J. Szigeti

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.

Creation of a coherent superposition of quantum states by a single frequency-chirped short laser pulse

Traditional schemes for coherent population transfer or generation of coherent superposition states in multilevel atoms or molecules usually utilize two or more laser beams with radiation bandwidth smaller than the frequency interval between the working levels. We show the possibility of creation of the coherent superposition of three metastable states of a four-level atom with tripodlike level structure using a single short frequency-chirped laser pulse. The bandwidth of the pulse envelope (without chirp) must be comparable to or exceed the frequency distance between the two metastable levels. No appreciable excitation of the atom takes place during the creation of the coherent superposition state, thus diminishing significantly the effect of decoherence due to the spontaneous decay of the excited state. The proposed method of creation of superposition states is robust against variations in the laser pulse parameters. Since this method does not require maintaining steady resonance with the atomic transitions (owing to the frequency chirp of the laser pulse), it is effective both in homogeneously and inhomogeneously broadened media.

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.

Stabilization and time resolved measurement of the frequency evolution of a modulated diode laser for chirped pulse generation

We have developed experimental methods for the generation of chirped laser pulses of controlled frequency evolution in the nanosecond pulse length range for coherent atomic interaction studies. The pulses are sliced from the radiation of a cw external cavity diode laser while its drive current, and consequently its frequency, are sinusoidally modulated. By the proper choice of the modulation parameters, as well as of the timing of pulse slicing, we can produce a wide variety of frequency sweep ranges during the pulse. In order to obtain the required frequency chirp, we need to stabilize the center frequency of the modulated laser and to measure the resulting frequency evolution with appropriate temporal resolution. These tasks have been solved by creating a beat signal with a reference laser locked to an atomic transition frequency. The beat signal is then analyzed, as well as its spectral sideband peaks are fed back to the electronics of the frequency stabilization of the modulated laser. This method is simple and it has the possibility for high speed frequency sweep with narrow bandwidth that is appropriate, for example, for selective manipulation of atomic states in a magneto-optical trap.

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.

Measurement of the vibrational energy-transfer rates in mixtures of polyatomic molecules

The pressure coefficient of the vibrational deexcitation of methanol molecules caused by collisions with NH3, CO2, SF6 molecules and He atoms is measured by infrared-far-infrared double-resonance spectroscopy. Large coefficients are measured for those buffer gases which increase the output power of far-infrared lasers. For other buffer gases such as NH3 and CO2 the measured coefficients are zero within the limits of experimental error. The relevance to measurements on the photoacoustic trace-gas analysis is discussed.