High-speed Chopper Research Articles

Development and characterization of a narrow-pulsed molecular beam system

A narrow-pulsed and velocity-controlled molecular beam system is constructed by using a high-speed chopper and a temperature adjustable pulsed valve. The duration of the hydrogen molecular beam pulse is reduced to approximately 6.3 µs and characterized using resonance-enhanced multiphoton ionization (REMPI) at a distance of ~193 mm downstream from the nozzle. To precisely determine the velocity of the hydrogen molecules, a pump-probe technique combining stimulated Raman pumping and REMPI is employed at a fixed distance (193 mm) with nanosecond lasers. By adjusting the temperature of the pulsed valve, the velocity of the hydrogen beam varies continuously from 1290 m/s to 3550 m/s. The system exhibits potential for multiple applications in the field of chemical reaction dynamics, including its potential to integrate with ion traps and surface scattering experiments.

A versatile molecular beam apparatus for cold/ultracold collisions

We have developed an apparatus capable of performing intrabeam and near-copropagating beam scattering experiments at collision energies from room temperature to below 1 K where interesting quantum phenomena can be observed. A detailed description of the major components of the apparatus, single and dual molecular beam valves, high speed chopper, and the discharge source, is presented. With the intrabeam scattering setup, a novel dual-slit chopper permits collision energies down to millikelvins with a collision energy spread of 20%. With the near-copropagating beam configuration, state-to-state differential cross sections for rotationally inelastic collisions of highly vibrationally excited NO molecules with Ar have been measured at broadly tunable energies documenting the versatility of the instrument. Future applications in stereodynamics and cold state-to-state collisions of vibrationally excited polyatomic molecules are briefly discussed.

DEM による水平型高速撹拌混合機に内蔵される超高速チョッパーの解析

DEM による水平型高速撹拌混合機に内蔵される超高速チョッパーの解析

100-kHz shot-to-shot broadband data acquisition for high-repetition-rate pump-probe spectroscopy.

Shot-to-shot broadband detection is common in ultrafast pump-probe spectroscopy. Taking advantage of the intensity correlation of subsequent laser pulses improves the signal-to-noise ratio. Finite data readout times of CCD chips in the employed spectrometer and the maximum available speed of mechanical pump-beam choppers typically limit this approach to lasers with repetition rates of a few kHz. For high-repetition (≥ 100 kHz) systems, one typically averages over a larger number of laser shots leading to inferior signal-to-noise ratios or longer measurement times. Here we demonstrate broadband shot-to-shot detection in transient absorption spectroscopy with a 100-kHz femtosecond laser system. This is made possible using a home-built high-speed chopper with external laser synchronization and a fast CCD line camera. Shot-to-shot detection can reduce the data acquisition time by two orders of magnitude compared to few-kHz lasers while keeping the same signal-to-noise ratio.

Dry Surface Modification of Adhesive Fine Particle by a Horizontal Shear Mixer with Super High Speed Chopper

Dry Surface Modification of Adhesive Fine Particle by a Horizontal Shear Mixer with Super High Speed Chopper

Development of a Novel Shear Mixer with Super High Speed Chopper and its Performance of Fine Particle Dispersion

Development of a Novel Shear Mixer with Super High Speed Chopper and its Performance of Fine Particle Dispersion

A Neutron Diffractometer for a Long Pulsed Neutron Source

Long pulsed neutron sources are being actively developed as small university based sources and are being considered for the next generation of high powered sources, such as the European Neutron Source (ESS) and the Spallation Neutron Source (SNS) second target station. New instrumentation concepts will be required to effectively utilize the full spectrum of neutrons generated by these sources. Neutron diffractometers, which utilize time-of-flight (TOF) techniques for wavelength resolution, are particularly problematic. We describe an instrument for a long pulsed source that provides resolution comparable to that obtained on short pulsed sources without the need of long incident flight paths. We accomplish this by utilizing high speed choppers to impose a time structure on the spectrum of incident neutrons. By strategically positioning these choppers the response matrix assumes a convenient form that can be deconvoluted from the measured TOF spectrum to produce the diffraction pattern of the sample. We compare the performance of this instrument to other possible diffraction instruments that could be utilized on a long pulsed source.

