Transverse Beam Profile Research Articles

Temporal laser beam construction under controlled mode filtering

The transient regime of a new type of free-running Nd: YAG laser including two spatial filters in order to synthesize an unusual transverse mode is studied. The laser is seeded by a short (30 ps) laser pulse, and a streak camera analyses the transverse beam profile after each cavity round-trip. The expected filtered mode is obtained after approximately 15 round-trips. This result shows the ability of this original filtering to be used by others emission regimes and gain mediums.

To the theory of induced scattering of electromagnetic waves by an electron beam in a regular waveguide

In this work, a formula for the amplification parameter (analogous to the Pierce parameter in travelling-wave tube theory) of regular waveguide modes is deduced in the case of induced scattering by an electron beam with an arbitrary electron density distribution in the cross section. The presence of an axial magnetic field transporting a beam along the interaction region is taken into account. Obtained results can be used for calculation of the amplification in free electron lasers with electromagnetic, magnetostatic or electrostatic pump and a definite beam transverse profile. In particular, expressions for amplification coefficients in the case of a thin-wall hollow electron beam propagating in a cylindrical waveguide, that takes into account the excitation of beam waves (space charge in TWT theory), are found. An analysis of the dependence of amplification on the guiding magnetic field value and hollow beam radius is carried out.

Simultaneous Observation of Spatial Hysteresis and Bistable Loop Variation in a Semiconductor Interferometer with Electronic Nonlinearity

Abstract We have experimentally investigated in detail the changing of reflected beam transverse profile and spatial variation of bistable loops in an InSb interferometer at 5·6 μm and 80 K. We have clearly observed spatial hysteresis and simultaneously recorded the great changes of bistable loop character in different parts of the beam. We established that at the fixed transverse coordinate the shape of loops in different interference orders can be different. Ring-structured transverse profiles were obtained with the number of rings equal to the number of observed interference orders. In the case of competition between electronic and thermal dispersive nonlinearities, the inversion of the initial phase of regenerative oscillations was also observed at the beam edge.

Propagation Effects Due to Nonlinear Absorption and Refraction Within a Thin Medium

Abstract External self-focusing and optical interference phenomena can appear in the transmitted field when a laser beam or pulse interacts with a thin nonlinear medium, and these effects have been investigated for different types of nonlinearity. Physical regimes have been considered where both nonlinear refraction and nonlinear absorption play important roles. Strong modulations in the transmitted irradiance can occur in principle when the incident-beam power is allowed to vary. This type of modulation does not generally die away as the power increases, moreover its amplitude and period will be markedly changed if the nonlinear absorption coefficient is allowed to increase. It is also shown that the shape of the incident transverse-beam profile strongly affects the nonlinear aberrations and interference effects, with implications for such applications as optical power limiting and transverse optical bistability.

Visible light emitted by electrons and positrons from an undulator

Currently in the SPS, visible light emitted by p and p in an undulator is used to measure profiles of the circulating beam. This paper shows how, when the SPS will operate as LEP injector, an identical undulator placed in another configuration can also be used to measure the lepton beam profiles. A simple method to separate the visible light from the high power X-rays is proposed. Power emitted in the visible range and the diffraction effects are calculated and should permit enough accuracy in the transverse beam profile measurements.

Near spherical illumination of ion-beam and laser targets

A procedure is developed for reducing energy-deposition asymmetry in spherical targets driven directly by ion or laser beams. This work is part of a strategy for achieving illumination symmetry in such targets, which is proposed as an alternative to those in the literature. This strategy allows an axially symmetric placement of beamlets, which would be convenient for some driver or reactor scenarios. It also allows the use of beam currents or energy fluxes and beam transverse profiles to help reduce deposition asymmetry with fewer beamlets. In the ideal limit of thin deposition layers and controlled beam profiles, at most six beamlets are needed for target symmetry.

Two-dimensional transverse solitary waves as asymptotic states of the field in a bistable optical resonator

The dynamical evolution of a two transverse dimensional laser beam profile is monitored as it switches from a low to a high transmission state in a bistable optical resonator. Sharp gradients appearing at the edge of the switched-on central portion of the beam initiate a train of solitary waves which grow inwards towards the beam center. In a high finesse resonator, strong saturation of the beam in the on state tends to stabilize the solitary waves as concentric rings. Unlike earlier one transverse dimensional studies, the central part of the beam undergoes a slow periodic oscillation, but the shapes of the outer rings are consistent with earlier one transverse dimensional predictions. Numerical evidence is provided for filamentation of lower amplitude rings.

