P.N. Ignácz

Absolute measurement of velocity distribution of neutrals in sodium laser blow-off beam

Abstract Absolute density measurements of Na blow-off atomic beam were carried out with time of flight laser induced fluorescence. The preferentially used 1 μm Na layer was in situ evaporated on a glass plate. The velocity distribution of the blow-off neutrals was investigated as a function of the Nd laser fluence. The measurements showed that the blow-off beam contains two atomic packets with different propagation velocities in the laser range of 10–20 J/cm 2 and one atomic packet elsewhere. The faster packet has a maximum total number of atoms at 3 J/cm 2 and its velocity can be well fitted with a power function of the irradiance with v ∼ I 0.29 .

Laser blow‐off plasma propagating in low‐pressure gas

The change of the properties of the plasma ball created in the laser blow‐off process is investigated in interaction with residual gas and buffer gas particles during its flight in space. The main process of the interaction is the collision leading to plasma particle loss. The center of mass velocity and the temperature of the ball weakly depend on the buffer gas density because the plasma is mainly collisionless.

Self-focusing of laser beam crossing a laser plasma

Abstract A crossed beam experiment was performed to clarify the mechanism of self-focusing in a laser produced spark. The plasma was created by one beam and self-focusing was observed in the weak probe beam which crossed the plasma. Experimental results show that the cause of self-focusing is the nonuniform heating mechanism.

Pre-excitation studies for rubidium-plasma generation

The key element in the Proton-Driven-Plasma-Wake-Field-Accelerator (PWFA) project is the generation of highly uniform plasma from Rubidium vapor. A scientifically straightforward, yet highly challenging way to achieve full ionization is to use high power laser which can assure the barrier suppression ionization (BSI) along the 10 m long active region. The Wigner-team in Budapest is investigating an alternative way of uniform plasma generation. The proposed Resonance Enhanced Multi-Photon Ionization (REMPI) scheme can be probably realized by much less laser power. In the following we plan to investigate the resonant pre-excitations of the Rb atoms, both theoretically and experimentally. In the following our theoretical framework is presented together with the status report about the preparatory work of the planned experiment.

Light scattering and self-focusing in laser produced sparks

Abstract The spectral and temporal behaviour of back-and forward-scattered laser light on laser produced spark are investigated. Short backscattered pulses from the plasma fronts appears just after the breakdown. Self-focusing caused by nonlinear heating begins when the plasma filament is already formed.

Interaction of sodium laser blow-off beam with resonant laser radiation

Abstract Sodium atomic beams produced by laser blow-off technique were investigated by counterpropagating resonant laser radiation. The light scattered perpendicularly to the beam and the produced ions were detected. The observed light signal could be decomposed into two fundamental constituents. One of them, following the incident pulse shape with no wavelength dependence, stemmed from the nonresonant scattering of light on the particles of the beam. The other constituent showed exponential decay with wavelength- dependent amplitude and with lifetime considerably longer than that of the excited state. This originated from the imprisonment of resonant radiation. From the observed lifetime the density of the ground state sodium atoms has been estimated. Multiphoton ionization of the neutral atoms of the beam was also investigated. The observed ion yield/laser intensity dependence is in good agreement with the theoretical predictions for three-step multiphoton ionization.

Role of power broadening in influencing maximum gain of far infrared gain material

The gain of methanol vapor is investigated at various pumping power densities. The behavior of the gain can be described by the four-level rate equation model if the power broadening of the transitions is taken into account phenomenologically. This phenomenologically modified rate equation model reflects the most important features of the density matrix calculations, and it also takes into account the vibrational relaxation, which is not considered in the three-level density matrix calculations. In such a way the pressure behavior of the far-infrared laser is explained by the power broadening of the gain in contrast to the usual explanation relating to the diffusion of the laser molecules. >

Coherent interaction of frequency-modulated laser pulses with Rb atoms

We investigate the behavior of Rb85 atoms in the field of a sequence of frequency-chirped short laser pulses for coherent manipulation of the Rb atomic beam. The analysis is based on numerical solution of equations for the density matrix elements of the hyperfine levels of the 5S1/2-5P3/2 transition in Rb85 atom. We analyze two different regimes of interaction including relatively short laser pulses (when the width of the pulse envelope spectrum is of order or exceeds frequency interval between the hyperfine levels resulting in effective mixing of them) and relatively long ones (when the ground hyperfine levels are resolved but the excited ones are not resolved). We show that in all regimes considered, the interaction of a frequency-chirped laser pulse with the multilevel Rb85 system is similar to the interaction with an effective two-level atom at sufficiently large peak intensities of the pulses. It allows us to perform efficient excitation of the multilevel atom by transferring populations of two hyperfine ground states to the excited ones and back to the ground states using a pair of frequency-chirped laser pulses. We present experimental results of utilizing this scheme of population transfer for coherent manipulation of an ensemble of Rb85 atoms in a magneto-optical trap.

Laser ablation/ionization mass spectrometry on tokamak deposition probes

Laser ablation-ionization mass spectrometric surface analysis was applied in plasma diagnostic experiments on the MT-1M tokamak. Non-intrinsic impurities were injected into the discharges using laser blow-off in order to study the impurity transport processes. High purity silicon samples were used as deposition probes in the edge plasma of the tokamak. The distributions of the impurities deposited on the probe surfaces during the discharges were measured after the exposures by laser ionization mass spectrometry. In the first experiments with sodium impurities the surface distributions showed a considerable poloidal dependence besides the well known decrease with increasing minor radius. The poloidal dependence showed mirror-like symmetry on the probes facing the ion and electron sides. In the second experimental series carried out with tungsten impurities even more localized deposit distributions were obtained. The results are explained assuming that the impurities propagate in confined packets along the magnetic field lines even for several toroidal turns.

Tokamak deposition probe analysis by laser ionization spectroscopy

Laser analytical methods are used for the determination of surface sodium impurity distribution on silicon samples. In the first experiments, the spots on the surface of the sample were atomized by ruby laser pulses and the sodium atoms were selectively ionized by resonant ionization, thus the simple ion detection yielded the sodium surface contamination density. The second method applies excimer laser ablation, and the generated ions are analyzed and detected by a reflectron-type time-of-flight mass spectrometer. The analytical arrangements are applied for tokamak plasma investigations. Silicon plates as deposition probes located in the edge plasma region are exposed to several tokamak discharges, which are contaminated by pulsed sodium impurity injections. Surface analysis of the samples enables determining the sodium impurity fluxes in the tokamak edge plasma region, which are characteristic of particle transport properties. The high sensitivity of the method applying resonance ionization enables the analysis of single discharges, whereas the application of mass spectrometry offers the possibility of detecting several impurity materials.