Anup Bhowmik

Analysis and simulation of a synthetic-aperture technique for imaging through a turbulent medium: erratum

Active imaging techniques offer the promise of improved signal-to-noise ratios for high-resolution imaging of distant objects. However, this promise has remained unfulfilled because such techniques typically require relatively high-power illumination. The proposed technique, referred to as Fourier telescopy, largely overcomes such limitations by encoding image information in the time domain, allowing very large total collector areas to be used. The basic approach combines long-baseline interferometry with phase closure to sample the object at high spatial frequency but low temporal frequency. A phase closure strategy is selected that maximizes the intermittency of transmission, further reducing illuminator power requirements. Error sources are analyzed, and simulation results are presented for imaging of remote objects. Asymptotic results are given for the lowest-order effects of atmospheric aberrations, and the limitations of the approximations are discussed.

First demonstration of a free-electron laser driven by electrons from a laser-irradiated photocathode

We report the results from the first operation of a free electron laser (FEL) driven by an electron beam from a laser-irradiated photocathode. The Rocketdyne/Stanford FEL achieved sustained oscillations, lasting in excess of three hours, driven by photoelectrons accelerated by the Stanford Mark III radiofrequency linac. A LaB6 cathode, irradiated by a tripled Nd: Yag mode-locked drive laser was the source of photoelectrons. The drive laser, operating at 95.2 MHz, was phase-locked to the 30th subharmonic of the S-band linac. Peak currents in excess of 125 A were observed and delivered to the Rocketdyne 2 m undulator which was operated as a stand-alone oscillator. Sustainable small-signal gain of 100% per pass was observed over a 2 h time period with periodic observation of small-signal gain as high as 150% per pass. Preliminary estimates of the electron-beam brightness deliverable to the undulator range from 3.5 × 1011 to 5.0 × 1011 A/(rad m)2.

A high quality permanent-magnet wiggler for the Rocketdyne/Stanford infrared free electron laser

Abstract A high quality, variable gap, variable taper, permanent-magnet wiggler has been built for infrared free electron laser (FEL) experiments to be performed at the Stanford Photon Research Laboratory. The design and characterization procedure used to assemble the wiggler is discussed. A simulated annealing code was used to minimize field errors arising from variations in the individual magnets. The computed electron trajectories associated with the measured magnetic fields are presented for a range of different operating points of the wiggler. These plots indicate a very high quality field over a large range of different wiggler operating regimes. Resultant trajectory wander over the 2 m long wiggler for a 40 MeV electron at a wiggler gap corresponding to 3.3 kG was calculated to be less than 25 μm. The ability to control trajectory wander and optical phase slip using the simulated annealing code suggests future extensions to extremely long wigglers.

A microwiggler free-electron laser at the Brookhaven accelerator test facility

Abstract We report the design and status of an FEL experiment at the Brookhaven National Laboratory Accelerator Test Facility. A 50 MeV high-brightness electron beam will be utilized for an oscillator experiment in the visible-wavelength region. The microwiggler to be used is a superferric planar undulator with a 0.88 cm period, 60 cm length and K = 0.35. The optical cavity is a 368 cm long stable resonator with broadband dielectric coated mirrors.

Closed-cavity solutions with partially coherent fields in the space-frequency domain

Closed or stable optical cavities, used frequently to determine the efficiency of high performance chemical laser nozzles, are designed primarily for maximum multimode power extraction from the medium. The very large (>500) Fresnel numbers associated with such cavities have in the past necessitated their analytical modeling by representing them as plane-parallel Fabry-Perot or rooftop cavities. In this paper, a rigorous 2-D scalar diffraction formalism of the closed cavity is presented in which quasi-monochromatic partially coherent fields in the space-frequency domain are used to obtain quasi-steady state but stable solutions using a simplified gain model. Small power fluctuations in the numerical iterative solution history that displays no monotonic increasing or decreasing trends are interpreted as the redistribution of energy from one degenerate set of high-order transverse modes into another. The degree of coherence in the second-order spatial correlation function (or the mutual coherence function) required of the input fields which permit such solutions is presented. Further, it is shown that the upstream/downstream coupling in this closed cavity occurs as a natural consequence of the physical model itself rather than through some artificial geometrical means, such as that introduced in the rooftop model. The axial variation in the resulting mode width is in excellent agreement with the Hermite-Gaussian distribution predicted for the particular geometry of interest. The computed closed-cavity power variation with mode width using a simplified gain model shows qualitative agreement with experimentally observed trends; quantitative agreement is poor and is ascribed to the rudimentary nature of the gain model. In the limiting case of small Fresnel numbers (NF ∼ 1) this procedure yields, in the bare cavity, the well-known fundamental mode of the cavity when appropriate symmetry constraints are applied.

