Pulsed Beam Source Research Articles

High-intensity pulsed beam source with tunable operation mode

The report presents the design of an electron and an ion pulsed accelerator. The powerful high-voltage pulse generator of the accelerator and the vacuum bushing insulator is able to change the polarity of the output voltage. The low-inductance matching transformer provides an increase in the DFL output impedance by 4 times. The generator based on a high voltage pulse transformer and a pseudo spark switch is applied for DFL charging. The high-impedance magnetically insulated focusing diode with Br magnetic field and the “passive” anode was used to realize the ion beam generation mode. The plasma is formed on the surface of the anode caused by an electrical breakdown at the voltage edge pulse; as a result, the carbon ion and proton beam is generated. This beam has the following parameters: the current density is about 400 A/cm2 (in focus): the applied voltage is up to 450 kV. The accelerator is designed for the research on the interaction of the charged particle pulsed beams with materials and for the development of technological processes of a material modification.

Plasma electron source for the generation of wide-aperture pulsed beam at forevacuum pressures

This article reports on design and application of wide-aperture pulsed beam source, based on hollow cathode discharge. The source is intended for electron beam generation in pressure range 2-15 Pa. Multi-aperture extraction system, used in a source, provided beam cross-section uniformity of 10% on diameter 40 mm. The limiting values of the current density, pulse duration, and accelerating voltage are 350 mA∕cm(2), 250 μs, and 10 kV, respectively. These parameters are sufficient for surface modification of various materials, including non-conducting matters.

Development of Supersonic Metastable Helium Pulsed Beam Source for Plasma Diagnostics

A supersonic metastable helium (2 1S) pulsed beam source, which consists of an electromagnetic valve, a collimation skimmer, and discharge electrodes for production of a plasma, has been constructed, which is essential for the direct measurement of electric fields in plasmas by means of polarization laser-induced fluorescence spectroscopy. A supersonic helium atomic beam with a short pulselength (∼300 µs), narrow divergence (∼1.1°), and high density of ∼1.4×1014 cm-3 has been achieved. In order to generate metastable atoms in this beam source, a Penning-type discharge was employed, which is suitable for producing stable plasma with high temperature, even at low gas densities, due to the particular configuration of electrodes together with a magnetic field. Spectroscopic observations indicated that the temporal behaviors of neutral atom and ion emissions were almost the same as that of the helium atom profile, and on increasing the discharge voltage applied between electrodes, the spectral intensity increased approximately linearly. Moreover, the fact that the ion emission can also be observed showed that it was probable that a high-temperature plasma was generated by the Penning discharge.

Design and characterization of late-mixing flash pyrolytic reactor molecular-beam source

We report on the design and characterization of an intense pulsed beam source for the generation of expansion-cooled radical species. The source combines both dual pulsed valve late mixing for coexpansion of reactive gases with a pyrolytic end nozzle as a flash kinetic reactor. As a demonstration of the source we present evidence of BrO and ClO radical production and subsequent photodissociation studies of these species using velocity map ion imaging. The radicals are characterized by a rotational temperature of 150±40K although substantially lower temperatures should be possible.

Rotatable source crossed molecular beams apparatus with pulsed ultraviolet/vacuum ultraviolet photoionization detection

A newly constructed universal crossed molecular beams apparatus for studies of bimolecular chemical reaction dynamics is described. The apparatus employs two rotatable molecular beam sources and a fixed ultrahigh vacuum (UHV) quadrupole mass spectrometer with electron impact or pulsed photoionization of reaction products. Electronically cold neutral supersonic transition metal atomic beams are produced in one of the rotatable sources using laser vaporization. The beams are characterized by laser induced fluorescence spectroscopy, photodepletion spectroscopy, and time-of-flight analysis. Photoionization of the ZrC2H2 products from the crossed beam reaction Zr+C2H4→ZrC2H2+H2 is carried out using the pulsed 157 nm radiation from a F2 excimer laser in the UHV region of the mass spectrometer. Compared to conventional electron impact ionization, 157 nm photoionization improves signal-to-noise ratios by more than two orders of magnitude for experiments using pulsed beam sources where cross correlation methods cannot be used. We also demonstrate the use of 1+1 resonance enhanced multiphoton ionization for state selective detection of nonreactively scattered transition metal atoms from decay of long-lived collision complexes. Due to the small reaction cross sections for Y+C2H6→YC2H4+H2 and Y+CD3CDO→DYCD3+CO, these reactions could not be studied using electron impact ionization. However, photoionization detection permitted direct studies of the reactions with excellent signal-to-noise ratios. The greatly improved sensitivity of the photoionization technique facilitates studies of transition metal systems not previously amenable to the crossed beams method.

