Z. Juhász

Guided transmission of slow Ne6+ ions through the nanochannels of highly ordered anodic alumina

A highly ordered hexagonally close-packed nanochannels array was prepared using the self-ordering phenomena during a two-step anodization process of a high purity aluminium foil. The anodized aluminium oxide, with pore diameters of about 280 nm and interpore distances of about 450 nm was prepared as a suspended membrane of about 15 mu m thickness on the aluminium frame to which it belongs. The Al2O3 capillaries were bombarded with 3 keV Ne6+ ions. The first results unambiguously show the existence of ion guiding observed at 5 degrees and 7.5 degrees tilt angles of the capillaries compared to the beam direction. To the best of our knowledge, such ion guiding effects of slow ions through hexagonally ordered nanochannels in alumina has not been reported previously.

State-selective electron-capture cross section measurements for low-energy collisions of He-like ions on H2

We measured state-selective absolute electron capture cross sections for collisions of He-like C4+, N5+ and O6+ ions with molecular hydrogen by means of photon emission spectroscopy at impact energies ranging from 4 keV amu(-1) down to energies as low as 5 eV amu(-1). In this low-energy region the total cross sections are found to be weakly dependent on the projectile energy, whereas the state-selective ones change drastically with varying impact energy. Up to now no experiments were available to benchmark theoretical predictions. In our method we combine for the first time decelerated projectile beams with the possibility of measuring state-selective electron capture. Compared with existing measurements for state-selective cross sections our deduced cross sections are in perfect agreement in the overlapping energy region (E much greater than 100 eV amu(-1)). Good agreement is also found on comparing our results with existing total cross sections. In comparison with theoretical calculations some remarkable and unexpectedly large discrepancies of orders of magnitude are found towards lower impact energies.

Nanochannel alignment analysis by scanning transmission ion microscopy

In this paper a study on the ion transmission ratio of a nanoporous alumina sample is presented. The sample was investigated by scanning transmission ion microscopy (STIM) with different beam sizes. The hexagonally close-packed Al(2)O(3) nanocapillary array, realized as a suspended membrane of 15 microm thickness, had pore diameters of approximately 215 nm and spacing of approximately 450 nm. When the proton beam size was limited to a single domain, a peak transmission ratio of 19% was observed as is expected from the geometry (approximately 19-20%). This result points out an almost perfectly parallel alignment of the capillaries within one domain. However, for larger beam scanning areas (sampling multiple domains) the transmission ratio was reduced to 5%. The STIM analysis over an area larger than the typical domain size revealed an overall capillary angular spread of approximately 2 degrees.

Fluctuations of melt temperature and growth rate during Bi4Ge3O12 growth

Abstract An experimental method is described to determine the transfer function and the spectral sensitivity relating the growth rate, i.e. the crucible weight, to the melt temperature during Czochralski growth. When investigating the transient behaviour of the growth rate, a significant difference was found between the growth processes at pulling rates of 2 mm/h and 0. The results of the spectral analysis are in good agreement with theoretical predictions suggesting decreasing sensitivity of the weight signal at increasing frequency of the temperature oscillation. The results prove that Fourier analysis of the crucible (or crystal) weight is a sensitive method for monitoring oscillations of the growth rate during Czochralski growth.

Real-time modelling and adaptive control of Czochralski growth

A controller based on experimental real-time modelling of the system was developed for Czochralski growth of Bi4Ge3O12 and LiTaO3 crystals. The dynamic behaviour of the growth process was characterized by a discrete-time linear ARMA (2, 3) model which relates the heating power to the crucible weight error. Different types of theoretically predicted behaviour were observed in our experiments. The pole and zero maps of the ARMA models show that unstable and oscillating behaviour may occur indicating different states of the system. The interpretation of the models in the time domain makes the effect of the meniscus force visible which has a particular importance during necking. The spectral sensitivity of the models can also be determined which is useful when adjusting the control parameters.

Experimental model of magnetic Czochralski growth

Abstract An experimental model of magnetic Czochralski growth has been built to investigate the hydrodynamic behaviour of the melt. The time dependence of temperature in molten gallium was measured and analyzed as a function of heating and magnetic field. The temperature fluctuations were damped when the magnetic field exceeded a threshold value dependent on heating temperature and rotation rate.

Variations of the transfer function during Bi4Ge3O12 growth

Abstract A technique based on the estimation of ARMA model parameters was developed for investigating the transfer function relating heating power to crucible weight signal for Czochralski growth. Dependence of the transfer function on crystal diameter, length and pulling rate in different phases of the growth process was investigated. Significant variations in the transfer function were observed during a normal BGO pulling process. The anomalous behaviour at large crystal cone angles is in good qualitative agreement with theoretical results.

Molecular and negative ion production by a standard electron cyclotron resonance ion source

Molecular and negative ion beams, usually produced in special ion sources, play an increasingly important role in fundamental and applied atomic physics. The ATOMKI-ECRIS is a standard ECR ion source, designed to provide highly charged ion (HCI) plasmas and beams. In the present work, H(-), O(-), OH(-), O(2)(-), C(-), C(60)(-) negative ions and H(2)(+), H(3)(+), OH(+), H(2)O(+), H(3)O(+), O(2)(+) positive molecular ions were generated in this HCI-ECRIS. Without any major modification in the source and without any commonly applied tricks (such as usage of cesium or magnetic filter), negative ion beams of several μA and positive molecular ion beams in the mA range were successfully obtained.

Dynamics of ion guiding through nanocapillaries in insulating polymers

We review recent studies of dynamic properties concerning the ion guiding through nanocapillaries etched in polyethylene terephthalate (PET) and polycarbonate (PC). Typical lengths of the capillaries were 10 μm with diameters ranging from ~100 - 400 nm. The temporal evolution of the intensity and the angular distribution of the transmitted ions were studied by measuring transmission profiles as a function of the charge inserted into the capillaries. Tilt angles of the capillaries axis with respect to the incident beam direction were 3° and 5°. The mean emission angle of the transmission profiles exhibit pronounced oscillatory structures both for PET and PC. However, for PC nearly an order of magnitude more charge is required to induce the oscillations. In contrast to PET, with capillaries in PC we observed a strong decrease of the profile intensities with irradiation time. This observation provides evidence for blocking effects on the ion transmission. The experimental results are interpreted by simulations of the ion trajectories guided in 3 dimensions by the electrostatic field within the capillaries. This field was determined from the charge deposited at the walls of the capillaries taking into account the removal of the charges by means of a non-linear conductivity law.

Enhanced fluctuations in driven lattice gases

Monte Carlo simulations are performed to study the enhanced density fluctuations in a square lattice gas with repulsive first and second neighbour interactions which is driven far from equilibrium by an external field. Regions with excess particles (and holes) travel (in opposite directions) with a velocity dependent on the concentration. In this steady state the power spectra of current-current correlation functions are proportional to ω−13 at low frequencies. This behaviour may be considered as a consequence of the peculiar concentration dependence of conductivity which has a sharp minimum related to the short range ordering.