Chopper System Research Articles

D33—a third small-angle neutron scattering instrument at the Institut Laue Langevin

D33 will be a third small-angle neutron scattering (SANS) instrument at the Institut Laue Langevin (ILL). Modern trends in materials science, physics and in particular nano-structured materials require that D33 should provide both high resolution and a wide dynamic range of measured scattering vector, q. In ‘monochromatic’ mode a velocity selector and flexible system of inter-collimation apertures will define the neutron beam. A double chopper system will enable a ‘time-of-flight’ (TOF) mode of operation allowing an enhanced dynamic q-range (qmax/qmin) and flexible wavelength resolution. Two large multitube detectors will extend the dynamic q-range further, giving qmax/qmin ∼ 20 in the monochromatic mode and massive qmax/qmin > 1000 in the TOF mode. Beam polarization and 3He spin analysis will facilitate and expand studies of magnetism and allow a more quantitative analysis of spin incoherent samples. The position of D33 will be such as to allow high magnetic fields at the sample position.

CO 2 laser beam modulating for surface texturing machining

Laser texturing is a novel technique that may be used to texture a cold roller in the process of manufacturing high quality steel sheets. With the aim of improving the quality of the textured roller by using a CO 2 laser, a new laser beam modulating device is proposed. An optical beam expander with a fast rotating chopper system is designed. The laser pulse is split into two parts by the chopper blades; one is the preheating pulse that is reflected onto optical loop mirrors; the other is the directly transmitted pulse that creates the craters at the preheated spots. The preheating beam focus spot size and position can be adjusted. The focusing characteristics and optical parameter compensation for the flying optics are investigated. The heat transfer and melt process of laser texturing are numerically simulated. The effects of the double pulses on the texturing are analyzed. The effect of preheating the sample ahead of the laser texturing pulse is examined. The surface profile and bump hardness show improvements by using this approach.

Analysis and prevention of corrosion in T o chopper systems

T o neutron choppers at Oak Ridge National Laboratory's Spallation Neutron Source will stop prompt pulses of high-energy neutrons outside the target moderator. Choppers are cooled internally by recirculated distilled water maintained at 15 °C. Numerous metallic components are used to fabricate the choppers and cooling system; water contacts many of these components, establishing a corrosion cell. T o chopper system corrosion products were investigated analytically. Results showed elevated concentrations of soluble and precipitated iron and copper in cooling water. A chemical-based corrosion strategy was implemented in a T o chopper system during mechanical tests not involving system irradiation. Chemical analyses showed that a chemical corrosion inhibitor stopped system corrosion for 6 months after its introduction. Corrosion cessation was beneficial, as mechanical tests were performed without interruption from precipitate wear on mechanical parts, flowmeter blockage, or a decrease in heat transfer capabilities. Aqueous chemical changes occurring in the cooling system upon inhibitor addition are discussed.

Generation of electron bunches at low repetition rates using a beat-frequency technique

Even at a continuous wave facility such as CEBAF at Jefferson Lab, an electron beam with long time intervals (tens of ns) between individual bunches can be useful, for example to isolate sources of background via time of flight detection or to measure the energy of neutral particles that cannot be separated with a magnetic field. This paper describes a demonstrated method to quickly and easily deliver bunches with repetition rates of 20 to 100 MHz corresponding to time intervals between 10 to 50 ns (respectively). This is accomplished by changing the ON/OFF frequency of the RF-pulsed drive laser by a small amount (f/f < 20%), resulting in a bunch frequency equal to the beat frequency between the radio frequencies of the drive laser and the photoinjector chopper system.

Correction of k0-factors for non-1/v elements based on neutron spectrum measurements

Abstract Neutron spectra were measured by the time of flight method using a rotating disk chopper system in the JRR-3M prompt gamma ray analysis system. Effective g-factors for non-1/v elements such as Cd, Sm and Gd were calculated from the measured neutron spectra and the neutron cross section data, and they was used for correcting the k 0-factors. The resulting k 0-factors for the cold and thermal neutron beams agreed within 10%. Furthermore, the prompt gamma ray emission probabilities of Cd, Sm and Gd were derived from the corrected k 0-factors using other relevant physical constants.

