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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

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

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

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

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

Comparison of electrostatic and time dependent simulation codes for modeling a pulsed power gun

Description: This paper is a result of a group of simulations used to determine the optimal parameters for a pulsed power electron gun. As electrostatic codes such as PBGUNS tend to be cheaper, easier to use, and have less stringent computational requirements than time dependent codes such as MAFIA, it was desirable to determine those regimes in which the electrostatic codes agree with time dependent models. It was also necessary to identify those problems that required time dependence, such as longitudinal variation in an electron bunch. PBGUNS was then used to perform the bulk of the optimization, with only those issues that required time dependence being resolved with MAFIA. Good agreement in transverse phase space values was found between the electrostatic code (PBGUNS) and the time dependent code (MAFIA) for a variety of pulse durations, even for pulse durations short compared to the electron transit time of the accelerating region. To obtain values for the longitudinal energy spread and the variation of the transverse phase space across the bunch, it was necessary to use MAFIA. The electrostatic codes have an advantage in terms of required computational resources and run time, and are therefore a good choice for modeling jobs in which the longitudinal energy spread is unimportant.
Date: June 1, 1998
Creator: Srinivasan-Rao, T.; Smedley, J.; Batchelor, K.; Farrell, J.P. & Dudnikova, G.
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

PERFORMANCE DATA OF A PULSED POWER PHOTO INJECTOR.

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 < 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
Creator: SMEDLEY,J.; SRINIVASAN - RAO,T.; TSANG,T.; FARRELL,J.P. & BATCHELOR,K.
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

EMITTANCE MEASUREMENTS WITH A PULSED POWER PHOTO INJECTOR.

Description: This paper describes measurements of beam spot size and emittance of electron beams from a pulsed power photo-injector operating at 150keV output energy. In these measurements, electron bunches with charge up to 20 pC were created by a 300 fs pulse duration Ti: Sapphire laser system illuminating a polished copper cathode. Images of the electron beam were captured at two locations downstream from a solenoid focusing magnet. The focal spot size was studied as a function of bunch charge and accelerating gradient. Beam waists down to 85 microns were obtained. The focal spot size was found to be dominated by spherical aberration at low beam charges, however the beam trajectory is in good agreement with simulation.
Date: May 12, 2003
Creator: SMEDLEY,J.; SRINIVASAN RAO,T.; TSANG,T.; FARRELL,J.P. & BATCHELOR,K.
Partner: UNT Libraries Government Documents Department

A high current, high gradient, laser excited, pulsed electron gun

Description: This paper describes a pulsed electron gun that can be used as an FEL, as an injector for electron linear accelerators or for rf power generation. It comprises a 1 to 5 MeV, 1 to 2 ns pulsed power supply feeding a single diode, photoexcited acceleration gap. Beam quality of a {approximately}1nC charge in {approximately}1 GV/m field was studied. Computations of the beam parameters as a function of electrode configuration and peak electron current are presented together with descriptions of the power supply, laser and beam diagnostics systems.
Date: June 1, 1998
Creator: Batchelor, K.; Farrell, J.P.; Dudnikova, G.; Ben-Zvi, I.; Srinivasan-Rao, T.; Smedley, J. et al.
Partner: UNT Libraries Government Documents Department

A SUB - PICOSECOND PULSED 5 MEV ELECTRON BEAM SYSTEM.

Description: Laser excited pulsed, electron beam systems that operate at energies from 1 MeV up to 5 MeV and pulse width from 0.1 to 100 ps are described. The systems consist of a high voltage pulser and a coaxial laser triggered gas or liquid spark gap. The spark gap discharges into a pulse forming line designed to produce and maintain a flat voltage pulse for 1 ns duration on the cathode of a photodiode. A synchronized laser is used to illuminate the photocathode with a laser pulse to produce an electron beam with very high brightness, short duration and current at or near the space charge limit. Operation of the system is described and preliminary test measurements of voltages, synchronization and jitter are presented for a 5 MeV system. Applications in chemistry, and accelerator research are briefly discussed.
Date: November 1, 2000
Creator: FARRELL,J.P.; BATCHELOR,K.; MESHKOVSKY,I.; PAVLISHIN,I.; LEKOMSTEV,V.; DYUBLOV,A. et al.
Partner: UNT Libraries Government Documents Department