Optical transition radiation proton beam profile monitor

For high energy and high intensity proton beams a new type of detector has been developed and tested in the CERN-SPS transfer lines. It uses the properties of transition radiation emitted, by the proton beam, when passing through a thin aluminium foil. Accurate transverse beam profiles can be obtained by means of this process. Compared to the classical secondary emission monitors the disturbance to the proton beam is reduced by an order of magnitude.

Spontaneous pulsing, period doubling, and quasi-periodicity in a unidirectional, single-mode, inhomogeneously broadened ring laser

Studies of a unidirectional ring laser using the 3.51–μm transition in xenon reveal a well-defined second threshold above the threshold for cw lasing at which spontaneous self-pulsing is observed. Higher excitation thresholds are found for transitions through a sequence of period doublings or to quasi-periodic behavior (two independent pulsing frequencies). Detuning the laser cavity from resonance does not lead to the dramatic complications in the pulsing structure that were observed in Fabry–Perot lasers. Instead with increasing detuning, we find a smooth reduction in complexity of the dynamical behavior that seems to correspond principally to the reduction in the gain and the laser intensity with detuning. We also find some evidence that transverse beam profiles cause qualitative and quantitative changes in the location and nature of the various transitions.

FMIT Diagnostic Instrumentation

The Fusion Materials Irradiation Test facility (FMIT) cw prototype accelerator has noninterceptive beamline instrumentation to measure beam parameters. The transverse emittances and beam profiles are measured with an array of photodiode sensors viewing light emitted from the beam region. Tomographic reconstructions of both spatial-density distributions and of transverse-emittance distributions are performed throughout a quadrupole focusing section. Beam bunches passing through capacitive probes produce bipolar waveforms whose zero crossing corresponds to the bunch's longitudinal centroid. By measuring the time required for a bunch to travel the known distance between two probes, velocity and energy are determined. A toroidal transformer measures the average ac beam current. Beam spill is measured by a set of movable jaws that intercept the beam edges. Each jaw contains a water flow channel whose flow rate and differential temperature are measured to derive a transverse power distribution. Beam centroid position is measured by a four-lobe, magnetic-loop pickup.

A transverse beam profile monitor for p-p studies in the CERN SPS

In the SPS collider, counter-rotating beams of protons and antiprotons will be stored at 270 GeV/c for many hours. Each beam will consist of up to 6 bunches of 1011 particles. A simple device has been developed which allows the transverse profile of each individual bunch to be measured in an almost nondestructive way. 1,2,3). A fine beryllium wire is passed quickly (/spl I.itilde/4m/s) through the beam. The profile is measured by detecting the high-energy secondary particles produced in the wire. Due to the strong directionality of secondary particle production the profile of each proton and antiproton bunch can be measured simultaneously using two scintillators placed either side of the wire and close to the beam pipe. In addition, profiles of single beams can be obtained by measuring the secondary electron current emitted by the wire. The device has been successfully tested during storage studies with protons.

Nonlinear-optical calculations using fast-transform methods: Second-harmonic generation with depletion and diffraction

The authors describe an analytical approach to nonlinear-optics calculations which is well adapted to obtaining accurate and efficient numerical results for steady-state nonlinear interactions including finite transverse beam profiles, beam-diffraction effects, and power-depletion effects. This general approach is applied to obtain new results for optical second-harmonic generation with both Gaussian and unstable-resonator transverse beam profiles, under large-signal conditions where pump depletion as well as diffraction effects are important.

Transverse profile evolution of pulses passing through TEA-CO2 amplifiers

Series of low gain TEA-CO2 amplifiers are used to study the evolution of beam transverse profile as short duration pulses are propagated through the amplifiers. Considerable distortion from the inp...

Recent Improvements in Beam Diagnostic Instrumentation

In high-current machines, such as LAMPF and the envisioned Hanford Engineering Development Laboratory (HEDL) Fusion Materials Irradiation Test (FMIT) Facility linac, hands-on maintenance is desired. Beam spill must be kept extremely low; therefore, attention must be given to beam fringes (tails). Beam matching to the structure becomes increasingly important. We describe equipment capable of accurately measuring transverse beam profiles over a range spanning more than four orders of magnitude and longitudinal phase profiles over ranges spanning more than three orders of magnitude. Errors in 100-MeV transverse emittance measurements are explored and experimental emittance measurements made with three different methods are compared. Advantages of one nondestructive method are developed.