Brookhaven Accelerator Test Facility Research Articles

Measurements of terahertz radiation generated using a metallic, corrugated pipe

A method for producing narrow-band THz radiation proposes passing an ultra-relativistic beam through a metallic pipe with small periodic corrugations. We present results of a measurement of such an arrangement at Brookhaven's Accelerator Test Facility (ATF). Our pipe was copper and was 5cm long; the aperture was cylindrically symmetric, with a 1mm (radius) bore and a corrugation depth (peak-to-peak) of 60µm. In the experiment we measured both the effect on the beam of the structure wakefield and the spectral properties of the radiation excited by the beam. We began by injecting a relatively long beam compared to the wavelength of the radiation, but with short rise time, to excite the structure, and then used a downstream spectrometer to infer the radiation wavelength. This was followed by injecting a shorter bunch, and then using an interferometer (also downstream of the corrugated pipe) to measure the spectrum of the induced THz radiation. For the THz pulse we obtain and compare with calculations: the central frequency, the bandwidth, and the spectral power—compared to a diffraction radiation background signal.

Plasma-based advanced accelerators at the Brookhaven Accelerator Test Facility

The Accelerator Test Facility at Brookhaven National Laboratory (BNL ATF) offers to its users a unique combination of research tools that include a high-brightness 70-MeV electron beam, a mid-infrared (λ = 10 μm) CO2 laser of terawatt power, and a capillary discharge as a plasma source. These cutting-edge technologies have enabled us to launch a new R&D program at the forefronts of advanced accelerators and radiation sources. The main subjects that we are researching are innovative methods of producing wakes in a linear regime using plasma resonance with the electron microbunch train periodic to the laser’s wavelength and so-called “seeded” laser wakefield acceleration (LWFA) that is driven and probed by a combination of electron and laser beams. We describe the present status of the ATF experimental program, including simulations and preliminary experiments; in addition, we review previous ATF experiments that were the precursors to the present program. They encompass our demonstration of longitudinal-and transverse-field phasing inside the plasma wave, plasma channeling of intense CO2 laser beams, and the generation of e-beam microbunch trains by the inverse FEL technique.

Surface-roughness wakefield measurements at brookhaven accelerator test facility.

An experiment has been carried out at the Brookhaven Accelerator Test Facility to investigate the effect of a surface-roughness wakefield in narrow beam tubes with artificially created bumps. The measurements show that the synchronous modes decay significantly due to the randomization of the roughness pattern. It is pointed out that this decay mechanism has not been investigated in the previous experiment at DESY and the investigators' conclusion does not apply for surface-roughness wakefields in real surfaces.

Experimental characterization of emittance growth induced by the nonuniform transverse laser distribution in a photoinjector

The emittance of a high-brightness electron beam from a photoinjector is affected by the transverse and longitudinal distributions of the laser beam illuminating the cathode. A nonuniform laser beam generates a nonuniform electron-beam distribution that experiences emittance growth on a time scale of the plasma period. Experiments were performed at the Brookhaven Accelerator Test Facility to investigate the emittance growth due to transversely nonuniform laser beams. Laser masks were fabricated to generate various laser distributions. Significant emittance growth was observed as the laser distribution deviated strongly from a uniform distribution. For cylindrically symmetric, nonuniform distributions, experimental results agree with PARMELA simulations. The emittance dependence on the bunch charge is linear as a function of the bunch charge for both uniform and nonuniform beams. For a uniform beam, the emittance measurements agree well with the predictions from PARMELA simulations, but the analytical approach overestimates the results. For nonuniform beams, analytical estimates are about 70% of the measurements. For noncylindrically symmetric, nonuniform beams, we observed that the emittance is linearly proportional to the rms laser nonuniformity and the best emittance for a perfectly uniform beam is extrapolated to be $1.07\ifmmode\pm\else\textpm\fi{}0.13\text{ }\mathrm{mm}\text{ }\mathrm{mrad}$ at 0.5 nC.

Submicron emittance and ultra small beam size measurements at ATF

This paper covers improvements to a photoinjector that lead to the realization and measurement of submicron normalized emittance for a 0.5 nC electron beam at the Brookhaven Accelerator Test Facility. A fitting procedure utilizing beam size measurements from multiple beam profile monitors along the transport line was used instead of more traditional techniques. Such a small emittance beam in combination with a tight focusing scheme allowed us to achieve 10 μm RMS beam sizes at 60 MeV. The behavior and limitations of different BPM screens is compared for such beams.

Gas breakdown limit and maximum acceleration gradient for inverse Cherenkov laser accelerator

Laser intensity thresholds for CO 2 laser-induced gas breakdown, such as tunneling, multiphoton, and cascade ionization have been estimated for the inverse Cherenkov accelerator experiment at the Brookhaven Accelerator Test Facility. The gas breakdown is dominated by cascade ionization and the maximum acceleration gradient is up to 300 MeV/m for a 3 ps CO 2 laser.

Vacuum laser acceleration using a radially polarized CO 2 laser beam

Utilizing the high-power, radially polarized CO 2 laser and high-quality electron beam at the Brookhaven Accelerator Test Facility, a vacuum laser acceleration scheme is proposed. In this scheme, optics configuration is simple, a small focused beam spot size can be easily maintained, and optical damage becomes less important. At least 0.5 GeV/m acceleration gradient is achievable by 1 TW laser power.