Verification Tests of Energy Storage System for DC Electrified Railways using EDLC

Power-source stabilization methods such as measures against voltage drop, power loading fluctuation and regenerative power lapse represent important issues in DC railway-feeding circuits. Providing an energy storage medium that uses power efficiently and reduces the problems mentioned above is therefore a major concern. The development of such a medium has attracted public interest, with several energy storage applications (including batteries and flywheels) already having been investigated. A wide range of large-scale electric double-layer capacitors [EDLCs] that are rapidly charged and discharged have been developed, and offer durability, maintenance-free features, low-pollution and high efficiency. For their fundamental study, we produced a prototype DC electrified railway energy storage system consisting of EDLCs, a diode bridge rectifier, a chopper system and a pulse-width modulated [PWM] converter, with useful information obtained from charge/discharge tests. Based on these results, two energy storage systems were built for DC 600 V and DC 750 V, and verification tests were conducted in conjunction with the Enoshima Electric Railway Company Ltd. and the Osaka Municipal Transportation Bureau. It was confirmed through verification tests that energy storage using electric double-layer capacitors provided effective functions to DC 600V and DC 750V railway systems. The tests proved to be a guideline for actual application in DC 1,500V or DC 3,000V railway systems.

Simple method enabling pulse on command from high power, high frequency lasers

A method for addressing individual laser pulses in high repetition frequency systems using an intracavity optical chopper and novel electronic timing system is reported. This “pulse on command” capability is shown to enable free running and both subharmonic pulse rate and burst mode operation of a high power, high pulse frequency copper vapor laser while maintaining a fixed output pulse energy. We demonstrate that this technique can be used to improve feature finish when laser micromachining metal.

Horizontal ToF-neutron reflectometer REFSANS at FRM-II Munich/Germany: First tests and status

The horizontal reflectometer REFSANS at FRM-2 in Munich/Germany has successfully been put into operation. It is dedicated to the comprehensive analysis of the air/water interface by means of specular and off-specular reflectivity as well as GISANS measurements [1,2]. In parts, novel components concerning the chopper system, the neutron optics, the detector and the data acquisition system have been developed to meet the requirements of the basic demands on REFSANS. An overview of the status of REFSANS as achieved in 2005 is presented.

Performance of TOF powder diffractometers on reactor sources

In 1998, a prototype of a time-of-flight (TOF) powder diffractometer was built at KFKI in Budapest in collaboration with the Hahn-Meitner-Institut (HMI) in Berlin. At a reactor source the neutron pulses are produced by a chopper system, which allows for shorter pulses than those obtained at pulsed spallation sources in the wavelength range most relevant for diffraction work, i.e. λ⩾0.7 Å. Furthermore, the chopper system provides an ideal symmetric line shape. First results proved the high potential of the approach, namely an excellent resolution of 1–5×10 −3 for Δ d/d was achieved. The prototype is presently rebuilt as a user instrument at the Budapest Neutron Centre. At HMI Berlin a new much more complex TOF powder diffractometer (EXED=extreme environment diffractometer) with higher resolution is under construction. It will benefit from variable resolution to achieve either ultrahigh resolution or very high intensities at conventional resolutions. EXED is devoted to studies under extreme sample conditions, for instance the TOF technique permits the access of a broad range of Q-values or d-spacing domains under scattering angle access strongly restricted by the use of highest field magnets. The whole instrument was simulated by Monte Carlo (MC) technique, and the simulations yield promising results.

