Cw Beam Research Articles

Multiple-charge beam dynamics in an ion linac

There is demand for the construction of a medium-energy ion linear accelerator based on superconducting rf (SRF) technology. It must be capable of producing several hundred kilowatts of CW beams ranging from protons to uranium. A considerable amount of power is required in order to generate intense beams of rare isotopes for subsequent acceleration. At present, however, the beam power available for the heavier ions would be limited by ion source performance. To overcome this limit, we have studied the possibility of accelerating multiple-charge-state (multi-Q) beams through a linac. We show that such operation is made feasible by the large transverse and longitudinal acceptance which can be obtained in a linac using superconducting cavities. Multi-Q operation provides not only a substantial increase in beam current, but also enables the use of two strippers, thus reducing the size of linac required. Since the superconducting (SC) linac operates in CW mode, space charge effects are essentially eliminated except in the ECR/RFQ region. Therefore an effective emittance growth due to the multi-charge beam acceleration can be minimized.

The Rossendorf radiation source ELBE and its FEL projects

The Forschungszentrum Rossendorf (FZR) is constructing a superconducting Electron Linac [F. Gabriel, J. Voigtländer, et al., ELBE Design Report 1998, http://www.fz-rossendorf.de/FWQ/report_d.htm; Annual Report 1996, FZR-179 (1997) 3; Annual Report 1997, FZR-215 (1998) 3] with high Brilliance and low Emittance (ELBE) which can deliver a 1 mA cw beam of 40 MeV. ELBE will be equipped with a free-electron laser (FEL) system for the production of infrared (IR) light in the range 5–300 μm and will thus cover the range from the infrared to the THz regime. The electron beam can also be used to generate X-rays, bremsstrahlung, positrons or fast neutrons.

Increase in weld penetration depth by two Nd: YAG laser beams combined

Two multi-kilowatts Nd: YAG laser beams are condensed separately and combined at surface of workpiece in order to increase penetration depth. In this system, one or two laser beams are inevitably inclined depending on size of condensing optics, thus, welding characteristics might be fairly complicated than that of a single laser beam welding. Welding characteristics are evaluated by penetration depth and penetration shape in bead-on-plate tests of stainless steel (304). This paper is intended as an investigation on the welding condition of the deepest weld penetration, especially on arrangement of condensing optics. The deepest weld penetration of 6.3 and 7.7 mm are obtained by two CW laser beams and two rectangularly modulated CW laser beams with total average power of four kilowatts at a welding speed of 5 mm/s, respectively. Deepened penetration depths of weld beads by laser beams in combination due to the increase in total laser power are obtained in comparison to penetration depths of 2.8 and 4.3 mm by a single CW laser beam and a single rectangular modulated CW laser beam with average powers of two kilowatts, respectively. Penetration depths of weld beads obtained with symmetric irradiation exceed those with asymmetric irradiation. The angle between two laser beams for the deepest penetration is 60 degrees for CW laser beams, while the angle is 30 degrees for modulated CW laser beams. The difference in the appropriate angles for the deepest penetration might be explained by different effect of keyhole bottom separation on penetration depths depending on waveform of laser beams. With asymmetric beam arrangement, a deeper penetration is obtained in backhand welding direction for the inclined laser beam than that in forehand direction both in welding using two CW laser beams in combination and two modulated CW beams in combination.

Nonlinear scattering of crossed ultrasonic beams in the presence of turbulence: Multiple scattering effects

