T. Srinivasan-Rao

Photoemission studies on metals using picosecond ultraviolet laser pulses

We report the quantum efficiency of various metals irradiated by 266‐nm, 4.66‐eV laser pulses of 10‐ps duration. The highest quantum efficiency obtained is 7.25×10−4 with samarium photocathodes. Current densities exceeding 66 and 21 kA/cm2 have been obtained from an area of 0.05 and 7 mm2, respectively. The maximum currents and current densities obtainable in these experiments are limited by the space charge. For surface fields exceeding 5×107 V/m on gold, the efficiency increases linearly with the field for the values investigated. Based on the quantum efficiency and optical damage threshold measurements, current densities exceeding 100 kA/cm2 seem feasible without damaging the photocathode.

Intense electron emission due to picosecond laser‐produced plasmas in high gradient electric fields

Picosecond laser pulses at a wavelength of 266 nm have been focused onto a solid metal cathode in coincidence with high gradient electric fields to produce high brightness electron beams. At power densities exceeding 109 W/cm2, a solid density plasma is formed and intense bursts of electrons are emitted from the target accompanied by macroscopic surface damage. An inferred ∼1 μC of integrated charge with an average current of ∼20 A is emitted from a radio‐frequency cavity driven at electric field gradients of ∼80 MV/m. In another experiment, where a dc extraction field of ∼6 MV/m is used, we observed an electron charge of ∼0.17 μC. Both results are compared with the Schottky effect and the Fowler–Nordheim field emission. We found that this laser‐induced intense electron emission shares many features with the explosive electron emission processes. No selective wavelength dependence is observed in the production of the intense electron emission in the dc extraction field. The integrated electrons give an ap...

Performance of the Brookhaven Photocathode RF Gun

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 {pi}-mode with a maximum cathode field of 100 MV/m. To achieve long lifetimes, the 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. The results are compared to models.

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.

High Current Energy Recovery Linac at BNL

We present the design and parameters of an energy recovery linac (ERL) facility, which is under construction in the Collider-Accelerator Department at BNL. This R&D facility has the goal of demonstrating CW operation of an ERL with an average beam current in the range of 0.1 - 1 ampere and with very high efficiency of energy recovery. The possibility of a future upgrade to a two-pass ERL is also being considered. The heart of the facility is a 5-cell 703.75 MHz super-conducting RF linac with strong Higher Order Mode (HOM) damping. The flexible lattice of the ERL provides a test-bed for exploring issues of transverse and longitudinal instabilities and diagnostics of intense CW electron beams. This ERL is also perfectly suited for a far-IR FEL. We present the status and plans for construction and commissioning of this facility.

PHOTOEMISSION FROM MG IRRADIATED BY SHORT PULSE ULTRAVIOLET AND VISIBLE LASERS

Quantum efficiency of diamond turned magnesium was measured with low intensity, 266 nm, 12 ps, p‐polarized radiation for various incident angles. The optimum quantum efficiency of 3×10−4 was obtained with p‐polarized light incident close to Brewster’s angle where the absorption is maximum. Investigations of the field dependence of the yield indicate that application of a high surface field is expected to lower the surface barrier by Schottky effect and increase the quantum efficiency further. Measurements of the electron yield with 632 nm radiation of 300 fs pulse duration indicate that at intensities exceeding 20 MW/cm2, the electric field due to the laser may become important and start contributing to the electron emission. Autocorrelation measurements at low intensities with this wavelength indicate that the electron emission is prompt.

Design of a high-brightness, high-duty factor photocathode electron gun

Abstract The proposed UV-FEL users facility at Brookhaven National Laboratory will require a phatocathode gun capable of producing short ( ps ) bunches of electrons at high repetition rates (5 kHz), low energy spread ( ), a peak current of 300 A (after compression) and a total bunch charge of up to 2 nC. At the highest charge, the normalized transverse emittance should be less than 7π mm mrad. We are presently designing a gun that is expected to exceed these requirements. This gun will consist of 3 1 2 cells, constructed of GlidCop-15, an aluminum oxide dispersion strengthened copper alloy. The gun will be capable of operating at duty factors in excess of 1%. Extensive beam dynamics studies of the gun were used to determine the effect of varying the length of the first cell, shaping the apertures between cells, and increasing the number of cells. In addition, a detailed thermal and mechanical study of the gun was performed to ensure that the thermal stresses were well within the allowable limits and that copper erosion of the water channels would not occur.

Design, construction and status of all niobium superconducting photoinjector at BNL

We present here the design and construction of an all niobium superconducting RF injector to generate high average current, high brightness electron beam. A 1/2 cell superconducting cavity has been designed, built, and tested. A cryostat has been built to cool the cavity to /spl sim/2 K. The RF system can deliver up to 500 W at 1.3 GHz to the cavity. A mode-locked Nd:YVO/sub 4/ laser, operating at 266 nm with 0.15 W average power, phase locked to the RF, will irradiate a laser cleaned Nb surface at the back wall of the cavity. Description of critical components and their status are presented in the paper. Based on DC measurements, QE of up to 10/sup -4/ can be expected from such cavity.

Surface-plasmon-enhanced multiphoton photoelectric emission from thin silver films

We report the enhancement of multiphoton photoelectric emission, from a thin silver film, by surface plasmons excited at the single-metal-vacuum interface. The surface plasmons are resonantly pumped by femtosecond laser pulses in the Kretchmann attenuated-total-internal-reflection configuration. A quantum yield approaching 2 x 10(-8) at a photon fluence of 10 MW/cm (2) from a two-photon process was observed, which is 5 x 10(3) higher than that of the bulk emission measured for the same film at the same power fluence.

Electro-optical measurements of picosecond bunch length of a 45 MeV electron beam

We have measured the temporal duration of 45 MeV picosecond electron beam bunches using a noninvasive electro-optical (EO) technique. The amplitude of the EO modulation was found to increase linearly with electron beam charge and decrease inversely with distance from the electron beam. The rise time of the temporal signal was limited by our detection system to ∼70 ps. The EO signal due to ionization caused by the electrons traversing the EO crystal was also observed. It has a distinctively long decay time constant and signal polarity opposite to that due to the field induced by the electron beam. The electro-optical technique may be ideal for the measurement of bunch length of femtosecond, relativistic, high energy, charged, particle beams.