Fast slit-beam extraction and chopping for neutron generator

High-intensity fast white neutron pulses are needed for pulsed fast neutron transmission spectroscopy (PFNTS). A compact tritium–tritium fusion reaction neutron generator with an integrated ion beam chopping system has been designed, simulated, and tested for PFNTS. The design consists of a toroidal plasma chamber with 20 extraction slits, concentric cylindrical electrodes, chopper plates, and a central titanium-coated beam target. The total ion beam current is 1A. The beam chopping is done at 30keV energy with a parallel-plate deflector integrated with an Einzel lens. Beam pulses with 5ns width can be achieved with a 15ns rise/fall time ±1500V sweep on the chopper plates. The neutrons are produced at 120keV energy. A three-dimensional simulation code based on Vlasov iteration was developed for simulating the ion optics of this system. The results with this code were found to be consistent with other simulation codes. So far we have measured 50ns ion beam pulses from the system.

The Effect of a Baler Chopping System on Fermentation and Losses of Wrapped Big Bales of Alfalfa

The improvement of wrapped big bale silage quality by increasing bale densities should be an important target. The objectives of this research were to evaluate the effects of the use of a cutting system (15 knives spaced 93 mm apart) installed behind the windrow pickup of a fixed chamber round baler. Three field trials were conducted near Turin, Italy (44°50′ N, 7°40′ E) by ensiling wrapped round bale of first‐ and second‐cutting alfalfa (Medicago sativa L.) at different dry matter (DM) concentrations. The DM and crude protein (CP) losses at baling, forage mean particle length, wet and dry bale weights, bale density, fermentation pattern, and the conservation losses were evaluated. The DM losses at baling ranged from 0.5 to 2.0% and from 0.7 to 4.7% of DM yield at cutting for unchopped and chopped treatments, respectively. The chopping system increased the percentage of stems shorter than 10 cm from 14 to 38% on a DM basis. Chopping increased DM bale density by about 4% in all the trials. After 140 d of conservation the final fermentation quality of silages was not consistently improved by chopping.

The development and beam test of an RF chopper system for J-PARC

The linac of the Japan Proton Accelerator Research Complex (J-PARC) is now under construction. It is a high-intensity linac with a peak H − current of 50 mA and a 1.25% duty factor. As a key component of the medium-energy beam-transport line of the linac, a chopper is necessary to obtain the required beam structure for the following 3-GeV rapid-cycling synchrotron, and also plays an important role in beam-loss control in such a high-intensity accelerator. Clean chopping requires a short rise time and a high deflecting field. An radio frequency deflector (RFD) was proposed as a chopper for the J-PARC linac, because of its merits of high deflecting field, compact structure, ease to manufacture and both reliability and flexibility of the total chopper operation. An RF chopper, consisting of two RFDs, has been developed. In a low-level RF test, the fundamental RF properties show good agreement with those of a simulation in the design. In a high-power test, the chopper works well without any discharge under 36 kW peak driving power. In a test of the coupled RFD system, the longer rise time of the second RFD and the delay of the second RFD excitation were observed. A beam test of the RF chopper was also successfully performed up to a peak current of 25 mA. The chopper system worked very stable during the beam test, and the sufficient separation between chopped and normal beam was obtained, with a short rise/fall time (10 ns) of the chopped beam.

High Intensity Neutron Spin Flip Chopper

We have proposed multi-stage spin flip chopper system to realize very high S/N ratio. Very high S/N ratios of about 3000 and 105 have been demonstrated for double and triple spin flip chopper systems, respectively. We here describe a double neutron spin flip chopper system applicable to pulsed source using polarizing bent guide tubes instead of simple magenetic mirrors, in order to obtain a high intensity and high S/N ratio. Simulation results show that it will realize spin flip chopper of high neutron intensity and S/N ratios.

Fast ion beam chopping system for neutron generators

Fast deuterium (D+) and tritium (T+) ion beam pulses are needed in some neutron-based imaging systems. A compact, integrated fast ion beam extraction and chopping system has been developed and tested at the Lawrence Berkeley National Laboratory for these applications, and beam pulses with 15ns full width at half maximum have been achieved. Computer simulations together with experimental tests indicate that even faster pulses are achievable by shortening the chopper voltage rise time. This chopper arrangement will be implemented in a coaxial neutron generator, in which a small point-like neutron source is created by multiple 120keV D+ ion beams hitting a titanium target at the center of the source.