The nonlinear scattering of two finite‐amplitude mutually perpendicular crossed ultrasonic beams—interacting in the presence of turbulence—generates a scattered sum frequency component that radiates outside the interaction region. In the absence of turbulence, virtually no scattering, at the sum frequency, is observed [M. S. Korman and R. T. Beyer, J. Acoust. Soc. Am. 84, 339–349 (1988); 85, 611–620 (1989)]. Here, two primary cw (f1=2.05 MHz and f2=1.95 MHz) beams are generated by 2.54‐cm‐diam circular plane array piston transducer units (T1 and T2). A 4‐MHz receiving unit (R) is of similar construction. All beam axes form a common plane and overlap region with the axis of a submerged circular water jet, which generates the turbulence. With R fixed, T1 and T2 rotate on radius arms—always keeping the beams perpendicular. Symmetry suggests an angle θ*, where θ*=0° defines forward scattering. This geometry allows a nonlinear forward scattering intensity component to exist without concern for a coherent component. Here, multiple scattering effects are needed to predict their results. Supplemental scattering experiments will be presented in an attempt to identify a transition from single (Born approximation) to multiple scattering. [Work supported by Naval Academy Research Council.]

Speed enhancement of photorefractive polymers by means of light-induced filling of trapping states

The fastest organic photorefractive materials show response times in the millisecond range. The origin of this limit is not yet fully understood. Charge-carrier generation and transport processes and reorientation of the nonlinear-optical chromophores may play an important role. We characterize a new photorefractive guest–host polymer by cw two-beam coupling, as well as by cw and pulsed four-wave mixing experiments. The latter were used to determine unambiguously the response time of the material. We show that a decrease of the response time by a factor of 1000 can be achieved by illuminating the sample with a strong cw beam during pulsed exposure. A model of the enhancement process backed by dc photoconductivity measurements is presented.

Ｎｄ：ＹＡＧレーザ２ビーム合成による溶込み深さの増大

Two multi-kilowatts Nd: YAG laser beams are condensed separately and combined at surface of workpiece in order to increase penetration depth. In this system, one or two laser beams are inevitably inclined depending on size of condensing optics, thus, welding characteristics might be fairly complicated than that of a single laser beam welding. Welding characteristics are evaluated by penetration depth and penetration shape in bead-on-plate tests of stainless steel (304). This paper is intended as an investigation on the welding condition of the deepest weld penetration, especially on arrangement of condensing optics. The deepest weld penetration of 6.3 and 7.7 mm are obtained by two CW laser beams and two rectangularly modulated CW laser beams with total average power of four kilowatts at a welding speed of 5 mm/s, respectively. Deepened penetration depths of weld beads by laser beams in combination due to the increase in total laser power are obtained in comparison to penetration depths of 2.8 and 4.3 mm by a single CW laser beam and a single rectangular modulated CW laser beam with average powers of two kilowatts, respectively. Penetration depths of weld beads obtained with symmetric irradiation exceed those with asymmetric irradiation. The angle between two laser beams for the deepest penetration is 60 degrees for CW laser beams, while the angle is 30 degrees for modulated CW laser beams. The difference in the appropriate angles for the deepest penetration might be explained by different effect of keyhole bottom separation on penetration depths depending on waveform of laser beams. With asymmetric beam arrangement, a deeper penetration is obtained in backhand welding direction for the inclined laser beam than that in forehand direction both in welding using two CW laser beams in combination and two modulated CW beams in combination.

Plume behaviour during welding with two Nd:YAG laser beams combined

Two different types of Nd:YAG laser oscillators are used simultaneously in order to increase the weld penetration depth. A rectangularly modulated CW beam emitted from a CW-pumped laser oscillator together with a pulse beam emitted from a pulse-pumped oscillator is irradiated onto stainless steel (304). This paper investigates the contribution of a pulsed beam to welding characteristics in two-laser-beam welding. A pulsed beam with high peak power is effectively combined with a modulated CW beam for adjusting the power density in order to obtain deep penetration. The plume is extinct during modulated CW beam irradiation after the emission of a pulsed beam under deep penetration conditions. On the basis of this phenomenon, keyhole behaviour could be considered and it is recognized that a pulsed beam with high peak power would play a significant role in enlarging the keyhole. A strong correlation between the penetration depth and the plume extinction time is apparent. Therefore, the keyhole might be temporarily but sufficiently enlarged and penetration might be consequently deepened by a pulsed beam with high peak power.

