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Accelerating structure with linear excitation

Description: The switched power linac (SPL) structures require a ring-shaped laser beam pulse of uniform intensity to avoid transverse field components of the accelerating field at the center. In order to also utilize the reflection of the outgoing EM wave, the switching element has to be very close to the outer edge of the structure to ensure nearly synchronous superposition at the beam hole with the original inward going wave. It is sometimes easier to produce linear (flat) laser beams, e.g., from powerful excimer lasers which have beams of rectangular cross section. Such flat beams could be used to excite linear photocathode switches or be used to produce flat electron beam pulses in electron sources. In this paper, an accelerator structure is proposed which may be considered a variant of the SPL disk structure, but could be used with linear beams. The structure utilizes a double parabolic horn. 8 refs., 9 figs.
Date: March 1, 1988
Creator: Fischer, J. & Srinivasan-Rao, T.
Partner: UNT Libraries Government Documents Department

Short-pulse high-current-density photoemission in high electric fields

Description: We present the experimental results of photoemission studies on thin wires of gold-coated tungsten, held at surface fields in the range of 10/sup 6/ to 3 x 10/sup 8/ V/m, and illuminated by 10 ps long, 4.66 eV photon laser pulses. The wire cathodes arranged coaxially in an anode experienced a surface-field enhancement of 10/sup 2/ to 10/sup 3/ over the applied voltage. We obtained current densities exceeding 10 kA/cm/sup 2/ from a 50 ..mu..m diameter wire, from a (50 x 400) ..mu..m/sup 2/ area, under partially space-change-limited conditions. The quantum efficiency for emission-limited cases was in the range of 10/sup -5/. For these cases results using 50 ..mu..m and 4 ..mu..m diameter wires indicated linear dependence of charge density with optical energy density. The emission also scaled linearly with the emitting area. For surface fields above 3 x 10/sup 7/ V/m, a twofold enhancement of emission was observed for a tenfold increase in the field. 7 refs., 9 figs.
Date: January 1, 1987
Creator: Fischer, J. & Srinivasan-Rao, T.
Partner: UNT Libraries Government Documents Department

UV photoemission studies of metal photocathodes for particle accelerators

Description: Photoemission from several metals was studied with 10 ps laser pulses at 266 nm. The yield was linear with energy and with area. Quantum efficiencies (/eta/) were determined (up to 10/sup /minus/3/ e/photons for samarium), and found to vary as (h..nu..-/phi/)/sup 2/. /eta/ also increased with the field. The field assisted efficiencies were calculated for some metals and confirmed by experiment for gold, up to surface fields of /approximately/3/times/10/sup 8/ V/m. High charge and current densities, close to 10/sup 5/ A/cm/sup 2/ from macroscopic areas, were measured or indicated. Results are then related to applications in accelerators. 18 refs., 15 figs., 4 tabs.
Date: September 1, 1988
Creator: Fischer, J. & Srinivasan-Rao, T.
Partner: UNT Libraries Government Documents Department

UV photoemission from metal cathodes for picosecond power switches

Description: Results are reported of photoemission studies using laser pulses of 10 ps duration and 4.66 eV photon energy on metal cathodes. These included thin wires, flat surfaces and an yttrium cathode with a grainy surface. The measurements of current density and quantum efficiency under low and high surface fields indicate that field assisted efficiencies exceeding 0.1% and current densities exceeding 10/sup 5/ A/cm/sup 2/ are obtainable. The results are compared to the requirements of switch power applications. 24 refs., 13 figs., 1 tab.
Date: January 1, 1989
Creator: Fischer, J.; Srinivasan-RAo, T. & Tsang, T.
Partner: UNT Libraries Government Documents Department

K2CsSb Cathode Development

Description: K{sub 2}CsSb is an attractive photocathode for high current applications. With a quantum efficiency of >4% at 532nm and >10% at 355nm, it is the only cathode to have demonstrated an average current of 35mA in an accelerator environment We describe ongoing cathode development work. for the energy recovery linac being constructed at BNL Several cathodes have been created on both copper and stainless steel substrates, and their spatial uniformity and spectral response have been characterized. Preliminary lifetime measurements have been performed at high average current densities (>1 mA/mm{sup 2}).
Date: October 1, 2008
Creator: Smedley,J.; Rao, T. & Wang, E.
Partner: UNT Libraries Government Documents Department

