therefore, to provide efficient transportation of hydrogen atoms, it is necessary to use materials with the lowest loss probability of H atoms, i.e., dielectrics.