BioCARS: a synchrotron resource for time-resolved X-ray science

BioCARS, a NIH-supported national user facility for macromolecular time-resolved X-ray crystallography at the Advanced Photon Source (APS), has recently completed commissioning of an upgraded undulator-based beamline optimized for single-shot laser-pump X-ray-probe measurements with time resolution as short as 100 ps. The source consists of two in-line undulators with periods of 23 and 27 mm that together provide high-flux pink-beam capability at 12 keV as well as first-harmonic coverage from 6.8 to 19 keV. A high-heat-load chopper reduces the average power load on downstream components, thereby preserving the surface figure of a Kirkpatrick-Baez mirror system capable of focusing the X-ray beam to a spot size of 90 µm horizontal by 20 µm vertical. A high-speed chopper isolates single X-ray pulses at 1 kHz in both hybrid and 24-bunch modes of the APS storage ring. In hybrid mode each isolated X-ray pulse delivers up to ~4 × 10(10) photons to the sample, thereby achieving a time-averaged flux approaching that of fourth-generation X-FEL sources. A new high-power picosecond laser system delivers pulses tunable over the wavelength range 450-2000 nm. These pulses are synchronized to the storage-ring RF clock with long-term stability better than 10 ps RMS. Monochromatic experimental capability with Biosafety Level 3 certification has been retained.

Energy Dependence of Hyperthermal Oxygen Atom Erosion of a Fluorocarbon Polymer: Relevance to Space Environmental Effects

The origin of the inconsistency in the erosion phenomena of fluorocarbon polymers between a ground-based atomic-oxygen test environment and the low Earth orbital space environment has been investigated. A detailed experiment was performed in order to study the effect of the high-energy component in the atomic-oxygen beam pulses on the erosion. The experiment was carried out with the combination of a polymer-coated quartz crystal microbalance and a high-speed chopper wheel installed in a laser-detonation atomic-oxygen facility. It was clearly observed that the mass-loss rates of both polyimide and fluorocarbon polymers depend on the impact energy of atomic oxygen. However, the energy dependence is more significant on the fluorocarbon polymer compared with that on polyimide. Collisions of atomic oxygen with translational energies higher than ∼5 eV induce a significant mass loss on the fluorocarbon polymer. These experimental findings agree with earlier measurements of the volatile products released during h...

Effect of Ultraviolet Radiation from an Oxygen Plasma on the Atomic Oxygen-induced Etching of Fluorinated Polymer

The contribution of extreme ultraviolet (EUV) and vacuum ultraviolet (VUV) from a laser-sustained plasma on mass loss phenomenon of fluorinated polymer in the ground-based laser-detonation atomic oxygen (AO) beam source was evaluated. The AO beam and EUV/VUV from an oxygen plasma were separated by a high-speed chopper wheel installed in the beam source. Mass changes of fluorinated polymer and polyimide were measured from the frequency shift of the quartz crystal microbalances during the beam exposures. It has been made clear that the fluorinated polymer is eroded by EUV/VUV exposure alone. In contrast, no erosion was detected for polyimide by EUV/VUV alone. The AO-induced erosion was measured for both materials even without EUV/VUV exposure. However, no strong synergistic effect was observed for the fluorinated polymer even under the simultaneous exposure condition of AO and EUV/VUV. Similar results were observed even in the simultaneous exposure condition of AO (without EUV/VUV from the laser plasma) and VUV from the 172 nm excimer lamp and D2 lamp. These experimental results suggest that the primary origin of the accelerated erosion of fluorinated polymer observed in the laser detonation AO source is not EUV/VUV from the laser-sustained plasma.

Chopper system for time resolved experiments with synchrotron radiation

A chopper system for time resolved pump-probe experiments with x-ray beams from a synchrotron is described. The system has three parts: a water-cooled heatload chopper, a high-speed chopper, and a millisecond shutter. The chopper system, which is installed in beamline ID09B at the European Synchrotron Radiation Facility, provides short x-ray pulses for pump-probe experiments with ultrafast lasers. The chopper system can produce x-ray pulses as short as 200 ns in a continuous beam and repeat at frequencies from 0 to 3 kHz. For bunch filling patterns of the synchrotron with pulse separations greater than 100 ns, the high-speed chopper can isolate single 100 ps x-ray pulses that are used for the highest time resolution. A new rotor in the high-speed chopper is presented with a single pulse (100 ps) and long pulse (10 micros) option. In white beam experiments, the heatload of the (noncooled) high-speed chopper is lowered by a heatload chopper, which absorbs 95% of the incoming power without affecting the pulses selected by the high speed chopper.