A 10-Hz Terawatt Class Ti:Sapphire Laser System: Development and Applications

Description: We developed a two stage Ti:Sapphire laser system to generate 16 mJ/80fs laser pulses at the pulse repetition rate of 10 Hz. The key deriver for the present design is implementing a highly efficient symmetric confocal pre-amplifier and employing a simple, inexpensive synchronization scheme relying only on a commercial digital delay-generator. We characterized the amplified pulses in spatial-, spectral-, and temporal-domains. The laser system was used to investigate various nonlinear optical processes, and to modify the optical properties of metal- and semiconductor-surfaces. We are currently building a third amplifier to boost the laser power to the multi-terawatt range.
Date: January 12, 2010
Creator: Sharma, A. K.; Smedley, J.; Tsang, T. & Rao, T.
Partner: UNT Libraries Government Documents Department

A high stability Nd:YAG photocathode drive laser

Description: A versatile, highly stable, reliable Nd:YAG laser is in operation at Brookhaven Accelerator Test Facility (ATF) for illuminating a metal photocathode RF electron gun. This system addresses stringent requirements on pulse duration, pulse timing jitter, pulse energy, spatial profile, and pointing imposed by electron accelerator experiments. This paper describes the oscillator operation, (which also includes control of a semicconductor switch for ATF`s high power CO{sub 2} laser), beam transport to the cathode, etc.
Date: December 31, 1995
Creator: Babzien, M.; Pogorelsky, I.; Srinivasan-Rao, T. & Fisher, A.
Partner: UNT Libraries Government Documents Department

Table top, pulsed, relativistic electron gun with GV/m gradient

Description: We present the design and performance characteristics of a compact high voltage pulser with 150 ps rise time, 0.2 to 2 ns adjustable flat top and up to 1 MV amplitude on a 80 Ohm load or up to 0.5 W on a 20 Ohm load, at 1 Hz repetition rate. Combination of a laser triggered SF{sub 6} and a liquid cap is used to form the fast rising pulse and maintain a low jitter between the laser, external trigger, and the high voltage pulsed output. The dark current and breakdown studies with this pulse applied between the electrodes of a diode indicate that fields up to 1 GV/m could be supported by stainless steel and copper cathodes without breaking down. The dark current from a conditioned cathode in a background pressure of 10{sup -7} Torr is below the detection limit of 0.5 mA of our system. Photoemission studies had been conducted with 300 kV applied between copper cathode and stainless steel anode separated by 2 mm. KrF laser of 5 eV photon energy and 20 ns FWHM was used to irradiate the cathode. In these preliminary measurements, 3 nC charge and corresponding quantum efficiency of 3.5 x 10{sup -4} have been obtained. Future plans include increasing the gradient to GV/m range, decreasing the laser pulse duration to ps and subps range and increasing the electron energy to a few MeV.
Date: October 1, 1996
Creator: Srinivasan-Rao, T. & Smedley, J.
Partner: UNT Libraries Government Documents Department

Relativistic electron beams from cathodes at 1 GV/m gradient

Description: In the past decade, there has been extensive research in the development of low emittance, high brightness electron injectors for linear collider and free electron laser applications. RF injectors with a few nC charge in a few ps, with an emittance of {approximately}1--5 {pi}mm mrad are operational in a number of facilities. In these devices, a laser beam irradiates a photocathode embedded in an RF cavity. The photoelectrons released by the laser are immediately accelerated to relativistic velocities, thereby reducing the space charge effects. The frequency of the RF and the design of the cavity are chosen to minimize the RF and space charge effects on the electron bunch so that low emittance, high brightness electron beam could be generated. Minimization of RF effects on emittnce growth require a low RF frequency while minimizing the space charge effects require high field and hence high RF frequency. The design is hence a compromise between these two conflicting requirements. Some of these limitations could be overcome by using a large pulsed electric field at the cathode rather than a RF field. The duration of the pulsed field should be chosen so that it is longer than the electron bunch length and the transit time in the accelerating region, but short enough to avoid breakdown problems. Development of a high brightness electron source using this scheme requires a pulse generator, a laser pulse of sufficient energy to trigger and synchronize the electrical pulses, and a short laser pulse to irradiate the photocathode and generate electron pulses to be accelerated. The designs of these components are described.
Date: May 1, 1998
Creator: Srinivasan-Rao, T.; Smedley, J. & Schill, J.
Partner: UNT Libraries Government Documents Department