A modified feed-forward control system at the Accelerator Test Facility

A modified feed-forward control system has been operated at the Brookhaven Accelerator Test Facility to control the phase and amplitude of two high power klystron rf systems used to power a photocathode rf gun and a traveling wave electron linac. The changes to the control algorithm include an improved handling of cross coupling between the amplitude and the phase channels, an improved calibration routine that allows for changes in the matrix elements due to the variable base-line and improved filtering. The modifications to the software include modularity, portability, and user-friendliness. Improvements to the hardware include a linearized phase and amplitude controller with dc biasing for an improved dynamic range. The feed-forward system can handle nonlinear and noninstantaneous systems. With simultaneous regulation of two channels, the phase and the amplitude fluctuations over a time span of more than 3 μS were reduced to less than ±0.2° and ±0.2%, from the initial ±2.7° and ±1.8%, respectively.

Experimental characterization of the high-brightness electron photoinjector

Abstract Operational experience of the emittance compensated photoinjector at the Brookhaven Accelerator Test Facility (ATF) is presented in this paper. The photoinjector has demonstrated the stability and reliability required for UV and X-ray FEL applications. The RF gun has been routinely running at more than 100 MV/m peak acceleration field; the laser system of the photoinjector has achieved 2% peak to peak energy stability, 0,5% point stability and better than 2 ps timing jitter. The highest measured quantum efficiency of the Cu cathode is 0.05%. The electron beam bunch length was measured to be 10 ps using a linac RF phase scan. The normalized rms emittance for 0.5 nC charge was measured, to be from 1 to 2 mm rad, which agrees with PARMELA simulations.

Controlled wake field acceleration via laser pulse shaping

We consider the interaction of high-intensity laser pulses with underdense plasmas and address the problem of the excitation of strong and stable wake plasma waves with regular electric fields to provide effective acceleration of charged particles over appreciably long distances. It is known that a relativistically strong laser pulse longer than the wavelength of plasma waves, propagating in a plasma is subject to self-modulation. This may result in a nonstationary behavior of the produced plasma wake field/particle dephasing, and reduced net acceleration. In this paper we present the results of 1(2/2)-D and 2(1/2)-D particle in cell (PIC) simulations which demonstrate that regular wake electric fields may be obtained by a properly shaped laser pulse (sharp steepening of its leading front). These results are relevant to the design of the 100 MeV laser wake field electron acceleration experiment that uses a terawatt picosecond CO/sub 2/ laser and is under construction at the Brookhaven Accelerator Test Facility.

Measurements on photoelectrons from a magnesium cathode in a microwave electron gun

The performance of a magnesium cathode in a one-and-half cell photocathode microwave gun was measured at the Brookhaven Accelerator Test Facility (ATF). The frequency quadrupled Nd:YAG laser (266 nm) was used to stimulate the photoelectron emission. For a normal incident laser pulse, the quantum efficiency of the magnesium at 10 −7 Torr was measured to be 5 × 10 −4, which is more than 20 times the value for copper under similar conditions. After more than 5000 h operation, and despite some optical damage on the cathode surface, no decrease in the quantum efficiency of the magnesium cathode has been observed. The Schottky effect of photoelectron emission was also studied. No electron saturation effect was observed for a train of laser pulses. The photoelectron beam bunch length was measured to be 11 ps FWHM using a microwave kicker cavity, in agreement with a streak camera measurement of the laser pulse length. The measured rms normalized emittance is 2.5 mm mrad for a bunch charge of 80 pC, and rms radius of the laser spot 0.2 mm, in agreement with a simple analytical model predictions.

Picosecond CO 2 laser for relativistic particle acceleration

A table-top 20-GW 50-ps CO 2 laser system is under operation at the Brookhaven Accelerator Test Facility. We compare laser performance with model predictions. Extrapolations suggest the possibility of compact terawatt CO 2 laser systems suitable as laser accelerator drivers and for other strong-field applications. Latest progress on an Inverse Cherenkov Laser Accelerator experiment is reported.

Performance of the Brookhaven photocathode rf gun

Abstract The Brookhaven Accelerator Test Facility (ATF) uses a photocathode rf gun to provide a high-brightness electron beam intended for FEL and laser acceleration experiments. The rf gun consists of 1 1 2 cells driven at 2856 MHz in π-mode with a maximum cathode field of 100 MV/m. To achieve long lifetimes, our photocathode development concentrates on robust metals such as copper, yttrium and samarium. We illuminate these cathodes with a 10 ps, frequency-quadrupled Nd: YAG laser. We describe the initial operation of the gun, including measurements of transverse and longitudinal emittance, quantum efficiencies, and peak current. These results are compared to models.

Status of the visible free-electron laser at the Brookhaven accelerator test facility

The 500 nm free-electron laser (FEL) at the accelerator test facility (ATF) of the Brookhaven National Laboratory is reviewed. We present an overview of the ATF, a high-brightness, 50-MeV, electron accelerator and laser complex which is a user's facility for accelerator and beam physics. A number of laser acceleration and FEL experiments are under construction at the ATF. The visible FEL experiment is based on a novel superferric 8.8 mm period undulator. The electron beam parameters, the undulator, the optical resonator, optical and electron beam diagnostics are discussed. The operational status of the experiment is presented.

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

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.