S. G. Kalmykov

Computational optimization analysis of a gas-jet target in a laser-plasma short-wave radiation source

A computational method is developed for optimizing the xenon gas jet used as the target in a laserplasma short-wave radiation source. The method is based on numerical hydrodynamic simulation of the jet flowing from the nozzle into vacuum, followed by computation of the optimization criterion describing the observed intensity of plasma glow. The application of this method permits an unambiguous and objective choice of optical experimental geometries and flow conditions; as a result, the radiation yield can be increased by several times. The calculated results are compared with available experimental data.

Shock waves in gas-jet target of a laser-produced-plasma short-wave-radiation source with two-pulse plasma excitation

Previous investigations of the laser plasma at two-pulse mode of its generation revealed long-living perturbations of the gas-jet target by the first pulse, leading to significant modulations of the plasma radiation. In the present paper, results of a numerical hydrodynamic simulation of the gas-jet target are reported which provide explanation of the observed phenomena. An impact of the first pulse (prepulse) upon the target results in formation of a dense quasi-spherical layer in it, with a low-density area inside. This layer expands with the time and drifts downstream with the gas flow. Depending on the time interval between pulses, the second laser pulse can either intersect the dense layer or pass through the low-density gas, whereby the observed modulations of the plasma emission can be explained.

The effect of a UV preionization pulse on short-wave radiation output from a laser-produced-plasma source with a Xe gas-jet target

Experiments aimed to raise the emissivity of a laser-produced plasma source with a Xe gas target in the far-UV spectral range are described. In these experiments, the main pulse of the IR Nd:YAG laser was preceded by a pre-ionization pulse of a UV KrF excimer laser. The consequences of applying the prepulse and its influence on the short-wavelength emission intensity were traced up to main-pulse delays of about 5 μs with respect to the prepulse. It is supposed that the main mechanism by which the prepulse affects the evolution of the plasma and its emission intensity is related to the density waves excited in the gas target by this pulse.

Laser spark propagation along the beam in stationary gases at low pressures

A study of space-time evolution of the laser spark induced with the Nd: YAG laser in stationary gases at low pressures has been realized with the aid of high-speed photography. A mechanism to explain light propagation along the beam is proposed. Calculated data are in a good agreement with the experimental ones. The article was translated by the authors.

The role of multiphoton ionization in the generation and heating of laser plasma

It is established that purely collisional mechanisms are insufficient and recourse to multiphoton ionization (MPI) mechanism is necessary, to explain the initial stage of laser plasma generation in a gas target. Based on simple assumptions, a numerical estimate from below is obtained for the MPI rate, which shows that this process is highly effective in the case of short-wavelength laser radiation of moderate intensity. It is suggested to use a combination of short-and long-wavelength lasers for increasing the efficiency of laser plasma as a source of extreme UV emission.

Start-phase ionization dynamics in the laser plasma at low gas target densities

In Xe laser-produced-plasma sources of short-wave radiation, the laser-energy-to-EUV conversion efficiency (CE) proves substantially less than theoretical expectations. In the present work, a calculated estimate has been made which indicates that a long period of the primary ionization, lasting up to a moment when high-Z ions appear to emit short-wave photons, can be one of main causes for this. During that period the plasma remains low-ionized and absorbs weakly the laser energy. The estimate above has been experimentally confirmed with spectroscopic data and those on the effective ion charge derived from measured absorption of the laser radiation in the plasma. A preionization of the gas target with an ultraviolet (UV) excimer laser pulse is proposed as a method to accelerate the ionization process and consequently, to enhance CE.

Ionization dynamics in the laser plasma in a low pressure gas target

In Xe-laser-plasma short-wave-radiation sources, the laser-energy-to-EUV conversion efficiency (CE) turns out to be substantially lower than theoretical expectations. An estimation made in the present work is evidence of what a long period of the primary ionization, lasting up to a moment when high-Z ions appear to emit short-wave photons, can be considered as a main cause for the low CE values. During that period the plasma remains low-ionized and absorbs weakly the laser energy. Data deduced from laser light absorption measurements confirm the estimation above. A preionization of the gas target with the UV excimer laser pulse is proposed as a method to accelerate the ionization process.

Study of the structure and parameters of a KrF excimer laser beam

For a KrF excimer laser with a confocal unstable resonator, energy distribution over the beam cross-section was measured with the aid of luminescence in the glass induced by the UV laser radiation. A complex fine structure of the beam has been found. In an area close to the focus, measurements were carried out by means of two different methods—via thermal paper blackening and via ablation craters on a glass surface. The complex beam structure above has been shown to persist also in the immediate vicinity of the beam waist. Measured cross-sectional waist dimensions varied significantly within a range from 0.3 to 0.6–1.3 mrad depending on the measuring procedure applied. Using a short-focus nonaberrational lens one could obtain a focal spot with linear dimensions of several tens of micrometers.

A study of the laser plasma in stationary gases at low pressures

Stretched along the laser beam and symmetrical relative to the focus forms of the laser spark induced with the Nd:YAG laser have been observed in stationary gases at low pressures. The spark length rose as the gas pressure increased amounting up to several millimeters. Based on shock waves, conventional mechanism of laser plasma propagation along the beam has been placed in question. Plasma temperature has been deduced from laser energy absorption in the plasma.

Computational simulation of laser plasma emission with shock-wave-affected density distribution in the gas-jet target

Based on the results of numerical fluid dynamics simulation, an imitation parameter has been constructed which simulates the observed intensity of the laser plasma emission in a short-wave range. Within the computational model frame, a high-temperature perturbation is created in the jet that generates a strong shock wave. The resultant complicated target structure and its evolution lead to nonmonotonic time variations of the simulation parameter. This result agrees well with the experimentally measured behavior of emission from the laser plasma formed on the target perturbed by an additional laser prepulse.