Experimental results of the ATF in-line injection system

Description: The initial experimental results of the Brookhaven accelerator test facility (ATF) in-line injector is presented. The ATF in-line injector employed a full copper RF gun with a pair of solenoid magnets for emittance compensation. The maximum acceleration field of the RF gun was measured to be 130 MV/m. The electron yield from the copper cathode was maximized using p- polarized laser and the Schottky effect. The quantum efficiency under optimum conditions was measured to be 0.04%. The measured electron bunch length was less than 11 ps, which agreed with the laser pulse length measurement using a streak camera. The normalized rms. emittance for 0.25 nC charge is 0.9 {plus_minus} 0.1 mm-mrad, which is almost four times smaller than the emittance predicted by the space-charge effect for a non-emittance compensation photocathode RF gun. The normalized rms for 0.6 nC charge was measured range from 1 to 3 mm-mrad. This measurement was first experimental demonstration of emittance compensation in a high-gradient, S-band photocathode RF gun.
Date: May 1, 1995
Creator: Wang, X.J.; Srinivasan-Rao, T. & Batchelor, K.
Partner: UNT Libraries Government Documents Department

Short-term bioassay of complex organic mixtures. Part II. Mutagenicity testing

Description: The feasibility of using short-term mutagenicity assays to predict the potential biohazard of various crude and complex test materials has been examined in a coupled chemical and biological approach. The principal focus of the research has involved the preliminary chemical characterizatiion and preparation for bioassay, followed by testing in the Salmonella histidine reversion assay system. The mutagenicity tests are intended to act as predictors of profound long-range health effects such as mutagenesis and/or carcinogenesis; act as a mechanism to rapidly isolate and identify a hazardous agent in a complex mixture; and function as a measure of biological activity correlating baseline data with changes in process conditions. Since complex mixtures can be fractionated and approached in these short-term assays, information reflecting on the actual compounds responsible for the biological effect may be accumulated.
Date: January 1, 1978
Creator: Epler, J.L.; Clark, B.R.; Ho, C.; Guerin, M.R. & Rao, T.K.
Partner: UNT Libraries Government Documents Department

Laser working group

Description: The laser working group considered options to deliver synchronized laser pulses of the required energy to the photocathode and laser triggered switches. These requirements actually decreased during the course of the workshop, and the values finally settled upon (<10 J in 100 fs at /approximately/250 nm for the photocathode and /approximately/10 mJ in 2 ps near either 250 nm or 1 m for the switches) were considered to be well within the state of the art. Some development work may be required, however, to provide a system that has the desirable characteristics of stability, ease of use and low maintenance. 9 refs., 1 fig.
Date: January 1, 1988
Creator: Bigio, I.J.; Kurnit, N.A.; Donaldson, W.R.; Geissler, K. & Srinivasan-Rao, T.
Partner: UNT Libraries Government Documents Department

High current photoemission with 10 picosecond uv pulses

Description: The quantum efficiency and the optical damage threshold of various metals were explored with 10 ps, 266 nm, UV laser pulses. Efficiencies for Cu, Y, and Sm were: 1.4, 5, and 7 {times} 10{sup {minus}4}, with damage thresholds about 100, 10, and 30 mJ/cm{sup 2}. This would permit over 1 {mu}C/cm{sup 2} or current densities exceeding 100 kA/cm{sup 2}. High charge and current densities of up to 66 kA/cm{sup 2} were obtained on 0.25 mm diam cathodes, and 21 kA/cm{sup 2} on a 3 mm diam yttrium cathode. The maximum currents were limited by space charge and the dc field. The experiments with small area illumination indicate that the emitted electrons spread transversely due to Coulomb repulsion and their initial transverse velocity. This increases the effective area above the cathode, reduces the space charge effect and increases emission density on the cathode. The quantum efficiency can be increased substantially by enhancing the field on the surface by either a suitable electrode geometry or microstructures on it. 14 refs., 12 figs., 3 tabs.
Date: June 1, 1990
Creator: Fischer, J.; Srinivasan-Rao, T. & Tsang, T.
Partner: UNT Libraries Government Documents Department