Acoustic streaming and temperature elevation in a high viscous fluid by irradiation of ultrasound beams

Sound energy losses due to viscosity and heat conduction in a fluid cause the medium movement as acoustic streaming and cause temperature elevation in beams. In most fluids, the buoyancy force by heat expansion is generally weaker than the driving force of the streaming, so the temperature field is basically determined by the heat transfer equation with convective terms. As some physical parameters such as viscosity depend generally on temperature, the sound field is changed with the irradiation time of cw beams. In addition to the viscosity, two other parameters of sound absorption and sound speed are taken account of their temperature dependency in the present theory. The hydrodynamic flow, forced convection heat transfer, and wave equations are simultaneously solved for axisymmetric ultrasound beams by a finite difference scheme. Numerical examples of the streaming velocity and temperature rise are given. They demonstrate that sound pressure amplitude is intrinsically changed with time and sound self-action is generated. The present method of numerical calculation is easily extended to the problems of focusing and multi-layered beam systems.

A new time-domain approach for nonlinear wave propagation: Comparison with the KZK equation approach in the case of an unfocused cw beam

A time-domain numerical model for simulating acoustic pulse focusing in soft tissue has already been presented. In this model, the main effects responsible for finite-amplitude wave propagation, i.e., diffraction, frequency-dependent attenuation, and nonlinearity, were taken into account. Here, a comparison between the results of this model with those of the KZK model is presented. Using this model, the acoustic pressure field of a plane circular transducer was simulated in water for a large range of cw-mode input excitation levels. The on- and off-axis pressure-time waveforms and their corresponding harmonic components were calculated using this model and the KZK model, and very good agreement was obtained between the two models. Furthermore, these results were verified by previously published experimental measurements. At present, all implementations of the KZK model are limited to planar or weakly focused sources. This study, along with this group’s previous work, reveals the validity of this finite-amplitude wave propagation model for a wide degree of source focusing (from a planar to a highly focused source), and in cw as well as PW modes. It is also shown that the computation time for this model is much less than for the KZK, using the same input parameters.

Fast spectral algorithm for modeling focused sound beams in a highly nonlinear regime

Numerical modeling of high-intensity focused sound beams is important for medical applications of ultrasound. The KZK equation, which may be modified to include relaxation in the time domain or arbitrary absorption and dispersion in the frequency domain, is widely used to model ultrasound beams in tissue. Numerical solutions are available in either the time or frequency domain, but they become time consuming at high intensities when thin shocks are developed. To speed up calculations, a known high-frequency asymptotic result for shocks can be incorporated in the frequency domain algorithm. In comparison with exclusively numerical schemes, the present approach substantially reduces the number of harmonics and thus the computation time [Yu. A. Pishchal’nikov, O. A. Sapozhnikov, and V. A. Khokhlova, Acoust. Phys. 42, 362–367 (1996)]. Focused cw beams radiated from Gaussian and piston sources are simulated in water and biological tissue. The accuracy of the asymptotic approach is verified via conventional numerical methods of solution. It is shown that the present asymptotic method provides accurate results with a considerable reduction in computation time. In combination with conventional numerical schemes, the developed asymptotic method provides an effective tool for investigating focused sound beams for a wide range of parameters. [Work supported by NIH PO1-DK43881, Fogarty Research Program, and CRDF.]

Dimer Destruction in a Cs Vapor by a Laser Close to Atomic Resonance

We demonstrate efficient depletion of the ground state dimers in a Cs vapor along the path of a Ti:sapphire cw beam. This arises from a quasiresonant process, assisted by collisions of ${\mathrm{Cs}}_{2}$ molecules with $\mathrm{Cs}(6P)$ excited atoms, taking place close to both ${D}_{1}$ and ${D}_{2}$ atomic transitions, the lower the density, the narrower the frequency range over which it occurs. We obtain the cross section of the collisions involved.