First operation of the Rocketdyne/Stanford free electron laser

Abstract A near infrared free electron laser (FEL) has been built and installed by Rocketdyne in the Stanford Photon Research Laboratory. The Rocketdyne/Stanford FEL utilizes a very high quality, 2 m long, permanent magnet planar wiggler whose gap may be continuously tuned, and magnetic field axially tapered by varying the gap at one end relative to the other. The laser is operated with an e-beam supplied by the Stanford Mark-III accelerator. A stable resonator with a broadband, dielectric coated element permits transmissive outcoupling over the 2.7–3.7 μm wavelength range. Results from initial operation of this laser are presented.

Design concept for a common rf accelerator driven free electron laser master oscillator/power amplifier

Abstract We present a conceptual design of a near infrared free electron laser master oscillator/power amplifier (FEL-MOPA) driven by a common radiofrequency accelerator. We present techniques for radiofrequency switching that permit exploration of the FEL-MOPA operation on single electron macropulses, in both the small signal and large signal regimes. Feasibility of this concept is shown by using, as an example, the parameters of the Stanford Mark-III oscillator [1] and the Rockwell tapered undulator [2], the latter in the amplifier configuration. Important physics issues of the FEL-MOPA addressed by this suggested experiment are (1) (synchronization and optimization of electron and laser pulses, and (2) gain-guided temporal and spatial evolution of the laser mode in the amplifier.

Initial results of operating the rocketdyne undulator in a tapered configuration

Abstract The near-infrared Rocketdyne/Stanford free electron laser (FEL) uses a very-high-quality precision undulator whose field strength and field taper are adjustable. The Rocketdyne undulator has been operated in both an amplifier configuration, as in the master-oscillator power amplifier (MOPA) experiments, and an oscillator configuration, as in the photocathode and tapered-undulator experiments. The tapered-undulator experiment was performed at the Stanford Photon Research Laboratory (SPRL) using an electron beam supplied by the Mark III rf-linac. During the experiment we observed sustained oscillations as the undulator magnetic-field taper was continuously tuned from 0% to 10%. We observed ∼1.2% extraction efficiency for a magnetic-field taper of 9.6%. During the same experiment we observed sustained oscillations as the undulator gap was continuously varied over 120 mil. Details of the experiment are presented.

Three Dimensional Modeling Of Free-Electron Lasers Using Rigorous Wave Propagation

The essential features of generalized numerical models developed at Rocketdyne to compute rigorously the 3 D transverse mode structure of free-electron laser oscillators are presented. The oscillator may consist of a conventional standing or a traveling wave cavity, or may be more complex, and contain intracavity grazing optical elements. Numerical resolution needed to (1) compute the gain due to a spatially distributed electron beam and (2) propagate the electromagnetic field when coupled to this gain are discussed with illustrative examples.

Comparison of DC electric field and tapered wiggler free electron laser efficiency enhancement schemes

A constant parameter wiggler free electron laser using a dc electric field for efficiency enhancement is compared to several tapered wiggler efficiency enhancement schemes. Analytical expressions for the efficiency in the plane wave, infinitesimally small radius electron beam limit are derived and compared. Numerical simulations for a Gaussian radiation field and finite radius electron beam with an output radiation wavelength of 2 μm are presented. For finite radius electron beams, the extraction efficiency using a dc electric field is somewhat greater than or equal to that using proposed tapering schemes. While the dc field offers flexibility in efficient, tunable laser design, the required field strengths for visible radiation are in the megavolt/meter range.