Separation and identification of mutagenic constituents of petroleum substitutes. [In coal- and shale-derived oils]

Description: A study combining chemical separations, mutagenicity testing, and spectroscopic identifications is underway to isolate and identify mutagens in coal- and shale-derived oils. Ether-aqueous partition combined with Sephadex LH-20 chromatography of the resulting neutral fraction is introduced as a preferred class fractionation procedure. The uniquely important role of polycyclic aromatic primary amines in the mutagenicity of petroleum substitutes is reviewed. Questions are raised concerning the role of polycyclic aromatic hydrocarbons in the mutagenicity of the neutral fraction of petroleum substitutes.
Date: January 1, 1980
Creator: Guerin, M.R.; Ho, C.H.; Rao, T.K.; Clark, B.R. & Epler, J.L.
Partner: UNT Libraries Government Documents Department

Photoemission using femtosecond laser pulses

Description: Successful operation of short wavelength FEL requires an electron bunch of current >100 A and normalized emittance < 1 mm-mrad. Recent experiments show that RF guns with photocathodes as the electron source may be the ideal candidate for achieving these parameters. To reduce the emittance growth due to space charge and RF dynamics effects, the gun may have to operate at high field gradient (hence at high RF frequency) and a spot size small compared to the aperture. This may necessitate the laser pulse duration to be in the subpicosecond regime to reduce the energy spread. We will present the behavior of metal photocathodes upon irradiation with femtosecond laser beams, comparison of linear and nonlinear photoemission, and scalability to high currents. Theoretical estimate of the intrinsic emittance at the photocathode in the presence of the anomalous heating of the electrons, and the tolerance on the surface roughness of the cathode material will be discussed.
Date: October 1, 1991
Creator: Srinivasan-Rao, T.; Tsang, T. & Fischer, J.
Partner: UNT Libraries Government Documents Department

Laser field assisted photoemission using femtosecond laser pulses

Description: Photoemission from Cu mirror at a laser fluence of 10[sup 11] W/cm[sup 2], 300 fs, pulse is investigated for various angles of incidence, intensities and polarizations. Electron emission is enhanced by [approximately] 20 from s to p polarization and by 4 on changing the angle of incidence from 0 to 73 degree.
Date: November 1, 1992
Creator: Srinivasan-Rao, T.; Fischer, J. & Tsang, T.
Partner: UNT Libraries Government Documents Department

Photoemission from Ag, Cu, and CsI

Description: Photoemission characteristics of three different cathodes, CsI, Ag film and Cu were investigated. CsI, upon irradiation by 213 nm, 10ps laser pulse yields a quantum efficiency of 4% at O.2[mu]J input energy. The saturation mechanism observed at higher input energies require further investigation. Ag film, upon irradiation by 630 nm, 300 fs laser emit prompt photoelectrons after absorbing 2 photons. There was no evidence of optical damage of the film up to 10[sup 11] W/cm[sup 2]. At low intensities, photoemission from Cu is a simple [nu]-e[sup [minus]] interaction, the nonlinearity of the process depending strongly on trace impurities. At higher intensities, there appears to be a change in the emission mechanism.
Date: June 1, 1992
Creator: Srinivasan-Rao, T.; Fischer, J. & Tsang, T.
Partner: UNT Libraries Government Documents Department

Multi-Alkali Photocathode Development at Brookhaven National Lab for Application in Superconducting Photoinjectors

Description: The development of a suitable photocathode for use in a high average current photoinjector at temperatures ranging from 273 K down to 2 K is a subject of considerable interest, and active research. The choice of photocathode material is often a trade-off made based on the quantum efficiency of the cathode material, the tolerance to adverse vacuum conditions, and the laser wavelength needed to produce photoelectrons. In this paper an overview of the BNL work to date on CsK{sub 2}Sb photocathodes on a variety of substrates, irradiated at multiple wavelengths, and at temperatures down to 170 K will be discussed. The application of this photocathode material into a SRF photoinjector will also be discussed.
Date: February 15, 2006
Creator: Burrill, A.; Ben-Zvi, I.; Pate, D.; Rao, T.; Segalov, Z.; /Brookhaven et al.
Partner: UNT Libraries Government Documents Department