On the resolution and intensity of a time-of-flight neutron reflectometer

In a neutron reflection experiment, the reflectivity is determined as a function of wave-vector transfer q = 4 π θ / λ , with θ the glancing angle and λ the neutron wavelength. At a continuous neutron source, it may be advantageous to use the time-of-flight technique to determine λ . Then, a chopper system is used to pulse the beam. We will discuss all contributions to both q resolution and intensity. Special attention is paid to the comparison of using a single- and double-disc chopper system. Examples of the performance of both systems are given. The double-disc chopper produces a burst time proportional to λ . By matching the different contributions to the resolution, the resulting intensity can be optimised.

Upgraded sinusoidal beam chopping system at CYRIC

The sinusoidal beam chopping system installed in the neutron time-of-flight (TOF) beam line at CYRIC was upgraded during the construction of the new K 110 AVF cyclotron. In order to achieve high stability of the RF phase under easy control, a new low-level RF electronics was designed and built using a direct digital synthesizer (DDS). The new control system for the beam chopper was also developed on the basis of programmable logic controllers (PLC). The performance tests were successfully carried out with proton beams from the cyclotron. The present beam chopping system has covered a wide dynamic range from 325.5 to 1000 ns for neutron TOF measurements at CYRIC.

Atomic fraction of hydrogen plasma in a negative hydrogen ion source

Hydrogen atoms effusing out from an extraction hole of a 6 cm diam and 6 cm long ion source was measured using a quadrupole mass spectrometer coupled to a neutral beam chopping system. The system background for atomic signal detection was negligible. As the discharge current was increased, the signal of atomic hydrogen increased almost linearly with the discharge current. Meanwhile the signal of molecular hydrogen slightly decreased with increasing discharge current. The signal to noise ratio of the present system is as low as 1/80 for typical operation of the ion source. The status of the system modifications to reduce residual hydrogen gas is reported.

Energy Storage System for DC Electrified Railway Using EDLC

How to stabilize power sources to minimize voltage drops, power load fluctuations, and regenerative power lapses has been a key problem area in DC feeding circuits. Energy storage media that use power efficiently and eradicate such problems have been attracting interest, with several energy storage applications, including batteries and flywheels, having already been investigated. A wide range of large-scale electric double-layer capacitors (EDLCs) quickly charged and discharged, offer durability, maintenance-free features, low pollution and high efficiency levels, have been developed. Our research compared batteries with EDLCs in terms of their ability to charge/discharge at DC 75V. For the latest study, we produced a prototype DC electrified railway energy storage system that consists of EDLCs, a diode bridge rectifier, a chopper system, and a pulse width modulated (PWM) converter, useful information being obtained from charge/discharge tests. This paper describes the characteristics and results of the energy storage system experiments.

Neutron spin flip chopper with very high S/N ratio for pulsed source

We propose a multiple neutron spin flip chopper system to improve considerably its S/N ratio. Evaluations of a double spin flip chopper system give a high S/N ratio and its experimental results demonstrated good performance. Multiple spin flip chopper systems with high S/N ratio will be applicable to a direct geometry inelastic spectrometer for pulsed source instead of mechanical choppers.

TOF-elastic resolution spectroscopy: time domain analysis of weakly scattering (biological) samples

Elastic resolution spectroscopy (ERS), a new application of the time-of-flight method, addresses the problem of weak scattering signals. Since only the maximum of the spectrum at zero energy transfer needs to be determined with sufficient statistical accuracy, significantly smaller samples, as compared to conventional spectroscopy, can be studied. The peak intensity is recorded versus the reciprocal width of the instrumental resolution function by adjusting the speed of the chopper system. This approach yields a close approximation to the intermediate scattering function in the time domain. A neutron ERS analysis of molecular motions in alanine-dipeptide crystals and hydrated myoglobin is presented.