The high-resolution time-of-flight spectrometer TOFTOF

The TOFTOF spectrometer is a multi-disc chopper time-of-flight spectrometer for cold neutrons at the research neutron source Heinz Maier-Leibnitz (FRM II). After five reactor cycles of routine operation the characteristics of the instrument are reported in this article. The spectrometer features an excellent signal to background ratio due to its remote position in the neutron guide hall, an elaborated shielding concept and an s-shaped curved primary neutron guide which acts i.a. as a neutron velocity filter. The spectrometer is fed with neutrons from the undermoderated cold neutron source of the FRM II leading to a total neutron flux of ∼ 10 10 n / cm 2 / s in the continuous white beam at the sample position distributed over a continuous and particularly broad wavelength spectrum. A high energy resolution is achieved by the use of high speed chopper discs made of carbon-fiber-reinforced plastic. In the combination of intensity, resolution and signal to background ratio the spectrometer offers new scientific prospects in the fields of inelastic and quasielastic neutron scattering.

RefloGrad/GradTOF: Neutron energy analysis for a very high-flux neutron reflectometer

The possibility of analyzing the neutron energy coupled with a position-sensitive detector enables to perform specular reflectivity measurements on continuous neutron sources without any monochromator or chopper and thus permits intensity gains up to 20 compared to conventional reflectometers equipped with a monochromator or a chopper. In this communication, we present a spectrometer design using an energy analysis performed with a permanent magnet Stern–Gerlach device. We also propose a modified version of the spectrometer in which we combine the above setup with a high-speed chopper in order to perform classical TOF measurements (at a high repetition rate, up to 200 Hz) while still being able to use up to 30% of the full white spectrum available in the neutron guide.

Location of frame overlap choppers on pulsed source instruments

A detailed study has been performed to investigate the effect of frame overlap in a cold neutron chopper spectrometer. The basic spectrometer is defined by two high-speed choppers, one near the moderator to shape the pulse from the moderator, and one near the sample to define energy resolution. Using ray-tracing timing diagrams, we have observed that there are regions along the guide where the trajectories of neutrons with different velocities converge temporally at characteristic points along the spectrometer. At these points of convergence, a frame overlap chopper would be totally ineffective, allowing neutrons of all velocities to pass through. Conversely, at points where trajectories of different velocity neutrons are divergent, a frame overlap chopper is most effective. An analytical model to describe this behaviour has been developed, and leads us to the counterintuitive conclusion that the optimum position for a frame overlap chopper is as close to the initial chopper as possible. We further demonstrate that detailed Monte Carlo simulations produce results which are consistent with this model.

An absolute measurement of optical reflectivity by modification of a WMS system

The absolute optical reflectivity of a solid sample can be measured by some modification of a wavelength modulating spectrometer (WMS) system. The arrangement of a feedback loop applied to the high voltage source of the photomultiplier tube in conjunction with a sample-and-hold circuit provides a reflectivity measurement free of the strong characteristic lines inherent to an arc lamp. Meanwhile, the application of a high speed chopper greatly increases the signal-to-noise ratio in both the absolute and derivative measurements.

The Scattering of Thermal Neutrons by Moderators

A comprehensive scheme for the analysis and use of thermal neutron scattering data is described. Experimental work was carrie out with a 4-rotor high speed chopper system operated in conjunction wlth a multicounter multlchannel neutron time-of-flight system. The results are reduced to a scattering law, which is a function S of two variables ( alpha and beta ) representing the momentum and energy transferred in the scattering process. The scattering law can be divided into interference and self-terms; for the application to neutron spectrum calculations it is sufficient to consider the lnterference term as a small correction to - the self-term. The self-term is the double Fourier transformation of the self-correlation function of Van Hove, which represents the motlon of an lndividual atom in the system. This function can be represented as a gaussian distribution of the atomic position plus correction terms. The width of this gaussian distribution is a function of time and its double differential with respect to time is the velocity correlation function for an atom in the system. Thus the velocity correlation function is the function which makes the major contribution to the scatterlng cross sections. In estimating the accuracy to which measurements, analysis, and calculationsmore » of the scattering law need be made, it is necessary to evaluate a sensitivity function for each neutron spectrum problem, and thus for each scattering law a variety of sensitivity functions need to be evaluated. Two slmple examples are discussed which show, separately, the effect of absorption and temperature gradients. (auth)« less