Performance of magnesium cathode in the S-band RF gun

Description: In this paper, the authors present the preliminary results of the performance of magnesium cathode in a high frequency RF gun. The quantum efficiency of Mg showed a dramatic improvement upon laser cleaning, increasing from 10{sup {minus}5} to 4 {times} 10{sup {minus}4} after two hours of cleaning, and to 2 {times} 10{sup {minus}3} after systematic cleaning. The cleaning procedure for this increase is described in detail. Charge measured as a function of the laser injection phase relative to the RF phase indicates that the temporal variation of the field on the cathode both due to the RF and the shielding effect of the emitted electrons play a critical role in the emission and extraction of electrons. A model that includes this variation is numerically fitted to the measured charge and the results are presented. The unexpected outcome of the fit was the low field enhancement factor (0.1) predicted by the model for the photoemission. The physical origin of this is still under investigation.
Date: May 1, 1997
Creator: Srinivasan-Rao, T.; Ben-Zvi, I.; Smedley, J.; Wang, X.J.; Woodle, M.; Palmer, D.T. et al.
Partner: UNT Libraries Government Documents Department

Experimental characterization of high-brightness electron photoinjector

Description: 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 a 0.5 nC charge was measured, to be from 1 to 2 mm-mrad, which agrees with PARMELA simulations.
Date: October 9, 1995
Creator: Wang, X.J.; Babzien, M.; Batchelor, K.; Ben-Zvi, I.; Pogorelsky, R.M.I.; Qui, X. et al.
Partner: UNT Libraries Government Documents Department

High-Yield Metal Materials for Photocathode RF Gun

Description: Significant progress has been made in both improving the quantum efficiency and understanding the physics process of photoemission of metal photocathode materials under high electric field. Metal material, copper and magnesium were installed on the photocathode RF gun, and experimentally investigated using frequency quadrupled Nd:Yag laser (266 nm). A systemic procedure was developed for preparing cathode; this involves diamond polishing, ultrasonic cleaning and high temperature vacuum bake out. Using laser cleaning and explosive electron emission cleaning, the highest quantum efficiency measured for copper and magnesium are 0.02% and 0.3%, respectively. The Schottky effect was experimental investigated, the quantum efficiency of the Cu is dominated by the Schottky effect since the work function of the copper is almost equal to the photon energy used (4.66 eV). The quantum efficiency of the magnesium is significant enhanced by the Schottky effect.
Date: November 1997
Creator: Wang, X. J.; Ben-Zvi, I.; Smedley, J.; Srinivasan-Rao, T.; Woodle, M.; Palmer, D. T. et al.
Partner: UNT Libraries Government Documents Department

ELECTRON BUNCH CHARACTERIZATION WITH SUBPICOSECOND RESOLUTION USING ELECTRO-OPTIC TECHNIQUE.

Description: In the past decade, the bunch lengths of electrons in accelerators have decreased dramatically and are in the range off a few millimeters. Measurement of the length as well as the longitudinal profile of these short bunches have been a topic of research in a number of institutions. One of the techniques uses the electric field induced by the passage of electrons in the vicinity of a birefringent crystal to change its optical characteristics. Well-established electro-optic techniques can then be used to measure the temporal characteristics of the electron bunch. The inherent fast response of the crystal facilitates the measurement to femtosecond time resolution. However, the resolution in experiments so far has been limited to 70 ps, by the bandwidth of the detection equipment. Use of a streak camera can improve this resolution to a few picoseconds. In this paper we present a novel, non-invasive, single-shot approach to improve the resolution to tens of femtoseconds so that sub mm bunch length can be measured.
Date: June 11, 2001
Creator: SEMERTZIDIS, Y.K.; CASTILLO, V.; LARSEN, R.; LAZARUS, D.M.; NIKAS, D.; OZBEN, C. et al.
Partner: UNT Libraries Government Documents Department

PHOTOINJECTED ENERGY RECOVERY LINAC UPGRADE FOR THE NATIONAL SYNCHROTRON LIGHT SOURCE.

Description: We describe a major paradigm shift in the approach to the production of synchrotron radiation This change will considerably improve the scientific capabilities of synchrotron light sources. We introduce plans for an upgrade of the National Synchrotron Light Source (NSLS). This upgrade will be based on the Photoinjected Energy Recovering Linac (PERL). This machine emerges from the union of two technologies, the laser-photocathode RF gun (photoinjector) and superconducting linear accelerators with beam energy recovery (Energy Recovering Linac). The upgrade will bring the NSLS users many new insertion device beam lines, brightness greater than 3rd generation lightsource's and ultra-short pulse capabilities, not possible with storage ring light sources.
Date: June 18, 2001
Creator: BEN-ZVI,I.; BABZIEN,M.; BLUM,E.; CASEY,W.; CHANG,X.; GRAVES,W. et al.
Partner: UNT Libraries Government Documents Department