Description: There has been a lot of interest in compact sources of high brightness, relativistic electron beams. One approach for developing such a source is to apply a high gradient that remains constant during the generation and acceleration of the electron beam. In this paper, we describe high voltage pulse generators that deliver up to 5 MV with 1 ns pulse duration. These devices are synchronizable to an external trigger with jitter of {approx}0.5 ns and can establish gradients in excess of 1 GV/m between two electrodes without breakdown. In the presence of field gradients up to 0.5 GV/m, electron beams of bunch lengths ranging from 1 ns to 0.3 ps and diameter &lt; 300 {micro}m have been generated by irradiating the cathode with UV lasers. Characteristics of these electron beams as well as those produced via field emission at gradients up to 1 GV/m will be discussed.
Date: November 12, 2002
Partner: UNT Libraries Government Documents Department

A novel, high gradient, laser modulated, pulsed electron gun

Description: This paper describes a high current, fast pulsed, laser excited, electron gun to operate at energies between 1 and 5 MeV. The authors present the design of the high voltage pulse generator, and the laser system, the experimental results obtained with copper cathode in fields > 1 GV/m and diode geometry optimized for the highest brightness using computer simulations. This electron source will generate an electron beam of brightness approaching 10{sup 16} A/m{sup 2} rad{sup 2}, which is 2 orders of magnitude greater than the present level of 10{sup 14} A/m{sup 2} rad{sup 2}, a parameter highly sought after for future linear colliders and short wavelength FELs. It will also be used to study properties of materials in the presence of high fields such as dark current emission and high voltage breakdown characteristics that will provide information critical to the development of high frequency accelerating structures. In addition, Bremsstrahlung radiation from these ultra short relativistic electrons, is expected to be an efficient source of x-ray photons for imaging transient effects in biological samples, microlithography and micromachining. These excellent beam qualities will be augmented for the first time by the simplicity and compactness of the device resulting in an efficient, affordable product with superior performance and unique capabilities.
Date: September 1, 1998
Creator: Batchelor, K.; Dudnikov, V.; Farrell, J.P.; Srinivasan-Rao, T. & Smedley, J.
Partner: UNT Libraries Government Documents Department

A high-power picosecond Nd:YAG/CO{sub 2} laser system for electron guns, laser acceleration and FEL

Description: Fourth-harmonic Nd:YAG pulses illuminating a microwave linear accelerator`s photoinjector generates electron bunches in trains for FEL experiments, or in a single pulse for laser acceleration. A multi-gigawatt CO{sub 2} laser switched by the ND:YAG fundamental delivers 50-ps pulses for Inverse Cherenkov, Inverse FEL, or Grating Linac electron acceleration experiments.
Date: December 31, 1994
Creator: Babzien, M.; Ben-Zvi, I.; Fisher, A.S.; Kusche, K.; Pogorelsky, I.V. & Srinivasan-Rao, T.
Partner: UNT Libraries Government Documents Department

Optimization of gun parameters for a pulsed power electron gun

Description: Extensive simulation work has been done to identify the optimal parameters for a pulsed power electron gun. PBGUNS, an electrostatic beam optics code, was used to optimize the electrode shape and the beam spatial distribution, including modeling the focusing effect of a curved cathode surface. MAFIA, a particle-in-a-cell code, was used to investigate those aspects that required time dependence, such as longitudinal energy spread. The range of agreement between the two codes was also investigated. The transverse phase space at a comparison plane was found to be very close (within 1% at low currents and 4% for higher currents), even for bunch lengths shorter than the gap transit time.
Date: July 1998
Creator: Srvinivasan-Rao, T.; Smedley, J.; Batchelor, K.; Farrell, J. P. & Dudnikova, G.
Partner: UNT Libraries Government Documents Department

Dark current measurements at field gradients above 1 GV/m

Description: In this paper, the authors report the results of dark current studies on copper cathodes and stainless steel anodes held at a field gradient > 1 GV/m. The field emission current is , 1 A for fields less than 1 GV/m. As the field is increased, the dark current increases rapidly to 150 A for applied fields of {approximately} 1.7 GV/m. Fowler-Nordheim plots in this range of applied fields indicate a field enhancement factor of 10--20 for a copper cathode with a work function of 4.6 eV.
Date: July 1998
Creator: Srinivasan-Rao, T.; Smedley, J.; Schill, J.; Batchelor, K. & Farrell, J. P.
Partner: UNT Libraries Government Documents Department