Mathematical model for predicting the molecular intensity of a solenoid-actuated pulsed beam source

A mathematical model for evaluating the molecular intensity produced by a solenoid-actuated pulsed beam source is described. The particular source studied here is used to introduce reactant gas pulses at various user-specified intervals into a fixed-bed microreactor system for transient catalysis studies where quantification of the inlet pulse intensity is required for data analysis. The model equations which describe the beam source performance utilize relationships from one-dimensional isentropic gas expansion, gas effusion through a small hole, and macroscopic mass balances around the valve orifice from which a closed-form algebraic expression for the pulse intensity is obtained. The parameters in this expression represent observables which include the ideal gas specific heat capacity ratio, the stagnation properties of the gas in the supply chamber of the beam source, and the beam valve driver settings. Comparisons between experimental and predicted values for the pulse intensities, where the latter are of the order 1017 molecules per pulse, show excellent agreement.

Adsorption calorimetry on single crystal surfaces: a new technique

An accurate adsorption calorimeter, with good sensitivity to 0.01 monolayers on metal single crystals, has been designed. Simultaneous measurements are made, using a pulsed beam source, of differential heats of adsorption and of sticking probabilities as a function of surface coverage. Results have been obtained for CO and O2 chemisorption, oxide film formation and CO/K coadsorption on {111}, {100} and {110} surfaces of nickel.

Boltzmann equation analysis of a pulsed molecular beam under non-equilibrium conditions

Time-of-flight spectra of He and Ar beams produced with a pulsed beam source of short open time (≈50 μs) are investigated as a function of the distance from the nozzle both experimentally and theoretically. The beam is found to be under non-equilibrium conditions even at a distance of longer than 1 m. The observed decrease of the contribution from slow molecules is explained on the basis of the Boltzmann equation with the effect of collisions within the beam.

Speed ratios greater than 1000 and temperatures less than 1 mK in a pulsed He beam.

Speed ratios v/\ensuremath{\Delta}v greater than 1000 and temperatures less than 1 mK have been achieved for the first time in a gas expansion into vacuum. The pulsed-beam source used in these experiments had an open time of 150 \ensuremath{\mu}sec, a nozzle diameter of 0.125 mm, and was operated with pure He gas at 294 K and pressures between 34 and 136 atm. Speed distributions for the collimated He beam were measured by time of flight. It is possible that such temperatures may be low enough to probe the low-energy limiting behavior predicted by Wigner in 1948 for the single-collision quantum dynamics of atoms and molecules.

ChemInform Abstract: DYNAMICS OF THE CHEMILUMINESCENT OXIDATION OF ZINC ATOMS BY NITROUS OXIDE

AbstractDie Dynamik von A wurde in einer neuen Molekularstrahl‐Apparatur aus Quellenkammer (Laser‐Verdampfung von einem Zn‐Film‐Target), Reaktionskammer (mit N2O‐ Gas und Photomultiplier) und Detektorkammer (mit Quadrupol‐Massenspektrometer) untersucht.

Dynamics of the chemiluminescent oxidation of zinc atoms by nitrous oxide

The dynamics of the chemiluminescent oxidation of zinc atoms by nitrous oxide to form ZnO* have been studied utilizing a laser vaporization pulsed beam source to produce the zinc reactant. The kinetic energy dependence of the chemiluminescent reaction cross section has a threshold at 1.6×10−19 J/molecule (1.0 eV) in the center of mass; this threshold value has been used to estimate the dissociation energy of ZnO as D≥4.48×10−19±0.32×10−19 J [2.8(±0.2) eV]. The reaction cross section increases with increasing kinetic energy in accord with the Arrhenius model from threshold to 3.2×10−19 J (2.0 eV), but then increases more rapidly. Above threshold, small amounts of thermal energy added to the N2O reactant increase the reaction cross section 70 times more than the equivalent energy supplied as relative kinetic energy. This energy selectivity is interpreted in terms of an electron jump reaction mechanism which is significantly enhanced by the ν2 bending vibrational mode of N2O.

Ten-microsecond pulsed molecular beam source and a fast ionization detector

We describe a pulsed gas valve which we have developed for use as a molecular beam source. In order to observe the performance of the pulsed beam source, we also have developed an ionization detector with a rise time of about 1 micros. The pulsed valve produces very intense supersonic molecular beam pulses of about 10 micros duration for light gases such as H2 and He, and of somewhat longer duration for heavier gases. As a new tool for the study of molecular collisions, the pulsed beam technique offers substantial advantages over the conventional continuous-beam method for experiments which are limited by the signal-to background ratio for scattered products.