Spatial Solitons and Induced Kerr Effects in Quasi-Phase-Matched Quadratic Media

We show that the evolution of the average intensity of cw beams in a quasi-phase-matched quadratic (or ${\ensuremath{\chi}}^{(2)}$) medium is strongly influenced by induced Kerr effects, such as self- and cross-phase modulation. We prove the existence of rapidly oscillating solitary waves (a spatial analog of the guided-center soliton) supported by the quadratic and induced cubic nonlinearities.

Near-field and far-field propagation of beams and pulses in dispersive media

We formulate an efficient exact method of propagating optical wave packets (and cw beams) in isotropic and nonisotropic dispersive media. The method does not make the slowly varying envelope approximation in time or space and treats dispersion and diffraction exactly to all orders, even in the near field. It can also be used to determine the partial differential wave equation for pulses (and beams) to any order as a power series in the partial derivatives with respect to time and space. The method can treat extremely focused pulses and beams, e.g., from near-field scanning optical microscopy sources whose transverse spatial extent in smaller than a wavelength.

Temperature dependence of planar channeling radiation in silicon, germanium, and beryllium between 12 and 330 K

The temperature dependence of planar channeling radiation of 62.8 MeV electrons has been studied for silicon, germanium, and beryllium. The measurements have been performed using an uncollimated low-emittance cw beam from the superconducting electron linac S-DALINAC at the Technische Hochschule Darmstadt. Energies and linewidths of transitions between transverse bound states have been determined in the energy range between 40 and 230 keV for silicon and beryllium at temperatures between 12 and 330 K, and for germanium between 12 and 223 K. From the shift of the transition energies with temperature the mean thermal vibrational amplitudes of the atoms transverse to the channeling planes are determined by comparison with calculations using the many-beam formalism. Within experimental errors no directional dependence of the vibrations is observed. For silicon a Debye temperature of (535.2\ifmmode\pm\else\textpm\fi{}8.5) K at 12 K and (519.0\ifmmode\pm\else\textpm\fi{}10.8) K at 300 K has been derived. For germanium an increase from (232.7\ifmmode\pm\else\textpm\fi{}12.8) K at 12 K to (292.0\ifmmode\pm\else\textpm\fi{}16.4) K at 223 K is observed. Planar channeling radiation spectra from a beryllium crystal taken at 12, 220, and 300 K have been analyzed the same way yielding a Debye temperature of (1060\ifmmode\pm\else\textpm\fi{}50) K.

Operation of a cw rf driven ion source with hydrogen and deuterium gasa)

We will describe the operation of a cw rf driven multicusp ion source designed for extraction of high current hydrogen and deuterium beams. The source is driven at 2 MHz by a 2.5 turn induction antenna immersed in the plasma. Bare stainless-steel and porcelain-coated Cu antennas have been used. The plasma load is matched to the rf generator by a variable tap N:1 transformer isolated to 46 kV, and an LC network on the secondary. With H2 gas the source can be operated at pressures between 5 and 60 mT with power reflection coefficients <0.01. The extracted ion current density with a porcelain-coated antenna is approximately given by 35 mA/cm2/kW with an 80 G dipole filter field for input powers from 3.5 to 6.6 kW. The current density remained constant for operation with a 6 and an 8 mm aperture. The source has been operated for 260 h at 3.6 kW with a single-porcelain-coated antenna. Mass spectrometer measurements of the extracted beam at this power show a species mix for H+:H+2:H+3:OH+ of 0.49: 0.04: 0.42: 0.04. The calculated beam divergence using the IGUN code is compared with the measured divergence from an electrostatic sweep emittance scanner designed for high-power cw beam diagnostics. Phase space measurements at 40 kV and 23 mA beam current result in a normalized rms emittance of 0.09 π mm mrad.

Resonant acoustic emission during laser welding of metals

We observed sharp resonances at certain frequencies in the acoustic and optical emission from metals during laser welding with a modulated laser beam. Strong emission is observed at high-order harmonics of the laser modulation frequency. Harmonics whose frequencies overlap with bands of vibrational frequencies corresponding to eigenmodes of the keyhole are greatly enhanced, relative to other harmonics whose frequencies lie outside these bands. The keyhole and its surrounding liquid layer act as a frequency selective amplifier for pressure fluctuations induced by changes in the interaction of laser radiation with the walls of the keyhole. When a CW beam is used, random fluctuations with frequencies within these allowed bands are amplified to produce a spectrum consisting of a large number of overlapping but discrete frequency components. When the laser beam is modulated, a forced response is elicited resulting in a much simplified emission spectrum. These results offer some new insights into the physical mechanisms that convert laser radiation to heat in laser welding, drilling and cutting applications.

Operation of a cw rf driven ion source with hydrogen and deuterium gas (abstract)

We will describe the operation of a cw rf driven multicusp ion source designed for extraction of high current hydrogen and deuterium beams. The source is driven at 2 MHz by a 2.5 turn induction antenna immersed in the plasma. Bare stainless-steel and porcelain-coated Cu antennas have been used. The plasma load is matched to the rf generator by a variable tap N:1 transformer isolated to 46 kV, and an LC network on the secondary. With H2 gas the source can be operated at pressures between 5 and 60 mT with power reflection coefficients <0.01. The extracted ion current density with a porcelain-coated antenna is approximately given by 35 mA/cm2/kW with an 80 G dipole filter field for input powers from 3.5 to 6.6 kW. The current density remained constant for operation with a 6 and an 8 mm aperture. The source has been operated for 260 h at 3.6 kW with a single-porcelain-coated antenna. Mass spectrometer measurements of the extracted beam at this power show a species mix for H+:H+2:H+3:OH+ of 0.49: 0.04: 0.42: 0.04. The calculated beam divergence using the IGUN code is compared with the measured divergence from an electrostatic sweep emittance scanner designed for high-power cw beam diagnostics. Phase space measurements at 40 kV and 23 mA beam current result in a normalized rms emittance of 0.09 π mm mrad.

Experimental and theoretical studies on a distributed temperature sensor based on Brillouin scattering

Results are reported from recent research on the use of the Brillouin gain/loss mechanism for distributed sensing. A theoretical model of the interaction of the pulsed and CW beams is described and compared with experiments. Results from a system with a 51 km sensing length are presented. We finally investigate issues related to the variation within the sensing fiber of the polarizations of the two beams.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Laser-Controlled Etching of (Al,Ga)As Epitaxial Layers

ABSTRACTWe report a systematic study of laser-controlled etching behaviors of AlxGa1−xAs epitaxial layers (x from 0 to 0.5) by varying parameters such as laser power, beam scanning speed, beam duty cycle, number of scans and different doping types and concentrations. The laser source was an argon ion laser emitting at 514.5 n-type layers were found to be significantly higher than those for p-type layers. Generally, higher etch rates were observed for layers with higher aluminium mole fraction. The etch rate was found to increase at an exponential rate with the laser power for CW beam, but this increase became linear when a chopped laser beam was used. The dependence of etch rates on different conditions can be largely accounted for by the differences in the built-in electric field at the surface as well as differences in the laser-induced local temperature rise in the layers.

Transient Raman spectroscopy of 15N-substituted bacteriochlorophyll a. An empirical assignment of T 1 Raman lines

15N-substituted bacteriochlorophyll a (BChl a) was extracted from the cells of Rhodobacter sphaeroides 2.4.1 grown in a medium containing 15N-ammonium sulfate and yeast concentrate. The T 1 Raman spectra of 14N-and 15N-BChl a were obtained as the difference spectra of high-power minus low-power of one-color, pump-and-probe measurements using 420 nm, 5 ns pulses. A set of empirical assignments of the T 1 Raman lines was made, based on shifts upon 14N→ 15N substitution. The S 0 Raman spectra of the two BChls were also obtained by using the 457.9 nm cw beam, and a set of assignments of the S 0 Raman lines was given for comparison.